New 1-Micron Laser Sources High Brightness Tools for Industrial Applications Manfred Berger, II-VI Development POLLASNET, Warszawa Dec. 11, 2006
Content 1. Introduction 2. Beamquality & Brilliance 3. Nd:YAG Lasers Rod Lasers Slab Lasers 4. Yb:YAG Disk Lasers 5. Yb - Fibre Lasers 6. Applications 7. Laser Safety 8. Future December 11, 2006 Manfred Berger POLLASNET 2
1.1 World Market for Lasers in Material Processing, 2005 Word Market for Lasers in Materials Processing, 2005 Total: EUR 1.70 Billion CO2 > 500W 36% CO2 + SSL = 79% CO2 < 500W 5% Diode 2% Excimer *) 19% SSL > 500W 7% SSL < 500W 31% *) incl. Microlithography OPTECH CONSULTING December 11, 2006 Manfred Berger POLLASNET 3
1.2 Evolution of the Beam Parameter Product for Industrial High Power Lasers (as of 2006) 1 µm Laser Wavelength: 1µm 10,6 µm 10.6 µm 4 kw Nd:YAG Lamp Pumped BPP > 20 mm*mrad 2 kw CO2 Slow Flow BPP : 4 mm*mrad 4 kw Nd:YAG Diode Pumped BPP >12 mm*mrad 5 kw CO2 Transverse Flow BPP >15 mm*mrad 4 kw Yb:YAG Disk Diode Pumped BPP : 8 mm*mrad 4 kw Yb:SiO2 Fibre Diode Pumped BPP : 4 mm*mrad 8 kw CO2 Fast Axial Flow BPP : 4 mm*mrad 0.5 kw Yb:YAG Disk Single Mode BPP : > 1 mm*mrad 2 kw Yb:SiO2 Fibre Single Mode BPP : 0.4 mm*mrad 8 kw CO2 Sealed Slab BPP : 4 mm*mrad December 11, 2006 Manfred Berger POLLASNET 4
1.3 Efficiencies and BPP of Industrial Laser Systems Disk and Fibre Laser show very high efficiency with low beam parameter product. Diode Lasers show highest efficiencies with lowest operating cost. Efficiency in % Fibre Disk DP-Rod LP-Rod HPDL with Fibre Beam Parameter Product in mm*mrad December 11, 2006 Manfred Berger POLLASNET 5
1.5 Performance of Laser-Systems for Industrial Manufacturing (2006) Rodlaser InnoSlab Disk laser Fibre laser December 11, 2006 Manfred Berger POLLASNET 6
1.6 Absorption Characteristics of Metals Fe 300 K December 11, 2006 Manfred Berger POLLASNET 7
1.7 Power Levels of DPSS-Lasers and Trend of MOOREs Law 100000 10000 1000 100 M2 < 1.1 Fibrelaser M2 > 1.1 Disk-/Fiberlaser Moores Law Fibrelaser M2 < 1.1 M2 > 1.1 Industrial CO2-Laser 10 1 1992 1994 1996 1998 2000 2002 2004 2006 2008 December 11, 2006 Manfred Berger POLLASNET 8
2.1 Caustic of a Gaussian Laser Beam 22w L 2w 0,G 22w L Θ 0,G Θ L Z R 0, G Z R L BPP = W 0, Θ0, G ΘL λl G = WL = = 2 2 π const December 11, 2006 Manfred Berger POLLASNET 9
December 11, 2006 Manfred Berger POLLASNET 10 2.2 Equations const π λ 2 θ W BPP q L 0,G 0,G G = = = = 2 2 2 2 2 L L L y x L BPP P M M P B λ λ K BPP BPP M G real 1 2 = = 2 2 ) ( 4 BPP P d F P B F L = π π L d fibre NA M = 2 2 3 2 0 4 f D K D f M d L + = π λ 2 2 2 1 sin n n NA = = α π λ L G G G BPP K M = = = 1 1 2 D f F =
3.1 Lamp pumped YAG-laser 4.4 kw December 11, 2006 Manfred Berger POLLASNET 11
3.2 Principle of a Lamp Pumped YAG-Laser December 11, 2006 Manfred Berger POLLASNET 12
3.3 Limits of Rod Laser, Focusability Thermal Stress, Thermal Lensing (Thermal properties of YAG) limit the available BPP For Distortion-free Lasing the Homogeneity - and also the Temporal Stability - of the Pump Beam Distribution has to be kept below 0.1%!!! Focusability Decreases with Laser Power since these Conditions are increasingly difficult to reach! December 11, 2006 Manfred Berger POLLASNET 13
3.4 Schematic Layout of a Nd:YAG Slab Laser Cavity December 11, 2006 Manfred Berger POLLASNET 14
3.5 Absorption Data Absorption (cm -1 ) 60 40 20 Comparison of 300K Absorption from Single Crystal and Ceramic Nd:YAG Samples 1% Single Crystal 1% CYAG 1.8% CYAG 4% CYAG 9% CYAG 798.8 808.7 VLOC, subsidiary of II-VI Incorporated 300K 0 770 790 810 830 Wavelength (nm) December 11, 2006 Manfred Berger POLLASNET 15
3.6 Conclusion Nd:YAG Lasers The maximum dissipated power in a laser rod is limited to 200 W/cm length and independent of the rod diameter The maximum extractable volume power density is limited to 100 Wcm -3 (typically 1kW/rod and approx. 10kW/slab) The typical high BPP (20 to approx. 80 mm*mrad) of slab or multi-rod kw-class Nd:YAG-lasers results in low focusability The Wall Plug Efficiency of rod-lasers is very low ( 3% for lamp pumped lasers to 10% for diode pumped systems) Rod lasers (lamp- or diode-pumped) suffer from thermal problems due to radial temperature gradients ( T 50 K) Focusability decreases with increased laser power Large diameter fibre delivery possible December 11, 2006 Manfred Berger POLLASNET 16
4.1 Disk laser, resonator configuration December 11, 2006 Manfred Berger POLLASNET 17
4.2 Principle of Yb:YAG-Disk Laser Heatsink Solder indium pump radiation Output- Coupler D D laser beam heat sink thin disk d d o.c. mirror pump radiation Pump-Beam December 11, 2006 Manfred Berger POLLASNET 18
4.3 Yb:YAG Disk Laser: Absorption-, Emission-Spectra Cross-Section in 10-20 cm 2 Pump- Diodes Wavelength in nm December 11, 2006 Manfred Berger POLLASNET 19
4.4 Pumping Efficiency of a Single Disk Output Power in W Efficiency (optical-optical) in % Pumping Power Density in kw / cm 2 December 11, 2006 Manfred Berger POLLASNET 20
4.5 Cavity design for a multiple-disk laser Cavity Kavitäten Mirror Pumpeinheiten Laser Endspiegel Mirror Fold Faltungsspiegel Mirror Partial Auskoppler Reflector Beam Delivery Fibre December 11, 2006 Manfred Berger POLLASNET 21
4.6 Output power scaling with number of disks December 11, 2006 Manfred Berger POLLASNET 22
4.7 Disk laser resonator length restriction Fundamental Mode Opteration of a confocal resonator is defined by: N F 2 wa = 1 λl N F Fresnel-number W a beam radius at the disk L resonator-length For a max. achievable power density of 50 W/mm 2 at the disk the beam radius is: w = P mm 50π P L expected fundamental mode laser power 2 L 2 a Consequently the resonator lenght as function of laser power is given by: 2 wa L = λ 6,37 10 3 PL λ [ mm] Resonator lenght scales with laser power and reaches 30 m at 5 kw laser power That long resonators are mechanically and optically not very stable December 11, 2006 Manfred Berger POLLASNET 23
4.8 Phase distortion and optical path difference due to temperature gradients in a Yb:YAG-disk The temperature gradient at the edge of the pump-spot results in a phase distortion since the index of refraction in hot Yb:YAG material is different to the cooled outer part of the disk. OPD Depending on the design the optical path difference is: 0,1µ m l 1µm This path difference reduces the efficiency of fundamental mode operation. ηg 0,9 0,5 η MM ηg η MM Ratio of TEM 00 - to multimode-efficiency An adaptive mirror in the disk cavity helps to compensate the phase distortion. December 11, 2006 Manfred Berger POLLASNET 24
4.9 Amplified stimulated emission within a Yb:YAG-disk Gain Reduction Lasing within the disk Remedy possible by: Disk diameter >> Pump spot diameter Special edge shaping to supress volume reflexion ASE limits the maximal power / disk to: 30kW < P < 1MW depending on the design L The resulting maximal extractable power density per disk is therefor: PL kw 10 2 A cm December 11, 2006 Manfred Berger POLLASNET 25
4.10 Conclusion Yb:YAG Disk Laser The maximum extractable volume power density per disk is 1 MW/cm 3 The extractable power density per disk is 10 kw/cm 2 ( 100 kw/disk possible) The low BPP of a multi-disk kw-class lasers results in good focusability (BPP 8mm*mrad) High Wall Plug Efficiency ( 25%) Conventional (folded) Resonator allows for tailoring of the BPP and very compact design (important features for material processing) Low resonator power-density (back-reflex insensitivity) Effective pumping ( 65% optical efficiency) December 11, 2006 Manfred Berger POLLASNET 26
5.1 Pump concepts for Fibre laser End-on-Pumping by high power diode laser Stacks Pumping by several small Stacks Pumping by many individual Single Mode Emitters December 11, 2006 Manfred Berger POLLASNET 27
5.2 Pump beam, power limitations Typical Data for the inner cladding: Fibre- Pumpbeam Core inner cladding D = 400 µm N.A. = 0,4 Laser Focusability of the pump source M 2 < 260 Independant of pump power D outer cladding But: The Brightness of the most advanced diode stacks is limited to: Brightness P MW = M M cm sr 1 2 2 2 2 x y λ Maximum pump power to be coupled into the fibre: P pump,max = 2.5 kw equals P pump,max =1.25 kw/fibre end Doubling of fibre diameter and NA results in: P pump > 40 kw December 11, 2006 Manfred Berger POLLASNET 28
5.3 Scaling of fibre laser output-power Scaling of output power by means of beam combination incoherent beam superposition total output power scales with number of modules max. theoretical beam quality scales with P 0 1/2 real beam quality approximately 2-5 times lower (due to losses) Beam quality : Example : Q = θ w Ptotal 60 modules,100 W each = 6 kw Max. beam quality = 8 * Q 0 2θ December 11, 2006 Manfred Berger POLLASNET 29
5.4 Low NA large mode area fibre design n active core pump core coating core <10 µm core NA = 0.1 0.2 absorption length > 20 m n active core double-clad large-mode-area fiber pump core coating core = 30.. 40 µm core NA = 0.06.. 0.08 absorption length < 10 m increased mode-field diameter reduced fiber length reduced nonlinearity December 11, 2006 Manfred Berger POLLASNET 30
5.5 Power limitations for active fibres 1. Available power per unit length of fibre Heat loss within the core material, Thermal flux through inner and out cladding, Heat transfer by air or water Temperature limits laser power per length unit For water cooling this limit amounts to: P l L W 1200 m For a 40 µm diameter core the max. extractable volume power density is therefor: P V L 1 MW cm³ core Pumpbeam inner cladding Out cladding December 11, 2006 Manfred Berger POLLASNET 31 Fibre- Laser
5.6 Limiting effects of quartz damage threshold and SRS Power density of fibre core and -endfaces Damage threshhold of quartz: E 1GW cm max 2 For a fibre core diameter of 40 µm results then: P L,max 9 kw Stimulated Raman Scattering SRS (non linear effect) limits the internal power: Max. power limited by SRS: P SRS 16 L A eff eff g R P SRS A eff L eff g R Power scaling possible by: increasingof thecorediameter Shortening of fibre length max. power out of the fibre effective area of the fibre core effective fibre length Raman gain coefficient December 11, 2006 Manfred Berger POLLASNET 32
5.7 Limitations of fundamental mode fibre lasers with present technology 12 Fibre Kerndurchmesser Core Diameter 40µm 10 damage threshold Power [kw] 8 6 4 2 1200 W/m extracted SRS threshold 0 0 2 4 6 8 10 12 14 16 18 20 22 24 Fiber length [m] 10 10 kw kw fundamental Grundmode mode Faserlaser fibremöglich laser is possible December 11, 2006 Manfred Berger POLLASNET 33
5.8 Evolution of fundamental mode output-power 3030 W Double clad fiber laser 2500 W CW output power [W] 2000 1800 1600 1400 1200 1000 800 600 400 200 step index fiber photonic crystal fiber 5 W 9,2 W 30 W 110 W 2000 W 1360 W 1300 W 800 W 1530 W 600 W 485 W 400 W 370 W 260 W 150 170 270 W W W 200 W 135 W 80 W 4 W 0 1992 1994 1996 1998 2000 2002 2004 2006 1900 W IPG SPI JDSU / SDL Jenoptik Nufern Crystal Fibre December 11, 2006 Manfred Berger POLLASNET 34
5.9 Design of multi-kw photonic chrystal fibre core diameter: 42 µm (~30 µm MFD) laser core NA: ~0.03 pump core diameter: 500 µm pump core NA: ~ 0.6 December 11, 2006 Manfred Berger POLLASNET 35
5.10 Conclusion Yb-fibre lasers The maximum extractable volume power-density in a single fibre is 1 MW/cm 3 ( 10 kw/fibre possible) Very low BPP results in very good focusability (BPP 5 mm*mrad) Very High Wall Plug Efficiency ( 30%) Single element laser diodes with fibre pigtail and / or laser-diode bars allow for efficient optical pumping and enable monolithic compact design of the laser including the beam delivery fibre. With the availability of Large Mode Area (LMA) fibres multi-kw fundamental mode lasers offer highest beam quality and brilliance. December 11, 2006 Manfred Berger POLLASNET 36
6.1 Application Space Beam Parameter Product (mm.mrad) 1000 100. 10 1 Printing Soldering Polymer welding Welding Microbending Rapid Prototyping Microsoldering Marking Microwelding Brazing Cutting 0.1 1 10 100 1000 10000 100000 1W 10W 100W 1kW 10kW Laser Power Hardening remelting Cladding Thick sheet welding Aerospace 1000 100 December 11, 2006 Manfred Berger POLLASNET 37 10 1 100kW M 2 Assuming 1090nm
6.2 Welding efficiency of CO 2 - vs YAG-laser December 11, 2006 Manfred Berger POLLASNET 38
6.3 Remote Welding December 11, 2006 Manfred Berger POLLASNET 39
6.4 Transmission of 5 kw power through 1 km MM fibre December 11, 2006 Manfred Berger POLLASNET 40
6.5 High speed cutting with fibre lasers (ILT Aachen) December 11, 2006 Manfred Berger POLLASNET 41
6.6 Comparison of fibre- and CO2-laser cutting edge quality (IWS Dresden) December 11, 2006 Manfred Berger POLLASNET 42
4 0 3 0 2 0 1 0 0 0 2 4 6 8 1 0 1 2 6.7 Hypothetical cutting performance with comparable laser parameters for CO2- and disk-/fibre lasers Cutting Performance CO2 (10,6 µm) vs. Fiber Laser ( ~ 1 µm) CO2 Fiber a. u. V [m/min] cross over zone a. u. Thickness [mm] December 11, 2006 Manfred Berger POLLASNET 43
6.8 Precission micro cuts in medical components December 11, 2006 Manfred Berger POLLASNET 44
7.1 Lambertian stray radiation and NOHD-limit for a 12 kw-co2-laser at 10.6 µm (Ing. Büro Göbel) December 11, 2006 Manfred Berger POLLASNET 45
7.2 Lambertian stray radiation and NOHD-limit for a 12 kw-fibre laser at 1070 nm December 11, 2006 Manfred Berger POLLASNET 46
FUTURE December 11, 2006 Manfred Berger POLLASNET 47
And/Or December 11, 2006 Manfred Berger POLLASNET 48
And/Or hν hν hν hν hν hν hν hν... the PHOTON FACTORY December 11, 2006 Manfred Berger POLLASNET 49
Acknowledgements I am very grateful for help and advise I received from the following organisations: - University of Stuttgart IFSW - University of Jena IAP - Fraunhofer Institute IOF Jena - Fraunhofer Institute ILT Aachen - Fraunhofer Institute IWS Dresden - Rofin Sinar Laser - IPG Photonics - Trumpf - Southampton Photonics - Primes - Ingenieurbüro Göbel, Darmstadt December 11, 2006 Manfred Berger POLLASNET 50