Narrow line diode laser stacks for DPAL pumping Tobias Koenning David Irwin, Dean Stapleton, Rajiv Pandey, Tina Guiney, Steve Patterson DILAS Diode Laser Inc. Joerg Neukum
Outline Company overview Standard products Motivation and applications VBG principles Design Stack design Temperature tuning Test results Power (LIV) Beam quality Spectral line width Wavelength tuning Stability Summary Outlook Scalability Different wavelength Next steps 2
DILAS Company Data COMPANY Founded 1994 340 employees worldwide ISO 9001-2008 certified Sites Mainz (HQ) Nanjing (Sub.) Tucson/AZ subsidiary of RofinSinarTechnologiesInc (Nasdaq: RSTI) DILAS Information PRODUCT RANGE Wavelength 450nm 2.3µm Single Bars Stacks (vertical & horizontal) Fiber coupled modules Turn key systems APPLICATIONS Pumpingof rod-, disk- and fiber laserin OEM and Scientific Materials Processing Medical Printing Industry Defence Projection and Display 3
Dilas products Single Emitters (m2k) Conduction-Cooled Diode Laser Bars Water-Cooled Vertical Stacks Water-Cooled Horizontal Stacks Components Conduction-Cooled QCW Stacks Fiber-Coupled Modules λ = 450nm to 2300nm P = 5W up to multiple kilowatts COMPACT Operating mode = CW, QCW Systems Fiber core diameter = 100µm, 200µm, 300µm, 400µm, 600µm, 800µm 4
Dilas fiber-coupled modules Fiber core diameter Commercial Maximum power Not fully qualified Demonstrated 100µm 50W 120W 180W 200µm 650W 850W 1KW 400µm 1KW 1.2KW 1.5KW 1mm 4KW 5KW N/A Fiber NA 0.2 for all modules shown All power levels shown for single wavelength 5
Dilas QCW products Available wavelength All Dilas wavelength (635nm to 2300nm) Typical pulse duration 100µs to milliseconds Peak power Up to 500W per bar (depending on fill factor and wavelength) Duty cycle <2-3% (typical) Fiber coupling 800um fiber core bars optimized for high peak power bars not suited for high brightness applications due to beam quality 6
Dilas high power stacks Available wavelength Average power All Dilas wavelength (450nm to 3200nm) Up to 200W per bar (depending on fill factor and wavelength) Cooling Micro channels Operating mode CW or pulsed Highest power per package size Lowest thermal resistance Maximum CW power per bar 7
Motivation / Applications Narrow line Applications High Energy Lasers (DPAL) Medical diagnostics Requirements Extremely narrow spectral line width ( 0.1nm) Tight tolerance of center wavelength (± 0.05nm) High optical power ( 100W/bar) 8
Motivation Why DPALs? Potential to achieve very high output powers in NIR spectral region Laser diode array Gas (vapor) laser gain medium Laser output beam High power conversion efficiency High output power Poor beam quality No stress birefringence No beam distortions by thermal effects No stress fractures No optical damages (i.e. in laser fibers) Reduced thermal focussing High average output power High beam quality Single aperture power scaling 9
Motivation Why DPALs? Further advantages of DPALs: Scalability to high power (cell size, pump power) Low quantum defect (reduction of waste heat) Excellent thermal management Lightweight packaging Closed cells (no vacuum pumping or discharge of chemicals) Directed energy& power beaming applications: Processing of photovoltaic cells Underwater communication Power supply for space stations or propulsion systems Laser weapons 10
Physical basics of alkali lasers Three-level laser with small quantum defect E/E 2 E 2 E 1 E D 2 line λ pump n 2 P 3/2 Fast quenching (collisional n 2 relaxation, buffer P 1/2 gas: He, CH 4, ethane etc.) D 1 line λ laser n: principal quantum number for the ground configuration of alkali atoms E 0 n 2 S 1/2 Neutral alkali atom (Cs, Rb, K, Na, Li): single valence s-electron 11
Physical basics of alkali lasers Summary of transition energies and wavelengths Laser n λ pump E 2 -E 0 E 1 -E 0 λ laser E E/E 2 entity (nm) (ev) (ev) (nm) (mev) Nd 3+ 808 1.5344 1.1652 1064 369.2 0.24 Yb 3+ 941 1.3176 1.2038 1030 113.8 0.086 Cs 6 852 1.4546 1.3859 894.3 68.7 0.047 Rb 5 780 1.5890 1.5596 794.8 29.4 0.0185 K 4 766 1.6171 1.6099 770.1 7.2 0.0044 Na 3 589 2.1044 2.1023 589.8 2.1 0.0010 Li 2 670 1.8479 1.8479 670.1 0.04 0.00002 12
Wavelengths locking with VBGs Laser Bar FAC lens SAC lens Feedback VBG VBG provides selective feedback Effective gain increased within selected wavelength range Critical design parameters (VBG): Index contrast Reflectivity / Diffraction Eff. Thickness Uniformity (in VBG and between VBGs) Critical design parameters (Diode) Epitaxy design-active region gain Cavity length Facet reflectivity VBG center wavelength is temperature dependent (3pm/ C) 13
Spectral width of ensemble Design challenge Center wavelength of VBG varies in manufacturing Power differences between bars cause further differences in central wavelength Individual bar spectra don t usually exactly overlap Spectral line width of ensemble broadens Solution Mount resistive heater to VBGs Adjust temperature of each VBG individually to overlap bar spectra Minimize spectral line width Use heaters to shift center wavelength of ensemble Avoid active cooling due to heat rejection complexities 14
Module design FACs SACs VBGs Heaters Standard 15-bar Stack with FAC lenses Front view of DPAL stack with FACs, SACs, and heated VBGs Rear view of DPAL stack showing 30 pin connector for heaters 15
Tuner design Heater Controller Tuning software Temperature controller to heat VBGs and tune wavelength Individual set-point for each laser bar (15 channels) Master channel to tune entire stack at once Stand alone or software controlled operation Save settings for various operating points 16
Final LI curve of 15-bar stack Optical Power 1000W Operating current 76.3A Operating Voltage 28.5V Water temperature 23 C Electrical-to-optical efficiency 46% 17
Beam profiles Far Field -Fast Axis Far Field Slow Axis Near Field 51.5mm Test Parameter Fast Axis Slow Axis Divergence FWHM 3.9mrad 22.7mrad Divergence 90% power content 4.6mrad 21.0mrad Near field beam size 90% power content 51.5mm ~10mm 3.6mm Note: Near field fill factor << 50% 10mm -> Brightness can be doubled by use of stripe mirrors 18
Output Spectrum with tuned gratings 50A (600W), 23 C 80A (1030W), 23 C Test Parameter 50A 80A Center wavelength (vacuum) 780.246nm 780.293nm Bandwidth (3dB or FWHM) 0.072nm (35.5GHz) 0.083nm (40.9GHz) Bandwidth (95% power enclosed) 0.136nm (67GHz) 0.166nm (81.8GHz) Wavelength drift of 47pm (23.2GHz) over 400W change in optical power 19
Wavelength tuning Test procedure Heaters initially off Tuning @ optimum to minimize spectral width Master setting for blue and red shift (no optimization) Tuning range: 0.155nm (76.4GHz) Test Parameter Heaters off Optimum Blue shift Red shift Center wavelength (vacuum) 780.037nm 780.254nm 780.174nm 780.329nm Bandwidth (3dB or FWHM) Bandwidth (95% power enclosed) 0.11nm (54.2GHz) 0.23nm (113.3GHz) 0.076nm (37.4GHz) 0.140nm (69.0GHz) 0.069nm (34.0GHz) 0.134nm (66GHz) 0.080 (39.4GHz) 0.146nm (71.9GHz) 20
Stability Test: 1hr burn-in @ 50A Temperature[ C] Temperature[ C] Spectral width [nm] Spectral width[nm] Stability Test Water temperature fluctuations due to PID controller of chiller Optical power varies with water temperature Center wavelength stable within 19pm (peak to valley Temperature change Induced change in water temp. (+/-5 C ) +/-20pm (+/-9.9GHz) change in center wavelength 21
Summary High power diode laser stack for Rubidium pumping VBGs to narrow spectrum Heaters to tune individual bar spectra Minimize spectral line width Tune center wavelength of ensemble Parameter Power Center wavelength (vacuum) Bandwidth (3dB) Bandwidth (95% enclosed power) Tunable range Center wavelength drift over operating temperature Fast Axis divergence Slow Axis divergence Heaters off 1030W @ 80A 600W @ 50A 780.29nm @ 80A 780.25 @ 50A 0.083nm (40.9GHz) @ 80A 0.072nm (35.5GHz) @ 50A 0.166nm (81.8GHz) @ 80A 0.136nm (67.0GHz) @ 50A 0.155nm (76.4GHz) +/-20pm (+/- 9.9GHz) over +/- 5 C 3.9mrad FWHM 22.7mrad FWHM 22
Outlook Planned Tasks Increase power per bar to 200W per bar Improve assembly process to minimize cost Develop closed loop heater control Transfer technology to different wavelengths Increase number of total bars to scale power 23
Contract# FA9451-12-D-0191 Thank you!
Thank you for your attention! CONTACT: Dr. Jörg Neukum DILAS Diodenlaser GmbH www.dilas.de Email: j.neukum@dilas.de Phone: +49 6131 9226 140