Coherent Beam Combining of 21 Semiconductor Gain Elements in a Common Cavity* SSDLTR 2012 12-SSDLTR-029 Wenqian Huang (Ronny), Juan Montoya, Steven Augst, Kevin Creedon, Jan Kansky, T.Y. Fan, Antonio Sanchez-Rubio MIT Lincoln Laboratory June 12, 2012 *This work is sponsored by High Energy Laser Joint Technology Office under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government.
Overview • Array of gain elements inside a common optical cavity – an old concept for scaling with diffraction limited beam quality • Scalability in earlier proof-of principle demos was hampered by the need to maintain phase across the array • Number of elements (3 - 6) • Combining efficiency (70% - 80%) • Power ~10 mW per element J. R. Leger, G. J. Swanson, W. Veldkamp, “Coherent laser addition using binary phase gratings,” Appl. Opt. 26 , 4391 (1987) In this work: Active control of the phase allows for scaling to 21 elements with excellent beam quality. SSDLTR 2012- 2 WH 06/12/2012
Master Oscillator Power Amplifier (MOPA) Configuration • MOPA configuration requires multiple amplification stages with isolators and mode matching optics, but has been a successful platform for coherent beam combining. 1xn Amplifier Transform Splitter Array Beam Lens DOE Sampler Isolator, Seed Output Mode-matching, source Beam Pre-amplifier Losses Phase Detector controller S. Redmond et al, Active Coherent beam combining of diode lasers , Optics Letters, Vol. 36, No. 6, 2011 MITLL has demonstrated coherent beam combining (CBC) of 218 semiconductor amplifier elements. SSDLTR 2012- 3 WH 06/12/2012
Power-Oscillator Configuration A power-oscillator is simpler and more compact than a MOPA implementation Laser Transform Array Output Lens Output DOE Coupler Beam Losses SSDLTR 2012- 4 WH 06/12/2012
Number Scaling of Passive CBC Cavities • Efficient CBC in passive cavities (no phase control of individual elements) does not scale well above ~ 8 elements – Arbitrary arm lengths cause random phase relationships BC efficiency for Random Phasing Experiment ●○ Theory ̶ ̶ Kouznetsov, et. al, Opt. Rev. 12 , 445 (2005) Scaling to higher number of elements can be achieved using active phasing SSDLTR 2012- 5 WH 06/12/2012
Active Phase-Control Power Oscillator Active phase-control allows for scaling beyond passive limits Laser Transform Array Output Lens Beam Output DOE Coupler Sampler Beam Losses Phase controller Detector SSDLTR 2012- 6 WH 06/12/2012
Stochastic Parallel Gradient Descent (SPGD) Phase-Control Algorithm SPGD Convergence • SPGD has enabled multiple CBC demonstrations at MIT LL On-Axis Intensity (arb. u.) 6 5 • SPGD is a hill climbing algorithm 4 N 4 » t channels - Does not require a reference beam or 3 ct f phase knowledge dither 2 1 • Optimizes zero order output of DOE -1.5 -1.4 -1.3 -1.2 -1.1 Time(s) 1xn Amplifier Transform Splitter Array Lens Beam DOE Sampler Seed source Output Beam Losses Phase controller Detector SSDLTR 2012- 7 WH 06/12/2012
Diode Arrays with Individually Addressable Elements Connector Low profile flex Flex print print cable cable Flex cable traces AlN subcarrier Coolant lines SCOWL array CuW bus Single bar cooler (SBC) Array on cooler • Stackable high density arrays were developed to demonstrate coherent combination of semiconductor amplifiers: – 21 individually addressable gain elements – 200-µm spacing – Precise position tolerances – Back facet HR-coated and front facet AR-coated • Each array is collimated with a spherical microlens array to increase the fill factor SSDLTR 2012- 8 SCOWL – Slab Coupled Optical Waveguide Laser WH 06/12/2012
21 Diode Array Combining Efficiency Measurement and Diagnostics Power Monitor f 1 =300mm Near Field f=500mm f 2 =75mm f 1 f 2 Spectrometer 21 Element 50/50 Laser Array 1x21 1% Beam Slit DOE Sampler Grating f=500mm SPGD Detector Slit • Cavity output ports allow for efficiency measurement (ƞ=P 0 /P T ) • Spatial filter (slit) prevents feedback from higher DOE orders • Intracavity diagnostics include: – Near-Field Spectrometer, Far-Field Camera, Power monitors Cavity designed with diagnostics for proof-of-principle SSDLTR 2012- 9 WH 06/12/2012
SPGD Control Loop Phase-Locks 21-Element Array Far-field images Random phase – no active control* With active phase control* *Color scales normalized to show peaks, horizontal cross sections (line profiles) are not normalized 0.9mm 0.9mm Combining efficiency = 5% Combining efficiency = 81% Combining efficiency = 81% 7mm 7mm Combining efficiency = 5% Intensity (a.u.) Intensity (a.u.) 200 30 150 100 20 50 10 0 0 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 x position (mm) x position (mm) • Random-phase combining efficiency is ~ 5%, consistent with incoherent beam combining of 21 beams Active phase-control enables scaling to large number of elements SSDLTR 2012- 10 WH 06/12/2012
Near-Field Spectrometer Results Random Phase Active Phase Control • Near field spectrometer illustrates that all elements operate at the same wavelength when SPGD is activated SPGD adjusts optical path lengths (phase) of each emitter to coherently combine the beams SSDLTR 2012- 11 WH 06/12/2012
Combined Power/ Efficiency 21-Element Combined Efficiency Estimates Output Power P 0 = 2.5 W, M 2 =1.11 Efficiency Cumulative Max Loss Mechanism Penalty (%) Efficiency (%) DOE splitting 10 90 efficiency Pointing Error 3 87 SPGD Dither 1 86 Aberrations 1 85 Amplitude 4 81 Variations Achieved record combining efficiencies of 81% for 21 semiconductor elements. Cavity not optimized to produce high power. SSDLTR 2012- 12 WH 06/12/2012
SPGD Convergence and Long-Term Stability Long-Term Stability with SPGD SPGD Convergence Frozen at Converged Values SPGD Signal (rel. units) 8 1.4 SPGD Signal (rel. units) 1.2 6 1.0 4 0.8 0.6 2 0.4 0.2 0 0 0 20 40 60 80 -40 -20 0 20 40 time (ms) time (minutes) • Experimentally observed convergence time ~ 4 ms • Once CBC is established and phases are held fixed at optimum values, the active phase control may be turned off and efficiency is self-sustaining SSDLTR 2012- 13 WH 06/12/2012
Summary • Power oscillators are more compact than MOPA lasers • Diode and bulk solid-state lasers are well-suited to power oscillator configurations – We have successfully demonstrated 21 diode element CBC in a power oscillator with an 81% combining efficiency Acknowledgements: • George Turner, Leo Missaggia for diode arrays • Shawn Redmond for SPGD discussions • SCOWL array and DOE development funded by DARPA MTO SSDLTR 2012- 14 WH 06/12/2012
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