INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES

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1 C ATA L O G 2 13

2 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Contents INTRODUCTION About LAYERTEC 2 Precision optics 3 Sputtering 4 Thermal and e-beam evaporation 5 Measurement tools for precision optics 6 Measurement tools for coating 8 PRECISION OPTICS How to specify substrates Standard quality substrates Aspheres, off axis and free form optics Special optical components Substrate materials for UV, VIS and NIR/ IR optics Transmission curves measurement tools for precision optics OPTICAL COATINGS Optical interference coatings Metallic coatings Metal-dielectric coatings Measurement tools for coatings SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Components for F 2 lasers Components for ArF lasers Components for KrF, XeCl and XeF lasers Components for ruby and alexandrite lasers Components for Ti : Sapphire lasers in the ns regime Components for diode lasers Components for Yb :YAG, Yb : KGW and Yb - doped fiber lasers Components for Nd :YAG/Nd:YVO 4 lasers Components for the second harmonic of Nd:YAG, Yb:YAG lasers Components for the third harmonic of Nd:YAG, Yb:YAG lasers Components for the higher harmonics of Nd:YAG, Yb:YAG lasers Components for weak Nd :YAG/Nd:YVO 4 laser lines Components for Ho:YAG and Tm:YAG lasers Components for Er :YAG lasers and the 3µm region

3 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS FEMTOSECOND LASER OPTICS Introduction to Femtosecond Laser Optics Standard femtosecond laser optics Broadband femtosecond laser optics OCTAVE SPANNING FEMTOSECOND LASER OPTICS Silver mirrors for femtosecond lasers High power femtosecond laser optics Components for the second harmonic of the Ti :Sapphire laser Components for the third harmonic of the Ti :Sapphire laser Components for the higher harmonics of the Ti :Sapphire laser Gires -Tournois- Interferometer (GTI) mirrors Optics for femtosecond lasers in the 11 16nm wavelength range SELECTED SPECIAL COMPONENTS Components for optical parametric oscillators (OPO) Broadband and scanning mirrors Filters for laser applications Thin film polarizers Low loss optical components Coatings on crystal optics METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS Front surface silver mirrors Front surface aluminum mirrors Special metallic coatings Cleaning of Optical surfaces 126 REGISTER 128

4 2 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES ABOUT LAYERTEC LAYERTEC, established in 199 as a spin off from the Friedrich-Schiller-Universität Jena, produces high quality optical components for laser applications in the wavelength range from the VUV (157 nm) to the NIR (~4µm). Since the beginning in the early 199 s LAYERTEC has worked for universities and research institutes worldwide and many important progresses in laser technology of the past years have been supported by LAYERTEC products. Today, a staff of more than 125 employees are working in the precision optics facility and coating laboratories of LAYERTEC. More than 3 coating machines are available to cover the wavelength range from the VUV to the NIR using sputtered and evaporated coatings made of fluorides and oxides, metallic and metal-dielectric coatings. LAYERTEC offers the full spectrum of design and manufacturing for a high flexibility to customize optical components for special applications with This catalog gives an overview about our production program and shows some highlights which represent innovative solutions of outstanding quality and which are intended to point out the capabilities of LAYERTEC for further developments. Please do not hesitate to contact LAYERTEC for an offer or for a discussion of your special problem even if your type of laser or your special field of interest is not explicitly mentioned in this catalog. Our company combines a precision optics facility and a variety of coating techniques (magnetron and ion beam sputtering, thermal evaporation, ion assisted e-beam evaporation) which enables LAYERTEC to control the quality of the optical components over the whole production process from grinding, polishing and cleaning of the substrates to the final coating process. an optimum of coating performance and cost efficiency. The variety in size and technology of our coating equipment allows a high-volume fabrication for serial products as well as a flexible prototype manufacturing for R&D groups in the industry and for research institutes.

5 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 3 PRECISION OPTICS The precision optics facility of LAYERTEC produces mirror substrates, etalons, retarders, lenses and prisms of fused silica, optical glasses like BK7 and SF1 and some crystalline materials, e.g. calcium fluoride. The polishing of fused silica and YAG has been optimized over the recent years. We are able to offer fused silica substrates with a surface rms roughness of.12 nm. High quality substrates for laser mirrors are characterized by Geometry and shape (diameter, thickness, wedge and radius of curvature) Surface roughness Surface form tolerance and Surface defects LAYERTEC offers substrates which are optimized for all of these parameters. The specifications of premium quality fused silica substrates with diameters up to 5mm are: Surface rms-roughness lower than.12 nm Surface form tolerance of λ/3 (633nm) Defect density as low as 5/1 x.1 (ISO 111) These parameters are not limited to a standard geometry but can also be achieved on substrates with uncommon sizes, shapes or radii of curvature. LAYERTEC substrates meet the demands for the production of components for Cavity Ring-Down Spectroscopy and EUV mirrors. LAYERTEC produces precision optics in a wide range of sizes. Typical diameters for the laser optics are between 6.35mm and 1mm, but sizes down to 2mm for a serial production of smallest laser devices as well as diameters up to 5mm for uncommon projects of high energy lasers or astronomical telescopes are possible.

6 4 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Sputtering Principle Magnetron Sputtering Ion Beam sputtering (IBS) In general, the term "sputtering" stands for the extraction of particles (atoms, ions or molecules) from a solid by ion bombardment. Ions are accelerated towards a target and collide with the target atoms. The original ions as well as recoiled particles move through the material, collide with other atoms and so on. Most of the ions and recoiled atoms remain within the material, but a certain fraction of the recoiled atoms is scattered towards the surface by this multiple collision process. These particles leave the target and may then move to the substrates and build up a thin film. The above mentioned ions are delivered by a gas discharge which burns in front of the target. It may be excited either by a direct voltage (DC-sputtering) or by an alternating voltage (RF-sputtering). In the case of DC-sputtering the target is a disk of a high purity metal (e.g. titanium). For RF-sputtering also dielectric compounds (e.g. titanium dioxide) can be used as targets. Adding a reactive gas to the gas discharge (e.g. oxygen) results in the formation of the corresponding compounds (e.g. oxides). This technique uses a separate ion source to generate the ions. To avoid contaminations, RF-sources are used in modern IBS plants. The reactive gas (oxygen) is in most cases also provided by an ion source. This results in a better reactivity of the particles and in more compact layers. Properties of sputtered coatings Because of the high kinetic energy (~1 ev), i.e. high mobility, of the film forming particles, sputtered layers exhibit An amorphous microstructure A high packing density (which is close to that of bulk materials) No external heating is necessary to produce oxide layers with minimum absorbance. These structural characteristics result in very advantageous optical properties such as: Low straylight losses High thermal and climatical stability of the optical parameters because in and out diffusion of water is prevented High laser induced damage thresholds High mechanical stability LAYERTEC has developed magnetron sputtering for optical coatings from a laboratory technique to a very efficient industrial process which yields coatings with outstanding properties especially in the VIS and NIR spectral range. The largest magnetron sputtering plant can coat substrates up to a diameter of 5mm. The main difference between magnetron sputtering and ion beam sputtering is that ion generation, target and substrates are completely separated in the IBS process while they are very close to each other in the magnetron sputter process.

7 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 5 Thermal and E-beam Evaporation Principle Thermal and electron beam evaporation are the most common techniques for the production of optical coatings. LAYERTEC uses these techniques mainly for UV-coatings. The evaporation sources are mounted on the bottom of the evaporation chamber. They contain the coating material which is heated by an electron gun (e-beam evaporation) or by resistive heating (thermal evaporation). The method of heating depends on the material properties (e.g. the melting point) and the optical specifications. The substrates are mounted on a rotating substrate holder on top of the evaporation chamber. Rotation of the substrates is necessary to ensure the coating uniformity. The substrates must be heated to a temperature of 15 4 C, depending on the substrate and coating materials. This provides low absorption losses and good adhesion of the coating to the substrates. Ion guns are used to get more compact layers. LAYERTEC is equipped with several evaporation plants covering the whole bandwidth of the above mentioned techniques from simple thermal evaporation to ion assisted deposition (IAD) using the APSpro ion source. APSpro is a trademark of LEYBOLD OPTICS GmbH Properties of Evaporation coatings The energy of the film forming particles is very low (~1eV). That is why the mobility of the particles must be enhanced by heating the substrates. The packing density of standard evaporated coatings is relatively low and the layers often contain microcrystallites. This results in relatively high straylight losses (some tenth of a percent to some percent, depending on the wavelength). Moreover, water vapour from the atmosphere can diffuse into the coatings and out of the coatings depending on temperature and humidity, resulting in a shift of the reflectance bands by ~1.5% of the wavelength. Shiftfree, i.e. dense, evaporated coatings can be produced by IAD using the APSpro ion source which provides very high ion current densities. Nevertheless, evaporated coatings have also high laser damage thresholds and low absorption. They are widely used in lasers and other optical devices.

8 6 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Measurement tools for precision optics Surface Form Measurement The precision optics facility of LAYERTEC is equipped with laser interferometers and special interferometer setups for plane, spherically and parabolically curved surfaces. For aspheric surfaces, LAYERTEC uses tactile and contactless metrology systems. In general, the form tolerance of spherical and plane optics with diameters up to Ø 1mm can be measured with an accuracy of λ/ 1 (633 nm). However, in many cases, a higher accuracy up to λ/ 3 is possible. Measurement protocols can be provided on request. Large Aperture Interferometry Especially for laser optics with large dimensions, LAYERTEC uses a high performance Fizeau interferometer and a Twyman-Green interferometer within the following measurement ranges: Plane surfaces: Ø 3mm with an accuracy up to λ/5 (633nm) and Ø 6mm better than λ/1 Spherical surfaces: Ø 6 mm with an accuracy better than λ/1 (633nm) Parabolic surfaces: Ø 3 mm full aperture measurement with an accuracy up to λ/1 (633nm) Contactless Metrology The contactless metrology measurement system LuphoScan, developed by Luphos GmbH, allows an ultra high precision measurement of distance and surface form. The unique system combines many advantages of other distance measurement systems without their disadvantages of a necessary contact, a small working distance or a tiny working range. This technology allows the determination of the topology of different objects down to the nanometer range. Here, high reflective polished objects as mirrors or metal coated substrates can be measured as well as transparent objects providing only weak reflectivity (glass lenses, substrates). Featuring the absolute measurement range it is possible to determine the height of structures of up to 1mm height on polished objects with a precision of ±5nm. Escpecially topological errors of aspheric surfaces can be exactly determined and used for a correction of the form parameters during the polishing process. Optotech OWI 15 HP interferometer system ADE Phaseshift MiniFIZ 3 large aperture interferometer Interferometry of large surfaces LuphoScan metrology system

9 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 7 Optical Profilometry Scanning Probe Microscopy Surface profiler A 3D optical surface profiler based on a white light interferometer is used to visualize the surface form and roughness of our substrates. The profiler is furthermore applied for the characterization of surface defects and other structures in the range of sizes from.5µm up to 1µm. LAYERTEC is equipped with a scanning probe microscope (atomic force microscope, AFM) to control the special polishing processes for surface roughness values below Sq.5nm as well as to provide inspection protocols on request. The Talysurf PGI 124 is a tactile surface profile measuring tool used to characterize strongly curved surfaces. A small tip is in contact with the surface and moves along a line, while its displacement is measured. The measurement principle is independent from surface topology or optical properties such as coatings or thin contaminations, which often prevent direct interferometry. The vertical accuracy depends on the gradient of the surface an can reach values of 2nm, which corresponds to ~ λ/2 (633nm). LAYERTEC uses this tool for measurements of small to mid-size non spherical surfaces up to a diameter of 2mm. Optical profiler Sensofar DI Nanoscope 31 AFM Surface defects visualized by the optical profiler AFM scan of an optical surface Taylor Hobson Talysurf PGI 124 Asphere

10 8 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Measurement tools for coatings Quality control is most important for production as well as for research and development. The standard inspection routines at LAYERTEC include interferometric measurements of the substrates and spectrophotometric measurements of the coated optics in the wavelength range between 12nm and 2µm. Spectrophotometer Standard spectrophotometric measurements in the wavelength range λ =12nm 2µm are carried out with UV-VIS-NIR spectrophotometers, VUV- and FTIR-spectrophotometers. Cavity Ring-Down (CRD) High reflectivities and transmission values in the range of R, T=99,5% 99,9999% are determined by Cavity Ring-Down time measurements. This method has a high accuracy and is an absolute measurement procedure. LAYERTEC comes with various CRD setups which cover a spectral range between 21 nm 18 nm without gaps. Group Delay (GD) & Group Delay Dispersion (GDD) Besides transmittance and reflectance, LAYERTEC is able to measure the phase properties of mirrors in the wavelength range between 25nm and 18nm by several white light interferometers. These setups can be used for the characterization of broadband femtosecond laser mirrors with positive or negative GDD as well as for measuring the GDD of GTI mirrors which reaches up to -1fs² in a narrow spectral range. Spectrophotometer Perkin Elmer Lambda 95 CRD measurement setup GDD measurement setup Z91182 edge/short side Z91182 center Exemplary mono-exponential CRD-curve of a highly reflecting mirror pair for 45nm with R = 99,995% measured using a resonator length L=228mm GDD spectra of GTI mirrors, see also pages 96, 97

11 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 9 Laser Induced Damage Threshold (LIDT) Defect Analysis Absorption & Scattering Losses LIDT measurements according to ISO standards and to our own procedures can be carried out (see pages 37, 38) with a new measurement setup at LAYERTEC. The following wavelengths are available: 266nm, 355nm, 532nm and 164nm. The pulse duration is 7ns at all four wavelengths. Measurements with other LIDT test conditions are carried out in cooperation with the Laser Zentrum Hannover (LZH) for example. LAYERTEC is equipped with a measurement system according to DIN EN ISO which enables us to detect, count and analyze defects on optical surfaces. The measurement of absorption and scattering losses of optical thin films and bulk materials are also available in cooperation with the Institute of Photonic Technology Jena e.v. (IPHT) and the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) Jena. Intra-Cavity Heating Measurement Setup Absorption losses in optical coatings lead to the heating of the coating and the substrate. At an average laser power of several kilowatts (cw) and higher even low absorption losses in the range of some parts per million cause significant heating of the optical component. LAYERTEC has built a heating measurement setup for the purpose of quality assurance and technology development on high-power optical components for the wavelength 13nm. LIDT measurement setup for pulsed laser sources Defect inspection system for optical components LIDT beamline Absorption measurement using a high power cw laser

12 1 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES

13 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 11 Precision Optics

14 12 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES How to specify substrates Price and quality of substrates are determined by material, shape, size, tolerances and polishing quality. Material The first decision is the material of the substrate. It should be free of absorption for all wavelengths of high transmission. If no transmission occurs a low cost material can be used, e.g. Borofloat (SCHOTT AG) for metallic mirrors. With respect to the surface form tolerance a low thermal expansion is beneficial. Although often specified otherwise in optical designs the thickness description means the maximum thickness of the substrate, i.e. the center thickness for plano-convex substrates and the edge thickness for plano-concave substrates. Consequently, the thickness of a wedged plate is measured on the thicker side. wedge and parallelism describe the angle between the optical surfaces while centering describes the angle between the optical surfaces and the side surfaces (see fig. 2). Shape bad substrate wedged substrate parallel substrate centred substrate The shape must be specified for both sides separately. Basically all combinations of plane, convex and concave surfaces are possible. A special role plays the wedge. A wedge (e.g. 3 arcmin) can be applied to any kind of surface (plane as well as convex or concave). For curved substrates there are different conventions for the sign of the radius. Sometimes "+" means convex and "-" means concave. Other users refer "+" and "-" to the light propagation. In this case "+" means "curvature with the direction of propagation", "-" means "curvature against the direction of propagation". To avoid confusion please specify concave or convex in words or by the acronyms CC or CX, respectively. Size The main decision should be about the size, i.e. the edge length or diameter. Small diameters are more favourable for the production. The sagitta heights become lower and it is easier to achieve a good form tolerance. wedge Figure 1: Conventions for the specification of the thickness of different types of substrates (schematic drawing) In order to achieve a good form tolerance one should take care for the ratio of diameter and thickness. As a rule of thumb the thickness should be the fifth part of the diameter. Of course, other ratios are possible but the production expenditure increases. Tolerances planoconcave planoconvex concaveconvex plane Besides size and material the tolerances are most important for manufacturing costs and therefore also for the price. Of course, the optics must fit into the mount, so that the diameter should not be larger than specified. The most common specification is + -.1mm. Mostly, the thickness is free in both directions. LAYERTEC usually specifies it with a tolerance of ±.1mm. There is a lot of confusion about the specification of wedge, parallelism and centering. Please note that Figure 2: Different kinds of plane substrates with respect to wedge and centering (schematic drawing) LAYERTEC standard substrates have a parallelism better than 5arcmin. Specially made parallels may have a parallelism as low as <1arcsec. Standard wedged substrates have wedges of.5 or 1. Larger wedge angles are possible depending on the substrate size. Normally, the 9 angle to the side surface has a precision of 2arcmin. Centering is an additional optics processing step which improves this accuracy to a few arcmin. Using the same nomenclature one can describe curved substrates. It should be distinguished between mirrors and lenses. The side surfaces of mirror substrates are just parallel. Nevertheless the direction of the optical axis can be inclined to the side surfaces. After centering the side surfaces are parallel to the optical axis. In that way the mirror substrate becomes a lens.

15 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 13 Surface form tolerance The surface form tolerance is normally measured by interferometers and is specified in terms of lambda, which is the reference wavelength. Without further statements the reference wavelength is λ = 546 nm. To avoid confusion, one must clearly distinguish between flatness, power and irregularity. In the following, flatness and irregularity shall be explained for a plane surface. Generally speaking, every real surface is more or less curved. Imagine that the "peaks" and "valleys" of a real surface are covered by parallel planes (see fig. 3). The distance between these planes is called the flatness. This flatness consists of two contributions. The first contribution is a spherical bend of the surface, which may be described by two best fitted spheres for the "peaks" and "valleys". The sagitta of this curvature with respect to an ideal plane is denoted as power. This spherical bend does not affect the quality of the reflected beam. It just causes a finite focal length. The second contribution is the deviation of the best fitted spheres, which is named irregularity. This is the most important value for the quality of the beam. λ/1 λ/1 λ/4 λ/4 The standard ISO 111 provides a sufficient opportunity for specifying the surface form tolerance. Having the best comparability with the measurement results all values are specified as numbers of interference fringes. They should be read as 1fringe = λ /2. In the drawings the surface form tolerance is allocated as item number three: 3 / power (irregularity) Example: A slightly bent (λ /4) optics which is regular (λ /1) would be specified as follows: 3 /.5 (.2) Using the optics only for transmission (e.g. laser windows) the power as well as the irregularity do not matter. If the optics has the same thickness all over the free aperture an uprightly transmitted beam is not affected. The deviation from this equality is defined in a similar way as the flatness. It is also measured in parts of the reference wavelength and called "transmitted wavefront". For instance, the window in fig. 3c has a flatness of λ /4 but a transmitted wavefront of λ / 1. Coating stress Thin substrates cannot withstand the coating stress. The coating will cause a spherical deformation. This means that a finite sagitta or power occurs. In case of circular substrates the irregularity is not affected by this issue. Taking the mentioned values seriously the flatness becomes poor. Nevertheless the quality of a normally incident beam is not affected. in MIL-O-1383 means the visibility of the biggest scratch compared to the corresponding one on a norm template. Actually "1" is the smallest scratch on this template. Thus better qualities cannot be specified seriously. The MIL norm does not specify a directly measured scratch width. Sometimes the number is interpreted as tenths of a micron, sometimes as microns. Actually a direct measurement never corresponds to the MIL norm. In contrast to the scratch, the dig number can be measured easily. The numerical value means the maximum dig diameter in hundredths of a millimeter. Maximum size digs can be one per 2mm of diameter. According to ISO 111 the defects are specified as item number 5. The grade number means the side length in millimeters of a quadratic area which is equivalent to the total fault area. So 5/1 x.25 means a surface fault area of.625 mm². Additionally, scratches of any length are denoted with a leading L. A long scratch with a width of 4microns would be specified as L 1 x.4. All these explanations are very simplified. For a detailed specification please read the complete text of the relevant standard. Please note: There is no direct conversion between MIL-O-1383 and ISO 111. All specifications in this catalog correspond to ISO 111. The mentioned scratch/dig values are a rough analog to MIL-O flatness power c) irregularity Defects Figure 3: Schematic drawing for the explanation of substrate properties: flatness of λ/1 spherical bending (power of λ/4) c) irregularity of λ/4, but transmitted wavefront of λ/1 MIL-O-1383 and ISO 111 are different standards for the description of optical elements. This often causes obscurities. Basically, one should distinguish between scratches and digs. The scratch number

16 14 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES STANDARD QUALITY SUBSTRATES The precision optics facility of LAYERTEC produces plane and spherically curved mirror substrates, lenses and prisms of fused silica, optical glasses like N-BK7 and SF14 and some crystalline materials, e.g. calcium fluoride and YAG. In the following you can find information on the specifications of our standard substrates. Please do not hesitate to contact us also for other sizes, shapes, radii and materials or for special components. Standard SPECIFICATIONS Plane substrates, parallels and wedges Standard plane substrates: wedge <5arcmin Standard parallels: wedge < 1 arcmin or wedge < 1arcsec Standard wedges: wedge = 3arcmin or wedge = 1deg Plano- concave and plano- convex substrates Standard radii: 25, 3, 38, 5, 75, 1, 15, 2, 25, 3, 4, 5, 6, 75, 1, 2, 3, 4, 5 mm t Materials Fused silica: Corning 798 or equivalent Fused silica for high power applications: Suprasil 3 / 31 / 32 or equivalent UV fused silica (excimer grade): SQ1 E-193 and SQ1 E-248 IR fused silica: Infrasil 32 or equivalent ULE Zerodur N-BK7 or equivalent CaF 2 : single crystal, random oriented, special orientations on request, excimer grade (248nm and 193nm) on request Sapphire: single crystal, random oriented, special orientations on request YAG: undoped, single crystal, random oriented Dimensions Fused silica, ULE, Zerodur, N-BK7 : diameter 6.35mm 58mm (¼inch 2inches) Calcium fluoride, YAG, sapphire: diameter 6.35mm 5.8mm (¼inch 2inches) Rectangular substrates and other diameters available on request Tolerances Diameter: +mm, -.1mm Thickness: ±.1 mm Clear aperture: central 85% of dimension Chamfer:.2.4mm at 45 r t t e r All trademarks mentioned are the property of the respective owners. t c Ø: diameter [mm] t e : edge thickness [mm] t c : center thickness [mm] t: thickness [mm]

17 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 15 Surface form tolerance (reference wavelength: 633nm) Material Standard Specification On request Fused silica ULE and Zerodur N-BK7 CaF 2 plane spherical plane spherical plane spherical plane Ø<26mm plane Ø <51mm spherical λ/1 λ/4reg. (typical λ/1reg.) λ/1 λ/4 (typical λ/1reg.) λ/1 λ/4reg. (typical λ/1reg.) λ/1 λ/4 λ/4reg. Sapphire λ/ 2 λ/ 1 YAG Si plane spherical plane spherical λ/1 λ/4reg. (typical λ/1reg.) λ/1 λ/4reg. (typical λ/1reg.) λ/3 λ/3 reg. (Ø<51mm) λ/3 λ/3 reg. (Ø<51mm) λ/3 λ/3 reg. (Ø<51mm) λ/1 λ/1 λ/1 reg. λ/2 λ/2 reg. (Ø<51mm) λ/2 λ/2 reg. (Ø<51mm) Surface quality Material Standard Roughness Standard Specification On request Fused silica < 2Å 5/1 x.25 L1 x.1 Scratch-Dig 1-3 ULE < 2Å 5/3 x.25 L1 x.1 Scratch-Dig 1-5 Zerodur < 4Å 5/2 x.4 L1 x.1 Scratch-Dig 1-5 N-BK7 < 3Å 5/2 x.4 L1 x.1 Scratch-Dig 1-5 CaF 2 < 4Å 5/3 x.25 L1 x.25 Scratch-Dig 2-3 Sapphire < 5Å 5/3 x.25 L2 x.25 Scratch-Dig 2-3 YAG < 3Å 5/1 x.25 L2 x.25 Scratch-Dig 2-3 Si < 3Å 5/3 x.25 L1 x.1 Scratch-Dig 1-5 < 1,2Å 5/1 x.1 L1 x.5 Scratch-Dig 5-1 < 2Å 5/1 x.1 L1 x.5 Scratch-Dig 5-1 < 3Å 5/2 x.25 L1 x.1 Scratch-Dig 1-3 < 3Å 5/2 x.25 L1 x.1 Scratch-Dig 1-3 < 3Å 5/3 x.16 L1 x.1 Scratch-Dig 1-2 < 3Å 5/3 x.16 L1 x.1 Scratch-Dig 1-2 < 3Å 5/1 x.1 L1 x.5 Scratch-Dig 5-1 < 2Å 5/1 x.1 L3 x.5 Scratch-Dig 5-1 All specifications according to ISO 111 (Ø 25mm). The mentioned Scratch-Dig values are approximately equivalent to MIL- O-1383

18 16 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES ASPHERES, OFF-AXIS AND FREE FORM OPTICS Basics Plane and spherical optics can be efficiently manufactured using traditional techniques of area grinding and polishing. The tool always works on a significant fraction of the substrate area at once. It is not or only hardly possible to manufacture surface geometries that differ from regular forms like planes, spheres, cylinders. Using ultra precision CNC machinery, surfaces can be processed zonally, i.e. the tool works on one point at a time. The possible surface forms and tolerances are only limited by the precision of the machine and the measurement equipment. In contrast to the areal techniques, zonal processing usually works with one single piece per run only. The non-spherical optics can be divided in three groups. Rotational symmetric non-spheric, off-axis and free form optics. ROTATIONAL SYMMETRIC NON SPHERICAL OPTICS (ASPHERES) Although the term "asphere" may stand for any nonspherical optics, it is often referenced to rotationally symmetric non spherical optics. They are described by the following equation (DIN ISO 111): r 2 z (r) = R (1 + k) z = displacement k = conic constant r = distance from axis R = radius of curvature A i = aspheric coefficients Neglecting the aspheric coefficients leads to a profile of conic sections: Ellipse: -1 < k < Sphere: k = Parabola: k = -1 z r 2 R 2 + A 4 r 4 + A 6 r 6 + OFF-AXIS SURFACES An off-axis surface can be seen as a section of a bigger on-axis surface. The focal point still is on the original optical axis but not the center of the section. It is located off axis. Off-axis surfaces derived from aspheres are described by the mentioned equation, the off-axis distance a and/or the off-axis angle α. Examples: Off-axis parabolas Off-axis ellipses Off-axis hyperbolas Off-axis spheres Off-axis cylinders Example: Off-Axis Parabola (OAP) z total diameter opening diameter inspection range sagitta height total thickness Ø f-eff α centering error tilting error Figure 2: Profile of an aspheric mirror center thickness r f-parent a r Figure 1: Aspheric lens Dimension Tolerances Total diameter 25 56mm <.1mm Inspection range < 55mm Total thickness < 1mm <.1mm Displacement < 5mm Centering error < 5µm Tilting error < 3'' Surface form tolerance Roughness Table 1: Production dimensions and tolerances < λ/4 (< λ/1 on request) < 3Å Figure 3: Schematic drawing of an off-axis parabola The focal length of the parent parabola (parent focal length) is measured from the vertex on the optical axis. For the off-axis parabola, an effective focal length f eff is introduced. The off-axis distance is measured from the optical axis to the middle of the OAP. The radius R denotes the radius of curvature in the vertex of the parent parabola. The conic constant k is -1.

19 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 17 In principle an off-axis substrate can always be mechanically cut from an on-axis substrate. The alternative way is a direct manufacturing. Depending on the size and tolerances, both ways are possible. Fig. 4 shows the common process of manufacturing a number of smaller OAPs from a parent parabola. FREE FORM SURFACES Free form surfaces are always customer specific, defined by a number of points or equations. No symmetry may be assumed. Free form surfaces are manufactured as single pieces. Table 2 shows LAYERTEC's production dimensions and tolerances for free form surfaces. MATERIALS The surface quality as well as the final tolerances depend very strongly on the material of the substrate. LAYERTEC has an optimized process for fused silica. In special cases materials like Zerodur, ULE or N-BK7 may be used. A Off-Axis Ø ±.1mm a ±.1mm Parent parabola Figure 4: Manufacturing off-axis parabolas as pieces of one "parent parabola" A d ±.2mm b ±.1mm d ±.1mm opening diameter Ø/2 dm ±.2 mm α d ±.1mm c ±.1mm a = off-axis distance Figure 6: Schematic drawing of an off axis ellipses b = width c = length α = off-axis angle d = distance between center beam and reference edge Figure 5: Basic properties of a free form substrate z a α r Measurement Measuring aspheric surfaces requires sophisticated devices. LAYERTEC applies 4 different measurement principles. Tactile measuring A tip has mechanical contact to the surface and its excursion is recorded. Measures one line at a time. Precision < 2nm on a 2mm line. Single point interferometer Contactless measurement of the surface, measuring the surface point by point. Precision < 5nm on a diameter of 42mm. Interferometer with reference surface Compares the surface to a well known reference surface. Precision < 5 nm, diameter 3 mm. Preferably concave surfaces. Interferometer with hologram Compares the surface to the wavefront which is generated by a computer generated hologram (CGH). Dimension Tolerances Ø <15mm.2mm (<2mm on request) b <15mm.1mm c <25mm.1mm α <45 d <6mm.1mm a Surface form tolerance Roughness < λ/2 (λ/4 on request) < 3Å Table 2: Dimensions and tolerances of free form substrates Figure 6: LuphoScan single point interferometer

20 18 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES SPECIAL OPTICAL COMPONENTS Etalons In optics, the etalon as a kind of a Fabry-Pérot interferometer is typically made of a transparent plate with two reflecting surfaces. Its transmission spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon. Etalons are widely used in telecommunications, lasers and spectroscopy for controlling and measuring the wavelength of laser sources. LAYERTEC offers etalons of various materials and customized diameters depending on the wavelength range. Subject to the diameter thicknesses down to 5 µm and a parallelism < 1 arcsec are possible. Do not hesitate to contact us for the customized diameter and thickness you need. Polishing of Crystals Besides the high quality optical coatings on crystals (see pages 116, 117) LAYERTEC supports the polishing of various types of crystals such as YAG, KGW, KYW, ZGP or LBO. Our polishing technology also allows us the careful handling and processing of small crystal sizes or extraordinary forms. Do not hesitate to contact us for your special problem. Thickness Ø = 5 mm Ø = 25mm Ø = 12.7mm Parallelism Fused Silica = 2µm = 13µm = 5µm < 1arcsec YAG = 2µm = 13µm = 5µm < 1arcsec CaF 2 = 3µm = 1µm < 5arcsec Waveplates LAYERTEC offers customer specific retardation plates made of crystalline quartz. Due to the minimal required thickness the below mentioned wave plates are available: LARGE SCALE OPTICS LAYERTEC has built up a production line for plane and spherical optics up to a diameter of 5mm. The production line also includes interferometers. Measurements for large optics are described on page 22. These optics can be coated using magnetron sputtering. The main products are large scale laser mirrors. A coating uniformity of ±.5% was demonstrated which enables also the production of large scale thin film polarizers and other complex coating designs. Order Ø = 38 mm Ø = 25mm Ø = 12.7mm Precision Parallelism λ/2 Available wavelenghts K= λ = 9nm ±1µm < 1arcsec K=1 λ = 9nm λ = 8nm λ = 35nm ±1µm < 1arcsec K=2 λ = 55nm λ = 5nm λ = 3nm ±1µm < 1arcsec λ/4 Available wavelenghts K=1 λ = 16nm λ = 94nm λ = 4nm ±1µm < 1arcsec K=2 λ = 62nm λ = 53nm λ = 3nm ±1µm < 1arcsec Figure 1: Mirror substrate with a diameter of 5mm Customized Prisms and Shapes In addition to the mentioned circular substrates LAYERTEC is able to produce a lot of different shapes. Besides rectangular substrates, wedges and prisms uncommon optics are possible. Typical issues are defined holes through the optics. So called D-cuts and notches can be produced.

21 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 19 SUBSTRATE MATERIALS FOR UV, VIS AND NIR/IR OPTICS Wavelength range free of absorption Refractive index at YAG (undoped) Sapphire BaF 2 CaF 2 Infrasil 1) Fused silica(uv) N-BK7 2) SF1 2) 4nm 4µm 4nm 4µm 4nm 1µm 13nm 7µm 3nm 3µm 19nm 2.µm 3) 4nm 1.8µm 4nm 2.µm 2nm nm nm µm µm µm µm Absorbing in the 3µm region no no no no yes yes yes yes Absorbing in the 94nm region For high power applications at 94nm we recommend the fused silica types SUPRASIL 3 1) and SUPRASIL 31/32 1) Birefringence no yes no no 4) no no no no Thermal expansion coefficient [1 6 K] 5) Resistance against temperature high high very low low high high medium medium gradients and thermal shock GDD fs² per mm TOD fs 3 per mm 4 nm nm nm nm , nm , nm nm nm nm nm ) Registered trademark of Heraeus Quarzglas GmbH & Co. KG 2) Registered trademark of SCHOTT AG 3) Absorption band within this wavelength range, please see transmission curve 4) Measurable effects only in the VUV wavelength range All values are for informational purposes only. LAYERTEC can not guarantee the correctness of the values given. 5) The values given here are rounded, because the measurements of different authors in the literature are inconsistent. Please note that the thermal expansion coefficient of crystals depends also on the crystal orientation.

22 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES 2 transmission curves Various types of fused silica and N-BK7 (9,5mm thick) Corning 798 Suprasil 3 Infrasil 32 N-BK 7 T [%] Absorption 7 T [%]

23 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 21 T [%] YAG undoped (3mm thick) 1 T [%] Sapphire (3mm thick) Calcium fluoride (3mm thick) Barium fluoride (3mm thick) T [%] 1 T [%]

24 22 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Measurement Tools For Precision Optics Surface Form Measurement For the measurement of surface form and regularity, the precision optics facility of LAYERTEC is equipped with laser interferometers and special interferometer setups for plane, spherically and parabolically curved surfaces. Additionally, a tactile measurement device (Taylor Hobson PGI 124 Asphere) is available for general aspheric and grinded surfaces. Besides the purpose of quality control, surface form measurement is a key function for the zonal polishing technology which is established at LAYERTEC. Abbreviations P-V: The peak-to valley height difference ROC: Radius of curvature of a spherically curved surface. : measuring wavelength of the laser interferometer (e.g. 633nm or 546nm). The P-V value is stated in a fractional amount of λ. The concrete value of λ is stated in our protocols. For detailed information about the standards concerning surface form measurement please refer to DIN EN ISO Accuracy of interferometric measurements Without special calibration procedures, the accuracy of an interferometric measurement cannot be better than the accuracy of the reference surface. With calibration the accuracy can be increased by factor 2 or more. Furthermore the precision is influenced by the size of the region (clear aperture) which is measured, and when it is about curved surfaces, by the radius of curvature itself. The accuracy values stated as "P-V better than " in the following articles are minimum guaranteed values. Very often accuracies in the region of λ/ 2 or better will be achieved. Standard measurements In general, the form tolerance of spherical and plane optics with diameters Ø 1mm can be measured with an accuracy of P-V better than λ/1 by using ZYGO Fizeau interferometers. To cover a measurement range of ROC = ±12mm over an aperture of Ø=1mm, LAYERTEC uses high precision JenFIZAR Fizeau objectives. In many cases, a higher accuracy up to P-V = λ/3 is possible. Measurement protocols can be provided on request. Large Radius Test (LRT) Surfaces with radii of curvature beyond ±12mm are tested with a special Fizeau zoom lens setup called Large Radius Test (LRT). This setup was developed by DIOPTIC GmbH in cooperation with LAYERTEC. Its operating range is ROC= ±1mm ±2.mm at working distances lower than 5mm. The accuracy is guaranteed P-V = λ/8 over Ø 1mm, but typically better than P-V λ/15. LRT has the advantages that only one Fizeau-objective is needed to cover a wide range of radii of curvature and that the working distance is kept small (thus disturbing air turbulences during the measurement are kept small). Large aperture interferometry Especially for laser optics with large dimensions, LAYERTEC uses high performance interferometers. A wavelength-shifting Fizeau interferometer (ADE Phaseshift MiniFIZ 3 ) is used for flat surfaces. LAYERTEC has enlarged the measurement aperture of the system with a special stitching setup. The measurement range of the system is: P-V up to λ/5 (633nm) at Ø 3mm with a full aperture measurement, P-V better than λ/1 (633nm) at Ø 6mm with a special stitching measurement setup. Figure 1 shows the height map of a flat surface with a diameter of Ø=52mm which was measured with the MiniFIZ 3 interferometer and the stitching configuration at LAYERTEC. The measurements of spherically curved concave surfaces are carried out with a Twyman-Green interferometer (PhaseCam 53 from 4D-Technology). This interferometer uses a special technology which allows measurement times in the region of a few milliseconds. Thus, the interferometer is insensitive to vibrational errors when measuring over long distances up to 2m between the device and the specimen. The measurement accuracy of the system is P-V better than λ/1 at Ø 6mm with a full aperture measurement (in case of concave surfaces). Figure 1: Height map of a flat surface with a diameter of Ø=52mm polished and measured at LAYERTEC. The P-V value is λ/1 over the full aperture (Ø=5mm inspection are after zonal correction.

25 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 23 Surface Roughness Measurement The surface roughness value of an optical surface is denoted as roughness parameter Rq or Sq. This parameter is also named "RMS roughness" because it is calculated as the root mean square of the surface height values. The letter "R" indicates that only a 2-dimensional roughness profile is basis of calculation according to ISO 4287/1. The letter "S" means a 3-dimensional measurement and calculation (3D-Reference ISO/DIS ). For the aim of measuring and stating roughness parameters of optical surfaces, the corresponding measurement device and the spectral range within which the optical surface should be used, has to be taken into account. The spatial resolution of the roughness measurement device plays an important role with respect to the roughness value. A higher spatial resolution allows the detection of smaller surface structures and higher spatial frequencies, respectively. Furthermore, the amount of stray light losses on the surface depends on the spatial frequencies of the surface structures and the wavelength of the light itself. Fig. 2 shows the spatial frequencies which influence the scattering losses in different spectral ranges and typical spatial resolutions of roughness measurement devices. Spatial Resolution Bandwidth 3D optrical surface profiler 25 x 25 µm 2 Generally, for the characterization of optical surfaces which are appointed to be used in the NIR, VIS and UV spectral range, surface roughness measurements should be accomplished with spatial resolutions which are equal or better than 1/µm to obtain roughness information over the entire relevant spatial frequency range. Fig. 3 clarifies the differences between an AFM measurement and an optical surface profiler measurement with respect to the calculated RMS roughness and the spatial resolution. LAYERTEC has available a phase shifting 3D optical surface profiler (Sensofar) and a scanning probe microscope (AFM) DI Nanoscope 31 for the purpose of measuring and analyzing the surface roughness of optical components. The optical profiler has a low spatial resolution <2/µm but the acquisition time of a measurement amounts only a few seconds. Thus, the device is used for the measurement of optics with lower surface roughness requirements and for the general inspection of the polishing processes. Surface defects and inhomogeneities can be characterized too. The AFM is used for the characterization of polished surfaces which have roughness values Sq<,5nm. This device has a very high spatial as well as vertical resolution, but the acquisition time is approximately 1 3 minutes. Therefore it is primarily used for the further development of the polishing processes. Our premium polish and especially our optics for UV applications with Sq <,2nm are checked for the reason of quality control in regular periods of time. The standard AFM measurement parameters at LAYERTEC are: Scan size of 1 x 1 µm² and a spatial resolution of 25/µm (see fig. 2). Measurement reports are available on request. * For more information on straylight losses please see: A. Duparré, "Light scattering on thin dielectric films" in "Thin films for optical coatings", eds. R. Hummel and K. Guenther, p , CRC Press, Boca Raton, 1995 AFM 1 x 1 µm 2 Scattering Losses λ = 193nm (DUV) Scattering Losses λ = 633nm AFM 1 x 1 µm 2,1,1, Spatial Frequency [1/µm] Figure 2: Spatial frequency resolution of AFM and 3D optical surface profiler at LAYERTEC for typical scan sizes. Additionally, the figure shows the spatial frequency ranges which influence the scattering losses in the VIS and DUV spectral range*. Figure 3: Surface roughness measurement of a super polished Low Loss substrate for an UV application made of fused silica. Both images show the same region on the surface, but they were recorded with different measurement devices (optical surface profiler and AFM). The color bar scaling is ±1nm for both images. At equivalent lateral pixel scaling, the AFM measurement with high spatial resolution shows fine structures which are relevant for stray light losses. The optical profiler measurement only shows a few pixels but no significant information about the surface roughness and structures. Moreover, the roughness value obtained from such undersampled measurements is too low, that means that the value seems to be too good.

26 24 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES

27 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 25 Optical Coatings

28 26 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES OPTICAL INTERFERENCE COATINGS The purpose of optical coatings is to change the reflectivity of optical surfaces. According to the materials and physical phenomena which are used one can in principle distinguish metallic and dielectric coatings. Metallic coatings are used for reflectors and neutral density filters. The reflectivity which can be achieved is given by the properties of the metal. Some of the most common metals for optical applications are described on page 31. Dielectric coatings use, however, optical interference to change the reflectivity of the coated surfaces. Another major difference is that the materials used for this kind of coatings show very low absorption. Using optical interference coatings the reflectivity of optical surfaces can be varied from nearly zero (anti reflection coatings) to nearly 1% (low loss mirrors with R>99.999%). However, these reflectivity values are achieved only for a certain wavelength or wavelength range. Basics The influence of a single dielectric layer on the reflectivity of a surface is schematically shown in fig. 1. An incident beam ( is split into a transmitted beam ( and a reflected beam (c) at the air-layer Incident beam ( (c) (d) Reflected beams interface. The transmitted beam ( is again split into a reflected beam (d) and a transmitted beam (e). The reflected beams (c) and (d) can interfere. In fig. 1 the wave is represented by the shading of the reflected beams. The distance from "light-to-light" or "dark-to-dark" is the wavelength. Depending on the phase difference between the reflected beams constructive or destructive interference may occur. The reflectivity of the interface between two media depends on the refractive indices of the media, the angle of incidence and the polarization of the light. In general, it is described by the Fresnel equations. n1cosα n 2 cosβ R s = ( n 1 cosα + n 2 cosβ ) n 2 cosα n 1 cosβ R p = ( n 2 cosα + n 1 cosβ ) R s... reflectivity for s-polarization R p reflectivity for p-polarization n1 refractive index of medium 1 n 2 refractive index of medium 2 α angle of incidence (AOI) β angle of refraction (AOR) Incident beam ( (c) (d) 2 2 Reflected beams For normal incidence (a = b = ) the formulae can be reduced to the simple term n 2 n 1 R = ( n 2 + n ) 1 The phase difference between the beams (c) and (d) is given by the optical thickness n t of the layer (the product of the refractive index n and the geometrical thickness t). Moreover, one must take into account that a phase jump of π, i.e. one half wave, occurs if light coming from a low index medium is reflected at the interface to a high index medium. For a detailed explanation of the physics of optical interference coatings we refer to the literature cited on page 32. To help our customers understand the optical properties of the coatings described in this catalog, we only present some rules of thumb: High index layers always increase the reflectivity of the surface. The maximum reflectivity for a given wavelength λ is reached for n t = λ/4. A layer with n t = λ/2 doesn t change the reflectivity of the surface for this wavelength λ. Low index layers always decrease the reflectivity of the surface. The minimum reflectivity for a given wavelength λ is reached for n t = λ/4. A layer with n t = λ/2 doesn t change the reflectivity of the surface for this wavelength λ. 2 b High index layer b Low index layer Substrate Substrate (e) (e) Figure 1: Schematic drawing to explain the interference effect of quarterwave layers of a high index material and a low index material (after [1])

29 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 27 Anti reflection coatings A single low index layer can be used as a simple AR coating. The most common material for this purpose is magnesium fluoride with a refractive index n =1.38 in the VIS an NIR. This material reduces the reflectivity per surface to R ~1.8 % for fused silica and to nearly zero for sapphire. Single wavelength AR coatings consisting of 2 3 layers can be designed for all substrate materials to reduce the reflectivity for the given wavelength to nearly zero. These coatings are especially used in laser physics. AR coatings for several wavelengths or for broad wavelength ranges are also possible and consist of 4 1 layers Mirrors and partial reflectors The most common mirror design is the so called quarterwave stack, i.e. a stack of alternating high and low index layers with an equal optical thickness of n t = λ/4 for the desired wavelength. This results in constructive interference of the reflected beams arising at each interface between the layers. The spectral width of the reflection band and the achievable reflectivity for a given number of layer pairs depend on the ratio of the refractive indices of the layer materials. A large refractive index ratio results in a broad reflection band while a narrow reflection band can be produced using materials with a low refractive index ratio. Air Thickness [nanometers] Substrate To visualize the effect of different refractive index ratios figure 3b compares the reflectivity spectra of quarterwave stacks consisting of 15 pairs of Ta 2 O 5 /SiO 2 and TiO 2 /SiO 2 for 8nm (n1/ n2=2.1/1.46 and 2.35/1.46, respectively). Assuming ideal coatings with zero absorption and scattering losses the theoretical reflectivity will approach R=1% with increasing number of layer pairs. Also partial reflectors with several discrete reflectivity values between R=% and R=1% can be manufactured using only a small number of layer pairs (see fig. 4). Adding some non-quarterwave layers to such a stack allows to optimize the reflectivity to any desired value. Figure 4 also shows that an increasing number of layer pairs results in steeper edges of the reflectivity band. This is especially important for edge filters, i.e. mirrors with smoothed side bands. Extremely steep edges require a large number of layer pairs which in turn results also in a very high reflectivity. Extremely high reflectivity values require very low optical losses. This can be achieved by using sputtering techniques. TiO 2 /SiO 2 Ta 2 O 5 /SiO 2 15 pairs 1 pairs 5 pairs 3 pairs 2 pairs 1 pair Figure 2: Schematic reflectivity spectrum of a single wavelength AR coating ("V-coating") ( and of a broadband AR coating ( Figure 3: Schematic drawing of a quarterwave stack consisting of layers with equal optical thickness of a high index material (shaded) and a low index material (no shading) (after [1]) (, reflectivity spectra of quarterwave stacks consisting of 15 pairs of Ta 2 O 5 /SiO 2 and TiO 2 /SiO 2 ( Figure 4: Calculated reflectivity of quarterwave stacks consisting of 1, 2, 3, 5, 1 and 15 layer pairs of Ta 2 O 5 / SiO 2 for 8nm

30 28 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Optical losses Light which impinges on an optical component is either reflected, transmitted, absorbed or scattered. From this basic point of view, the energy balance can be written in the simple equation R + T + A + S =1 with R Reflectivity, T Transmission, A Absorption and S Scattering In precision optics and laser physics absorption and scattering are summarized as optical losses, because the absorbed and scattered part of the incoming light can no longer be used as carrier of information or as an optical tool. In practice the reflectivity which can be achieved depends on the absorption and scattering losses of the optics. Scattering losses increase drastically with decreasing wavelength. The basic phenomena are described by the Mie theory (scattering by particles with diameters in the order of λ, S~1/λ 2 ) and by the Rayleigh Wavelength range materials Coating technology ~ 2 nm fluorides evaporation >96.% ~ 25 nm oxides IAD >99.% sputtering >99.7% ~ 3 nm oxides IAD >99.5% sputtering >99.9% ~ 35 nm oxides IAD >99.8% sputtering >99.95% VIS oxides IAD >99.9% sputtering >99.95% Low loss mirrors VIS oxides sputtering >99.99% NIR oxides IAD >99.9% sputtering >99.98% Low loss mirrors NIR oxides sputtering >99.998% theory (scattering by particles with diameters <λ, S~1/λ 4 ). Depending on the surface and bulk structure scattering losses are a mixture of Mie scattering and Rayleigh scattering at the same time. The strong dependence of the scattering losses on the wavelength is the reason why scattering losses are a huge problem in the UV range while they are less important in the NIR. Scattering losses critically depend on the microstructure of the coatings. This causes a dependence on the coating technology used. Usually, coatings produced by evaporation techniques show significantly higher scattering losses than coatings produced by magnetron sputtering or ion beam sputtering. Absorption in optical coatings and substrates is mainly determined by the band structure of the materials. Common oxide materials show bandgaps of 3 7eV which correspond to absorption edges in the NUV and DUV, while fluorides have bandgaps of 9 1eV resulting in absorption edges in the VUV spectral range (for examples please see page 2ff). Some materials show Reflectivity also absorption bands apart from the basic absorption edge as for instance the absorption band of Si-O-H-bonds in fused silica around 2.7µm. Another reason for absorption losses at wavelengths apart from the absorption edge are defects in the layers which form absorbing states in the band gap of the materials. These may result from contaminations or from the formation of substoichiometric compounds. That s why optical coatings must be optimized with respect to low contamination levels and good stoichiometry. This kind of absorption losses also increases with decreasing wavelength. Table 1: Reflectivity of HR mirrors in different spectral regions (for AOI= ) All kinds of losses depend on the thickness of the layer system. Each layer pair increases the theoretical reflectivity. However, in practice it also increases the optical losses. Thus, especially for evaporated coatings with relatively large scattering losses there is an optimum number of layer pairs which generates the maximum reflectivity. Stress Another effect which limits the number of layers is the stress in the coatings. This stress results from the structure of the layers but also from different thermal expansion coefficients of substrates and coatings. Mechanical stress may deform the substrates but it may also result in cracks in the coatings or in a reduced adherence of the coatings. Stress can be limited by material selection and the optimization of process temperature, deposition rate and, in case of ion assisted and sputtering processes, ion energy and ion flux. Angle shift A special problem of interference coatings is the angle shift. This means that features shift to shorter wavelengths with increasing angle of incidence. Turning an optical component from AOI= to AOI=45 results, for instance, in a shift of the features by about 1%. That s why the angle of incidence must always be known to design any optical coating. Moreover, polarization effects must be taken into account at non-normal incidence (see below). Please note that the angle of incidence varies naturally, if curved surfaces are used. Lenses in an optical system always have a range of acceptance angles which is determined by the shape of

31 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 29 Polarization effects the lens and by the convergence or divergence of the beam. AR coatings can be improved significantly, if these items are known. Besides the shift, broadband AR coatings often show an increased reflectivity at AOI 3 (see fig. 5. The angle shift offers, however, also the possibility to angle tune an interference coating. This is especially useful in the case of filters and thin film polarizers. These optics show extremely narrow spectral ranges of optimum performance which may decrease the output and increase the costs drastically, if the specifications for wavelength and AOI are fixed. Angle tuning (see fig. 5 is in these cases the best way to optimize the performance and to minimize the costs AOI= AOI= AOI=1 AOI=2 AOI=15 AOI=3 AOI= Figure 5: Angle shift and change of reflectivity of a broadband AR coating at AOI= (red), 2 (blue) and 3 (green) ( and angle tuning of a narrow band filter for 8nm ( Besides the angle shift polarization effects appear at non-normal incidence. For optical interference coatings it is sufficient to calculate the reflection coefficients for s- and p-polarized light. The reflectivity of unpolarized light is calculated as the average of Rs and Rp. To explain the meaning of the terms "s-polarization" and "p-polarization" one must first determine a reference plane (see lower part of fig. 6). This plane is spanned by the incident beam and by the surface normal of the optic. "S-polarized light" Reflectance of an uncoated glass surface (R vs. angle of incidence) Θ B incident beam s+p-polarization Θ B transmitted beam s+p-polarization Θ B surface normal AOI [deg] reflected beam s-polarization Θ B is that part of the light which oscillates perpendicularly to this reference plane ("s" comes from the German word "senkrecht" = perpendicular). "P-polarized light" is the part which oscillates parallel to the reference plane. Light waves whose plane of oscillation is inclined to these directions are split into a p-polarized and an s-polarized part. The upper part of fig. 6 shows the reflectivity of a glass surface vs. AOI for s- and p-polarized light. The reflectivity for s-polarized light increases with increasing angle of incidence while the reflectivity for p-polarized light decreases first, reaches R=% at the "Brewster angle" and increases again for angles of incidence beyond Brewster angle. p-pol. In principle, the same is true for dielectric mirrors. For AOI the reflectivity for s-polarized light is higher and the reflection band is broader than for p-polarized light. : Brewster angle In the case of edge filters, where one of the edges of the reflectance band is used to separate wavelength regions of high reflectivity and high transmission, tilting results in a polarization splitting of the edge. This means that the angle shift is different for s- and p-polarized light. This results in a broadening of the edge if unpolarized light is used. Figure 6: Definition of the terms "s-polarized light" and "p-polarized light" and reflectance of an uncoated glass surface vs. angle of incidence for s- and p-polarized light

32 3 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES T [%] p-pol. unpol. AOI= Figure 7: Polarization splitting of an edge filter. Please note that the edges of the reflectance bands are steep for s- as well as for p-polarization even at AOI=45, but are located at different wavelengths. As a result, the edge of the reflectance band for unpolarized light is considerably broadened. Documentation of coating performance at LAYERTEC LAYERTEC provides a data sheet for transmission and/or reflectivity with each delivered optical component. The standard procedure is to measure the transmission of the optics at AOI= and to calculate the reflectivity at the desired AOI from this measurement. Sputtered optical coatings for the VIS and NIR exhibit extremely low straylight and absorption losses (both in the order of some 1 5 ). This has been confirmed by direct measurements of straylight and absorption as well as by highly accurate reflectivity measurements (e.g. by Cavity Ring-Down spectroscopy). With these very small optical losses, the reflectivity of sputtered mirrors can be determined by measuring the transmittance T and the simple calculation: and NIR by transmission measurements is much more accurate than direct reflectivity measurements. Please note that this method can only be applied because the optical losses are very small (which is one of the advantages of sputtered coatings). The method is also used for evaporated coatings in the NIR, VIS and near UV spectral range, where the optical losses of only 1-3x1 3 can be included into the reflectivity calculation. In the deep UV range, the coatings usually show straylight losses in the order of some , depending on the wavelength. That is why, especially fluoride coatings for wavelengths < 22nm are delivered with direct reflectivity measurements. Direct reflectivity measurements are also necessary for low loss mirrors. LAYERTEC has a Cavity Ring- Down setup for spectrally resolved measurements in the wavelength range between 21 18nm. The data sheets can also be downloaded from the LAYERTEC website. Fig. 8 shows the download window for data sheets. To avoid mistakes registration with the part#, order# and batch# is required. Dielectric broadband coatings The first step to broadband mirrors and output couplers is to use coating materials with a large refractive index ratio (see above). The bandwidth can be further increased by special coating designs which apply also non-quarterwave layers. The easiest way is to combine two or more quarterwave stacks with overlapping reflectance bands. However, this results in increased optical losses at the wavelengths where the bands overlap. Moreover, such multiple stack designs cannot be used for femtosecond lasers because they induce pulse distortion. LAYERTEC offers special all-dielectric broadband components for fs-lasers up to bandwidths of one octave, i.e. 55nm 11nm (see pages 84 85). Larger bandwidths can be achieved using metals. However, the natural reflectivity of metals is limited to 92 99% (see the following paragraphs). It can, however, be increased by dielectric overcoatings. For such ultra broadband metal-dielectric mirrors see page 19. R =1% T. In a normal spectrophotometer, the transmission can be measured with an accuracy of about.1...2% (depending on the absolute value), whereas reflectance measurements in spectrophotometers mostly have errors of about.5%. Thus, the determination of the reflectivity of sputtered coatings in the VIS Figure 8: Download measurement report from the Layertec website

33 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 31 METALLIC COATINGS Aluminum Metals are the most common materials for mirror fabrication. Polished metals, especially gold, copper and bronze, were used as mirrors already in the ancient world. In the middle ages, mirrors with relatively constant reflectivity in the visible spectral range were fabricated using tin foils and mercury which were put on glass. The era of thin film metal coatings on glass began in the 19th century when Justus von Liebig discovered that thin films of silver can be manufactured using silver nitrate and aldehyd. Mirrors for applications in precision optics and laser physics are produced by evaporation or sputtering techniques. LAYERTEC uses magnetron sputtering for the fabrication of metallic coatings. This results in coatings with extremely low straylight losses. Moreover, also transparent, i.e. very thin metal coatings can be produced with high accuracy. For detailed information about our metallic mirrors and neutral density filters please see pages and Fig. 9 gives an overview about the reflectivity of the most common metals In the following we give some hints for the use of these metals and about the role of protective coatings: Silver Highest reflectivity in the VIS and NIR LAYERTEC produces protective layers by magnetron sputtering. These layers with very high packing density make silver mirrors as stable as mirrors of other metals (e.g. aluminum). Lifetimes of 1years in normal atmosphere were demonstrated. The use of protective layers is mandatory, because unprotected silver is chemically unstable and soft See separate data sheets on pages and Gold,2,3,4,5,6,7,8, wavelength [µm] Cu Similar reflectance as silver in the NIR Chemically stable, but soft Protective layers are necessary to make gold mirrors cleanable We recommend to use Au Ag Al protected silver mirrors instead of protected gold, because the sputtered protective layers overcome the insufficiency of silver and make it a better choice because of the broader wavelength range, the slightly higher reflectivity and the more favourable price. See separate data sheet on page 125 Relatively high and constant reflectance in the VIS and NIR Highest reflectance in the UV Surface oxide layer absorbs in the deep UV A protective layer is recommended, because aluminum is soft See separate data sheet on pages Chromium Medium reflectivity in the VIS and NIR (R~4% 8% depending on the coating process) Hard, can be used without protective layer Good adhesive layer for gold and other metals on glass substrates Protective layers Improve cleanability and chemical stability Influence the reflectance of the metal Even very thin sputtered layers can be used for chemical protection of the metal because of high atomic density of the layers. Such layers show minimal influence on the VIS and NIR reflectivity of the metal Mechanical protection and cleanability can, however, only be reached by relatively thick protective layer systems Optimization of the protective layer system for the wavelength of interest is particularly necessary in the UV Figure 9: Reflectance of several metals versus wavelength (taken from [2])

34 32 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES METAL-DIELECTRIC COATINGS Metal-dielectric coatings In general, all layer systems consisting of metals and dielectric layers can be called "metal dielectric coatings". The most familiar ones are metal-dielectric filters consisting of transparent metal layers which are separated by a dielectric layer. These filters are characterized by extremely broad blocking ranges which result from the reflectivity and absorption of the metallic layers. The spectral position of the transmission band is determined by the optical thickness of the dielectric spacer layer. AOI= AOI= In this catalog we want to draw the attention of the reader, however, to metal-dielectric reflectors. Metals and metallic coatings show an extremely broadband natural reflectivity which is, however, restricted to about 9 % in the UV spectral range (aluminum), 96% in the VIS (silver) and 99% in the NIR (gold and silver). Moreover, most of the metals must be protected by dielectric coatings to overcome limitations of chemical (silver) or mechanical stability (aluminum, silver, gold). More strictly speaking, almost all metallic mirrors are metal-dielectric coatings. The protective coatings always influence the reflectivity of the metals. Single dielectric layers of any thickness lower the reflectivity in most parts of the spectrum. However, multilayer coatings on metals can increase the reflectivity of the metallic coating. The bandwidth of enhanced reflectivity can also be optimized for extremely broad spectral ranges as can be seen in fig. 9. For more examples please see pages 86 87, and Figure 1: Reflectance spectra of a protected silver mirror ( and a metal dielectric silver mirror (, both optimized for high reflectivity in the VIS for use in astronomical applications Literature: [1] P. W. Baumeister "Optical coating technology", SPIE press monograph, PM 137, Washington 24 [2] H. A. Macleod "Thin film optical filters", A. Hilger, Bristol, 1986 [3] A. J. Thelen "Design of optical interference coatings", McGraw Hill, New York 1989 [4] N. Kaiser, H.K.Pulker (eds.) "Optical interference coatings", Springer Verlag Berlin Heidelberg, 23

35 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 33 MEASUREMENT TOOLS FOR Coatings Spectrophotometry Standard spectrophotometric measurements in the wavelentgh range λ = 19 nm 32 nm are carried out with commercial spectrophotometers PERKIN ELMER Lambda 15 PERKIN ELMER Lambda 95 PERKIN ELMER Lambda 75 PERKIN ELMER Lambda 19 ANALYTIK JENA specord 25 plus. Cavity Ring-Down (CRD) Measurement High reflectivity and transmission values in the order of R, T = 99,5%... 99,9999 % are determined by Cavity Ring-Down Time measurements. This method has a high accuracy, e.g. R = (99,995 ±,1)%, and it is an absolute measurement procedure. R 1 = 99,99 % R 2 = 99,99 % LAYERTEC comes with various CRD systems which were developed in cooperation with research institutes and universities. A schematic representation of the CRD method is shown in fig. 2. intensity For measurements beyond this wavelength range, LAYERTEC is equipped with an FTIR spectrometer (λ = 1µm 2µm) and a VUV spectrophotometer (λ = 12nm 24nm). Please note that the absolute accuracy of spectrophotometric measurement amounts,2...,4% over the full scale measurement range R, T =...1 %. For measurements with higher precision, a self-constructed setup in the limited range T =,1,1% with an accuracy up to,2ppm is available. laser pulse I in L cavity I I 1 I 2 I 3 time Figure 2: Schematic representation of the CRD method Measurement Technologies computations, measurements report digital oscillograph FTIR- Spectrometer 1µm 2µm 1 21 Spectrophotometer Standard 19nm 32nm VUV 12nm 24nm Cavity Ring-Down Measurement 21nm 18nm laser source fast photodetector 12,1,1, ,9 99,99 99,999 99,9999 Reflectivity, Transmission [%] mode matching optics Ring-Down cavity Figure 1: Measurement technologies and their range for reflectivity and transmission measurements at LAYERTEC Figure 3: Schematic representation of a Cavity Ring-Down setup Figure 4: Exemplary mono-exponential CRD-curve of a highly reflecting mirror pair for 45nm with R = 99,995% measured using a resonator length L=228 mm

36 34 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES A laser pulse is coupled into an optical cavity consisting of two highly reflecting mirrors. The intensity of the light behind the cavity is measured. At the beginning, the intensity increases during the pulse duration. Then it decreases exponentially with the time constant τ according to With R 1, R 2 and R 3 being the reflectivities of the mirrors 1, 2 and 3, respectively, and RM 12, RM 23 and RM 13 being the measured geometric means of the reflectivities for the pair of mirrors with the corresponding numbers, three measurements of mirror pairs provide 1, 99,95 99,9 99,85 99,8 99,75 AOI= with I T = I exp (- τ t ) τ = L c (1 RM) where c is the speed of light and L is the cavity length. RM is the geometric mean of the mirror reflectivities and can be derived from the measurement of the time constant by RM = (R 1 R 2 ) = 1 L cτ The accuracy of the measurement mainly depends on the accuracy of the time measurement and the measurement of the cavity length. Please note that errors of beam adjustment will always lower the decay time and/or will cause non mono-exponential Ring-Down curves. Thus, in the case of a monoexponential decay (fig. 4), stochastic errors cannot result in overstated reflectivity values. Compared to a reflectivity measurement in a spectrophotometer, CRD has two main advantages: It is applicable for very high reflectivities (and transmission values when using an enhanced measurement setup). It is impossible to get measurement values which are higher than the real ones. The reflectivity of single mirrors can be derived from the pairwise measurements of a triplet of mirrors. (1) (2) (3) RM 12 = R 1 R 2 RM 23 = R 2 R 3 RM 13 = R 1 R 3 RM 12 RM 13 R 1 = RM 23 RM 23 RM 12 R 2 = RM 13 RM 13 RM 23 R 3 = RM 12 In practice this method is often used to determine the reflectivity of a set of reference mirrors. Knowing the reflectivity of a reference mirror, the reflectivity of a specimen mirror can directly be derived using equation (3). Broadband Cavity Ring-Down setup and applications LAYERTEC has used CRD for the qualification of low loss mirrors for some years. Though there was the limitation that only discrete wavelengths, either generated by solid state lasers or diode lasers, (4) Solving this system of equations the mirror reflectivities can be calculated by (5) 99,7 99,65 99, Figure 5: Reflectivity spectrum of a negative dispersive broadband mirror for the wavelength region 65 11nm with R>99,9%. The measurement was performed by using an optical cavity consisting of 2 identical mirrors were used. The increasing demands concerning the optical properties of spectrally broadband mirrors required a measurement system for a spectral range over several hundreds of nanometers with a very high accuracy for measuring high reflectivities. So LAYERTEC has developed a novel spectrally broadband Cavity Ring-Down Time measurement system in cooperation with the Institute of Photonic Technology (IPHT) Jena e.v.*. An optical parametric oscillator (OPO), which is pumped by the third harmonic of a Nd:YAG laser is used as light source. By the use of an additional wavelength range extension towards the ultraviolet region, a spectral measurement range from 21 18nm is covered without gaps. Photomultipliers and avalanche diodes are used as detectors. The Ring-Down cavity can consist of two or three cavity mirrors. A two mirror cavity is used for reflectivity measurements at angle of incidence (Fig. 5 shows such a measurement).

37 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 35 The mirrors are mounted on precision rotary stages. So the setup can be used for wavelength scans at constant angle or for angle resolved measurements with a constant wavelength (see fig. 6). If the reflectivity of two mirrors is known, the reflectivity of the third mirror can be calculated. When the incidence of light is not perpendicular to the sample, the linear polarization of the OPO output beam can be rotated to set up perpendicular (s-) or parallel (p-) polarized light with respect to the sample over the entire spectral measurement range. mirror 3 Furthermore the system can be used for the measurement of high transmission values T > 99,5 %. For this purpose, the transmission sample is placed between both cavity mirrors. Since the sample is an additional optical loss between the cavity mirrors, the transmission value can be calculated if the reflectivity of the mirrors is known. For measurements at a defined angle of incidence, the sample can be tilted in the range of 75 with respect to the optical axis of the cavity (Fig. 7). Wavelength resolved measurements as well as angle resolved measurements are possible. The latter is very useful for the determination of the optimum angle for Thin-Film-Polarizers (TFP). Measurements and reports can be provided on request. The broadband setup is permanently under further development. The measurement functions and the performance increase steadily. * S. Schippel, P. Schmitz, P. Zimmermann, T. Bachmann, R. Eschner, C.Hülsen, B. Rudolph und H. Heyer: Optische Beschichtungen mit geringsten Verlusten im UV-VIS-NIR-Bereich, Tagungsband Thüringer Grenz- und Oberflächentage und Thüringer Kolloquium "Dünne Schichten in der Optik" 7. 9.September 21, Gera; S mirror 1 2Θ mirror 1 mirror 2 mirror 2 (sample) Θ sample p-pol. 1, 99,99 99,98 99,97 99,96 99,95 99,94 99,93 99,92 99,91 99, angle of incidence [ ] Figure 6: Schematic representation of a V-cavity ( CRD reflectivity measurement ( with variable angle of incidence, but with fixed wavelength 164nm on a special separator for intra cavity frequency doubling. A V-shaped CRD cavity is used for the measurement. For the reason of analyzing the polarization dependency of the separator exactly, the measurement was performed at parallel (p-polarization) and perpendicular (s-polarization) polarization with respect to the sample (mirror 2) R,T [%] 1, 99,75 99,5 99,25 99, 98,75 98,5 98,25 Reflectivity Transmission p-pol. 98, Figure 7: Schematic representation of a cavity for T-measurement (, CRD measurement ( of TFP for 39nm: blue curve - Rs(V-cavity), red curve - Tp (two mirror cavity)

38 36 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Laser Induced Damage Threshold (LIDT) Damage in cw and ns laser optics is mainly related to thermal effects such as increased absorption either intrinsic absorption in the coating materials or absorption by defects or poor thermal conductivity and low melting temperatures of the coatings. High power coatings require both the controlling of the intrinsic properties of the coating materials and the reduction of defects in the layers. Laser damage of picosecond and femtosecond laser optics is mainly caused by field strength effects. High power coatings for these lasers require very special coating designs. The determination of the laser induced damage threshold (LIDT) according to the standards ISO (cw-lidt and 1 on 1 LIDT, i.e. single pulse LIDT), ISO (S on 1, i.e. multiple pulse LIDT) and ISO (LIDT for a certain number of pulses) requires laser systems operating under very stable conditions, precise beam diagnostics as well as online and offline damage detection systems. This is the reason, why only a limited number of measurement systems with only a few types of lasers is available (e.g. for 164nm at Laserzentrum Hannover). For some of the most prominent laser wavelengths such as e.g. Argon ion lasers (488nm or 514nm), there is no measurement system available and no certified LIDT data can be provided. LIDT MEASUREMENT SETUP AT LAYERTEC LAYERTEC has developed its own LIDT measurement setup for in-house measurements with the aim to optimize the coatings concerning their stability against laser damage. The light source is a q-switched Nd:YAG laser which can emit the wavelengths 164, 532, 355 and 266nm. The pulse duration is about 4-1 ns and the repetition rate is 1 Hz at all 4 possible wavelengths. A close-to-gaussian shaped beam profile is generated by focusing the laser beam with a lens. The spot size is in the region of 2µm...1µm (1/e² radius). The accurate value depends on the wavelength and the focal length of the lens. The setup fulfils the requirements of ISO It has an online detection system based on a digital Sample stage Motorized positioning along x-, y- and z-axis. Angle of incidence - 75 CCD Spatial beam profile measurement Obj. CCD camera with fast image processing to inspect the sample with respect to damage after every laser pulse. Online beam profile measurements and the determination of the energy density are carried out with a CCD camera beam profiler in combination with calibrated energy measuring heads with single pulse resolution. A motorized 3-axis stage and a sample holder for multiple pieces allow automated measurements at angles of incidence in the range of...7 either on reflecting or transmitting samples. The linear polarization of the laser beam is set up to p- or s-polarization with the help of wave plates and a spectrally broadband polarizer for the desired wavelength. The measurement setup is schematically shown in fig. 1. Online damage detection system The 1-on-1-LIDT (i.e. 1 pulse on 1 site of the sample) is not representative for the normal operation conditions. However, these values can be used for comparison of different coatings and for optimization procedures. Moreover, the 1 on 1 values are directly related to the more practical S-on-1-LIDT (LIDT for a given number "S" of pulses on the same site of the sample) and can be interpreted as upper limit of the LIDT. Laser systems with high repetition rates (some khz) require lifetime tests expressed by LIDT values for high numbers of pulses. Nd:YAG Laser / 532 / 355 / 266nm 1Hz, 4-1ns, 164nm Energy meter and temporal beam profile measurement λ 2 BBO polarizer Figure 1: Nanosecond Nd:YAG Laser LIDT measurement setup at LAYERTEC.

39 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 37 LAYERTEC tests samples according to its selfdeveloped procedure (please see next paragraph "LAYERTEC LIDT Testing Procedure...") because the ISO standards deliver wrong values at damage thresholds which are higher than 3 J/cm². But, if there is an explicit request for a measurement according to ISO , we have the ability to do this. In this case, to reduce the expenditure of time, we apply only 1 or 1 pulses at each measurement position: Figure 2: Damage probability of an anti reflection coating for 355nm after 1 pulses (pulse duration 7ns, 1Hz repetition rate) according to ISO This measurement was accomplished at LAYERTEC. This is not a test for longtime stability but the LIDT result after 1 or 1 pulses is more realistic in comparison with a 1-on-1 LIDT result. Our measurements are primarily intented to compare LAYERTEC coatings among themselves for the purpose of coating and technology development. We provide LIDT results on request, but please note that these results are only valid for the specific measurement conditions (pulse duration, wavelength, number of pulses, beam shape, repetition rate). LAYERTEC LIDT TESTING PROCEDURE FOR PULSED LASER SOURCES During the last two years LAYERTEC has gained a lot of experience by applying the LIDT testing procedures according to ISO standards. It became clear that the measurements are cost- as well as time-intensive and the results often are questionable. Significantly lowered damage thresholds and simultaneous strongly distorted damage probability distributions were observed in many cases. After applying technical troubleshooting on the measurement setup without success, other reasons appeared to be responsible for the assumed measurement errors. As mentioned above, we use a relatively large gaussian-shaped laser spot to measure the damage threshold of our coatings. The spot size amounts up to one millimeter (1/e² diameter) with the benefit of applying a uniform energy density distribution in a relatively large irradiated surface area at each measurement position. Large spot sizes demand high laser energy and peak power to reach the appropriate fluence which finally causes destruction of the testing position. For example, coatings with damage thresholds above 5J/cm² require several hundreds of millijoules laser pulse energy to get destructed. In this case, large amounts of debris are generated and deposited within a circular zone of several millimeters in diameter around the damaged position. If an adjacent test position is located within this zone, its damage threshold is significantly reduced by the influence of debris. This systematic error can be avoided by choosing large distances between the measurement positions, with the limitation of reduced availability of measurement positions. According to our experience, when very high damage thresholds above 1J/cm² are reached, the distance between adjacent positions should exceed 1 millimeters. Furthermore, the ISO standard assumes a symmetric distribution of the damage probability. We observe this behavior only at average damage thresholds below 3J/cm². Threshold probability distributions with average damage values above 3J/cm² are significantly distorted along the direction of lower values. The main reasons for this phenomenon are imperfections in the coating and sometimes the surface quality itself, supposed that the influence of debris can be neglected. Contrary to ISO standards, significant low thresholds must not be treated as statistical outliers strictly, rather they have to be taken into account. Otherwise, damage threshold measurements would provide wrong values. On the basis of the mentioned problems we made the conclusion, that LIDT tests according to ISO standards are no more viable for our coatings with high damage thresholds. That is why we developed our own LIDT measurement method, which is well suited to measure the minimal damage threshold of optical coatings for high-power or high-energy laser applications. This procedure requires 4 7 testing positions, in a distance of approximately 1mm to each other on the sample. Wherever applicable, four identical samples with 25mm in diameter are used to get measuring positions per testing procedure. Every position is irradiated with stepwise increasing energy densities, whereby the energy values are subdivided into 5 energy classes. Generally, 1 laser pulses are applied at each energy class for the consideration of accumulation effects in the coating. The starting energy has to be low enough to prevent any laser induced damage. Then the energy is increased until laser induced damage occurs at the testing position. Figure 3: Sample stage of the LIDT measurement setup

40 38 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES All positions on the sample are irradiated in this way until every position is destructed. For the purpose of measurement analysis, the highest, the average and the lowest measured damage threshold is reported. A further statistical analysis is not carried out. An exemplary LIDT measurement report is shown in fig. 4. All damage threshold values measured at LAYERTEC GmbH, which are stated in this catalog, are determined according to the LAYERTEC LIDT testing procedure for pulsed laser sources. The LIDT values given in this catalog are own measurements but also measurements which were taken by our partners Laser Zentrum Hannover, Laser Labor Göttingen and Friedrich-Schiller- Universität Jena. The limited number of measurement facilities and the need for lifetime tests for practical applications make it necessary to include also measurements, lifetime tests or cumulative irradiation tests of several customers into this catalog. Please take into account that these values cannot be compared with a standardized LIDT measurement, because the laser parameters given there are those without damage. Moreover, there is always an uncertainty of these values, especially with respect to the determination of the spot size. Errors in the order of about 3% must be taken into account. Nevertheless, we think that information on parameters of successful operation of our optics will certainly help to decide to use LAYERTEC optics. Sometimes, however, tests at the customers laser system will be necessary. LAYERTEC supports such tests at the customers facility by a considerable discount for the test pieces. Figure 4: Exemplary LIDT measurement report

41 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 39 Intra cavity Heating Measurement Setup Absorption losses in optical coatings lead to the heating of the coating and the substrate. At an average laser power of several kilowatts (cw) and higher, even low absorption losses in the range of some parts per million cause significant heating of the optical component. LAYERTEC has built a heating measurement setup for the purpose of quality assurance and technology development on high-power optical components for the wavelength 13 nm. An Yb:YAG thin disc laser is used to generate a high power laser beam (fig. 1). The setup consists of the laser disc and the pump-chamber, the sample (e.g. a highly reflecting mirror) which works as a folding mirror, a second folding mirror, the output coupler, a laser power meter and a pyrometer for the temperature measurement on the sample. The beam spot size on the irradiated sample surface area is 1,5mm (1/e²) in diameter. Very high laser power about 12kW (cw) inside the resonator is achieved by choosing an output coupler with a relatively low transmission value. Under these conditions, the peak power fluence on the sample is approximately 15 MW/cm². Laser Disc Sample (HR-Coating) Folding Mirror Output Coupler Power- Meter Pyrometer Figure 1: Intra cavity heating measurement setup for HR-mirrors at 13nm For example, the irradiated zone (1,5 mm in diameter) of a HR-coating with an absorption loss of 5ppm at 13 nm is heated to a temperature of about 8 C when applying a power of 8kW. Generally, coatings with a setup-specific heating temperature lower than 1 C can be used for high power applications. Please note that the average temperature of the optical component which is measured, is clearly lower than the temperature within the small irradiated zone on the coating. For the purpose of realizing absolute absorption measurements, it is possible to calibrate the setup with a set of samples with well known absorption. The absorption measurement of the calibration samples was accomplished by the LIDT measurement setup at the Institute of Photonic Technology (IPHT) in Jena. Defect Inspection System For Coatings LAYERTEC is equipped with a measurement system to count and classify defects in optical coatings. The system is able to inspect coated surfaces completely. It detects defects in the dimension down to 4μm. Small optical components as well as large optics with a diameter up to Ø 6 mm can be analyzed. Small and medium sized pieces are placed in a special sample holder magazine which enables the automated measurement of a large number of pieces in a single inspection run (fig. 2). Measuring small defects is very challenging, because the necessary microscope objectives have a very short depth of focus and require a precise adjustment and positioning. Another importance is the proper lighting. Depending on the geometry of the defect, it can only be seen when it is illuminated in a certain angle. Finally our broad bandwidth of different geometries calls for a very flexible controller software, quick in defining new geometries while avoiding collisions. We continously improve the system, enabling it for the inspection of cylindrical and aspherical optics and reducing the measurement effort and time. When the inspection procedure is accomplished, the defects are sized and classified according to DIN EN ISO Measurement reports of each inspected piece are generated. An exemplary inspection report is shown in fig. 3. Figure 2: Laser mirrors are placed in a special holder magazine for automated defect inspection at LAYERTEC. Figure 3: Simplified inspection report of a laser mirror. The report shows the sizes and the coordinates of large defects on the coated surface. Furthermore, all defects which were found, are shown in a histogram plot.

42 4 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES

43 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 41 Selection of optical components for common laser types

44 42 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Components for F 2 Lasers MIRRORS OUTPUT COUPLERS AND LENSES VARIABLE ATTENUATORS Measurement Measurement Measurement R T AOI = R=5±3% R=25±2% AOI = R, T [%] T [%] Measurement Rs Ts Rp Tp AOI = 45 Figure 2: Measured reflectance spectra of standard output couplers with R = 5 ± 3% and R = 25 ± 2 % (rear side uncoated) Tp Ts Tr λ = 157nm R, T [%] Figure 1: Measured reflectance and transmittance spectra of a laser mirror (AOI =, and a turning mirror (AOI = 45, for 157nm Laser mirrors: R = % at AOI = Turning mirrors (AOI = 45 ) : R s > 97 % R p > 9 % R r > 92 % High quality mirror substrates, windows and lenses of CaF 2 (193nm excimer grade quality, HELLMA Materials GmbH) Please note that 157nm excimer grade CaF 2 can no longer be offered. The market for this kind of material is too small for the huge effort which has to be made by the crystal manufacturers to test the material for this quality standard. Thus, all optics for F 2 lasers will be manufactured from 193 nm excimer grade material in the future. PR coatings with tolerances of ± 2 % for R = % ± 3 % for R = % and ± 2 % for R = % Development and production of customer specific components like beamsplitters and variable attenuators on request T [%] angle of incidence [deg] Figure 3: Measured transmittance spectra of a variable attenuator at different AOI ( and transmittance vs. AOI (; the transmittance varies from T > 75 % at AOI = to T < 5 % at AOI = 45

45 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm ALUMINUM MIRRORS for F 2 Lasers TECHNICAL DATA OF STANDARD F 2 LASER COMPONENTS Measurement AOI = Measurement AOI = Figure 4: Reflectance spectrum of a protected Al mirror ( and an enhanced Al mirror for 157nm ( Protected Al mirrors (optimized for 157nm): R=74 78% Dielectrically enhanced Al mirrors: up to R=94% at AOI= For more information on Al mirrors see pages Coating Spectral performance Lifetime tests HR (, 157nm) R = % 2 x x 1 9 pulses* HR (45, 157nm) R = % (random pol.) PR (, 157nm) R = 5 ± 3 % 2 x x 1 9 pulses* PR (, 157nm) R = 25 ± 2 % 2 x x 1 9 pulses* Attenuator T = 67 ± 3 % 5 x 1 7 pulses**, no damage Attenuator T = 33 ± 3 % 1 x 1 8 pulses***, no damage Beam splitter T = 2 ± 3 % 1 x 1 8 pulses***, no damage AR (, 157nm) % * Energy density: 25mJ/cm², repetition rate: 8Hz, pulse duration: 15ns; tested at COHERENT AG, München ** Energy density: 15mJ/cm², rep. rate: 2Hz, pulse duration: 2ns; tested at Institut für Photonische Technologien (IPHT) Jena *** Energy density: 2mJ/cm², rep. rate: 5Hz, pulse duration: 2ns; tested at Institut für Photonische Technologien (IPHT) Jena COMPONENTS FOR THE FIFTH and Sixth HARMONIC of ti:sapphire Lasers Measurement Rs Rr Rp AOI = 45 p-pol. AOI = Figure 5: Reflectance (measured, and GDD - spectra (calculated, of a turning mirror for 16nm (AOI = 45 ) Mirrors and separators for the 16nm range and the 133nm range are produced by coating techniques which were developed for F 2 laser coatings. For more information please see pages

46 44 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR ArF lasers MIRRORS OUTPUT COUPLERS AND LENSES VARIABLE ATTENUATORS Measurement Measurement Measurement R T AOI = PR 25% PR 5% AOI = λ = 193nm R, T [%] T [%] Measurement Measurement R T AOI = 45 AOI = Tp Tr Ts λ = 193nm ,5 3 T [%] , , , angle of incidence [deg] Figure 1: Measured reflectance and transmittance spectra of a cavity mirror ( and of a turning mirror (AOI=45 ) for 193nm for unpolarized light ( Figure 2: Measured reflectance spectra of output couplers with R(,193 nm) = 5 ± 3% and R (,193 nm) = 25 ± 2% (rear side uncoated) ( and of a CaF 2 window coated on both sides with a fluoride AR coating for 193nm ( Figure 3: Measured transmittance spectra of a variable attenuator at different AOI ( and transmittance vs. AOI (; the transmittance varies from T > 88 % at AOI = to T < 2 % at AOI = 45 All fluoride systems guarantee high reflectivities and high damage thresholds High quality mirror substrates, windows and lenses of CaF 2 (193nm excimer grade, HELLMA Materials GmbH) and fused silica Development and production of customer specific components such as beamsplitters and variable attenuators on request PR coatings with tolerances of ± 2% for R = % ± 3% for R = % ± 2% for R = % and ± 1% for R > 9 % Single wavelength AR coating with residual reflectivities of R <.25% at AOI = and R<.6% at AOI = 45 (unpolarized light) Broadband and multiple wavelength AR coatings Attenuators with different transmission ranges on request Attenuators can be delivered with AR coated compensation plates of CaF 2 or fused silica

47 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm ALUMINUM MIRRORS TECHNICAL DATA OF STANDARD ArF LASER COMPONENTS Measurement AOI = Coating / reflectance Substrate Damage threshold* Lifetime test Fluoride coatings AR (, 193nm) R<.25% CaF J/cm pulses, no damage** AR (, 193nm) R<.25% fused silica 2 3 J/cm 2 PR (, 193nm) R=25% CaF J/cm pulses, no damage** PR (, 193nm) R=5% CaF J/cm pulses** HR (, 193nm) R>96% CaF J/cm pulses **, no damage 4 x 1 9 pulses ***, no damage HR (45, 193nm) R>95% (random polarized) CaF J/cm 2 Measurement p-pol AOI = Figure 4: Measured reflectance spectra of a protected Al mirror optimized for 193nm ( and an enhanced Al mirror for 193nm, AOI = 45 ( Enhanced aluminum mirrors: Rp > 93% Rs > 98% Rr > 96% For more information on aluminum mirrors see pages * 1 - on -1, 14 ns; measurements were performed at Laser Labor Göttingen, Laser Zentrum Hannover and at Friedrich - Schiller - Universität Jena ** Energy density: 55mJ/cm², repetition rate: 1kHZ, pulse duration 15ns; tested at COHERENT AG, München *** Energy density: 8mJ/cm², repetition rate: 1kHz, pulse duration: 12ns; tested at COHERENT AG, München Components for the 2 nm range Measurement Rs Rr Rp AOI = 45 p-pol. AOI = Figure 5: Reflectance (measured, and GDD - spectra (calculated, of a turning mirror for 2nm (AOI = 45 ) Mirrors and separators for the 2nm range are produced by coating techniques which were developed for ArF laser coatings. For more information please see pages

48 46 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR KrF, XeCl and XeF LASERS Cavity Mirrors Output Couplers Windows and Lenses AOI = AOI = AOI= AOI= Figure 1: Reflectance and transmittance spectra of a 38nm cavity mirror Oxide coatings for high mechanical stability Coatings can be produced by IAD, magnetron sputtering or IBS Figure 3: Reflectance spectrum of an output coupler for 38 nm R(, 38nm) =5±3% PR coatings with tolerances of ±2% for R = % ±3% for R = % ±2% for R = % and ±1% for R > 9% Figure 5: Reflectance spectra of an AR coating for 38nm for AOI= 3 High quality mirror substrates, windows and lenses of fused silica FLUORINE RESISTANT CAVITY MIRRORS FLUORINE RESISTANT Output Couplers FLUORINE RESISTANT Window AOI = AOI = AOI = Figure 2: Reflectance spectrum of a fluoride KrF cavity mirror Fluoride coatings and CaF 2 substrates for high stability against fluorine and chlorine Laser mirrors (R > 98 % at 248 nm and 38 nm, R>96% at 351nm) Figure 4: Reflectance spectrum of a fluoride output coupler with R(, 248nm) =5±3% PR coatings with tolerances of ±2% for R = % ±3% for R = % and ±2% for R = > 9% Figure 6: Reflectance spectrum of a fluoride AR coating for 248nm High quality mirror substrates, windows and lenses of CaF 2 (248nm excimer grade or UV quality, HELLMA Materials) Extended lifetimes at high energy densities at 248nm

49 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm, 38 nm, 351 nm TURNING MIRRORS ATTENUATORS LIDT AND LIFETIME DATA Measurement Rs Rp AOI = 45 AOI= AOI= 2 AOI= 3 AOI= 4 LIDT of oxide coatings: 1, 99,8 99,6 99,4 99,2 99, 98,8 98,6 98,4 98,2 98, T [%] Coating HR (, 248nm) IAD HR (45, 248nm) IAD LIDT* [J/cm 2 ] 1 (1-on-1) 5(1-on-1) 1 (1-on-1) Figure 7: Reflectance spectra of a turning mirror for 38nm produced by IBS, reflectance measurement in s- and p-polarization by CRDS Reflectivity of turning mirrors (AOI = 45 ): Coating Rs [%] Rp [%] 248nm IAD >99,5 >99, 248 nm sputtering > 99,8 > 99,6 38nm IAD >99,8 >99,5 38 nm sputtering > 99,9 > 99,7 351nm IAD >99,9 >99,7 351 nm sputtering > 99,95 > 99,9 T [%] λ = 38nm 45 AOI [deg] Figure 8: Transmittance spectra of a variable attenuator for 38nm at different AOI ( and transmittance vs. AOI (; the transmittance varies from T < 1% at AOI = to T > 9% at AOI = 4 (unpolarized light) Lifetime of fluoride coatings: Coating Lifetime HR (, 248nm) 2 x 1 8 pulses** PR (, 248nm) = 5% 2 x 1 8 pulses** AR (, 248nm) 2 x 1 8 pulses*** HR (, 38nm) 2 x 1 8 pulses*** HR (, 351nm) 2 x 1 8 pulses*** PR (, 351nm) = 25% 2 x 1 8 pulses*** Reflectivity of laser mirrors (AOI = ): Coating R [%] 248nm IAD > 99, 248 nm sputtering > 99,7 38nm IAD > 99,7 38 nm sputtering > 99,9 351nm IAD > 99,9 351 nm sputtering > 99, R( 1%) T( 1%) T( 1%) AOI = Figure 9: Transmittance spectrum of a sputtered attenuator for 38nm with exactly adjusted and thermally stable transmittance of T =.2% at AOI = 45 (unpolarized light) * Measurements were performed at Laser Labor Göttingen and at Friedrich-Schiller Universität Jena ** Energy density: 1mJ/cm 2, rep. rate: 1Hz, pulse duration: 15ns; tested at COHERENT AG, München *** Energy density: 55mJ/cm 2, rep. rate: 1Hz, pulse duration: 15ns; tested at COHERENT AG, München

50 48 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR RUBY AND ALEXANDRITE LASERS BEAM COMBINERS Ruby and Alexandrite Lasers are especially used for medical laser applications and work at 694nm and 755nm, respectively. LAYERTEC offers a wide range of laser optics for both wavelengths with high laser induced damage thresholds and long lifetimes. Besides typical combinations with wavelengths for the alignment of the optical system (e.g. 694nm + 633nm) a special feature of LAYERTEC products are the variety of combinations with other common wavelengths used for medical applications in the same device, but from different laser sources (e.g. 532nm + 694nm) AOI = 45 CAVITY MIRRORS R(-1%) R(99-1%) AOI = TURNING MIRRORS AOI = R(-1%) R(99-1%) AOI = Figure 1: Reflectance spectra of cavity mirrors for 694nm ( and 755nm ( Reflectivity: R> R>99.9% at AOI= High damage thresholds (8 MW / cm 2, 35ns pulse length) AOI = Figure 2: Reflectance spectra of turning mirrors for 694nm ( and 755nm ( Reflectivity: R>99.5% at AOI=45 for random polarized light integrated pilot laser beam alignment (e.g. at 63 65nm) High damage thresholds (8 MW/cm 2, 35ns pulse length) AOI = Figure 3: Reflectance spectra of special beam combiners for 694nm and 633nm: PRr(45,694nm) = 99.+,3% + Rr(45,633nm) < 35% Rr(45,63 64nm) > 35% + Rp(45,694nm) <.3% Precisely adjusted degree of reflectivity by using sputtering technology Integrated pilot laser beam alignment (e.g. at 635nm) High performance and cost-optimized solutions with special designs

51 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm, 755 nm Output Couplers Windows AND LENSES COMPONENTS FOR COMBINING RUBY LASERS WITH OTHER HIGH POWER LASERS AOI = AOI = Rr(-1%) Rr(9-1%) AOI = Figure 4: Reflectance spectrum of an output coupler for the ruby laser: PR(,694nm) =2±2% AOI = Figure 5: Reflectance spectrum of an output coupler for the alexandrite laser: PR(, 755nm) =85±2% Output couplers with precisely adjusted degree of reflectivity AOI=3 AOI=3 p-pol. AOI= Figure 6: Reflectance spectra of AR coatings for 694nm and 755nm: AR(,694nm) <.2%, AR( 3, 755nm) <.5% AR coatings with residual reflectivities R<.2% on the rear side of output couplers as well as on both sides of lenses and windows made of fused silica Figure 7: Reflectance spectrum of a triple wavelength turning mirror for 532nm, 694nm and 164nm (for unpolarized light) R>99% at all three wavelengths (AOI=45, unpolarized light) High laser damage thresholds AOI = 12 Figure 8: Reflectance spectrum of a triple wavelength AR coating for 532nm, 694nm and 164nm

52 5 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Components for Ti : Sapphire lasers in the ns regime Pump MIRRORS Turning MIRRORS On these pages we present optical components for Ti:Sapphire lasers which are operated with ns pulses. Please note that all of these components are optimized for smooth group delay (GD) spectra in order to achieve wide tuning ranges. However, these components are not optimized for group delay dispersion (GDD). Such optics which are necessary for fs pulses are introduced on pages 74ff. Cavity MIRRORS 1, 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, 55 AOI = GD [fs] R ( -1%) R (99-1%) AOI= AOI = Figure 2: Reflectance ( and GD ( spectra of a broadband pump mirror GD [fs] 1, 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, p-pol. AOI= p-pol. AOI= Figure 3: Reflectance ( and GD ( spectra of a broadband turning mirror GD [fs] Special features: Very high reflectance of the mirrors (R > 99.9 % R > % depending on the design) Spectral tolerance: 1% of center wavelength Center wavelength, bandwidth and reflectivity of partial reflectors according to customer specification Figure1: Reflectance ( and GD ( spectrum of a broadband laser mirror

53 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm bandwidth Output Couplers and Beamsplitters Special Components Tolerances: Standard output couplers (bandwidth: nm): R = 1 7 % ± 2.5 % R = 7 9 % ± 1.5 % R = 9 95 % ±.75 % R = % ±.5 % R > 98 % ±.25 % Tolerances: Broadband output couplers (bandwidth: 2 6 nm): R = 1 7 % ± 3 % R = 7 9 % ± 2 % R = 9 95 % ± 1 % R = % ±.5 % T [%] AOI = standard broadband AOI= unpol. p-pol. AOI=45 Figure 6: Transmittance spectrum of a narrow band intra cavity filter for 86nm which is used to select one wavelength from the Ti:Sapphire spectrum AOI = p-pol. AOI = 45 AOI = GD [fs] 2,8,7,6, ,4,3,2, Figure 4: Reflectance spectra of a standard and a broadband output coupler ( and of an output coupler with a special reflectance profile which enables the compensation of the amplification characteristics of the laser (; see also B. Jungbluth, J. Wueppen, J. Geiger, D. Hoffmann, R. Poprawe: "High Performance, Widely Tunable Ti:Sapphire Laser with Nanosecond Pulses" in: Solid State Lasers XV: Technology and Devices, Proc. of SPIE Vol. 61, 61-2, San Jose 26 Figure 5: Reflectance ( and GD ( spectra of a broadband beam splitter PRr(45, 65 15nm) = 5 ± 3% Figure 7: Reflectance spectrum of a broadband anti reflection coating with extremely low residual reflectivity: AR (,7 9nm) <.5%

54 52 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR DIODE LASERS SCANNING MIRRORs Thin Film Polarizers Diode lasers are widely used for measurement applications, as pilot lasers, for pumping of solid state lasers and for direct materials processing. Although diode lasers do not require external resonator optics and are mostly coupled to fibers, many applications require high quality beam steering optics such as beam combiners or scanning mirrors which are shown on the following pages. For more information on pump mirrors for solid state lasers and combiners for diode lasers please see also pages and TURNING MIRRORS 1 99,8 99,6 99,4 99,2 99, 98,8 98,6 98,4 98,2 98, Figure 1: Reflectance spectra of a broadband turning mirror which can be used for all diode lasers between 88nm and 98 nm (AOI = 45, s - and p - polarization) Rs Rp AOI = 45 AOI=22 AOI=45 AOI= AOI=22 AOI=45 AOI= Figure 2: Reflectance spectra of a scanning mirror for diode lasers between 85 and 94 nm combined with R > 5 % between 63 and 67 nm (pilot laser): HRr (22 58, nm) > 99.3 % + Rr (22 58, nm) > 5 % Rs Rp AOI = Figure 3: Reflectance spectra of a thin film polarizer for 94 nm, AOI=55 Rp ( 1%) Rp ( 1%) Rs ( 1%) Rs (99 1%) AOI = Figure 4: Reflectance spectra of a broadband thin film polarizer for 94 97nm: HRs(45,94 97nm) > 99.9% + Rp(45,94 97nm) < 1% Scanning mirrors with other specifications on request For more information and examples on scanning mirrors please see pages and Thin film polarizers are especially useful for polarization coupling of high power laser diodes For high power 94nm radiation we recommend to use SUPRASIL 3 or SUPRASIL 31/32 as substrate material, because standard fused silica shows an absorption band around this wavelength (see page 2)

55 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm, nm CONVENTIONAL STEEP EDGE COMBINERS FOR DIODE LASERS SPECIAL STEEP EDGE COMBINERS FOR UNPOLARIZED LIGHT Windows and lenses Tp Ts Tr AOI = 22,5 Tp Ts Tr AOI = 45 AOI = T [%] 1 8 T [%] 1 8 5, 4,5 4, 3, , 2, , 1, ,, T [%] Figure 5: Transmittance spectra of a conventional steep edge filter HR (98 nm) > 99.9 % and HT (915 nm) > 95 % which is used as combiner for pump laser diodes at 915 nm and 98 nm; AOI = 22.5 ( and AOI = 45 ( At AOI = 22.5 the conventional steep edge filter separates 915nm and 98nm for p - and s - polarized and thus also for unpolarized light To preserve the steep edge at AOI = 45 the radiation must be polarized and only one polarization can be used. Unpolarized light changes the slope of the edge significantly Figure 6: Transmittance spectra of a special steep edge filter HRr (45, Tp Ts Tr AOI = 45 98nm) > 99.8% + HTr (45, 94nm) > 97% 2, Filters of this type can be used as separators or combiners for s - and p - polarized light even at 45 incidence The cut on / cut off edges for the two polarizations show only a spectral distance of about 1 nm Consequently, these filters can be applied as combiners for unpolarized light of 94 nm and 98 nm diodes at AOI = 45 High quality substrates and lenses of fused silica SUPRASIL 3 or SUPRASIL 31/32 substrates on request Broadband and multiple wavelength AR coatings according to customers specification c) 1,75 1,5 1,25 1,,75,5 AOI =, AOI = 5, 4,5 4, 3,5 3, 2,5 2, 1,5 1,, Figure 7: Reflectance spectra of broadband AR coatings for several types of laser diodes: AR (, 6 8 nm) <.5 % (, AR (,79 15 nm) <.5% ( and of a broadband AR coating which is combined with an AR coating for a pilot laser: AR (, nm) <.5 % (c)

56 54 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR Yb :YAG, Yb : KGW AND Yb-DOPED FIBer LASERS Edge FILTERS and pump Mirrors Special Output Couplers Lasers on the basis of Yb doped crystals or fibers have gained increasing importance over the recent years. High power cw lasers were developed on the basis of Yb:YAG as well as with Yb doped fiber. Yb:YAG and Yb:KGW lasers can also be operated as high power ns, ps or fs lasers. MIRRORS AOI = Rp Rs Rr AOI = Figure 1: Reflectance spectra of a HR cavity mirror ( and a HR turning mirror ( Lasers with extremely high output power (e.g. >1kW cw) are often based on Yb:YAG. LAYERTEC has developed different coating designs for handling extraordinary high fluences. The designs are optimized either for cw-radiation or ns-pulses or ps-pulses. T [%] Figure 2: Transmittance spectrum of a steep edge short wavelength pass filter with HR(, 13nm) > 99.9% and HT(,88 98nm) > 99.5% (rear side AR coated) T [%] AOI = AOI = Figure 3: Transmittance spectrum of a steep edge long wavelength pass filter with HR (, nm) > 99.8 % and HT (,13 12 nm) > 97 % for use as output mirror of a fiber laser (rear side AR coated) Special features: Short wavelength pass filters with very steep edge for use as pump mirror for solid state lasers on the basis of Yb-doped materials (e.g. Yb:YAG, Yb:KGW, Yb-doped fiber) Also useful for Nd-doped and Yb-Nd-co-doped materials AOI = Figure 4: Reflectance spectrum of an output mirror for a fiber laser which blocks the diode radiation at 98nm and has a partial reflectivity R =1% for 13 11nm (rear side AR coated) Special features: Transmittance T > 99% at 88nm 99nm, reflectance R > 99.9% at 13nm, i.e. transition from the high transmittance range to the high reflectance range within 4% of the laser wavelength Superior laser damage thresholds (1MW/cm 2 cw at 164nm*) Thermal and climatical stable * Measured with a high power fiber laser at Institut für Angewandte Physik, Friedrich-Schiller-Universität Jena

57 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm THIN FILM POLARIZERS PICOSECOND LASERS ON THE BASIS OF Yb-DOPED MATERIALS Rs( 1%) Rp( 1%) Rp( 1%) AOI = Figure 5a: Reflectance spectra for s- and p-polarized light of a broad band thin film polarizer showing an bandwidth of 25nm with Rp <.2% (AOI=55 ) Rs( 1%) Rp( 1%) Rp( 1%) AOI = Figure 5b: Reflectance spectra for s- and p-polarized light of a narrow band thin film polarizer which is optimized for very low Rp values and easy angle tuning for the optimization of the polarizer performance (AOI=55 ) Picosecond lasers, i.e. lasers with pulse lengths of some hundred fs to 1ps can be built on the basis of Yb:YAG-, Yb:KGW- and Yb:KYW. These lasers enable materials processing without unwanted thermal effects such as melting, which results in unprecedented accuracy of the processes. Moreover, picosecond lasers do not require chirped pulse amplification which reduces the costs compared to fs-lasers and the laser crystals do not show thermal lensing which enables high output power. Recently, it was demonstrated that lasers with an average power of 4W (77fs, 1MHz) are possible on the basis of Yb:YAG slab crystals. Measurement c) T [%] 1 99,8 99,6 99,4 99,2 Tp (theory) Tp (CRD-measurement) Figure 5c: Calculated and measured transmission spectra for s- and p-polarized light of a narrow band thin film polarizer according to the design shown in fig. 5b (AOI=55 ). It is clearly visible that Tp >99.8% is reached with a bandwidth of 15nm and that Tp >99.9% can be achieved within a bandwidth of 5nm. The spectral position of this transmission maximum can be adjusted to any wavelength between 135nm and 145nm by angle tuning. Thin film polarizers are key elements for regenerative amplifiers in ns- and ps-lasers. LAYERTEC has optimized its polarizer designs for high laser induced damage thresholds. Figure 5 shows as examples a broadband polarizer with Rp(55 ) <.2% within a bandwidth of 25 nm in the wavelength range of Yb-doped fiber lasers (fig.5 and a narrow band polarizer which is optimized for very low Rp values Measurement AOI = 55 d) T [%] 1 99,8 99,6 99,4 99,2 λ = 142 nm AOI [deg] Figure 5d: Transmission spectrum Tp vs. AOI at 142 nm measured at the polarizer shown in fig.5c at a single wavelength (fig.5. Figure 5c shows a comparison of the calculated transmission spectrum for p-polarized light and a measurement in a CRD setup, figure 5d shows Tp vs. AOI for the same polarizer (measured at 142 nm). These measurements prove that Tp > 99.9% can be reached by angle tuning. This is especially important for intra cavity applications. Tp Picosecond laser optics require specially designed optics to achieve high laser damage thresholds. For detailed information please see pages For GTI mirrors which are often used for pulse compression from the ps range down to a few hundred fs please see pages AOI = 16 Figure 6: Reflectance spectrum of a special output coupler with high transmission for the pump radiation: PR(,14nm)=98% + R(, nm)<2%. Moreover, this output coupler exhibits GDD <2fs² around 14nm

58 56 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR Nd :YAG / Nd :YVO 4 LASERS CAVITY MIRRORS TURNING MIRRORS, SEPARATORS AND COMBINERS ALIGNMENT AND PROCESS MONITORING AOI = Rs Rp AOI = 45 random AOI= , 99, , , , , AOI = R(-1%) R(99-1%) AOI= random AOI= Figure 1: Reflectance spectrum of a high power cavity mirror ( and a pump mirror HR(,164nm) > 99.9% + R(,88nm) <2% ( HR cavity mirrors with R>99.9% Typical reflectivity: R>99.95% For special demands we guarantee R>99.99% (delivery with cavity ring down measurement) Spectral bandwidth of about 7nm, 1-on1 - LIDT >5MW/cm² (cw) and >5J/cm² (1ns) Pump mirrors HR (,164nm)>99.9% + R (, 88nm) <2% Figure 2: Reflectance spectra of a high power turning mirror ( and a separator for the second harmonic from the fundamental HR(,164nm)>99.9% + R(,532+88nm) < 3% ( HR turning mirrors with R>99.9% for s- and p-polarization Optics for the harmonics of the Nd:YAG / Nd:YVO 4 laser are presented on pages Figure 3: Reflectance spectra of a turning mirror for the laser beam with a partial reflector for the pilot laser and high IR transmission for process monitoring ( and silver based turning mirror for 13nm with R>8% for a pilot laser in the red spectral range (

59 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm THIN FILM POLaRIZERS Beam splitters and output couplers NON-POLARIZING BEAM SPLITTERS AOI = % (164nm) AOI = Rp = 66 ± 1, % Rp = 5 ± 2, % Rp = 33 ± 3, % AOI = 45 Rs = 66 ± 1, % Rs = 5 ± 2,% Rs = 33 ± 3, % % 6% 45% % 3 2 Katalog_Layertec_ Measurement AOI = 45 random p-pol. p-pol. AOI = Figure 4: Reflectance spectra of a standard thin film polarizer ( and of a special thin film polarizer ( Figure 5: Reflectance spectra of several output couplers with different degrees of reflectivity ( and a common 1:1 beam splitter for random polarization ( Figure 6: Calculated reflectance spectra of 3 types of nonpolarizing beam splitters for AOI = 45 ( and measured reflectance spectra of the 5% beam splitter ( Thin film polarizers which work at the Brewster angle exhibit a considerably broader bandwidth than those which work at AOI=45 : (Tp/Ts)=1 AOI= 55 ~45nm AOI= 45 ~2nm Beam splitters and output couplers can be produced with precisely adjusted degree of reflectivity: Reflectance Tolerance R > 95% ±.5% R = % ± 1% R = % ± 2% Beam splitters with Rs ~ Rp ( Rs-Rp < 1.5%) for AOI=45 and different degrees of reflectivity Common types: Rs,p = 66 ± 1% Rs,p = 5 ± 2% Rs,p = 33 ± 3% All non-polarizing beam splitters with rear side AR (Rs~Rp.6%)

60 58 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR THE second HARMONIC OF Nd : YAG, Nd : YVO 4 AND Yb:YAG LASERS DUAL WAVELENGTH Cavity MIRRORS SEPARATORS FOR THE SECOND HAR- MONIC FROM THE FUNDAMENTAL WAVE The harmonics of Nd:YAG, Nd:YVO 4 and Yb:YAG lasers are widely used for materials processing as well as for measurement applications. Moreover, the second harmonic of these lasers is often used as pump source for Ti:Sapphire lasers. On these pages we introduce optics for 532 nm: dual wavelength mirrors, separators, thin film polarizers and nonpolarizing beam splitters, but also cavity optics for compact diode pumped lasers of different configurations. Coatings for 515 nm are available as well. All designs are calculated according to customer specification R(-1%) R(99-1%) AOI= Figure 2: Reflectance spectra of a dual wavelength cavity mirror with high transmittance for the pump wavelength (88nm) R(-1%) R(99-1%) AOI= DUAL WAVELENGTH TURNING MIRRORS 1, 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, 4 p-pol. AOI= Figure 1: Reflectance spectra of a dual wavelength turning mirror HRs+p(45, nm) >99.9% R(-1%) R(9-1%) AOI= Figure 3: Reflectance spectrum of a HR mirror for 164nm which is also an output coupler for 532nm: HR(, 164nm) > 99.9% + R(, 532nm ) = 99±.3% Rs(-1%) Rs(99-1%) Rp(-1%) Rp(99-1%) AOI= Figure 4: Reflectance spectra of separators for the second harmonic from the ground wavelength: (: HR(,164nm) > 99.9% + R(, nm) < 3% (:HRs+p(45, 532nm) > 99.9% + Rs + p(45, nm) < 2% These optics are useful for "green" lasers Separators with different features are available according to customer specifications.

61 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 59 THIN FILM POLARIZERS COMPONENTS FOR THE SECOND AND THIRD HARMONIC 515 nm, 532 nm COATINGS ON NONLINEAR OPTICAL CRYSTALS p-pol. AOI = 55 p-pol. AOI= 45 AOI = Figure 5: Reflectance spectra of a thin film polarizer for 532nm The transmission of thin film polarizers for p-polarized light can be measured with high accuracy by a modified cavity Ring-Down setup. NON-POLARIZING BEAM-SPLITTERS p-pol. AOI= Figure 6: Reflectance spectra of a non-polarizing beamsplitter for 532nm with Rs=Rp=5±2% ( Rs- Rp <3%) p-pol. AOI= Figure 7: Reflectance spectra of a separator for the third harmonic from the second harmonic and the fundamental wave ( and of a dual wavelength turning mirror for 355nm and 532nm ( For common specifications of separators for the harmonics in the UV spectral range please see table on page 63. Please do not hesitate to contact us for separators or mirrors with other angles of incidence. Figure 8: Reflectance spectrum of a dual antireflection coating on KTP for 532nm and 164nm Figure 9: Reflectance spectrum of a dichroic mirror on KTP: HR(,532nm)>99,98% + R(,164nm)<,2% optimized for very high transmission at 164nm AOI = Nonlinear optical crystals are the key elements for frequency conversion. LAYERTEC offers a variety of coatings on crystals like KTP and lithium niobate For more information about coatings on crystals see pages

62 6 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR THE THIRD HARMONIC OF Nd:YAG, Nd:YVO 4 AND Yb:YAG LASERS Standard Components The third harmonic of Nd:YAG, Nd:YVO 4 and Yb:YAG lasers has gained increasing importance in the field of materials processing as well as for measurement applications and as pump source for optical parametric oscillators. On these pages we introduce optics for 355nm: single and multiple wavelength mirrors, separators, thin film polarizers and anti reflection coatings. The coating designs shown here are calculated for 355nm, but designs for 343nm are available as well. All designs are calculated according to customer specification. Turning Mirrors Special Separators Windows and Lenses Rs( 1%) Rs(9 1%) Rp( 1%) Rp(9 1%) AOI=45 p-pol. AOI= p-pol , 4, , 7 7 3, , 5 5 2, , 3 3 1, ,, Figure 1: Reflectance spectra of a turning mirror HRs(45,355nm)>99.9% + HRp(45,355nm)>99.5% Standard Separators Figure 3: Reflectance spectra of a special separator which is especially optimized for low reflectance at 164nm: HRs(45, 355nm) >99.9% + HRp(45, 355nm) >99.5% + Rp(45, nm)<2% + Rs(45,532nm)<5% +Rs(45, 164nm) <2% Figure 5: Reflectance spectra of a single wavelength AR coating for 355nm optimized for AOI = 3 p-pol. AOI=45 p-pol. AOI= 45 AOI = , 4,5 4, 3,5 3, 2,5 2, 1,5 1,, Figure 2: Reflectance spectra of a standard separator for the third harmonic from the second harmonic and the fundamental: HRs(45, 355nm) >99.9% + HRp(45, 355nm) >99.5% + Rp(45, nm)<2 + Rs(45,532nm)<5% + Rs(45,164nm) <1% Figure 4: Reflectance spectra of special separators for the third harmonic from the second harmonic and the fundamental wavelength: Fluoridic Separator HRs (45, 355nm) > 95% + Rp (45, 532nm) < 2% + Rs,p (45, 164 nm) < 2% Fluoridic separators show an extended lifetime at high power densities. Figure 6: Reflectance spectrum of a triple wavelength antireflection coating on fused silica for 355nm, 532nm and 164nm

63 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm, 355 nm SPUTTERED COMPONENTS Multiple Wavelength Mirrors SEPARATORS WITH HIGH TRANSMISSION IN THE UV RANGE Thin Film Polarizers p-pol. AOI= 45 p-pol. AOI= 45 p-pol. AOI = Figure 7: Reflectance spectra of a triple wavelength turning mirror for 355nm, 532nm and 164nm Figure 9: Reflectance spectra of a special separator for the third harmonic from the second harmonic: HRs+p(45,532nm) >99.8% + Rs+p(45,355nm) <2% Figure 1: Reflectance spectra of a thin film polarizer for 355 nm: HRs (55, 355 nm) > 99,5% + Rp (55, 355 nm) < 2% p-pol. AOI= Figure 8: Dual wavelength turning mirror for 355nm and 532nm Due to the low straylight losses of sputtered components a transmission of T > 98 % is achieved for this type of separators. TECHNICAL DATA OF MIRRORS AND SEPARATORS The transmission of the p-polarized light can be optimized by angle tuning. Tilting the polarizer by ±2 shifts the minimum of Rp to longer or shorter wavelengths which can improve the polarization ratio significantly. Type of coating Standard Sputtered Mirror for AOI = R > 99.5% R > 99.9% Turning mirror Rs > 99.7%, Rp > 99% Rs > 99.95%, Rp > 99.8% Separator AOI = 45 Rs (355nm) > 99.7% Rp (355nm) > 99% Rs (532nm) < 5% Rp (532nm) < 2% Rs (164nm) < 1%, Rp (164nm) < 2% Rs (355nm) > 99.9% Rp (355nm) > 99.7% Rp (532nm) < 2% Rp (532nm) < 1% Rs (164nm) < 2%, Rp (164nm) < 1%

64 62 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR the higher HARMONICS OF Nd : YAG AND Nd : YVO 4 LASERS Separators for the fourth harmonic Separators for the fifth harmonic The harmonics of Nd:YAG and Nd:YVO 4 lasers are widely used for materials processing as well as for measurement applications. On these pages we introduce dual wavelength mirrors and separators for the fourth (266nm) and fifth harmonic (213nm). All designs are calculated according to customer specification p-pol. AOI= 45 Measurement unpol. p-pol. AOI= 45 Multiple Wavelength Mirrors p-pol. AOI= 45 Figure 3: Reflectance spectra of a separator for the fourth harmonic from the longer wavelength harmonics and the fundamental AOI= p-pol. AOI= Figure 1: Reflectance spectra of a dual wavelength turning mirror for 266nm and 355nm Rs(-1%) Rp(9-1%) AOI= Figure 2: Reflectance spectra of a four wavelength turning mirror: HRs(45, 266nm + 355nm) > 99%+ HRs(45,532nm + 164nm) > 99.9% Figure 4: Reflectance spectrum of a special separator for the second harmonic from the fourth harmonic: HR (, 532nm) > 99.95% + R (, 266nm) < 5% Figure 5: Measured reflectance spectra of a turning mirror for the fifth harmonic ( and of a separator for the fifth harmonic from the long wavelength harmonics and the fundamental (; fluoridic coatings on CaF 2 For high power applications we recommend fluoridic coatings on calcium fluoride which are produced according to the technology of ArFexcimer laser mirrors.

65 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm, 266 nm Special Separators Thin Film Polarizers Rr Tr AOI= 45 Rs Rp AOI = 56 p-pol. AOI = 56 R,T [%] Figure 7: Reflectance spectra of thin film polarizers for 266nm ( and 213nm (: HRs (56, 266nm) >98% + Rp (56, 266nm) <5%, Tp (56, 266nm) ~ 95% HRs(56,213nm)>97% + Rp (56, 213nm) <5%, Tp (56, 213nm) ~ 75% R,T [%] Rr Tr AOI= 45 Sputtering techniques enable us to offer thin film polarizers also for the fourth and fifth harmonic of the Nd:YAG laser Figure 6: Reflectance spectra of separators for the fourth and fifth harmonics: HRr (45, 266nm) > 98% + Rr(45, 213nm) < 1% for unpolarized light oxide coatings optimized for low straylight losses fluoride coatings for high laser induced damage thresholds The fifth harmonic at 213nm is a critical wavelength for oxide coatings, because the absorption edge of aluminum oxide begins in this wavelength range. Aluminum oxide is, however, the only high index oxide material which can be used for 213nm. Compared to fluorides, oxide coatings are hard and show low straylight losses. Fluorides exhibit higher LIDT values and extended lifetime at high and medium power applications. Common specifications of separators for the harmonics in the UV spectral range: Separator type Type Reflectance at center wavelength [%] Reflectance at the corresponding longer Nd:YAG wavelengths [%] 266nm 355nm 532nm 164nm Rs Rp Rs Rp Rs Rp Rs Rp Rs Rp 3 rd harmonic, 355nm standard > 99.7 > 99 < 5 < 2 < 1 < 2 sputtered > 99.9 > 99.8 < 2 < 1 < 2 < 1 4 th harmonic, 266nm standard > 99.7 > 99 < 5 < 2 < 1 < 2 < 1 < 2 sputtered > 99.7 > 99 < 5 < 1 < 2 < 1 < 2 < 1 5 th harmonic, 213nm* standard > 97 > 93 < 5 < 2 < 1 < 2 < 1 < 2 < 1 < 2 Table 1: Common specifications of separators for the harmonics in the UV *Fluoridic coating on CaF 2

66 64 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR WEAK Nd:YAG OR Nd:YVO 4 LASER LINES Neodymium doped crystals show laser transitions at different wavelengths. Tables 1 and 2 give an overview about the laser wavelengths of the most common Nd doped materials Nd:YAG and Nd:YVO 4. Laser lines Nd :YAG Second harmonic 946 nm 473 nm 164 nm 532 nm 1123 nm 561 nm 1319 nm 659 nm Table 1: Laser lines and corresponding wavelengths of the second harmonic of Nd:YAG Laser lines Nd :YVO 4 Second harmonic 915 nm 457 nm 164 nm 532 nm 134 nm 67 nm Table 2: Laser lines and corresponding wavelengths of the second harmonic of Nd:YVO4 As can be seen, a variety of laser lines in the VIS and NIR can be obtained from these crystals. This phenomenon is used to build compact diode pumped solid stated lasers with a variety of wavelengths which are used for measurement applications as well as for projection systems (RGB lasers). The strongest laser transition in both materials is the 164nm line. Efficient laser radiation at the other wavelengths is only possible by suppressing this line. LAYERTEC offers a variety of laser mirrors for this application. Compact laser designs include also the pump diode (88nm) and a unit for the second harmonic generation. This is the reason why coatings for Nd:YAG or Nd:YVO 4 wavelengths apart from 164nm mostly show several spectral regions of high transmission as well as of high reflection. In the following we present some examples of such coatings. All coatings are designed according to customer specifications, because the specifications depend on the laser design. All examples on these pages are for Nd:YAG wavelengths. Coatings for Nd:YVO 4 can be designed and produced as well Figure 1: Reflectance spectrum of a dual wavelength mirror for a weak laser line and its second harmonic with high transmission for the pump wavelength and the strongest laser line: HR(, 473nm) > 99.85% + HR(, 946nm) > 99.95% + R(, 88nm) < 2% + R(, 164nm) < 5% Feature Suppression of the strongest laser line HR mirror for the weak laser line High transmission for the pump wavelength HR mirror for the second harmonic of the weak laser line Reflectivity R(,164nm) < 5% R(,946nm) > 99.95% R(,88nm) < 2% AOI= HR(,473nm) > 99.85%

67 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm, 946 nm, 1123 nm, 134 nm R(-1%) R(99-1%) AOI= Rs Rp AOI = 56 R(-1%) R(99-1%) AOI= Figure 2: Reflectance spectrum of a dichroic mirror with high transmission for the pump wavelength which also suppresses the 164nm line: HR(, 1123nm) > 99.9% + R(, 561nm) < 2% + R(, 88nm) < 1% + R(, 164nm) < 5% Figure 3: Reflectance spectra of a thin film polarizer with high transmission for the pump wavelength and the second harmonic which also suppresses the 164nm line: HRs(56, 1123nm) > 99.9% + Rp(56, 1123nm) < 5% + Rs,p(56, nm) < 1% + Rs,p(56,164nm) < 5% Figure 4: Reflectance spectrum of a dichroic mirror with high reflectance for the NIR wavelengths and high transmission for the corresponding second harmonic wavelengths: HR(, nm) > 99.9% + R(, nm) < 2% Feature Reflectivity Feature Reflectivity Feature Reflectivity HR mirror for the weak laser line HR(, 1123nm) > 99.9% HR for s-polarized light of the weak laser line HRs (56, 1123nm) > 99.9% Broadband HR mirror for several laser lines HR(, nm) > 99.9% Suppression of the strongest laser line High transmission for the pump wavelength R(, 164nm) < 5% R(, 88nm) < 1% Suppression of p-polarized light of the weak laser line Suppression of the strongest laser line Rp(56, 1123nm) < 5% Rs+p(56,164nm) < 5% High transmission for the second harmonics of these laser lines R(, nm) < 2% High transmission for the second harmonic of the weak laser line R(, 561nm) < 2% High transmission for the pump wavelength High transmission for the second harmonic of the weak laser line Rs+p(56, 88nm) < 1% Rs+p(56, 561nm) < 1%

68 66 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR Ho :YAG AND Tm :YAG LASERS Turning MIRRORS Output Couplers Ho :YAG and Tm :YAG lasers emitting at wavelengths of 21 nm and 21 nm are widely used for medical applications. LAYERTEC offers optical coatings for this wavelength range with high laser induced damage thresholds and long lifetimes. 1, 99,9 99,8 99,7 99,6 99,5 p-pol. AOI= AOI = Cavity MIRRORS 99,4 99,3 99,2 7 6 AOI = 99,1 99, , 99,9 99,8 99,7 99,6 99,5 99,4 99,3 1, 99,9 99,8 99,7 99,6 99,5 p-pol. AOI= 45 Figure 3: Reflectance spectrum of an output coupler with R = 82 %±1 at 21 nm^ Output couplers with precisely adjusted degrees of reflectivity: 99,2 99,1 99, R ( -1%) R (99-1%) AOI= 99,4 99,3 99,2 99,1 99, Figure 2: Reflectance spectra of turning mirrors for 21nm ( and 21nm ( Reflectance Tolerance R > 95% ±.5% R = % ± 1% R = 1 %... 8% ± 2% The coating materials are chosen to guarantee high laser induced damage thresholds. To achieve the highest reflectivity for p-polarization mirrors for 21 nm and 21 nm should be specified separately. Figure 1: Reflectance spectra of a cavity mirror ( and a pump mirror ( which has a region of high transmittance around 88nm. HR cavity and pump mirrors with R>99.9% High laser induced damage thresholds Turning mirrors with R>99.9% for s-polarized light and R>99.8% for p-polarized light at AOI=45 High laser induced damage thresholds

69 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm, 21 nm Edge Filters Thin Film Polarizers Windows and Lenses R ( -1%) R (99-1%) AOI= Rs ( -1%) Rp ( -1%) Rs (99-1%) AOI = 55 AOI = , 1,75 1,5 1, , ,75,5, Figure 4: Reflectance spectra of a cavity mirror for 21 nm which supresses the 21 nm line R ( -1%) R (99-1%) AOI= Figure 6: Reflectance spectra of a thin film polarizer for 21nm (Rs>99.8%, Rp<2%, AOI=55 ) Separation of the s - and p-polarized component of the light (s-polarized light is reflected and p - polarized light is transmitted) TFPs can be produced for AOI > 4, but polarization is most efficient and appears in a broad wavelength range if AOI 55 (Brewster angle) is used AOI = 1,,9,8,7,6,5,4,3,2, Figure 5 Reflectance spectra of a steep edge filter for the separation of the 21 nm and 21 nm lines c) AOI = Lens materials according to customer specifications Infrasil, sapphire, undoped YAG and calcium fluoride can be applied Special AR coatings for high index materials as GGG on request Figure 7: Reflectance spectra of typical anti reflection coatings: Single wavelength AR coating for 21nm ( Broadband AR coating 21nm- 21nm ( Dual wavelength AR coating for the pump and laser wavelength (88nm+21nm) (c)

70 68 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Components for Er :YAG Lasers and the 3µm region Pump Mirrors Turning Mirrors Er:YAG lasers are widely used in medical applications, especially in dermatology. The most important feature of this laser is the high absorption coefficient of water for the 294 nm radiation. This makes surgical applications easier but is also a challenge for the optical coatings which must be completely free of water. Coatings produced by magnetron sputtering proved to be ideally suited for this application. Cavity Mirrors AOI = Figure 2: Reflectance spectrum of a HR cavity mirror with a HT region between 8 nm and 11 nm Figure 3: Reflectance spectrum of a turning mirror unpol. AOI = AOI = Reflectivity of cavity mirrors and pump mirrors: R>99.9% at AOI= Pump mirrors with high transmittance between 8nm and 11nm for pumping with a Nd:YAG laser or a diode laser Reflectivity of turning mirrors: R > 99.8% at AOI=45 for random polarized light High laser induced damage thresholds (4J/cm² at 4µs) Figure 1: Reflectance spectrum of a HR cavity mirror HR(,294nm) > 99.8%

71 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm Beam Combiners and Pilot Mirrors Output Couplers and Lenses Components for other Lasers Around 3µm unpol. AOI= unpol. AOI= Figure 4: Reflectance spectra of a dual wavelength turning mirror ( and a separator/combiner for 294 nm and a pilot laser between 63 nm and 655nm ( AOI= Figure 5: Reflectance spectra of output couplers with R = 7 ± 1% and R = 84 ± 1 % Output couplers with precisely adjusted degrees of reflectivity (tolerances of ±1% at reflectivities between 7 % and 9 %) 1,,8,6,4 AOI = The fundamental effect which is especially used for the medical application of lasers emitting light between µm is the strong absorption of water in this wavelength range. Between 2.6 µm and 2.8 µm the absorption of water is still stronger than at 2.94 µm (Er :YAG laser) which makes lasers working in this wavelength range (e. g. the Er : Cr :YSGG laser) promising candidates for future applications. However, the strong absorption of water is also the most serious problem with respect to laser damage. Therefore, it is essential to keep the layer system free of water. LAYERTEC uses magnetron sputtering for the production of coatings for the 3 µm region. The high atomic density of sputtered layers which is close to that of bulk material suppresses the diffusion of water into the layer systems. This enables LAYERTEC to offer also coatings for the critical µm region. Fig. 7 shows as an example a HR mirror centered at 2.8 µm with a reflectivity R > 99.7 %. Designs for beam splitters and pilot mirrors are calculated according to customer specifications, Figure 6: Reflectance spectrum of an AR- coating for 2.94µm on sapphire AR coatings with residual reflectivities R <.2 % on the rear side of output couplers as well as on lenses and windows made of sapphire, undoped YAG, CaF 2 or Infrasil (for substrate materials see pages 2 21) AOI = 1 99,8 99,6 99,4 99,2 99, 98,8 98,6 98,4 98,2 98, Figure 7: Reflectance spectrum of a HR mirror for 2.8 µm with R > 99.7 %

72 7 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES

73 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 71 Femtosecond Laser Optics

74 72 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES INTRODUCTION TO FEMTOSECOND LASER OPTICS Short pulse lasers are used in numerous applications such as time resolved spectroscopy, precision material processing and large bandwidth telecommunication. Driven by these applications, recent developments in this field are directed to lasers generating higher output power and shorter pulses. Nowadays most of the work in short pulse physics is done with Ti:Sapphire lasers, but also dye lasers and solid state lasers on the basis of other transition metal or rare earth metal doped crystals such as Yb:KGW are used for the generation of femtosecond pulses. The reproducible generation of sub-1fs-pulses is closely connected with the development of broadband low loss dispersive delay lines consisting of prism or grating pairs or of dispersive multilayer reflectors. The spectral bandwidth of a pulse is related to the pulse duration by a well known theorem of Fourier analysis. For instance, the bandwidth (FWHM) of a 1fs gaussian pulse at 8 nm is 11nm. For shorter pulses, the wavelength spectrum becomes significantly broader. A 1fs pulse has a bandwidth of 17nm. If such a broad pulse passes through an optical medium, the spectral components of this pulse propagate with different speeds. Dispersive media such as glass impose a so called "positive chirp" on the pulse, meaning that the short wavelength ("blue") components are delayed with respect to the long wavelength ("red") components (see schematic drawing in fig. 1). A similar broadening can be observed if a pulse is reflected by a dielectric mirror and the bandwidth of the pulse is larger or equal to the width of the reflection band of the mirror. Also broadband mirrors consisting of a double stack system cause pulse broadening, because the path lengths of the spectral components of the pulse are extremely different in these coatings. In the sub-1fs-regime it is essential to control the phase properties of each optical element over the extremely wide bandwidth of the fs-laser. This holds not only for the stretcher and compressor units, but also for the cavity mirrors, output couplers and the beam propagation system. In addition to the power spectrum, i.e. reflectance or transmittance, also the phase relationship among the Fourier components of the pulse must be preserved in order to avoid broadening or distortion of the pulse. A mathematical analysis of the phase shift which is applied to a pulse passing through a medium or being reflected by a mirror (see insert on page 73) shows that the main physical properties which describe this phenomenon are the group delay dispersion (GDD) and the third order dispersion (TOD). These properties are defined as the second and third derivative of the reflected phase with respect to the frequency. Especially designed dielectric mirrors offer the possibility to impose a "negative chirp" on a pulse. Thus, the positive chirp which results from crystals, windows etc. can be compensated. The schematic drawing in fig. 2 explains this effect in terms of different optical path lengths of blue, green and red light in such a negative dispersion mirror. Figure 2: Optical path lengths of blue green and red light in a negative dispersion mirror (schematic drawing) LAYERTEC offers femtosecond laser optics with different bandwidths. This catalog shows e.g. optics for the wavelengths range of the Ti:Sapphire laser in three chapters, each representing a characteristic bandwidth of the optics: standard components with a bandwidth of about 12nm, broadband components (bandwidth about 3nm) and octave spanning components. Figure 1: Broadening of a pulse by propagation through an optical medium (schematic drawing) Each of these chapters shows low dispersion laser and turning mirrors, negative dispersion mirrors or mirror pairs, output couplers and beam splitters of the corresponding bandwidth. Moreover, we want to present silver mirrors for fs applications which are the components with the broadest low-gdd wavelength region available.

75 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 73 Please note that the GDD spectrum of a dielectric negative dispersion mirror is not a continuous flat graph. All types of negative dispersion mirrors exhibit oscillations in the GDD spectrum. These oscillations are small for standard bandwidths. However, broadband and ultra broadband negative dispersion mirrors exhibit strong GDD oscillations. A considerable flattening of these oscillations can be achieved by using mirror pairs consisting of mirrors with slightly shifted GDD oscillations which compensate each other. These mirror pairs are especially designed for this compensation behaviour. Fig. 3 shows a schematic drawing of such a mirror pair and the corresponding GDD spectra. GDD-spectrum of mirror GDD-spectrum of mirror 1 average GDD-spectrum GDD and TOD Figure 3: Schematic drawing of a negative dispersion mirror pair If a pulse is reflected by a dielectric mirror, i. e. a stack of alternating high and low refractive index layers, there will be a phase shift between the original and the reflected pulse resulting from the time which it takes the different Fourier components of the pulse to pass through the layer system of the mirror. In general, the phase shift Φ(ω) near the center frequency ω may be expanded in a Taylor series for frequencies near ω : Φ (ω) = Φ (ω ) + Φ (ω ) (ω ω ) + Φ (ω ) (ω ω ) 2 /2 + Φ (ω ) (ω ω ) 3 /6 + The derivatives are, respectively, the Group Delay (GD) Φ (ω ), the Group Delay Dispersion (GDD) Φ (ω ) and the Third Order Dispersion (TOD) Φ (ω ). More strictly speaking, this expansion is only useful in an exactly soluble model, for the propagation of a transform limited Gaussian pulse and for pure phase dispersion. For extremely short pulses and combinations of amplitude and phase dispersion numerical calculations may be necessary. Nevertheless, this expansion shows clearly the physical meaning of the single terms: Assuming the phase shift is linear in frequency (i. e. GD, GDD = and TOD = over the pulse bandwidth), the reflected pulse is delayed in time by the constant group delay and, of course, scaled by the amplitude of reflectance R. The pulse spectrum will remain undistorted. If GDD, two important effect are observed: The reflected pulse is temporarily broadened. This broadening effect depends only on the absolute value of the GDD. LAYERTEC offers "low GDD mirrors", i. e. mirrors with GDD < 2 fs² over a given wavelength range, which are needed to preserve the pulse shape when the pulse is reflected by these mirrors. Moreover, the pulse becomes "chirped", i. e. it changes its momentary frequency during the pulse time. This effect depends on the sign of the GDD, so that the momentary frequency may become higher (up -chirp, GDD > ) or lower (down - chirp, GDD < ). This allows to compensate positive GDD effects of nonlinear optical elements by using negative GDD mirrors. The TOD determines also pulse length and pulse shape (distortion of the pulse) and becomes a very important factor at pulse lengths of 2fs and below. It is also possible to use negative dispersion mirrors with high values of negative GDD for pulse compression. These so called Gires-Tournois-Interferometer (GTI)- mirrors (see pages 96 97) are successfully used in Ti:Sapphire lasers, Yb:YAGand Yb:KGW-oscillators and Er:Fibre lasers. Pulse compression in Yb:YAGand Yb:KGW-oscillators provides pulses of some hundred femtoseconds pulse length. For each wavelength components with different amounts of negative GDD are presented. Besides these optics for the spectral range of the Ti:Sapphire fundamental and for the very promising Yb:YAG- and Yb:KGW-lasers we also offer optics for the harmonics of this radiation down to the VUV wavelength range, optics for femtosecond lasers in the 15nm-range and especially designed optics for high power ultra short pulse lasers. LAYERTEC has own capabilities for design calculation and also for GDD-measurements in the wavelength range from 25 17nm. References: H.Holzwarth, M.Zimmermann, Th.Udem, T.W.Hänsch, P.Russbüldt, K.Gäbel, R.Poprawe, J.C.Knight, W.J.Wadsworth and P.St.J. Russell; Optics Letters Vol. 26, No.17 (21), p Y.-S.Lim, H.-S.Jeon, Y.-C.Noh, K.-J.Yee, D.S.Kim, J.-H. Lee, J.-S. Chang, J.-D.Park; Journal of the Korean Physical Society Vol.4, No.5 (22), p G. Tempea, V.Yakovlev and F. Krausz; "Interference coatings for Ultrafast Optics" in N.Kaiser, H.K.Pulker (eds.) "Optical Interference Coatings ", Springer-Verlag Berlin Heidelberg 23, p and the references therein

76 74 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES STANDARD FEMTOSECOND LASER OPTICs The coatings shown here are calculated for a bandwidth of 12 15nm in the wavelength range between 6 nm and 1 nm Very high reflectivity of the mirrors (R > %) Spectral tolerance 1% of center wavelength LIDT = J/cm² depending on the coating design In-house design calculation and GDD measurement capabilities Center wavelength, GDD and TOD according to customer specification All components for 12 are supplied with measured GDD spectra Standard Mirror AOI = Pump Mirror AOI = Turning Mirror AOI = 45 AOI = R( 1%) R(99 1%) AOI = Rs Rp AOI = 45 1, 99, , 99,9 99, ,7 7 99,7 99,6 6 99,6 99,5 5 99,5 99,4 4 99,4 99,3 3 99,3 99,2 99,1 99, ,2 99,1 99, GDD s GDD p Figure 1: Reflectance ( and GDD ( spectra of a standard low dispersion femtosecond laser mirror Figure 2: Reflectance ( and GDD ( spectra of a standard low dispersion pump laser mirror Figure 3: Reflectance ( and GDD ( spectra for a standard low dispersion turning mirror (bandwidth Rr (45 ) > 99,9% ~ 16nm) All types of mirrors are also available with negative GDD (e.g. - 4 fs²). For high dispersive mirrors see page 76.

77 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm bandwidth Reflectance and transmittance of output couplers and beam splitters can be adjusted according to customer specifications Tolerances for output couplers: R =1 7 % ± 2.5 % R = 7 9 % ± 1.5 % R = 9 95 % ±.75 % R = % ±.5 % R > 98% ±.25 % Standard AR coatings: AOI= : R<,2% AOI=45 : Rs or Rp<,2% In case of p-polarization uncoated rear side possible Rp(fused silica, 45 ) ~,6% Output Coupler AOI = beam Splitters for p-polarized Light at AOI = 45 beam Splitters for s-polarized Light at AOI = 45 R=98±,25% R=95±,5% R=9±,75% AOI = Rp=66±3% Rp=5±3% Rp=33±3% AOI = 45 Rp=66±3% Rp=5±3% Rp=33±3% reflected beam transmitted beam reflected beam transmitted beam reflected beam transmitted beam Figure 4: Reflectance ( and GDD ( spectra of several standard output couplers (the GDD spectra are calculated for R=98%, but they are similar for all degrees of reflectivity) Figure 5: Reflectance ( and GDD ( spectra of several standard beam splitters for AOI = 45 and p - polarized light (the GDD spectra are calculated for R = 5%, but they are similar for all degrees of reflectivity) Figure 6: Reflectance ( and GDD ( spectra of several standard beam splitters for AOI = 45 and s - polarized light (the GDD spectra are calculated for R = 5%, but they are similar for all degrees of reflectivity)

78 76 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES STANDARD FEMTOSECOND LASER OPTICs Recent advances in design calculation and process control enable LAYERTEC to offer high dispersive mirrors for pulse compression in advanced Ti:Sapphire lasers. These mirrors and mirror pairs show spectral bandwidths of 1nm 3nm and negative GDD values of some hundred fs². These mirrors can be used for pulse compression. Compared to prism compressors high dispersive mirrors reduce the intra-cavity losses resulting in higher output power of the laser. Negative dispersion mirror AOI = HIGH DISPERSIVE MIRROR AOI = HIGH DISPERSIVE MIRROR pair AOI = AOI = AOI = mirror A mirror B AOI = 1, 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, ,8 99,6 99,4 99,2 99, 98,8 98,6 98,4 98,2 98, ,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, mirror A mirror B Figure 1: Reflectance ( and GDD ( spectra of a standard negative dispersion mirror with GDD = 4 ±1fs 2 Measurement c) Measurement c) average Measured and calculated GDD-curves match very well. This proves the reliability of the coating process Figure 2: Reflectance (, calculated ( and measured (c) GDD spectra of a high dispersive mirror with a bandwidth of 12nm and a GDD of -45 ± 1fs² in the 8nm range Figure 3: Reflectance (, calculated ( and measured average (c) GDD spectra of a high dispersive mirror pair with a bandwidth of 2nm and an average GDD of -12 ± 4fs² per bounce in the 8nm range

79 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm bandwidth This very special type of optical coatings can be used to compensate the third order dispersion which results from laser crystals, substrates or dispersive elements as MIRROR With NEGATIVE Third ORDER DISPERSION prisms or gratings. Positive as well as negative TOD can be achieved with this type of coatings. All coatings are optimized for nearly constant TOD which means TOD MIRRORS with POSITIVE THIRD ORDER DISPERSION oscillations in the order of some hundreds of fs 3. Please note that without TOD optimization these oscillations are in the order of some thousands of fs 3. MIRROR PAIR WITH OPTIMIZED THIRD ORDER DISPERSION AOI= AOI = mirror A mirror B AOI= 1 99,9 1 99,9 1 99,9 99,8 99,8 99,8 99,7 99,7 99,7 99,6 99,6 99,6 99,5 99,5 99,5 99,4 99,4 99,4 99,3 99,3 99,3 99,2 99,2 99,2 99,1 99, ,1 99, ,1 99, mirror A mirror B average c) c) c) mirror A mirror B average TOD [fs 3 ] TOD [fs 3 ] TOD [fs 3 ] Figure 4: Reflectance (, GDD ( and TOD (c) spectra of a mirror optimized for nearly constant negative third order dispersion Figure 5: Reflectance (, GDD ( and TOD (c) spectra of a mirror optimized for nearly constant positive third order dispersion Figure 6: Reflectance (, GDD ( and TOD (c) spectra of a mirror pair optimized for nearly constant negative third order dispersion.

80 78 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES STANDARD FEMTOSECOND LASER OPTICs MIRRORS WITH DIFFERENT TOD VALUES Thin Film Polarizers for AOI = 55 Thin Film Polarizers for AOI = 65 type A type B type C AOI = p-pol. AOI = 55 p-pol. AOI = Figure 1: GDD spectra of three negative dispersive mirrors with different TOD values, i.e. different slope of the GDD curves Increasing the GDD slope which means increasing the absolute TOD results in a lower bandwidth and stronger GDD and TOD oscillations. Center wavelength and amount of TOD according to customer specifications In the wavelength range of the Ti:Sapphire laser the bandwidth of single mirrors with optimized TOD is limited to about 15nm GDD(Rs) GDD(Tp) Figure 2: Reflectance ( and GDD ( spectra of a standard TFP (AOI=55 to use the Brewster angle for the transmitted p-pol. light) The bandwidth of thin film polarizers can be extended if large angles of incidence are used. As shown in fig.4 a polarizer bandwidth as large as 1 nm can be achieved. However, this is combined with a reduced reflectivity for the s-polarized light. c) GDD(Rs) GDD(Tp) ,9,8,7,6,5,4,3,2, Figure 3: Reflectance ( and GDD ( spectra of a TFP (AOI=65 to achieve a low GDD for Rs and Tp, bandwidth ~ 4nm), (c) rear side ARp(65,75-85nm)

81 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 79 Thin Film Polarizers for AOI = 8 Non-Polarizing Beamsplitters for AOI = nm bandwidth Anti Reflection Coatings p-pol. AOI = 8 p-pol. AOI = 45 AOI = ,9,8,7,6,5,4,3,2, p-pol. p-pol. p-pol. AOI = 45 GDD(R) [fs 2 ] GDD(R) [fs 2 ] ,5 2, 1,5 1,, c) p-pol. c) p-pol. c) AOI = 45 GDD(T) [fs 2 ] GDD(T) [fs 2 ] ,8 1,6 1,4 1,2 1,,8,6,4, Figure 4: Reflectance ( and GDD (b, c) spectra of a TFP (AOI=8, lower Rs-value to achieve a low GDD for Rs and Tp, bandwidth ~ 15nm) Figure 5: Reflectance ( and GDD (b, c) spectrum of a non-polarizing beam splitter Figure 6: Reflectance of different AR-coatings ( AOI=, ( AOI=45 Rs=Rp~,5%, (c) AOI=45 Rs<,2% (only possible for s- or p-pol. light) Broadband anti reflection coatings for AOI = or a single polarization at AOI > with R <.1% on request.

82 8 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES BROADBAND FEMTOSECOND LASER OPTICS The coatings shown here are calculated for the wavelength range 7 1nm. Similar coatings are available for 6 9nm or 65 95nm Very high reflectivity of the mirrors (R > 99.8 % R > % depending on the design) Center wavelength, bandwidth, GDD and TOD according to customer specification Spectral tolerance 1% of center wavelength LIDT.1J/cm² In-house design calculation and GDD measurement capabilities Components are delivered with measured GDD spectra MIRROR PAIR for AOI = turning MIRROR for s-polarized Light at AOI = 45 turning MIRROR for p-polarized Light at AOI = 45 mirror A mirror B AOI= AOI=45 AOI= ,9 1 99,9 1 99,9 99,8 99,8 99,8 99,7 99,7 99,7 99,6 99,6 99,6 99,5 99,5 99,5 99,4 99,4 99,4 99,3 99,3 99,3 99,2 99,2 99,2 99,1 99, ,1 99, ,1 99, mirror A mirror B average Figure 1: Reflectance ( and GDD ( spectra of a negative dispersion laser mirror pair Figure 2: Reflectance ( and GDD ( spectrum of a broadband turning mirror for s - polarized light Figure 3: Reflectance ( and GDD ( spectrum of a broadband turning mirror for p - polarized light Mirror pairs show a very smooth average GDD spectrum, although the single broadband mirrors exhibit strong GDD oscillations. Pump mirror pairs (i.e. mirror pairs with at least one mirror showing high transmission around 5nm) are also available. (see page 82)

83 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm bandwidth OUTPUT COUPLERs For AOI = BEAM SPLITTERs for p-polarized Light at AOI = 45 BEAM SPLITTERs for S-polarized Light at AOI = 45 R = 95 ± 1.5% R = 8 ± 3% R = 7 ± 3% AOI= Rp = 67 ± 3% Rp = 5 ± 3% Rp = 33 ± 3% AOI=45 Rs = 67 ± 3% Rs = 5 ± 3% Rs = 33 ± 3% AOI= transmitted beam reflected beam reflected beam transmitted beam reflected beam transmitted beam c) c) ARp (45 ) Rp (45, uncoated rear side) c) ARs(45 ) Rs(45, uncoated rear side) 2, 1, 1 9 1,5, ,,5 5 4,5, Figure 4: Reflectance ( of broadband output couplers, figure ( shows the GDD spectra for the 8% output coupler, figure c: reflectance spectrum of a broadband AR coating AR(, 68-13nm) <.5% Figure 5: Reflectance ( of broadband beam splitters for p-polarization, figure ( shows the GDD spectra for the 5% splitter, figure c: reflectance spectrum of a broadband AR coating for p-polarized light Figure 6: Reflectance ( of broadband beam splitters for s-polarization, figure ( shows the GDD spectra for the 5% splitter, figure c: reflectance spectrum of a broadband AR coating for s-polarized light

84 82 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES BROADBAND FEMTOSECOND LASER OPTICS NEGATIVE DISPERSION PUMP MIRROR PAIR for AOI = MIRROR Pair WITH positive average GDD Thin Film Polarizers for AOI = 7 mirror A R(-1%) mirror B R(-1%) AOI = mirror A R(99-1%) mirror B R(99-1%) Katalog_Layertec_ mirror 1 mirror 2 AOI = 3 1, 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, p-pol. AOI = mirror A mirror B average mirror 1 mirror 2 average GDD Rs GDD Tp Figure 1: Reflectance ( and GDD ( spectra of a negative dispersion pump mirror pair (mirror 2 without HT-option) Figure 2: Reflectance ( and GDD ( spectra of a broadband mirror pair with positive average GDD for s-polarized light at AOI=3 c) 6 p-pol Figure 3: Reflectance ( and GDD ( spectra of a TFP (AOI=7, lower Rs-value to achieve a "zero" GDD for Rs and Tp, bandwidth~3nm), figure (c): rear side AR coating for s-polarized light. Please note that this coating results in R~15% for the p-polarized component. As AR coating for the p-polarization we advise the use of the design from fig.3a.

85 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm bandwidth High NEGATIVE DISPERSION MIRROR PAIR for AOI = MIRROR PAIR WITH OPTIMIZED THIRD ORDER DISPERSION for AOI = mirror A mirror B AOI = 1, 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, mirror A mirror B , 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 mirror A mirror B AOI = 99, mirror A mirror B average Special features: GDD of high dispersive mirrors between -5fs² and -5fs² Very high reflectivity Measured LIDT.1J/cm² Center wavelength, bandwidth and GDD according to customer specification Please note that bandwidth and GDD are closely connected. A high value of negative GDD results in a very narrow bandwidth Spectral tolerance 1% of center wavelength In-house design calculation and measurement capabilities (GDD 25 18nm, R-measurement by CRD 21 17nm) Measurement c) average c) mirror A mirror B average TOD [fs 3 ] Figure 4: Reflectance (, calculated ( and measured average (c) GDD spectra of a high dispersive mirror pair with a bandwidth of 25nm and an average GDD of -18 ± 4fs² per bounce in the 8 nm range Figure 5: Reflectance (, GDD( and TOD (c) spectra of a mirror pair optimized for broadband low third order dispersion The mirror pair shows very smooth GDD and TOD spectra, although the single mirrors exhibit considerable GDD and TOD oscillations.

86 84 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES octave spanning FEMTOSECOND LASER OPTICS The coatings shown here are calculated for the wavelength range of one octave (e.g nm). Similar coatings are possible for other wavelength ranges. NEGATIVE DISPERSION LASER MIRROR PAIR for AOI = Center wavelength, bandwidth, GDD and reflectance of output couplers and beam splitters according to customer specification Spectral tolerance 1% of center wavelength NEGATIVE DISPERSION Pump MIRROR PAIR for AOI = LIDT ~.1 J/cm² In-house design calculation and GDD measurement capabilities Components are supplied with measured GDD spectra NEGATIVE DISPERSION TURNING MIRROR PAIR for p-polarized Light AOI = 45 mirror A mirror B 1, 99,8 99,6 99,4 99,2 99, 98,8 98,6 98,4 98,2 98, mirror A R(-1%) mirror B R(-1%) AOI = mirror A R(99-1%) mirror B R(99-1%) mirror A mirror B AOI = mirror A mirror B average mirror A mirror B average mirror A mirror B average Figure 1: Reflectance ( and GDD ( spectra of an ultra broadband negative dispersion laser mirror pair Figure 2: Reflectance ( and GDD ( spectra of an ultra broadband negative dispersion pump mirror pair Figure 3: Reflectance ( and GDD ( spectra of an ultra broadband turning mirror pair for p-polarized light Mirror pairs designed by LAYERTEC show a very smooth average GDD spectrum, although the single broadband mirrors exhibit strong GDD oscillations. The pump mirror pair consists of two mirrors which both show a region of high transmission around 5nm.

87 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm Bandwidth OUTPUT COUPLERs for AOI = BEAM SPLITTERs For p-polarized Light at AOI = 45 BEAM SPLITTERs for S-polarized Light at AOI = 45 AOI = AOI = 45 AOI = Rp [%] Rs [%] reflected beam transmitted beam reflected beam transmitted beam transmitted beam reflected beam c) c) AOI = 45 c) AOI = 45 5, 4,5 4, 3,5 3, 2,5 2, 1,5 1,, Figure 4: Reflectance ( and GDD ( spectra of an ultra broadband output coupler with R = 85 ± 3 %, (c) rear side octave spanning AR coating Rp [%] 5, 4,5 4, 3,5 3, 2,5 2, 1,5 1,, Figure 5: Reflectance ( and GDD ( spectra of an ultra broadband beamsplitter for p - polarized light with Rp = 5 ± 4 %, (c) rear side octave spanning AR coating for p-polarized light Rs [%] Figure 6: Reflectance ( and GDD ( spectra of an ultra broadband beamsplitter for s-polarized light with Rs=5±5%, (c) rear side octave spanning AR coating for s-polarized light

88 86 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES silver mirrors for femtosecond lasers SILVER MIRRORS optimized for femtosecond laser applications AOI= Figure 1: Reflectance ( and GDD-spectrum ( of a silver mirror optimized for use with fs-lasers in the wavelength range 6 1nm (AOI= ) unpol. p-pol. AOI= p-pol Figure 2: Reflectance ( and GDD-spectra ( of a silver mirror optimized for use with fs-lasers in the wavelength range 6 1nm (AOI=45 ) Special features: High reflectance in the VIS and NIR Very broad reflectance band with GDD ~ fs 2 Silver mirrors with defined transmission (e.g..1%) show high LIDT-values (see table) and the same R and GDD values as shown in fig. 1 and 2 Extremely low straylight losses (TS ~ 3x1 5 in the VIS and NIR) Lifetimes of more than 1 years in normal atmosphere were demonstrated Highly stable optical parameters because of sputtered protective layers Good cleanability (tested according to MIL-M- 1358C 4.4.5) Stock of standard components: Standard and fs - optimized protected silver on substrates with Ø =12.7mm, Ø = 25mm and Ø = 5mm: Plano, Plano /concave and plano /convex with a variety of radii between 1mm and 1 mm Coating Reflectance* Wavelength range LIDT [J/cm 2 ]** fs- optimized protected silver R= % 6 1nm.38 Enhanced silver 8nm R>99% 7 9nm.37 Broadband enhanced silver R= % 6 12nm.24 Partially transparent silver R=96.5% 98.5% 6 1nm.22 Other sizes, shapes, radii and coatings for other wavelength ranges on request * For unpolarized light at AOI=45 ** Measurements were performed at Laser Zentrum Hannover according to ISO measurement conditions: pulse duration: 15 fs, 3 pulses, repetition rate 1kHz, λ=8 nm

89 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm SILVER MIRRORS WITH ENHANCED REFLECTIVITY The reflectance of silver mirrors can be enhanced by a dielectric overcoat. The bandwidth of the enhanced reflectivity must be exactly specified. Outside this band the reflectivity of the mirror may be lower than that of a standard silver mirror. For use with fs-lasers, the dielectric overcoat must be optimized for high reflectivity and low GDD. The following figures show examples for silver mirrors with enhanced reflectivity at a specified wavelength (fig. 3 and 4) and over the wavelength range of the Ti:Sapphire laser (fig. 5). Also enhanced silver mirrors can be designed for a defined transmission (e.g. T=.1%). enhanced 3 enhanced 2 enhanced 1 standard AOI= unpol. p-pol. AOI= 45 unpol. p-pol. AOI= enhanced 3 enhanced 2 enhanced 1 p-pol. p-pol Figure 3: Reflectance ( and GDD ( spectra of silver mirrors with different designs for enhanced reflectivity around 85nm (AOI = ) Figure 4: Reflectance ( and GDD ( spectra of silver mirrors with enhanced reflectivity around 8nm (AOI = 45 ) Figure 5: Reflectance ( and GDD ( spectra of silver mirrors with enhanced reflectivity in the wavelength range 6 12nm (AOI = 45 )

90 88 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES HIGH POWER FEMTOSECOND LASER OPTICS Coatings for femtosecond laser optics are usually optimized for special reflectance, transmittance and GDD spectra. The laser induced damage threshold (LIDT) of such coatings is less important for the majority of the applications. Nevertheless, high power fs lasers are under investigation and the output power of fs lasers has increased enormously over the recent years (see. Ref. 1 and the references therein). Thus, the LIDT of fs laser optics becomes more and more important. LAYERTEC has carried out detailed investigations of the LIDT of optics for fs and ps lasers.(*) As shown in the table on page 89 we found large differences in the LIDT values of standard, broadband and ultrabroadband optics and our high power mirrors. Standard low GDD mirrors (see page 74 fig. 1) show a LIDT value of.3j/cm 2 while negative GDD mirrors, broadband and ultrabroadband components (all other examples on page 74 85) have an LIDT of about.1j/cm 2. We distinguish between the different high power mirrors because they are based on different material combinations and designs. These designs require different efforts with respect to the preparation of the coatings and show different amounts of stress. The spectral bandwidth of these coatings is nearly the same. An example is shown in fig. 1. It was also found that LAYERTECs optimized silver mirrors have LIDT values which are higher than that of standard components in the fs range. Silver mirrors are advantageous because of their extremely broad zero GDD reflectance band with a reflectivity of up to 98.5% at normal incidence. Even silver mirrors with a defined transmission of.1% show higher damage thresholds than all dielectric ultra broadband components. For more information on silver mirrors see pages , 99,8 99,6 99,4 99,2 99, 98,8 98,6 98,4 98,2 98, 6 AOI= Figure 1: Reflectance spectrum of a high power fs laser mirror HR (, 8 nm) R > 99.9 % Our investigations proved that the LIDT of femtosecond laser optics depends on the coating materials and on the coating designs. The difference in the LIDT values of standard low GDD components and the other optics mentioned above results from the different coating designs used. Especially the broadband and negative GDD designs result in low damage thresholds. Only the designs for standard low GDD components reached higher LIDT values. In all of these cases materials with a high contrast of the refractive indices were chosen in order to achieve large bandwidths. Higher LIDT values can be achieved by using other coating materials. However, mirrors made of these materials have only a bandwidth of about 8nm because of the lower contrast of the refractive indices. Nevertheless, this bandwidth is enough for pulse lengths as low as 15fs. All high power designs are optimized for GDD<2fs 2. 1) E.Innerhofer, T.Südmeyer, F.Brunner, R.Häring, A.Aschwanden and R.Paschotta, C.Hönninger and M.Kumkar, U.Keller, "6-W average output power in 81-fs pulses from a thin-disk Yb:YAG laser", OPTICS LETTERS Vol.28, No.5, p * In cooperation with the Laser Zentrum Hannover and the Friedrich-Schiller Universität Jena. Most of this work was done within the framework of the German joined project PRIMUS which was supported by the Bundesministerium für Forschung und Technologie.

91 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm bandwidth OVERVIEW ABOUT LASER INDUCED DAMAGE THRESHOLDS OF FEMTOSECOND LASER OPTICS METALLIC HIGH POWER MIRRORS Coating Reflectance at 8nm LIDT[J/cm 2 ] * 5fs 15fs 1ps Bare gold 97.5% fs-protected silver 98.5% Enhanced silver (8nm) 99.7%.37 Enhanced silver (6 12nm) 98.5%.24 Partially transparent silver ( T( 8nm) =.1%) 98.5%.22 Negative dispersion mirrors > 99.9% ~.1 Broadband low GDD mirrors > 99.9% ~.1 Standard low GDD mirrors > 99.9%.3.55 High power mirror for ps pulses > 99.9% High power mirror (special type) > 99.8% Single wavelength AR coating <.2% 1.2 ** Broadband AR coating <.5% 1.2 ** fs-optimized silver Figure 3: Reflectance spectra of bare gold and fs-optimized silver (optimized for high reflectance at 8nm) gold GDD OF HIGH POWER FEMTOSECOND LASER MIRRORS AOI= * Measurement conditions: 3 pulses, repetition rate 1kHz, λ=8nm; measurements were performed at Laser Zentrum Hannover and Friedrich-Schiller-Universität Jena according to ISO ** Self focussing effects may destroy the substrate while the AR coating is still stable high power standard low GDD fs-optimized Ag Figure 4: Group delay dispersion (GDD) of standard and high power dielectric mirrors and fs-protected silver mirrors Figure 2: Laser induced damage of a coated surface

92 9 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPonents FOR THE SECOND HARMONIC OF THE Ti:SAPPHIRE LASER DUAL WAVELENGtH MIRRORS The second harmonic of the Ti:Sapphire laser provides fs-pulses in the NUV and VIS spectral range. This offers a variety of applications in spectroscopy as well as in materials science. Optics for these very special applications must be optimized for both high reflectivity and low dispersion. Also negative dispersion mirrors for pulse compression are of interest. Special features: Very high reflectivity (R > 99.9 %) Center wavelength and bandwidth according to customer specifications Spectral tolerance 1% of center wavelength R(-1%) R(99-1%) AOI= Rs(-1%) Rp(-1%) Rs(99-1%) Rp(99-1%) AOI= p-pol c) c) p-pol Figure 1: Reflectance ( and GDD (b,c) spectra of a fs optimized dual wavelength mirror for 4nm + 8nm for AOI= Figure 2: Reflectance ( and GDD (b,c) spectra of a fs optimized dual wavelength turning mirror for 4nm + 8nm for AOI=45

93 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm SEPARATORS FOR THE SECOND HARMONIC FROM THE fundamental for AOI = 45 NEGATIVE DISPERSIon MIRROR PAIR FOR THE 4 nm SPECTRAL RANGE at AOI= p-pol. AOI= 45 Rs(-1%) Rp(-1%) Rs(99-1%) Rp(99-1%) AOI= 45 mirror A mirror B AOI = ,75 99,5 99, , ,75 98,5 98,25 98, p-pol. AOI= 45 p-pol. mirror A mirror B average Figure 3: Reflectance- ( and GDD ( spectra of a separator HR 4 nm + HT 8nm (AOI = 45 ) Figure 4: Reflectance- ( and GDD ( spectra of a separator HR 8 nm (optimized for p - polarization) + HT 4 nm (AOI = 45 ) Figure 5: Reflectance- ( and GDD ( spectra of a negative dispersion mirror pair for 35nm 48nm with an average GDD varying from -3fs² at 35nm to fs² at 48 nm (TOD optimized) Reflectivity R > 99.9% for s - and p - polarization in the reflection band Transmission T > 95% for s - and p - polarization in the transmission band These components work for p - and s - polarization, but the performance can be optimized if the polarization is clearly specified Bandwidth of the 8 nm reflection band >2nm for p - polarization All separators exhibit GDD < 2fs² in the transmission band Prototype production according to customer specifications In house design calculation and measurement capabilities

94 92 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPonents FOR THE third HARMONIC OF THE Ti:SAPPHIRE LASER DUAL WAVELENTGH TURNING MIRRORS for AOI = 45 TRIPLE WAVELENTGH TURNING MIRRORS for AOI = 45 BROADBAND LOW DISPERSION MIRRORS for AOI = 45 p-pol. AOI= 45 p-pol. AOI= 45 AOI= ,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, p-pol. AOI= 45 p-pol. AOI= 45 AOI= Figure 1: Reflectance ( and GDD ( spectra of a fs-optimized dual wavelength turning mirror for 27 nm + 45 nm Figure 2: Reflectance ( and GDD ( spectra of a fs-optimized turning mirror for the 266 nm 4 nm 8 nm wavelength regions Figure 3: Reflectance ( and GDD ( spectra of a broadband negative dispersion mirror HRs (45, 325nm 6 nm) with Rs > 99.7% and low GDD Please note that this triple wavelength turning mirror exhibits GDD < 5 fs² in all three wavelength regions of interest.

95 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm SEPARATORS FOR THE THIRD HARMONIC FROM THE SECOND HARMONIC AND THE FUNDAMENTAL WAVE for AOI = 45 NEGATIVE DISPERSION MIRROR PAIR for AOI = p-pol. AOI= 45 p-pol. AOI= 45 mirror A mirror B AOI = ,75 99,5 99,25 99, 98,75 98,5 98,25 98, p-pol. AOI= 45 AOI= 45 mirror A mirror B average AOI= Figure 4: Figure 4: Reflectance ( and GDD ( spectra of a standard separator HR 27nm+ HT 45+81nm Figure 5: Reflectance ( and GDD ( spectra of a broadband separator with high reflectivity for s-polarized light throughout the wavelength range of the third harmonic of the Ti:Sapphire laser and high transmission for p-polarized light in the VIS and NIR: HRs (45, 25 33nm) > 95% + Rp(45, 44 1 nm) < 3% Figure 6: Reflectance ( and GDD ( spectra of a broadband negative dispersion mirror pair HR (, 275 nm 4 nm) with R > 99% and an average GDD of -1 fs² per bounce

96 94 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COMPONENTS FOR THE HIGHER HARMONICS OF THE Ti:SAPPHIRE LASER TURNING MIRRORS AND SEPARATORS FOR THE FOURTH HARMONIC at AOI = 45 Measurement unpol. p-pol. AOI= The fourth, fifth and sixth harmonics of the Ti:Sapphire laser provide fs-pulses in the DUV/VUV range. These offer a variety of applications in spectroscopy as well as in materials science. Optics for these very special applications must be optimized for high reflectivity and low dispersion. Mirrors and separators for the wavelength range 125nm 215nm consist of fluoridic layer systems COMPONENTS FOR THE FIFTH HARMONIC at AOI = 45 on CaF 2 substrates while components for longer wavelength can be made of oxides. For mirrors we recommend substrates with a thickness of 3mm or 6.35mm to achieve a good flatness of the mirror. For special separators we offer substrates of fused silica or calcium fluoride as thin as 1mm or.5mm. COMPONENTS FOR THE sixth HARMONIC at AOI = 45 Measurement Measurement p-pol. AOI= 45 unpol. p-pol. AOI= 45 unpol. AOI= c) p-pol. AOI= 45 s p AOI= 45 p-pol. AOI= Figure 1: Reflectance (measured, a and calculated, and GDD (calculated, c) spectra of a separator for the fourth harmonic from the longer wavelength harmonics and the fundamental wave (AOI=45 ) Figure 2: Reflectance (measured, and GDD (calculated, spectra of a turning mirror for 16nm (AOI=45 ) Figure 3: Reflectance (measured for unpolarized light, and GDD (calculated, spectra of a turning mirror for 133nm (AOI=45 )

97 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm Bandwidth of the Reflectance and Low GDD Range of Standard Components Depending on the wavelength range of the fundamental wave the coatings described in the data sheets on pages can be used for the center wavelengths given in the following table All coatings are optimized for broad reflection bands, high reflectivity and low GDD Component Wavelength range P - polarization S - polarization GDD < 2fs 2 GDD < 2fs 2 Turning mirror or separator 3rd harmonic 25 nm 33 nm 3nm (R > 99 %) 5nm (R > 99 %) Dual wavelength turning mirror 25 nm 33 nm 15nm (R > 99 %) 26nm (R > 99 %) 37 nm 5 nm 34nm (R > 99 %) 72nm (R > 99 %) Turning mirror or separator 4th harmonic 18 nm 25 nm 5nm (R > 93%) 15nm (R > 97%) Turning mirror or separator 5th harmonic 14 nm 18 nm 4nm (R > 9%) 12nm (R > 97%) Turning mirror or separator 6th harmonic 125 nm 14 nm 8nm (R > 85 % unpolarized) BROADBAND REFLECTORS FOR THE 2 25 nm WAVELENGTH RANGE Advanced sputtering techniques enable LAYERTEC to produce broadband mirrors and separators for the wavelength range of the fourth harmonic of the Ti:Sapphire laser. These components consist of oxidic coatings on fused silica substrates. Please note that oxidic coatings show considerable absorption losses between 2nm and 215nm. That is why we can only specify R > 8 9 % in this wavelength range. Nevertheless, this is the only way to produce broadband low dispersion components for the UV with high transmission in the VIS and NIR. Rs [%] AOI= AOI= 45 AOI= GDD (R) AOI = GDD (R) [fs 2 ] Figure 4: Reflectance ( and GDD ( spectra of a broadband mirror HRs(45, nm) > 9% Figure 5: Reflectance ( and GDD ( spectra of a broadband separator HR(,2 245nm) > 8% + R(, 3 1nm) <1%

98 96 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES GIRES-TOURNOIS-INTERFEROMETER (GTI) MIRRORS GTI - MIRRORS FOR Yb:YAG - and Yb:KGW- LASERS Gires -Tournois - Interferometer mirrors are used for pulse compression in short pulse lasers such as Yb:YAG - or Yb: KGW- lasers. LAYERTEC offers GTI mirrors also for the Ti:Sapphire wavelength range and for other femtosecond lasers in the NIR spectral range. Compared to prism compressors GTI mirrors reduce the intra-cavity losses resulting in higher output power of the laser. Special features: GDD values according to customer specification Very high reflectivity Measured LIDT.1J/cm² Center wavelength, bandwidth and GDD according to customer specification Please note that bandwidth and GDD are closely connected. A high value of negative GDD results in a very narrow bandwidth Spectral tolerance 1% of center wavelength In house design calculation and measurement capabilities (GDD 25 17nm, R-measurement by CRD 21 18nm) fs -13fs fs -5fs -1fs Figure 1: GDD spectra of GTI-mirors for 14nm with different GDD values GDD AOI= Figure 2: GTI-mirror with nearly constant TOD GDD AOI= Figure 3: GTI-mirror with positive GDD Reflectivity [%] measured by CRD GDD AOI= Figure 4: GDD spectrum of a rear side GTI mirror with GDD ( - 1,13nm) ~-7fs² The mirror is irradiated through the substrate which has an AR coating on the front side. Rear side GTI mirrors are insensitive against surface contaminations which sometimes distort the GDD spectrum of common front side GTI mirrors

99 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm OPTICAL LOSSES OF GTI MIRRORS COATING HOMOGENITY AND REPRODUCIBILITY OF GTI MIRRORS without absorption losses with typical absorption losses AOI= 1 99,99 99,98 99,97 99,96 99,95 99,94 99,93 99,92 99,91 99, Figure 5: Calculated reflectance spectra of a GTI mirror without absorption losses (green) and with typical absorption losses (red) unpol, AOI= Figure 7: Calculated GDD spectrum of a GTI mirror: GDDr(6,12-16nm) ~-1±2fs² Fig. 8 and 9 show that all GTI mirrors which were produced in these batches meet the specifications given in fig.7. The variance between the mirrors from one batch and between the batches is much smaller than the specification allows. The excellent reproducibility even of complex coating designs such as GTI mirrors is the basis for the use of GTI mirrors in industrial short pulse lasers. Measurement Measurement Measurement 1 99,99 99, , , , , , ,92 99,91 99, Figure 6: Measured reflectance spectrum of a GTI mirror (design as calculated in fig.5) Figure 8: Measured GDD spectra of 6 GTI mirrors which were coated in the same batch according to the design of fig.7 Figure 9: Measured GDD spectra of 6 GTI mirrors which were coated in a second batch according to the design of fig.7 The GTI mirrors offered by LAYERTEC show high reflectance values (e.g. R>99.98% at 13nm). The reflectance can be measured exactly by CRD. The high reflectivity proves that the absorption losses in the GTI mirrors are very small. This results also in a very small thermal lens which is caused by these GTI mirrors if they are used in high power lasers.

100 98 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES OPTICS FOR FEMTOSECOND LASERS IN THE nm WAVELENGTH RANGE NEGATIVE DISPERSION LASER AND PUMP MIRRORS For AOI = Although Ti:Sapphire lasers are currently the most important femtosecond lasers many applications require femtosecond pulses at considerably longer wavelengths. Several lasers emitting light between 11nm and 16nm have been developed in the recent years, such as the Cr:Forsterite laser ( nm) or the Er:Fiber laser (155nm). On these pages we present some examples of coatings such as negative dispersion mirrors and mirror pairs. Special features: Very high reflectivity of the mirrors (R>99.8% R > 99.99% depending on the design) Center wavelength, bandwidth, GDD and TOD according to customer specification Spectral tolerance 1% of center wavelength LIDT ~.1 J/cm² In-house design calculation and measurement capabilities (GDD 25 nm 17 nm, R 21 nm 18nm) 1, 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, AOI= R = ( -1%) R= (99-1%) AOI= Figure 1: Reflectance ( and GDD ( spectrum of a negative dispersion laser mirror (GDD ~ 15 fs² for nm) Figure 2: Reflectance ( and GDD ( spectrum of a negative dispersion pump mirror: HR(, nm) > 99.8% + R(, 12 17nm) < 5%, GDD ( nm) ~ 6 fs² GTI Mirrors For AOI = -2 fs 2-35 fs 2-5 fs 2 AOI= -2 fs 2-35 fs 2-5 fs 2 1, 99,8 99,6-1 99,4 99,2-2 99, -3 98,8 98,6-4 98,4 98,2 98, Figure 3: Reflectance ( and GDD ( spectra of GTI mirrors for 15 nm with different GDD values

101 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 99 BROADBAND NEGATIVE DISPERSION MIRROR PAIRS For AOI = BROADBAND NEGATIVE DISPERSION TURNING MIRRORS For AOI = nm Separator/Combiner with negative GDD For AOI = 45 mirror A mirror B AOI= AOI= 45 p-pol. AOI= ,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, , 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, Rp [%] mirror A mirror B average p-pol. p-pol Figure 4: Reflectance ( and GDD ( spectra of a broadband negative dispersion mirror pair; single mirrors with R > 99.7 % (mirror A) and R>99.85 % (mirror B) Figure 5: Reflectance ( and GDD ( spectrum of a broadband negative dispersion turning mirror for p - polarized light Figure 6: Reflectance ( and GDD ( spectrum of a beam combiner HRp(45,15 2nm) + HT(45, 8nm) which shows negative GDD in the reflection band Specially designed mirror pairs show a very smooth average GDD spectrum, although the single broadband mirrors exhibit strong GDD oscillations. Pump mirror pairs (i.e. mirror pairs with one mirror showing high transmission at the pump wavelength of the respective laser type) are also possible. Please note the large bandwidth of this mirror. Low dispersion turning mirrors are available with bandwidths of about 2 nm for p-polarization and of about 4 nm for s-polarization in this wavelength range.

102 1 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES

103 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 11 Selected Special Components

104 12 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES components FOR OPTICAL PARAMETRIC OSCILLATORS (OPO) Cavity Mirrors for AOI = Mirrors for OPOs are optimized for a good separation of the pump laser, signal and idler wavelengths as well as for a broad reflectance band of the signal wavelengths and a wide range of high transmittance for the idler wavelengths. Moreover, most of the optics show smooth group delay (GD) and group delay dispersion (GDD) spectra. Thus, wide tuning ranges for the signal and the idler wavelengths can be achieved. Moreover, this enables to operate OPOs with fs pulses. Broadband output couplers are also available. Center wavelength and tuning range can be adjusted according to customer specifications. All OPO coatings are produced by magnetron sputtering. This guarantees that the optical parameters are thermally and climatically stable, because the coatings are dense, free of water and adhere strongly to the substrate in spite of the extreme coating thickness in the order of 2 3µm. This makes sputtered OPO coatings ideal for application in harsh environments. GD, GDD AOI= 1, 99,9 99,8 99,7 99,6 99,5 99,4 99,3 99,2 99,1 99, GD [fs] Figure 1: Reflectance (, GD and GDD ( spectra of a broadband HR mirror for the signal wavelength: HR(,1 16nm) > 99.9% GD, GDD AOI= GD [fs] Figure 2: Reflectance (, GD and GDD ( spectra of a dual HR mirror for the signal and idler wavelengths: HR(,14 18nm) > 96% + HR(,29 49nm) > 93% This dual wavelenght mirror shows smooth GD spectra for signal and idler, while only the broadband mirror for the idler wavelengths is GDD optimized.

105 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 13 Pump mirrors and separators AOI = R(-1%) R(99-1%) AOI= R(-1%) R(99-1%) AOI= R(-1%) R(99-1%) AOI= GD, GDD GD [fs] Figure 3: Reflectance (, GD and GDD ( spectra of an OPO pump mirror This type of mirrors separates the pump and signal wavelengths while suppressing the idler wavelength: R(, 7 85nm <1%) + HR(, 9 16nm >99.8% + R(, 18 5nm) < 6% GD, GDD GD [fs] Figure 4: Reflectance (, GD and GDD ( spectra of a separator for the signal and idler wavelengths Edge filters separating signal and idler wavelengths can be used as broadband outcoupling mirrors for the idler: HR (, 11 16nm) > 99.8 % + R(, nm) < 1% These filters can also be provided with a band of high reflectance or high transmission for the pump wavelengths or for the second harmonic of the signal wavelengths We recommend undoped YAG or sapphire as substrate material if high transmission for the idler wavelengths is required (see also page 21 for transmission curves) Figure 5: Reflectance spectrum of a broadband mirror for the NIR: HR (, 23 4 nm) > 99 % AOI= Figure 6: Reflectance spectrum of a broadband mirror for the NIR: HR(, 25 35nm) > 99% + HT(,16 193nm)

106 14 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES components FOR OPTICAL PARAMETRIC OSCILLATORS (OPO) Output couplers for AOI = Beam Splitters for AOI = 45 Special output couplers for AOI = R = 91± 2% R=75±4% AOI= AOI= 45 AOI= Rp [%] reflected beam transmitted beam Figure 1: Reflectance ( and GDD ( spectra of different broadband output couplers for the signal wavelengths. Please note the smooth GDD spectra. The GDD spectra shown are calculated for the 75% output coupler, but the spectra for other reflectivities are very similar. reflected beam transmitted beam Figure 2: Reflectance ( and GDD ( spectra of a broadband beam splitter for p-polarized signal and idler radiation: Rp (45, 11 24nm) = 5 ± 5% Figure 3: Special output coupler: R(, 14 17nm) < 3% + PR(, 2 315nm) = 9±3% AOI= Figure 4: Special output coupler: R(, 1 11nm) < 3% + PR(,135 2nm) = 98 ±5%+R (,22 5nm) < 2 % The reflectivity of output couplers and beam splitters can be adjusted according to customer specifications. The output couplers for the signal wavelengths (fig. 3) can suppress the idler and vice versa (fig. 4). These output couplers may also have a pump window.

107 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 15 TURNING MIRRORS AND SEPARATORS for AOI = 45 p-pol. AOI= 45 Rp(-1%) Rp(99-1%) AOI= 45 p-pol. AOI= 45 1, 99, , , , , , , , ,1 99, GDp [fs] GDDp [fs 2 ] GDp [fs] GDDp [fs 2 ] GDs [fs] GDDs [fs 2 ] GD, GDD 4 2 GD, GDD 4 2 GD, GDD Figure 5: Reflectance (, GD and GDD ( spectra of a turning mirror HRp(45, 145 2nm) >99.8% Figure 6: Reflectance (, GD and GDD ( spectra of a separator for signal and idler Figure 7: Reflectance (, GD and GDD ( spectra of a separator for the signal and idler with high transmission for the pump radiation Turning mirrors and separators for pump wavelengths, signal and idler are key components of OPOs. On this page we present some examples of such optics. The spectral position of the reflection and transmission bands can be adjusted according to customer specification. Please note that the GD and GDD can only be optimized for s- or for p-polarization while the reflectivity is usually very high for both polarizations. A broad reflectance band for the signal wavelengths is combined with a broad transmission band for the idler: HRp(45, 145 2nm) > 99.8% + Rp(45, 225 4nm) < 1%. This separator can be used as incoupling mirror for the pump radiation: HRs(45,77 93nm)>99.8%+Rp(45,51 55nm) < 1% + Rp(45, nm) < 1%.

108 16 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES components FOR OPTICAL PARAMETRIC OSCILLATORS (OPO) Ultra broadband components for AOI = 45 Edge filters for AOI = 45 p-pol. AOI = 45 p-pol. AOI = 45 AOI= Rs [%] p-pol. AOI = Figure 1: Reflectance spectrum of an ultra broadband beam combiner HRs(45, 2 5nm) > 98% + Rp(45, 633nm) < 2% This beam combiner can be used to couple a pilot laser into the beam line. Please note the very low reflectivity at 62 65nm. Figure 2: Reflectance spectrum of an ultra broadband separator for signal and idler wavelengths HRr(45, 1 25nm) >98% + Rr(45, 44 5nm)<5% Figure 3: Edge filter for the idler and signal wavelength range with high transmission for the pump wavelength: HRs (45, nm) > 99.9% + Rs (45, 2 27nm) < 1%+ Rs (45, 164nm) < 1% p-pol. AOI = Figure 4: Broadband edge filter for the idler wavelength range with high transmission for the pump wavelength: HRs (45, 33 42nm) > 99.9% + Rs (45, 45 49nm)< 6 %+ Rs+p (45, 164 nm) < 5%

109 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 17 Special mirrors AOI = COATINGS ON NONLINEAR OPTICAL CRYSTALS AOI = AOI= AOI= AOI= Figure 5: Special pump mirror: PR (, 164nm) = 94 ± 2% + HR(, nm) > 99.9% + R(, 4 49nm) < 3% Figure 7: Reflectance spectrum of an AR coating on lithium niobate: R(,164nm) <.5% + R(, nm) <1% Figure 9: Reflectance spectrum of an AR coating on lithium niobate: R(,164nm) <.5% + R(, nm) <.5% + R(,315 37nm) <2% AOI =11 AOI= AOI= Figure 6: Special mirror: R (11, nm) < 1% + PR (11, 9 1nm) = 98 ±.5% + HR (11,128 16nm) > 99.9% Figure 8: Reflectance spectrum of an AR coating on lithium niobate: R(,191 23nm) <.5% + R(,32 42nm) <1% Figure 1: Reflectance spectrum of a double reflector with two regions of high transmittance on lithium niobate HR(, nm) > 99,8% + R(, nm) <1% All coatings according to customers specification.

110 18 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES BROADBAND AND SCANNING MIRRORS BROADBAND MIRRORS LAYERTEC produces broadband and scanning mirrors according to customers specifications. Full dielectric and metal-dielectric coating designs are used. In the following we present some examples which are designed for very broad wavelength regions or extremely large ranges of incidence angles. Broadband mirrors are widely used to reflect light of lasers emitting in a broad wavelength range such as e.g. Ti:Sapphire lasers, dye lasers, or for combination of different diode lasers. Special mirrors are also available which cover the whole visible spectrum, the near ultraviolet and considerable parts of the near infrared spectral regions. We recommend such mirrors as universal turning mirrors for nearly all types of laser diodes. Broadband mirrors for the NIR range are especially useful for reflecting the idler wavelengths of optical parametric oscillators or for special fs-application. In combination with fused silica as substrate material a large blocking range from 23 6nm can be achieved. Other NIR materials such as sapphire and YAG are possible alternatively, but can be especially used for high power applications to control the thermal conductivity if the absorption of water around 2.8µm or the coating material itself leads to an increase of the temperature p-pol. AOI= Figure 1: Reflectance spectra of an ultra broadband mirror for the NUV, VIS and NIR; R (, nm) >99 % Rs (45, nm) >99 % + Rp (45, nm) >97 % AOI = 1 99,95 99,9 99,85 AOI = 99, Measurement 1 99,95 99,9 99,85 AOI = 99, Figure 2: Reflectance spectra of a broadband mirror HR(,4 14nm)>99.9%, calculated design ( and broadband CRD measurement ( Please note the good conformity between calculation and measurement. The CRD measurements are limited by the water absorption in the 75 78nm and 12 14nm region. For these regions, measurements in vacuum are required.

111 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm SCANNING MIRRORS LAYERTEC offers scanning mirrors for high power laser applications as well as for special demands on the wavelength spectra and the angular range. Scanning mirrors are optimized for high reflectance for one wavelength or a certain wavelength region at a wide range of incidence angles. Our coating technology provides industrial solutions for weightreduced scanning mirrors but also special mirrors with uncommon sizes up to 5mm for the research with cw and pulsed high power lasers. 1, 99,99 99,98 99,97 99,96 99,95 99,94 99,93 99,92 99,91 99,9 λ = 164nm angle of incidence [deg] AOI = 22 AOI = 45 AOI = AOI = 22 AOI = 45 AOI = Figure 4: Reflectance vs. AOI of a wide angle scanning mirror for polarized Nd :YAG laser radiation: HRs ( 9, 164 nm) > 99.9 % These mirrors are ideal as scanning mirrors for s-polarized light or to enhance the reflectivity of optical gratings. AOI = 22 p-pol. AOI = 22 AOI = 45 p-pol. AOI = 45 AOI = 58 p-pol. AOI = AOI = 22 AOI = 45 AOI = Figure 3: Reflectance spectra of a silver based scanning mirror with enhanced wavelength range for laser diodes in the NIR: HRr (22 58, 8 1nm) > 99% + PRr (22 58, 63 67nm) > 5% Figure 5: Reflectance ( and GDD ( spectra of a scanning mirror for femtosecond laser pulses from a Ti : Sapphire laser: HRr (22 58, nm) > 99.5 %, GDD < 2 fs² The broad low GDD wavelength range of these mirrors makes it possible to use them in femtosecond laser applications. Fore more information or more examples on broadband and scanning mirrors please see pages 5 53 (optics for Ti:Sapphire and diode lasers), pages 74ff (femto second laser optics) and, especially for scanning mirrors, page (silver mirrors).

112 11 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Filters for Laser Applications ANGLE TUNING OF NARROW BAND FILTERS VARIABLE FILTERS FOR LASER APPLICATIONS AOI= AOI= T [%] T [%] 1 8 T [%] AOI= AOI=5 AOI =1 AOI =15 c) d) T [%] T [%] center , , ,5 8 82, position [mm] Figure 1: Transmission spectra of a narrow band filter for ~ 8nm; spectral overview, transmission at AOI =, 5, 1 and 15 Figure 2: Transmittance spectra of a laterally variable filter for the wavelength range of the Ti:Sapphire laser taken on the short wavelength side (, in the center ( and on the long wavelength side of the filter (c). Center wavelength vs. position on the filter (d) Narrow band filters with FWHM of 1nm and maximum transmission of T > 8% An FWHM of 5pm with maximum transmission of T=5% was demonstrated Blocking: T<1 3, block band: ~2nm in the Ti:Sapphire range Useful to select one wavelength from the spectrum of the Ti:Sapphire laser Special features: Linear variation of the filter wavelength with the position on the filter Similar designs for the VIS range (4 7nm) and for the NIR range (up to 18 nm) Blocking: T < 1 3 ; block band: ~2 nm in the Ti : Sapphire range Maximum transmittance: 9 %; FWHM: 1nm Shape: rectangular; size: 1 2 mm long, 5 1 mm broad Spectral tolerance 1% of center wavelength. The spectral position of the transmission band may vary by 1% between the coating runs while the bandwidth remains unchanged. The spectral performance of the filter can be optimized by tilting the filter. Tilting results in a shift of the transmission band towards shorter wavelengths. Thus, the spectral position of a filter with the transmission band at longer wavelengths than required can be adjusted for its best performance by angle tuning. If angle tuning is possible, the specifications for the filter can be less stringent which increases the output and reduces the price

113 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 111 STEEP EDGE FILTERS NARROW BAND REFLECTANCE FILTERS nm DUAL WAVELENGTH FILTER WITH BROAD BLOCKING RANGE p-pol. unpol. AOI=22,5 AOI= AOI= T [%] T [%] T [%] p-pol. unpol. AOI=22,5 Figure 4: Transmittance spectrum of a narrowband reflectance filter for 55 nm AOI= Filters of this type are ideal for the blocking of a single laser line while preserving a high and relatively constant transmission over the whole visible range Figure 3: Transmission spectra (detail, a and spectral overview, of a steep edge short wavelength pass filter for use as combiner for laser diodes at 635nm and 67nm (HRr(22.5, 67nm) > 99.9%+HTr (22.5, 635nm) >98%, rear side AR coated) For more information on combiners for diode lasers see page 53. For steep egde filters used as pump mirrors for solid state lasers on the basis of Yb-doped materials (e.g. Yb:YAG, Yb:KGW, Yb-doped fibers) see page 54. Special features: Spectral width of the reflectance band: 3% (e.g. T<1% from nm) T<.1% at the center wavelength T > 9% throughout the visible spectral range Filters for laser applications require excellent spectral quality and high damage thresholds Spectral position of cut on/cut off wavelengths or reflectance bands according to customer specification Sizes and shapes: Edge filters can be produced on round or rectangular substrates up to diameters of 38.1mm (1.5 inch). Also the production of miniature size filters (e.g. 3 x 3mm 2 ) is possible. Narrow band reflectance filters are limited to diameters of 25.4mm (1inch) High thermal and climatical stability c) AOI= Figure 5: Reflectance spectra of a dual wavelength filter for 164nm and 157nm with a broadband blocking range from the UV to 21nm: Front side coating ( Rear side coating ( Sum (c) Double side coating reduces the mechanical stress. Blocking in the UV/VIS is done by a color glass.

114 112 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Thin Film Polarizers TECHNICAL TERMS To answer some frequently asked questions and to help our customers to specify thin film polarizers we first want to give the definitions of the most important technical terms. Light is a transversal wave, i.e. the vector of the electric field oscillates perpendicular with respect to the propagation direction of the light. Natural light (from the sun or from a lamp) is mostly "unpolarized" or "random polarized". This means that the oscillation planes of the electric field vectors of the single light waves are randomly distributed, but always transversal with respect to the direction of propagation. The term "linear polarized light" means, that there is only one plane of oscillation. There are different optics which can polarize light, e.g. crystal polarizers which split the light into an unpolarized "ordinary beam" and a polarized "extraordinary beam" or thin film polarizers. To explain the meaning of the terms "s-polarization" and "p-polarization" one must first determine a reference plane (see fig.1). This plane is spanned by the incident beam and by the surface normal of the mirror (or polarizer). "S-polarized light" is that part of the light which oscillates perpendicularly to this reference plane ("s" comes from the German word "senkrecht" = perpendicular). "P-polarized light" is the part which oscillates parallel to the reference plane. Light waves whose plane of oscillation is inclined to these directions can be described as a p-polarized and an s-polarized part. As can be seen from fig.1, the reflectivity for s-polarized light increases with increasing angle of incidence while the reflectivity for p-polarized light decreases first, reaches R = at "Brewster angle" and increases for angles of incidence beyond Brewster angle. In principle, the same is true for dielectric mirrors. Thin film polarizers separate the s-polarized component of the light from the p-polarized component using the effect that the reflectivity for s-polarized light is higher and the reflection band is broader than for p-polarized light. Thus, there is always a wavelength range, where Rs is close to 1% while Rp is close to zero. Special coating designs are used to make this wavelength range as broad as possible and to maximize the polarization ratio Tp/Ts. Very low values of Tp (<.5%) can be measured very exactly using a special cavity ring down setup. The TFP is inserted into a cavity thus introducing additional losses which are equal to 1%-Tp. Using this method one can also determine the most favourable AOI for each TFP. Thin film polarizers (TFPs) are key components in a wide variety of applications, e.g. in regenerative amplifiers. LAYERTEC produces thin film polarizers on plane substrates (dimensions according to customer specifications) for wavelengths between 26nm and 25nm. All TFPs are optimized for high laser induced damage thresholds. Although there is no certified measurement available, we know from several customers that the LIDT of a TFP is approximately one third of the LIDT of a high reflecting mirror for the same wavelength which is coated using the same technology. Reflectance of an uncoated glass surface (R vs. angle of incidence) Θ B incident beam s+p-polarization Θ B transmitted beam s+p-polarization Θ B surface normal AOI [deg] reflected beam s-polarization : Brewster angle Figure 1: Explanation of the terms "s-polarized light" and "p-polarized light" and reflectance of an uncoated glass surface vs. angle of incidence for s- and p-polarized light Θ B p-pol.

115 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 113 STANDARD THIN FILM POLARIZERS SPECIAL THIN FILM POLARIZERS Rp [%] 1 Rs(-1%) Rs(99-1%) Rp(-1%) AOI = Rp (53 ) Rp (55 ) Rp (57 ) TFPs can be produced for AOI > 4, but polarization is most efficient and appears in a broad wavelength range if AOI = 55 (Brewster angle) is used Typical polarization ratios Tp/Ts: standard: > 5 (AOI = 45 or 55 ) An extended wavelength range of polarization which shows, however, a limited polarization ratio can be obtained by choosing AOI beyond Brewster angle Special designs with a polarization ratio of Tp/Ts up to 1 are possible High laser induced damage thresholds (useful for intra cavity applications) It is favourable to design the laser so that the polarizers can be tilted by ± 2 to adjust the polarizer for its best performance The standard design can be applied for wavelengths between 26nm and 25nm p-pol. AOI =7 9 1 p-pol. AOI= Figure 2: Reflectance spectra of a standard TFP for 13nm at AOI = 55 (Brewster angle) for s- and p-polarized light (, reflectance spectra of the same design for AOI = 53, 55 for p-polarized light and 57 (angle tuning lowers Rp at 13nm from.25% to <.1% thus giving the chance to optimize the polarization ratio) Figure 3: Broadband TFPs for the wavelength range of the Ti:Sapphire laser with different bandwidths and different polarization ratios, working at AOI = 7 ( and AOI=8 ( Rs (-1%) Rp(-1%) Rp(-1%) AOI= Figure 4: Broadband TFP for the 8nm region This special design provides an extremely broad polarizing wavelength range (~1% of the center wavelength) with Tp/ Ts = 3 1.

116 114 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Low Loss optical Components HR MIRRORS R > 99.99% in the VIS and NIR spectral range R > % was demonstrated at several wavelengths between 1 16nm Mirrors with defined transmission (e.g. T =.2%) For cavity ring down time spectroscopy it is favourable to adjust the transmission to the value of the scattering and absorption losses (T= S + A), see fig.1 All mirrors for CRD experiments are delivered with rear side AR coating. Wedged substrates on request. Special polished plane and spherically curved fused silica substrates (see page 15) rms - roughness: 1.5Å/2Å Surface quality: 1-5 scratch-digs, 5/1 x.1 (DIN EN ISO 111) Coating technique: magnetron sputtering, ion beam sputtering Optical parameters are stable against changes of temperature and humidity Attractive prices for small and medium numbers of substrates per coating run Very high reflectivities also for complex coating designs, e.g. GTI laser mirrors with R > 99.95% (see pages 96 97) Vacuum packaging or packaging under nitrogen cover gas in dust free boxes Measurement T, L [%] losses L transmission T reflection R,1 1,9 99,999,8 99,998,7 99,997,6 99,996,5 99,995,4 99,994,3 99,993,2 99,992,1 99,991 99, Measurement T, L [%] losses L transmission T,1 1,9 99,999,8 99,998,7 99,997,6 99,996,5 99,995,4 99,994,3 99,993,2 99,992,1 99,991 99, Figure 1: Reflectance, transmittance and loss spectra of low loss mirrors for 155 nm, optimized for highest reflectance (transmission ~ ) ( and designed for T S + A ( Please note that the reflectivity of the mirrors in fig.1a and 1b is nearly the same. However, the extremely low transmission of the mirror in figure 1a makes CRD measurements very difficult. reflection R Measurement P4116 P112 P7121 P316 P213 W31237 Q ,95 99,9 99,85 99,8 99,75 99,7 99,65 99,6 99,55 99, Measurement Q1112 Q413 Q514 V213A2 F51152 Z111 O413B3 A11128 Z895 Z ,998 99,996 99,994 99,992 99,99 99,988 99,986 99,984 99,982 99, Figure 2: Reflectance spectra of a variety of low loss mirrors for the UV ( and for the VIS-NIR spectral range (; all measurements were performed at the CRD setup which is described on pages Please note that these mirrors are specially designed for relatively high transmission.

117 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm Direct measurements of optical losses Type of losses VIS NIR Scattering Typical: 2 3ppm <1ppm Measured: 633nm*, 2 532nm** Absorption 1 2ppm*** <1ppm*** Total <5ppm <2ppm * Measurement performed at Jenoptik L.O.S. GmbH, Jena ** Measurement performed at Fraunhofer Institut IOF Jena *** Measurement performed at Institut of Photonic Technology (IPHT) e.v. Jena CAVITY RING DOWN TIME MEASUREMENTS AND REFERENCE DATA Measurement Wavelength R max [%] T [%] Loss [ppm] L =1-R-T Measured at 248nm LAYERTEC GmbH 266nm LAYERTEC GmbH 355nm LAYERTEC GmbH 4nm LAYERTEC GmbH 55nm LAYERTEC GmbH 633nm Westsächsische Technische Hochschule Zwickau, Germany 66nm Universität Heidelberg, Germany 798nm LAYERTEC GmbH 84nm LAYERTEC GmbH 13nm LAYERTEC GmbH 115nm LAYERTEC GmbH 1392nm TIGER OPTICS, USA (R measurement) Layertec GmbH (T measurement) 155nm IPHT Jena, Germany 235nm University of Grenoble, France 325nm University of Grenoble, France 4nm 99.9 Universität Bielefeld, Germany Table 1: Reflectivity and transmission values of LAYERTEC low loss mirrors; Reflectance measured by Cavity Ring-Down spectroscopy, AOI= AOI= 1, 99, ,999 99, ,998 99, , Measurement T [ppm] AOI= Figure 3: Measured reflectance ( and transmission ( spectrum of a low loss mirror for the wavelength range nm

118 116 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES COATINGS ON CRYSTAL OPTICS Laser applications using crystal optics have reached a high standard in industry and research. Optical coatings on crystals are an essential part of modern laser designs and cover the wide range from single wavelength AR coatings on laser and nonlinear optical crystals up to complex multilayer coatings providing several high-reflectance and high-transmittance wavelength ranges and thus replacing external laser mirrors. LAYERTEC has a lot of experience in coatings on laser crystals. Our coatings are used in industrial high power Q-switched and cw lasers of several laser manufacturers. The quality of coatings on crystals depends on the coating technique as well as on the surface quality of the crystal. All coatings are produced by magnetron sputtering thus showing very low straylight losses and a high thermal and climatical stability of the optical parameters. The rapid progress in crystal growth techniques resulted in a wide variety of new crystals for laser applications, e.g. laser crystals as several tungstanates and vanadates or nonlinear optical crystals like RTP. Each crystal type requires optimized polishing procedures and coating techniques. Besides the optical properties, especially the thermal expansion coefficients and the surface quality after storage and transport influence the coating quality. Especially hygroscopic crystals like LBO or BBO require special pretreatments to achieve high damage thresholds and long lifetime of the coatings. Thus, coatings on new crystals always require experimental investigations to find the best coating procedures. Using the special LAYERTEC coating technology also coatings on crystals with extraordinary dimensions and difficult sizes are possible. The following table gives an overview about the crystals which have already been coated at LAY- ERTEC and the types of layer systems which have been tested successfully. Crystal Type AR/BBAR Single HR optional with HT Double HR/BBHR optional with HT a-sio 2 x x x BBO BiBO x x CaCO 3 CTA x x x Nd:GdVO 4 x x x Nd:GGG x x Nd:Cr:GSGG x x KTA x x KTP x x x Yb:KGW, Yb:KYW x x x LBO LiNbO 3 LMA x x x Nd:LSB x x x RDP Ruby x x Ti:Sapphire x x x Spinell x x x Cr:YAG x x Er:YAG x x Ho:YAG x x Nd:YAG, Yb:YAG x x x Nd:YALO (YAP) x YLF x x x Nd:YVO 4 x x x ZGP ZnSe x x x Every coating run will be performed with detailed measurement data sheets. Do not hesitate to contact us for a discussion or an offer of your special coating problem.

119 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm COATINGS ON DOPED LASER CRYSTALS COATINGS ON NONLINEAR OPTICAL CRYSTALS AOI= AOI= AOI= 1 9 5, 4,5 5, 4,5 8 4, 4, 7 6 3,5 3, 3,5 3, 5 2,5 2, , 1,5 2, 1,5 2 1, 1, , , Figure 1: Reflectance spectrum of a dual HR mirror for 532nm and 164nm with a HT region around 88nm for pumping with a laser diode (on Nd:YAG) Figure 3: Reflectance spectrum of a triple wavelength AR coating on KTP: AR(,164nm nm + 34nm) <.5% Figure 5: Reflectance spectrum of a dual wavelength AR coating on ZGP: AR(,25nm) <1% + AR(,43nm) <.2% AOI= AOI= AOI= 1, 5, 4,5 1 9,75 4, 3,5 3, 8 7 6,5 2,5 5, , 1,5 1,, Figure 2: Reflectance spectrum of an AR coating for an Yb:KYW crystal: AR(,13nm)<.2% + AR( -3,98nm)<.2% Please note the large acceptance angle for the pump radiation Sputtered coatings on laser rods, discs and slabs with High laser induced damage thresholds for critical industrial applications of Q-switched and cw lasers Low residual reflectivities (e.g. R<.1% at 88nm on Nd:YAG) Broadband and multiple wavelength AR coatings Complex HR and HR/HT-coatings for compact laser designs,(e.g. HR( nm)+HT(88nm) on Nd:YVO 4 for diode pumped and frequency doubled "green" lasers) Coating of crystals with variable or extraordinary sizes and shapes Coating of the full aperture of small crystals Figure 4: Reflectance spectrum of an AR coating for PPSLT: AR(,2nm) <.2% + AR(,34 44nm) <1.5% Figure 6: Reflectance spectrum of a dichroic mirror on KTP: R(,532nm) <1% + HR(,164nm) >99.95% Broadband and multiple wavelength AR coatings Complex HR and HR/HT-coatings for compact laser designs, (e.g. HR(164nm) + HT(532nm) on KTP for frequency doubled Nd:YAG or Nd:YVO 4 lasers) Coating of crystals with variable or extraordinary sizes and shapes Coating of the full aperture of small crystals

120 118 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES

121 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 119 Metallic Coatings For Laser And Astronomical Applications

122 12 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES FRONT SURFACE SILVER MIRRORS Broadband SILVER MIRRORS FOR THE VIS AND NIR SILVER MIRRORS FOR Use in Femtosecond Lasers c) AOI= AOI= AOI= Figure 1: Reflectance spectra of three standard types of protected silver mirrors: optimized for 6nm 1nm R > 98% optimized for 6nm 2nm 6 75nm R > 96%, 75 2nm R > 98% c) optimized for 1nm 1nm R > 98% Optical properties: R > 98% throughout the specified wavelength range (except type R = 94 97% in the VIS outside the specified wavelength range R > 97% in the NIR outside the specified wavelength range Special features: Silver has the highest reflectance of all metals in the VIS and NIR Sputtered protective layers yield very stable optical parameters Lifetimes of more than 1 years in normal atmosphere were demonstrated although unprotected silver is chemically unstable The high atomic density of sputtered coatings guarantees that also very thin protective layers (~2nm) provide a good protection against the atmosphere The thickness of the protective layer can be used to optimize the reflectance of the mirrors for different wavelength ranges (see fig.1) Sputtered silver mirrors show extremely low straylight losses (TS ~ 3 x1 5 in the VIS and NIR) Silver mirrors with defined transmission (e.g. T =.1%) on request (see page 86) Mechanical stability of protected silver mirrors is tested according to MIL- M-1358C Maximum diameter: 5mm, especially for astronomical applications unpol. p-pol. AOI= p-pol Figure 2: Reflectance ( and GDD ( spectra of a silver mirror optimized for use with fs-lasers in the wavelength range 6 1nm (AOI=45 ) Silver mirrors are ideal for use in femtosecond laser system because of their extremely broad low GDD reflection band. For more examples see pages 86 87)

123 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm SILVER MIRRORS WITH ENHANCED REFLECTIVITY The reflectivity of silver mirrors can be enhanced for selected wavelengths or wavelength regions by a dielectric overcoat. Fig. 2 5 show examples for such silver mirrors with enhanced reflectivity. Such mirrors combine very high reflectivity at the wavelengths of interest with a relatively high reflectance throughout the VIS which makes them ideal for use with a pilot laser. AOI= AOI= Figure 3: Reflectance spectra of an enhanced silver mirror which shows R 98% throughout the visible spectral range: AOI =, AOI = 45, unpolarized light Enhanced silver mirrors of this type are useful for applications in astronomical devices AOI= Figure 4: Silver mirror with enhanced reflectivity at 425 and 85nm (R > 99.5%) AOI= Figure 5: Silver based turning mirror for 13nm with R > 8% for any pilot laser in the red spectral range Especially the mirror in fig. 4 was tested for very high LIDT values of cw- and ns-radiation. This makes it a cost effective alternative for all-dielectric mirrors for high power Yb:YAG- or Nd:YAG-lasers AOI = AOI = 22 AOI = 45 AOI = 45 AOI = AOI = Figure 6: Reflectance spectra of a silver based scanning mirror for laser diodes in the NIR: HR(22 58, 85 94nm) > 99.3% combined with R >5% between 63 and 67nm For more information on enhanced silver mirrors see pages 56, and

124 122 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES FRONT SURFACE ALUMINUM MIRRORS BROADBAND MIRRORs FOR THE UV, VIS AND NIR Aluminium MIRRORs FOR Multiple Wavelength and Femtosecond Applications bare Al standard AOI= unpol. p-pol. AOI= 45 enhanced 2 enhanced 1 AOI= Figure 1: Reflectance spectra of bare aluminum and of a standard protected aluminum mirror Optical properties: Bare aluminum: R>8% at 193nm R=92% at 248nm R>85% from 2nm to 95nm (R>9% from 23nm to 6nm) R>9% for λ>1µm Standard mirror: R=82 92% from 24nm to 55nm R=85 92% from 55nm to 95nm R>92% for λ>1µm p-pol Figure 2: Reflectance ( and GDD ( spectra of an aluminum mirror optimized for R > 85% at 266nm, 4nm and 8nm (AOI=45 ) enhanced 2 enhanced Figure 3: Reflectance ( and GDD ( spectra of aluminum mirrors with different designs for enhanced reflectivity for the third harmonic of the Ti:Sapphire laser (AOI= ) Aluminum is the metal with the highest reflectivity in the UV spectral range. Besides this, aluminum has also a high and relatively constant reflectivity in the VIS and NIR. The minimum in the reflectance curve around 8nm is due to a phonon resonance and can only be overcome by a dielectric overcoating. The reflectivity in the VIS and UV spectral range can be influenced by the coating technologies. In the case of protected aluminum mirrors the position of the minima and maxima of the reflectance depends on the design of the protective layer system and on the angle of incidence (AOI). Please specify AOI and the wavelengths of interest to optimize the reflectance as far as possible. Fig. 2 shows the reflectance spectra of a mirror optimized for high reflectivity at 266nm, 4nm and 8nm at AOI = 45. Maximum diameter: 5mm, especially for astronomical applications Special features: High reflectivity in the wavelength range specified Extremely low straylight losses of protected aluminum mirrors (TS < 1 4 at 633nm, TS < 1 3 at 248nm, TS 5 x 1 3 at 193nm) Standard mirrors can be cleaned using ethanol or acetone and are resistant to moderate abrasion (tested according to MIL-M-48497A and ) All mirrors are resistant to humidity (tested according to MIL-M-1358C 4.4.7) Highly stable optical parameters because of sputtered SiO 2 protective layer

125 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS nm PROTECTED AND ENHANCED ALUMINUM MIRRORS FOR THE DUV AND VUV opt. 157nm opt. 193nm AOI= AOI= AOI= 45 p-pol. AOI= Rr [%] Figure 4: UV optimized Al: reflectance spectra of Al mirrors optimized for 157nm and 193nm (AOI= ) AOI= Figure 5: Reflectance spectra of two types of enhanced Al mirrors for 157nm: R > 8% for AOI = 45, R > 94% Figure 6: Reflectance spectra of an aluminum mirror with enhanced reflectivity for 193nm (AOI = 45, Rr > 95%) Reflectance at 193nm can be improved up to Rr>95% This kind of mirrors can also be used as scanning mirror for AOI=45 5 with Rr>93% Reflectance in the VIS= 6 8%. This can be used for a pilot laser. VUV optimized mirrors should be treated with extreme care. Optical properties: Special coating design depending on the wavelengths of interest Optimized for157nm: R =74 78% for 157nm (R >7% from 15 to 2nm) Optimized for193nm: R =75 8% for 193nm Optimized for248 nm: R >9% for 248nm Reflectance at 157nm can be further improved by dielectric overcoatings (up to R>94%) Reflectance in the VIS: R = 6 8%. This can be used for a pilot laser Especially mirrors with R = 85 9% can be used at a wider range of AOI than all dielectric mirrors of this reflectivity

126 124 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES SPECIAL METALLIC COATINGS ChROMIUM COATINGS FOR OPTICAL APPLICATIONS BROADBAND NEUTRAL DENSITY FILTERS FOR THE NIR Chromium coatings are used for lithographic process applications and other special optical applications. LAYERTEC offers chromium coatings with extremely low pinhole density on mask blanks and silicon wafers. Typical substrates sizes are 6 inch x 6 inch, but uncommon sizes up to diameter 5mm are also possible. LAYERTEC uses specialized coating processes of the sputtering technique for a very efficient industrial production, which are able to optimize and control the intrinsic properties of chromium coatings such as: Low pinhole density High optical density Low mechanical stress High electrical conductivity Besides the availability of efficient coating plants LAYERTEC has preserved its capabilities for flexible production of small volumes such as OEM components or components for research and development. Do not hesitate to contact us for your special request. T [%] T 6% T 36% T 12% AOI= Figure 1: Transmittance spectra of broadband neutral density filters with different transmittance values Broad wavelength range of nearly constant transmittance (1nm to 1µm) Substrate: BaF 2 Other transmittance values on request

127 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 125 Solderable coatings Soldering is one of the most important mounting techniques for optics which need a very good thermal contact to a heat sink. LAYERTEC has developed several coating designs for solderable coatings on the basis of gold and other metals coatings. A very special problem are solderable coatings on components for high power applications. Extreme care must be taken to avoid metallic contaminations on the optical surfaces while the solderable coating has to cover the whole side surface of the substrate. Fig. 2a and b show a pump mirror which is coated with a solderable layer system on the top side and with dielectric coatings on the front and rear surfaces. a GOLD MIRRORS FOR THE NIR Spectral Range protected bare gold Figure 3: Reflectance spectra of protected and unprotected gold mirrors Optical properties: Bare gold: R > 97% from 7nm to 1µm R > 98% for λ >1µm Protected: R > 96% from 7nm to 2µm R > 98% for λ > 2µm AOI= 4 1 nm PARTIALLY TRANSMISSIVE GOLD LAYERS FOR OPTICAL AND NON-OPTICAL APPLICATIONS Gold coatings with a thickness in the range of about 1nm to about 5nm can be used as partial reflectors or attenuators in the NIR spectral range. Fig. 4 shows the transmission spectra of gold layers with different thickness. Moreover, thin gold layers are also useful for nonoptical applications. An example of such an application is the generation of single electron pulses. Thin metallic layers are irradiated with femtosecond laser pulses. This results in the release of electrons. These electron pulses are generated with the repetition rate of the femtosecond laser. Recent investigations showed that the pulse length of these electron pulses can be compressed to the attosecond range using a microwave cavity 1,2. Magnetron sputtering allows the reproducible production of partially transmissive gold layers in the mentioned thickness range. The optical parameters of these coatings are very stable because gold is chemically inert. Please note that gold layers are soft and can easily be damaged mechanically. b Special features: High reflectance in the NIR and IR Extremely low straylight losses (TS < 1 4 at 633nm) Gold mirrors are chemically stable and can thus be used without protective layer Unprotected gold is soft and is easily scratched if it is touched Protected mirrors can be cleaned (tested according to MIL-M-1358C 4.4.5) T [%] nm 2nm 4nm Figure 4: Transmission of partially transparent gold coatings on sapphire Figure 2: Pump mirror which is coated with a solderable layer system on the top side Literature: 1 A.Gliserin, A.Apolonski, F.Krausz, P.Baum; "Compression of single electron pulses with a microwave cavity", New Journal of Physics 14(212) 7355 (18pp) 2 M.Aichelsburger, F.O.Kirchner, F.Krausz and P.Baum; PNAS vol.17 no

128 126 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Cleaning of Optical surfaces Prerequisites: Preparation of the cleaning tissue: Cleaning of the optical surface: An air blower Optical cleaning tissue (e.g. Whatman ) Nonslip tweezers (e.g. with cork) Grab the tissue as 1 Spectroscopy grade 5 shown in fig.8 9 acetone* Pre-cleaning: Clean hands with soap or use clean gloves (latex, nitrile) Fold a new tissue along the long side several times (5, 6) Fold across until you have a round edge (7) Blow off dust from all sides of the 2 sample (2) 6 1 Moisten the tissue with acetone (9) A wet tissue will result in streaks Hold the sample with tweezers (16) Slide the curved tissue from one edge of the sample to the other (1 12) The tissue may be folded around and used again once Repeat steps 9 12 with a new tissue until the sample is clean Moisten tissue with acetone (3) Remove coarse soil from the edge and the chamfer (4) * Compared to alcohol we find acetone to be the better solvent as it significantly reduces the formation of streaks

129 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 127 Hints Small samples: Storage: Cleaning of concave surfaces: Put sample onto a concave polished glass support to pick it up easily (13) Use special tweezers Fingerprints on sputtered coatings (14): Moisten the surface by breathing upon it It works best to store the samples on a polished curved glass support (15) Clean the support like an optical surface before use Holding the tissue: Use the tweezers to hold the moistened tissue (16) 14 Slide (acetone) moistened tissue over the surface as long as the water film is visible Use a less often folded tissue that can be slidely bent (17) Clean analog to fig.9 12 Use your thumb to gently press the tissue onto the curved surface (18, 19) Use tissue only one time A concave support helps holding the sample (2) Exception: Never do this with hygroscopic materials (CaF 2 ) 19 2

130 128 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES Register Absorption Basics 28 Measurement 9, 39 Values 115 Aluminum Basics 31 Coatings 43, 45, 122ff Astronomical applications 121 Aspheres 16, 17 Barium fluoride Properties 19 Spectrum 21 BK7 Standard specifications 14ff Properties 19 Spectrum 2 Calcium fluoride (CaF2) Standard specifications 14ff Properties 19 Spectrum 21 Cavity Ring-Down (CRD) Standard specifications 33 Measurement Tool 8, 34 Coatings for 114 Chromium Basics 31 Coatings 124 Crystals Substrate 14, 15 Polishing 18 Coatings on 59, 67, 69, 79, 17, 116ff Damage Defects See LIDT See surface quality Edge filter UV 63 VIS 111 NIR 53ff, 55 IR 67, 16 Electron beam evaporation 5 Etalons 18 Filters See edge filters Narrow band 51, 11ff Flatness How to specify 12 Standard specifications 14, 15 Fused silica Standard specifications 14, 15 Properties 19 Spectrum 2 Gold Basics 31 Coatings 89, 125 Group delay dispersion (GDD) Basics 72ff Measurement 8 Ion beam sputtering (IBS) 4 Ion assisted deposition (IAD) 5 Large scale optics 18 Laser induced damage threshold (LIDT) Basics 36, 88 Measurement 9, 36ff UV 45, 47 VIS 48 NIR 86, 89 IR 68 Losses, optical Basics 28 Measurement 8 Values 115 Low loss optical components 114 Magnetron sputtering 4 Metallic coatings Basics 31 Metal-dielectric coatings Basics 32 UV 43, 45, 122ff NIR 56, 87, 12 LIDT 86 Non polarizing beam splitter VIS 59 NIR 57 Polarization Basics 29, 112 Roughness Measurement 7, 23 Sapphire Standard specifications 14ff Properties 19 Spectrum 21 Scanning mirrors 19, 121 Scattering Basics 28 Measurement 9 Values 115 SF1 Properties 19 Silver Basics 31 Coatings 56, 86ff, 12ff Special polishing 15 Substrate materials 14ff, 19ff Substrates How to specify 12 Standard specifications 14, 15 Surface form Measurement 6, 22 How to specify 12ff Standard specifications 15 Surface quality Measurement 9, 39 How to specify 13 Standard specifications 15 Thermal evaporation 5 Thin film polarizer (TFP) Basics 112ff UV 61, 63 VIS 59 NIR 52, 57, 78ff, 82 IR 67 Triple Wavelength Mirrors 61, 92 Wave plates 18 YAG, undoped Standard specifications 14ff Properties 19 Spectrum 21

131 FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS Substrate Materials LAYERTEC Mirrors* Common Lasers 294 nm Er :YAG laser p nm Ho :YAG laser p nm Ho :YAG laser p nm Tm :YAG laser p nm Er : Fibre laser p.98, 114 YAG Sapphire CaF 2 IR- fused silica Fused silica BK7 Standard mirrors Broadband mirrors Special broadband mirror OPO optics Diode lasers p.52 LiSAF Yb : Fibre p. 54 Ti :Sapphire p. 5,74ff Ar Lines Ti :Sa - SHG p. 9ff 134 nm Nd :YVO 4 laser p nm Nd :YAG laser p nm Fosterite laser 11 nm Yb : Glass laser 164 nm Nd :YAG laser p nm Yb :YAG laser p nm Nd :YAG laser p nm Nd :YVO 4 laser p nm Ti:Sapphire laser p. 74ff 755 nm Alexandrite laser p nm Ruby laser p nm HeNe laser 578 nm Cu -Vapour laser 532 nm Nd :YAG - SHG p nm Nd :YAG -THG p nm HeCd laser 38 nm XeCI laser p nm Nd :YAG - FHG p nm KrF laser p nm ArF laser p nm F 2 laser p. 42 *Bandwidths of selected LAYERTEC mirrors

132 Interference Optics The colors of a partially oxidized bismuth crystal result from interference effects. These effects are also the active principle of optical coatings. Dreamtime phone: +49() /744-, fax: +49() /744-4 Ernst-Abbe-Weg 1, MELLINGEN, GERMANY e- mail: info@layertec.de, internet: