High Power Dense Spectral Combination Using Commercially Available Lasers and VHGs Christophe Moser, CEO Moser@ondax.com Contributors: Gregory Steckman, Frank Havermeyer, Wenhai Liu: Ondax Inc. Christian Wessling, Martin Traub, Dieter Hoffman Fraunhofer Institute for Laser Technology, Aachen, Germany Stefan Heinemann Fraunhofer USA Center for Laser Technology, Plymouth, MI, U.S.A Ondax,Inc. Page 1 AHPSL conference Sept 17-19 2007
Content: Introduction: Coarse vs. Dense Wavelength Multiplexing (DWM) Volume Holographic Gratings for High Power Lasers DWM DWM Architecture 1kW Fiber Coupled Unit Fabrication and Reliability of Volume Holographic Gratings Applications Conclusions Ondax,Inc. Page 2 AHPSL conference Sept 17-19 2007
970 971 980 Coarse vs. Dense λ Multiplexing COARSE λ multiplexing Commercially Available High Power Laser Diode Wavelengths 80X 91X 94X 97X DENSE λ multiplexing Ondax,Inc. Page 3 AHPSL conference Sept 17-19 2007
Challenges for Dense λ Muxing Broad Linewidth: 2.5 4nm MUXING GRID High Temperature Dependence: ~0.3nm/ o C Ondax,Inc. Page 4 AHPSL conference Sept 17-19 2007
Requirement for Dense λ Muxing Laser Diodes Combiner Linewidth 10 X improvement over state of the art laser diodes i.e ~ 0.2nm Bandpass Temperature Dependence 10 X improvement over state of the art laser diodes i.e ~ <0.03nm/ o C 20 X improvement Over State of The Art dichroics i.e < 0.5nm Ondax,Inc. Page 5 AHPSL conference Sept 17-19 2007
VHG-FAC for Laser Linewidth Reduction Volume Holographic Gratings (VHGs) in glass are critical for narrowing the spectral linewidth of High Power Laser Diodes 1,30E+07 HPSL VHG-FAC Intensity [a.u.] 1,10E+07 9,00E+06 7,00E+06 5,00E+06 I = 90A I = 90A locked P = 70,1 W ( free running * ) P = 67,4 W (locked) VBG with 90% efficiency 3,00E+06 1,00E+06 940 950 960 970 980 990 1000 l [nm] * without FAC lens VHGs in FAC lenses improve off-the-shelf diode lasers: 10 X in linewidth : 4nm 0.2nm 30 X in temperature dependence: 0.3nm/ o C 0.01nm/ o C Ondax,Inc. Page 6 AHPSL conference Sept 17-19 2007
VHG-FAC Working Principle Commercially Available VHG-FAC Combines Collimation And Wavelength Narrowing Into One Ondax,Inc. Page 7 AHPSL conference Sept 17-19 2007
Linewidth Reduction Results 30nm 6nm 16000 16000 Intensity [a.u.] 14000 12000 10000 8000 6000 15A 25A 35A 45A 55A Intensity [a.u.] 14000 12000 10000 8000 6000 15A 25A 35A 45A 55A 4000 4000 2000 2000 0 960 965 970 975 980 985 990 960 965 970 975 980 985 990 wavelength [nm] Wavelength [nm] 0 972 973 974 975 976 977 978 972 973 974 975 976 977 978 wavelength [nm] Wavelength [nm] Lineshape without VHG-FAC Lineshape with VHG-FAC Wavelength Stabilization of HPDL Array - Fast Axis Collimation Optic with Integrated VHG : C. Schnitzler, S. Hambuecker, O. Ruebenach, V. Sinhoff, G. Steckman, L. West, C. Wessling, D. Hoffman, Ingeneric GmbH, Ondax, Inc., Fraunhofer ILT, Photonics West, Jan. 2007 Ondax,Inc. Page 8 AHPSL conference Sept 17-19 2007
Effects of Bar Smile and VHG Alignment Linewidth narrowing independent of bar smile and up to 3 o FAC misalignment 14000 1nm 12000 10000 Optimum 1 2 3 Rolling 1 o int 8000 6000 4000 2000 0 972 973 974 975 976 977 978 Rolling 3 o 972 973 974 975 976 977 978 Wavelength wavelength [nm] Off-the-shelf High Power Laser Diodes are collimated and linewidth-narrowed to 0.2nm and collimated with a single VHG-FAC. These sources meet the requirement for dense wavelength multiplexing. Ondax,Inc. Page 9 AHPSL conference Sept 17-19 2007
VHG for Dense λ Muxing (1) Spectral combining of high power linewidth-reduced laser diodes Volume Holographic Gratings (VHGs) in glass provide a solution for narrow wavelength combiners: λ2 λ1 <1nm High Power Laser λ1 High Power Laser λ2 Ondax,Inc. Page 10 AHPSL conference Sept 17-19 2007
Properties of VHG Combiners VHG combiners work in reflection VHG combiners have sub-nanometer reflection bandwidth Diffraction efficiency depends strongly on angle of incidence for >90% combining efficiency, use FAC with less than 2 mrad rest divergence and 15 degree angle of incidence BC Ondax,Inc. Page 11 AHPSL conference Sept 17-19 2007
Summary: Dense Wavelength Multiplexing Off-the-shelf High Power Laser Diodes VHG-FAC Transform off-theshelf High Power Laser Diodes into suitable sources for DWM VHG Combiner ~90% Efficiency Ondax,Inc. Page 12 AHPSL conference Sept 17-19 2007
Architecture with DWM Dense Wavelength Multiplexing (DWM) Modules λ1 Repeat DWM with 80Xnm, 90Xnm, 94Xnm, 97Xnm and combine with coarse dichroic filters DWM λn λ1 λn DWM Geometrical Stacking Polarization Multiplexing Beam Transformation Coarse WM Fiber Coupling λ1 DWM λn Ondax,Inc. Page 13 AHPSL conference Sept 17-19 2007
1kW Fiber Coupled with DWM 70W bars 975nm 978nm DWM 975nm 978nm DWM Geometrical Stacking Polarization Multiplexing Beam Transformation 975nm 978nm 80W bars 908nm 911nm DWM DWM Coarse wavelength Multiplexing 952 Watts Fiber Coupled D=600um NA=0.175 908nm 911nm DWM Geometrical Stacking Polarization Multiplexing Beam Transformation 908nm 911nm DWM Ondax,Inc. Page 14 AHPSL conference Sept 17-19 2007
1kW Fiber Coupled with DWM Geometry of the Laser Diode Bars 0.5% AR Coated Front Facet Extended Locking Range (20nm) Ondax,Inc. Page 15 AHPSL conference Sept 17-19 2007
1kW Fiber Coupled with DWM 2 Wavelength Combiner Block High Power Laser with VHG VHG Combiner Ondax,Inc. Page 16 AHPSL conference Sept 17-19 2007
1kW Fiber Coupled with DWM Average combining efficiency for 6 units 908/911: 89.5% Average combining efficiency for 6 units 975/978: 79.7% Ondax,Inc. Page 17 AHPSL conference Sept 17-19 2007
1kW Fiber Coupled with DWM Ondax,Inc. Page 18 AHPSL conference Sept 17-19 2007
1kW Fiber Coupled with DWM DWM Laser Diodes Facility Unit (Water Cooling, Current Drivers): 1000 x 600 x 1200mm 3 (LxWxH) Ondax,Inc. Page 19 AHPSL conference Sept 17-19 2007
Outlook Multi-kW with DWM By multiplexing more wavelengths, the power in the fiber can be increased without increasing the fiber core or its numerical aperture. With the same performance VHGs used in the 1kW demo, up to 10 sources can be densely multiplexed with a cumulative efficiency of 70%. Goal Multi-kW fiber coupled power can thus be achieved from a 600 µm core and 0.175 N.A. fiber Cumulative combining efficiency [%] 100 90 80 70 60 50 40 30 20 10 Based on performance for 1kW demo η transmission 97% η diffraction 85% η λ locking 93% η transmission 98.5% η diffraction 90% η λ locking 95% 0 2 4 6 8 10 12 14 16 18 20 Number of beam sources Ondax,Inc. Page 20 AHPSL conference Sept 17-19 2007
VHG Fabrication and Test Wafer Fabrication and Automated 100% Wafer Testing Example of a wafer measurement from the database. All optical parameters of VHG measured at high spatial resolution. 35mm 150 µm spatial resolution 45mm efficiency Efficiency [%] Ondax,Inc. Page 21 AHPSL conference Sept 17-19 2007
VHG Damage Threshold High Damage Threshold : 3.85 J/cm 2 or 175 MW/cm 2 Pulsed 1064nm 20Hz Rep. Rate 20ns Pulse Width 0.5mm Spot Size Samples tested with AR coating meeting MIL-C-675 Ondax,Inc. Page 22 AHPSL conference Sept 17-19 2007
VHG Environmental Reliability VHGs Kept at 150 o C Show No Measurable Degradation Over Time 1.48660 Index variation Grating strength ( n) 1.48658 1.48656 3.40 Refractive Index 1.48654 1.48652 1.48650 1.48648 1.48646 1.48644 1.48642 n (10-4 ) 3.30 3.20 3.10 1.48640 0 2000 4000 6000 8000 10000 12000 Time (hrs) 0 2000 4000 6000 8000 10000 12000 Time (hrs) Ondax,Inc. Page 23 AHPSL conference Sept 17-19 2007
1 Applications of Kilowatt Laser Diode Systems 0.1 DPSSL Printing CO 2 BPF 0.01 0.001 0.0001 Thermal Marking Diode Laser Soldering Selective Laser Powder Remelting Metal Sheet Cutting Deep Penetration Welding AD AD AD 0.00001 Plastics Welding Brazing Cleaning Melting Hardening 0.000001 0.1 1 10 100 1000 10000 100000 BPF: Beam Parameter Factor Power (W) Efficiency * Beam Quality Ondax,Inc. Page 24 AHPSL conference Sept 17-19 2007
Summary Volume holographic gratings (VHG) are enabling Dense Wavelength Multiplexing with High Power Lasers Diodes by: narrowing the emission spectra of commercially available high power laser diodes and combining them Commercially available VHGs are wafer produced and 100% tested. FAC-VHGs incorporate collimation and line narrowing to reduce packaging costs. 952 W coupled in 600µm, NA:0.175 fiber with DWM by Fraunhofer Institute, Aachen: Power loss due to wavelength narrowing: 5-10% Combining efficiency at 908/911: 90% Combining efficiency at 975/978: 80% cause by mismatch of the slow axis divergence (90% achievable) Multi-kW fiber coupled power by adding Dense and Coarse wavelength combining Ondax,Inc. Page 25 AHPSL conference Sept 17-19 2007