ISO INTERNATIONAL STANDARD

Transcription

1 INTERNATIONAL STANDARD ISO First edition Lasers and laser-related equipment Test methods for laser beam widths, divergence angles and beam propagation ratios Part : General astigmatic beams Lasers et équipements associés au lasers Méthodes d'essai des largeurs du faisceau, angles de divergence et des facteurs de limite de diffraction Partie : Faisceau astigmatiques générau Reference number ISO :5(E) OSI 5

2 IS O:(5E) ii ISO 5 Allr ithgsr esedevr

3 Contents Page Foreword...iv Introduction...v 1 Scope...1 Normative references Terms and definitions Coordinate sstem Test principles General Spatial second order moments of the Wigner distribution Second order moments of the Wigner distribution Derived quantities Measurement arrangement and test equipment General Preparation Control of environment Detector sstem Beam-forming optics and optical attenuators Measurement of the second order moments General Measurement of the second order moments of power densit distributions Measurement of all second order moments of the Wigner distribution Determination of effective beam propagation ratio Determination of intrinsic astigmatism Determination of the twist parameter Test report...1 Bibliograph...15 I SO 5 All irthgs ersedevr iii

4 Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normall carried out through ISO technical committees. Each member bod interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closel with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part. The main task of technical committees is to prepare International Standards. Draft International Standards adopted b the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval b at least 75 % of the member bodies casting a vote. ISO was prepared b Technical Committee ISO/TC 17, Optics and photonics, Subcommittee SC 9, Electro-optical sstems. ISO consists of the following parts, under the general title Lasers and laser-related equipment Test methods for laser beam widths, divergence angles and beam propagation ratios: Part 1: Stigmatic and simple astigmatic beams Part : General astigmatic beams Part 3: Intrinsic and geometrical laser beam classification, propagation, and details of test methods (Technical Report) iv I SO 5 All irthgs ersedevr

5 Introduction The propagation properties of laser beams can be characterized b ten independent parameters when appling the method of second order moments (see ISO/TR ). Most laser beams need few parameters for a complete description due to their higher smmetr. Lasers emit beams which are stigmatic or simple astigmatic due to their resonator design. Part 1 of ISO describes the measurement methods for stigmatic and simple astigmatic beams while this part of ISO deals with the measurement procedures for general astigmatic beams. This part of ISO is applicable to beams of unknown tpe. Beam characterization, based on the method of second order moments as described in Part 1 and this part of ISO 11146, is onl valid within the paraial approimation. The theoretical description of beam characterization and propagation as well as the classification of laser beams is given in ISO/TR , which is an informative Technical Report. The procedures for background subtraction and offset correction are also given in ISO/TR In ISO 11146, the second order moments of the power (energ) densit distribution function are used for the determination of beam widths. If problems are eperienced in the direct measurements of these quantities, other indirect methods of measurement of second order moments ma be used as long as comparable results are achievable. In ISO/TR , three alternative methods for beam width measurement and their correlation with the method used in this part of ISO are described. These methods are: variable aperture method; moving knife-edge method; moving slit method. The problem of the dependence of the measuring result on the truncation limits of the integration area was investigated and evaluated b an international interlaborator eperiment carried out in The results of this interlaborator testing were taken into consideration in this document. The International Organization for Standardization (ISO) draws attention to the fact that it is claimed that compliance with this document ma involve the use of a patent concerning the determination of beam characteristics b measuring along the beam caustic of the transformed beam produced b a lens as described in 5.3 and 5.4. ISO takes no position concerning the evidence, validit and scope of this patent right. The holder of this patent right (U.S. No. 5,67,1) has assured ISO that he is willing to negotiate licences under reasonable and non-discriminator terms and conditions with applicants throughout the world. In this respect, the statement of the holder of this patent right is registered with the ISO. Information ma be obtained from: Coherent Inc. 51 Patrick Henr Drive Santa Clara, CA USA Attention is drawn to the possibilit that some of the elements of this document ma be the subject of patent rights other than those identified above. ISO shall not be held responsible for identifing an or all such patent rights. I SO 5 All irthgs ersedevr v

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7 Lasers and laser-related equipment Test methods for laser beam widths, divergence angles and beam propagation ratios Part : General astigmatic beams 1 Scope This part of ISO specifies methods for measuring beam widths (diameter), divergence angles and beam propagation ratios of laser beams. This part of ISO is applicable to general astigmatic beams or unknown tpes of beams. For stigmatic and simple astigmatic beams, ISO is applicable. Within this part of ISO 11146, the description of laser beams is accomplished b means of the second order moments of the Wigner distribution rather than phsical quantities such as beam widths and divergence angles. However these phsical quantities are closel related to the second order moments of the Wigner distribution. In ISO/TR , formulae are given to calculate all relevant phsical quantities from the measured second order moments. Normative references The following referenced documents are indispensable for the application of this document. For dated references, onl the edition cited applies. For undated references, the latest edition of the referenced document (including an amendments) applies. ISO 11145, Optics and optical instruments Lasers and laser-related equipment Vocabular and smbols ISO :5, Lasers and laser-related equipment Test methods for laser beam widths, divergence angles and beam propagation ratios Part 1: Stigmatic and simple astigmatic beams IEC 614:199, Power and energ measuring detectors, instruments and equipment for laser radiation 3 Terms and definitions For the purposes of this document, the terms and definitions given in ISO 11145, ISO , IEC 614 and the following appl. NOTE The, and z-aes in the following definitions refer to the laborator sstem (as described in Clause 4). Here and throughout this document the term power densit distribution refers to continuous wave sources. It might be replaced b energ densit distribution in case of pulsed sources. 3.1 generalized beam diameter d g measure of the etent of the power densit distribution of a beam in a cross-section at an aial location z, derived from the centred second order moments b I SO 5 All irthgs ersedevr 1

8 dg = + (1) NOTE This definition is similar to the beam diameter defined in ISO or ISO But in this contet the definition is not restricted to circular power densit distributions. 3. generalized beam waist location z,g position where the generalized beam diameter reaches its minimum value along the ais of propagation 3.3 generalized Raleigh length z R,g distance along the beam ais from the generalized beam waist where the generalized beam diameter is a factor of larger than the generalized beam waist diameter 3.4 Wigner distribution phase space distribution representing a laser beam in a transverse plane at location z NOTE The Wigner distribution is a function of two spatial and two angular coordinates, giving the amount of beam power propagating through the point (,) in the direction (Θ, Θ ). 3.5 spatial first order moments of the Wigner distribution, subset of the first order moments, which can be directl obtained from measured power densit distribution b ( z) = E(,,) z d d E(,,)d z d () and ( z) = E(,,) z d d E(,,)d z d (3) where E(,,z) is the power densit distribution at the specific plane z = constant. 3.6 second order moments of the Wigner distribution,,, Θ, Θ, Θ Θ, Θ, Θ, Θ, Θ ten second order moments of the Wigner distribution of the beam at location z NOTE 1 The ten second order moments contain information on the following phsical beam properties: beam size and orientation, divergence angles and their orientation, radii of curvature of the phase paraboloid and their orientation and the twist parameter. Details on these relations are given in ISO/TR I SO 5 All irthgs ersedevr

9 NOTE In ISO , the three spatial second order moments are defined as σ, σ and σ. In this part of ISO and ISO/TR , the angular brackets are used to emphasize the coordinates of the moments. This means that σ =, σ = and σ =. NOTE 3 Three angular moments Θ, Θ and Θ Θ are independent of z. The other seven second order moments are, in general, functions of z. 3.7 spatial second order moments of the Wigner distribution,, subset of the second order moments, which can be directl obtained from measured power densit distribution b ( z) = E(,,)( z ) d d Ez (,,)d d (4) ( z) = E(,,)( z ) d d Ez (,,)d d (5) and ( z) = E(,,)( z )( )d d Ez (,,)d d (6) 3.8 beam matri P smmetric and positive definite 4 4 matri containing all ten second order moments of the Wigner distribution and its elements and given b Θ Θ Θ Θ P = (7) Θ Θ Θ Θ Θ Θ Θ ΘΘ Θ 3.9 effective beam propagation ratio M eff invariant quantit related to the focusabilit of a general astigmatic beam, defined as: I SO 5 All irthgs ersedevr 3

10 M eff 4π 1 = det( ) 4 λ P (8) where det (P) is the determinant of matri P NOTE 1 The effective beam propagation ratio far field localization of the beam. M eff is an invariant related to the overall beam spread or the near and NOTE For simple astigmatic beams, the effective beam propagation ratio is the geometric mean of the beam propagation ratios of the principal aes of the beam: M eff = M M. For stigmatic beams M eff = M. 3.1 intrinsic astigmatism a degree of how close to a stigmatic beam the general astigmatic beam can be transformed b using lenses and free spaces ( Θ Θ ) Θ Θ ( ΘΘ Θ Θ ) ( eff ) 8π a = M + + W (9) λ NOTE Beams are classified according to their intrinsic astigmatism a, which is an invariant quantit. A beam with a = is called intrinsic stigmatic, a beam with a > is called intrinsic astigmatic. For simple astigmatic beams ( )( ) a = 1 M M. More details are given in ISO/TR twist parameter t parameter related to the rotational properties of the phase front of a beam, and also to the orbital angular momentum carried b the beam t = Θ Θ (1) NOTE The twist parameter is invariant under propagation through free space and spherical lenses. It might be altered under propagation through clindrical lenses. 3.1 principal aes of a power densit distribution aes of the maimum and minimum beam etent based on the centred second order moments of the power densit distribution in a cross-section of the beam [ISO :5] NOTE The aes of maimum and minimum etent are alwas perpendicular to each other orientation of a power densit distribution ϕ angle between the ais of the laborator sstem and that of the principal ais of the power densit distribution which is closer to the ais [ISO :5] π π NOTE From this definition it follows that < ϕ < for ϕ π 4 ; if ϕ = ±π 4, ϕ is defined as the angle between 4 4 the ais and the major principal ais of the power densit distribution. 4 I SO 5 All irthgs ersedevr

11 3.14 beam widths dσ, dσ etent of a power densit distribution in a cross-section of the beam at an aial location z along that principal ais which is closer to the - or -ais of the laborator coordinate sstem, respectivel, based on the centred second order moments of the power densit distribution NOTE If the principal aes make the angle π/4 with the and -aes of the laborator coordinate sstem, then d σ is b convention the larger beam width. [ISO :5] 4 Coordinate sstem The, and z aes define the orthogonal space directions in the laborator aes sstem and shall be specified b the user. The z ais shall approimatel coincide with the direction of the beam. The and aes are transverse aes, usuall horizontal and vertical, respectivel. The origin of the z-ais is in a reference plane defined b the manufacturer, e.g. the front of the laser enclosure. 5 Test principles 5.1 General The following test principles are valid for general astigmatic beams. For stigmatic and simple astigmatic beams ISO ma be applied. 5. Spatial second order moments of the Wigner distribution Spatial second order moments are obtained b acquisition of power densit distributions b means of spatiall resolving detectors, correcting the measured profiles and calculating the first and second order moments. 5.3 Second order moments of the Wigner distribution For the determination of all ten second order moments two different measurement set-ups are required. Eight of the ten second order moments and the sum ( Θ Θ ) + are obtained b acquisition of power densit distributions along the propagation ais z in different planes near the generalized waist position, calculating the three spatial second order moments of each measured power densit profile and fitting three independent parabolas to them. The difference ( Θ Θ ) is obtained from the spatial moments of a power densit distribution acquired behind a clindrical lens. 5.4 Derived quantities The effective beam propagation ratio M eff, the intrinsic astigmatism a, and the twist parameter t are obtained from the second order moments of the Wigner distribution according to the Equations (8) to (1). I SO 5 All irthgs ersedevr 5

12 6 Measurement arrangement and test equipment 6.1 General The test is based on the measurement of the cross-sectional power densit distribution of the entire laser beam. 6. Preparation The optical ais of the measuring sstem should be coaial with the laser beam to be measured. Suitable optical alignment devices are available for this purpose (e.g. aligning lasers or steering mirrors). The aperture of the optical sstem should accommodate the entire cross-section of the laser beam. Clipping shall be smaller than 1 % of the total beam power or energ. The attenuators or beam-forming optics should be mounted so that the optical ais runs through the geometrical centres. Care shall be taken to avoid sstematic errors. Reflections, interference effects, eternal ambient light, thermal radiation or air draughts are all potential sources of error. An evaluation shall be made to determine if the entire laser beam reaches the detector surface. For testing this, apertures of different widths can be introduced into the beam path in front of each optical component. The aperture that reduces the output signal b 5 % should have a diameter less than,8 times the aperture of the optical component. 6.3 Control of environment Suitable measures such as mechanical and acoustical isolation of the test set-up, shielding from etraneous radiation, temperature stabilization of the laborator, choice of low-noise amplifiers shall be taken to ensure that the contribution to the total probable error of the parameter to be measured is low. Care should be taken to ensure that the atmospheric environment in high-power laser beam paths does not contain gases or vapours that can absorb the laser radiation and cause thermal distortion in the beam to be measured. 6.4 Detector sstem Measurement of the cross-sectional power densit distribution requires the use of a detection sstem with high spatial resolution and high signal-to-noise-ratio. The accurac of the measurement is directl related to the spatial resolution of the detector sstem and its signal-to-noise ratio. The latter is important for laser beams with low power densities at larger diameters (e.g. for diffracted parts of the laser beams). For piel based detector sstems the spatial resolution, should be at least 1/ of the smaller beam width. In practice, noise in the wings of the power densit distribution E(,,z) ma readil dominate the second order moment integral. Thus, it is usuall necessar to appl background correction procedures. Refer to ISO/TR for further details. The radiation detector sstem shall be in accordance with IEC 614:199, in particular with Clauses 3 and 4. Furthermore, the following points shall be noted. Care shall be taken to ascertain the damage thresholds of the detector surface so as to make sure that these thresholds are not eceeded b the laser beam. It shall be confirmed, from manufacturers' data or b measurement, that the output quantit of the detector sstem (e.g. the voltage) is linearl dependent on the input quantit (laser power). An wavelength dependenc, non-linearit or non-uniformit of the detector or the electronic device shall be minimized or corrected b use of a calibration procedure. 6 I SO 5 All irthgs ersedevr

13 When using a scanning device for determining the power densit distribution function, care shall be taken to ensure that the laser output is temporall stable during the whole scanning period. When measuring pulsed laser beams, the trigger time dela of sampling as well as the measuring time interval pla an important role because the beam parameters ma change during the pulse. Therefore it is necessar to specif these parameters in the test report. 6.5 Beam-forming optics and optical attenuators If the beam cross-sectional area is greater than the detector area, a suitable optical sstem shall be used to reduce the beam cross-sectional area on the detector surface. The change in magnification shall be taken into account during the evaluation procedure. Optics shall be selected appropriate to wavelength. An attenuator ma be required to reduce the laser power densit at the surface of the detector. Optical attenuators shall be used when the laser output-power or power densit eceeds the detector's working (linear) range or the damage threshold. An wavelength, polarization and angular dependenc, nonlinearit or non-uniformit, including thermal effects of the optical attenuator, shall be minimized or corrected b use of a calibration procedure. None of the optical elements used shall significantl influence the relative power (energ) densit distribution. 7 Measurement of the second order moments 7.1 General Before the measurements are started, the laser shall warm up for at least 1 h (unless otherwise stated b the manufacturer) to achieve thermal equilibrium. The measurements shall be carried out at the operating conditions specified b the laser manufacturer for the tpe of laser being evaluated. 7. Measurement of the second order moments of power densit distributions Spatial second order moments are calculated from measured and corrected power densit distributions. The corresponding integrations are carried out on a subset of the measured data, called the integration area, because otherwise crucial noise in the data ma dominate the integrals. In man cases, a proper choice of the integration area is important for obtaining reliable results. The following procedure relates the size and position of the integration area to the size and position of the measured power densit distribution, which are initiall unknown. Hence, an iterative procedure is required. All integrations according to Equations () to (6) are performed on a rectangular integration area which is centred to the beam centroid, defined b the spatial first order moments, orientated parallel to the principal aes of the power densit distribution, and sized three times the beam widths d σ and d σ (see Figure 1). Since the beam's centroid, orientation and widths are initiall unknown, the procedure starts with an approimation for the integration area. The approimation should include the beam's etent, orientation and position. Using this integration area, initial values for the beam position, size and orientation are obtained which are used to redefine the integration area. From the new integration area, new values for the beam size, orientation and position are calculated. This procedure shall be repeated until convergence of the results is obtained. I SO 5 All irthgs ersedevr 7

14 The orientation of the principal aes of a power densit distribution, or the azimuthal angle ϕ, can be obtained from the second order moments of the power densit distribution b ( ) 1 ϕ( z) = arctan (11) for ; for =, ϕ(z) is obtained as: ϕ π = (1) 4 ( z) sgn( ) where sgn( ) = (13) The beam widths in the direction of its principal aes are given b 1 1 ( ) dσ z γ ( ) 4 = ( ) 1 1 ( ) dσ z γ ( ) 4 = + + ( ) (14) (15) where γ = sgn = ( ) (16) Equations (14) and (15) are valid onl for. For = the shall be replaced b ( ) 1 dσ ( z ) = + + (17) ( ) 1 dσ ( z ) = + (18) These calculations are performed for each measurement and the mean values and the standard deviations of the beam widths and the azimuth angle are calculated. 8 I SO 5 All irthgs ersedevr

15 Ke 1 outline of integration area Figure 1 Integration area 7.3 Measurement of all second order moments of the Wigner distribution For sufficient and balanced accurac of all ten second order moments, power densit profiles at no less than twent equall distributed z locations within a range of at least three generalized Raleigh lengths before and after the generalized beam waist shall be acquired. If the generalized beam waist is not accessible for direct measurement an artificial waist shall be created b means of an aberration-free focusing lens or focusing sstem. From the acquired power densit distributions at distance z from the reference plane or the rear principal z z z shall plane of the focusing sstem used, respectivel, the spatial moments ( ), ( ) and ( ) be calculated according to 7.. The second order moments Θ, Θ, and the quantit,,, Θ, Θ, Θ Θ, s = Θ + Θ (19) in the reference plane or the rear principal plane of the focusing sstem used, respectivel, are obtained b fitting three independent parabolas to the spatial moments in the measurement planes according to ( z) = + z Θ + z Θ () ( z) = + zs + z Θ Θ (1) I SO 5 All irthgs ersedevr 9

16 ( z) = + z Θ + z Θ () The proper choice of measurement plane positions requires that the distance of the generalized beam waist position from the reference position or the rear plane of the focusing sstem z,g and the generalized Raleigh range z R,g are determined using: z,g = Θ Θ + Θ + Θ (3) ( Θ Θ ) + + zr,g = + ( + ) Θ Θ Θ Θ (4) To determine the second order moments Θ and Θ, a clindrical lens with a focal length f c shall be placed at the distance z = z,g + f c from the reference plane or the rear plane of the focusing sstem in horizontall focusing orientation (see Figure ). The power densit distribution in the rear focal plane of the clindrical lens shall be acquired. The spatial second order moment calculated according to 7. and denoted as. The clindrical lens shall be h rotated to verticall focusing orientation (see Figure 3). The power densit distribution in the rear focal plane shall be acquired again, the spatial second order moment calculated according to 7. and denoted as. v NOTE A focal length of f c z R,g of the clindrical lens is recommended. This ensures a proper beam size in the rear focal plane of the clindrical lens. The second order moments c Θ and ( ) Θ in the plane z = shall then be calculated using s 1 Θ = + (5) f v h ( ) s 1 Θ = (6) f v h c If a focusing sstem has been used to create an artificial waist, the following procedure shall be performed to obtain the ten second order moments in the reference plane from the alread obtained ten second order moments. Arrange all the ten second moments in a beam matri P (see Clause 3). Calculate the beam matri P ref of the reference plane according to P ref = S P S T (7) where S is the inverse geometric optical sstem matri representing the ra propagation from the reference plane to the rear principal plane of the focusing sstem and S T is the transposition of matri S. Assuming that a thin spherical lens has been chosen and the origin of the z ais has been placed in the centre of the lens, this sstem matri is given b 1 I SO 5 All irthgs ersedevr

17 L 1 L f L 1 L f S = (8) 1 1 f 1 1 f where L f is the distance between the reference plane and the lens; is the focal length. If another focusing element has been used, e.g. a thick spherical lens or a sstem of lenses, see ISO/TR on how to obtain the sstem matri S. The desired second order moments in the reference plane are the elements of the matri P ref. Figure Clindrical lens in horizontall focusing orientation Figure 3 Clindrical lens in verticall focusing orientation 8 Determination of effective beam propagation ratio The effective beam propagation ratio is calculated according to Equation (8), where P is the so-called beam matri which contains all second order moments of the Wigner distribution which are determined as given in Clause 7. Further details are given in ISO/TR Determination of intrinsic astigmatism Another invariant during propagation is the intrinsic astigmatism a which is defined in Equation (9). The beams are classified due to their intrinsic astigmatism a. A beam with a = is called intrinsic stigmatic, a beam with a > intrinsic astigmatic. For the determination of the intrinsic astigmatism it is necessar to measure all ten second order moments of the Wigner distribution as described in Clause 7. Further details are given in ISO/TR I SO 5 All irthgs ersedevr 11

18 1 Determination of the twist parameter From the second order moments of the Wigner distribution determined according to Clause 7 the twist parameter t is calculated using Equation (1). 11 Test report The following information shall be included in the test report: a) General information 1) reference to this part of ISO (ISO :5); ) date of test; 3) name and address of test organization; 4) name of individual performing the test. b) Information concerning the tested laser 1) laser tpe; ) manufacturer; 3) manufacturer s model designation; 4) serial number. c) Test conditions 1) laser wavelength(s) at which tested; ) temperature in K (diode laser cooling fluid) (onl applicable for diode lasers); 3) operating mode [continuous wave (cw) or pulsed]; 4) laser parameter settings: output power or energ, current or energ input, pulse energ, pulse duration, pulse repetition rate; 5) mode structure; 6) polarization; 7) environmental conditions. 1 I SO 5 All irthgs ersedevr

19 d) Information concerning testing and evaluation 1) test equipment: camera, moving pinhole ; ) detector and sampling sstem: response time of the detector sstem, trigger dela of sampling (for pulsed lasers onl), measuring time interval (for pulsed lasers onl); 3) beam forming optics and attenuating method: tpe of attenuator, tpe of beam splitter, tpe of focusing element; 4) other optical components and devices used for the test (polarizer, monochromator, etc.); 5) other relevant parameters or characteristics of the test which have to be chosen (aperture setting, reference plane, reference ais, laborator sstem). I SO 5 All irthgs ersedevr 13

20 e) Test results 1) The ten second order moments of the Wigner distribution: Focusing element used for creating an artificial generalized waist Focal length At a distance from reference plane of Focal length of clindrical lens Parameter Mean value Standard deviation Θ Θ ΘΘ Θ Θ Θ Θ ) Beam propagation invariants and twist parameter (according to Clauses 8 to 1) Parameter Mean value Standard deviation Effective beam propagation ratio M eff Intrinsic astigmatism a Twist parameter t If required, phsical beam parameters such as beam widths and beam divergence angles ma be obtained from the measured second order moments of the Wigner distribution and added to the test report. See ISO/TR for the relationship between the second order moments of the Wigner distribution and the phsical beam parameters. 14 I SO 5 All irthgs ersedevr

21 Bibliograph [1] ISO/TR , Lasers and laser-related equipment Test methods for laser beam widths, divergence angles and beam propagation ratios Part 3: Intrinsic and geometrical laser beam classification, propagation and details of test methods I SO 5 All irthgs ersedevr 15

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