High-Power, High-Brightness Laser Beam Combining* IEEE Photonics Society Laser Workshop Nov 7, 2012 T. Y. Fan *This work was sponsored by HEL-JTO under Air Force contract FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government.
Outline Introduction Wavelength (Spectral) Beam Combining Coherent (Phased Array) Beam Combining IEEE 12-2.ppt
Continuing Interest in High Power with Good Beam Quality Applications continue to drive interest in high power with good beam quality and high efficiency Diode lasers with near-diffraction-limited output are Watt-class devices Incoherent combination of arrays have enabled diode-laser systems with multi-kw output, but with poor beam quality Beam combining approaches that enable arrays to have good beam quality (high power and brightness) are becoming viable after decades of trying IEEE 12-3.ppt
Brightness and Beam Quality Key beam quantities are the area of the beam A and its farfield solid-angle divergence W AW l 2 Brightness (radiance) is power per unit area per unit solid angle B = P/(AW) ~ P/(M 2 l) 2 For ideal Gaussian, B = P/l 2 Good beam quality and high brightness give highest power on target for a given aperture A Laser Beam Propagation 2q W = πq 2 Beam Quality Definitions in Common Use M 2 = 1 is an ideal Gaussian (M 2 1) Strehl ratio = 1 is an ideal tophat (S 1) Power in the bucket (PITB) ~ M 2 Fiber étendue AW = π 2 D 2 (NA) 2 / 4 S ~ 1/(PITB) 2 ~ 1/(M 2 ) 2 IEEE 12-4.ppt
Laser Beam Combination Approaches Conventional side-by-side Overlapping far fields Beam quality proportional to N 1/2 Brightness no greater than single element Wavelength beam combining (WBC) Inherently multi-wavelength No far-field sidelobes Similar to wavelength-division-multiplexing in fiber optic communications Brightness scales as f g N (f g <1) Coherent beam combining (CBC) Phasing to narrow far field and increase intensity Requires phase control to much better than l Brightness goes as f f N (f f <1) Highest spatial and spectral brightness possible IEEE 12-5.ppt
Simplest 2-Element Implementations Wavelength Combining (WBC) Coherent Combining (CBC) P Series 2P Filled Aperture (Near-field interference) P 2P P Dichroic Combiner N elements requires N-1 beamcombining optics (all different) 50/50 Beamsplitter Changing number of elements requires changing beam-combining optic P Parallel Tiled Aperture (Far-field interference) Additional elements can use same beam-combining optic Changing number of elements requires changing transmitting aperture size IEEE 12-6.ppt
Wavelength and Coherent Combining Characteristics Property Wavelength Coherent Brightness scaling f g N (f g 1) f g is grating efficiency f f N (f f 1) f f is fill factor Spectrum Multi wavelength, varies with N Single wavelength, invariant with N Relative spectral control Frequency (<few GHz) Phase (<l/8 modulo 2p) Polarization Does not matter Control needed Near-field pattern Invariant with N May vary with N Far-field pattern Invariant with N May vary with N Relative brightness for fraction F elements operational (F 1) F F 2 IEEE 12-7.ppt
Outline Introduction Wavelength (Spectral) Beam Combining Coherent (Phased Array) Beam Combining IEEE 12-8.ppt
Wavelength (Spectral) Combining Early demonstrations limited to a few elements with difficult spectral and mechanical control Early Series Implementation Challenge for large N is finding a simple implementation Grating-based (parallel) external cavity approaches are scalable to large N (1000 s of elements) Utilize optical feedback for spectrum control Combining of 1400 diode lasers demonstrated (Aculight) K. Nosu et al., Electron. Lett., vol. 15, 414 (1979) IEEE 12-9.ppt
Parallel WBC with External Resonator Output Mirror Output Beam Laser Array Relation among array dimension, transform lens focal length, spectral spread, and grating dispersion d f Dl (db/dl) Grating Lens or Mirror... l 1 l 2 l N d Dl l N l 1 db/dl is grating dispersion f f Use optical feedback for wavelength control of elements and geometry to overlap beams in near and far fields Beam quality is that of a single element Scaling to 100s 1000s of elements Example: d = 4 cm, and f = 40 cm, and db/dl of 2 rad/µm, then Dl = 50 nm For element center-to-center spacing of 200 µm, the number of elements is 200 in the 4 cm IEEE 12-10.ppt
Nearly Ideal WBC of Large Laser Arrays Wavelength 15 nm Beam Quality (M 2 ) Nearly ideal beam combining of a large (10 s of elements) laser array 100-element slab coupled optical waveguide laser (SCOWL) array at ~980 nm Single-mode elements 100-µm laser pitch Highest brightness diode array demonstrated to date 50-W output power with M 2 ~ 1.2 Output Far-Field M 2 = 1.2 Output Spectrum 10 mm Position Along Array Output Power Chann et al. (2005) Huang et al. (2007) IEEE 12-11.ppt
E f fic ie n c y 1-D WBC of Diode Elements in 2-D Array Broad Area COTS Diode Bars Bar 1 Bar 3 Vertical Diode Stack (3 bars shown) Cylindrical Lens Grating 195 W CW at 915 nm from 100/0.22 fiber using 5-bar stack (with Alfalight Inc) 2x improvement possible with polarization multiplexing 14-nm bandwidth 1.0 WBC dimension Front view Output Beams Output coupler P o w e r (W ) 1 6 0 1 2 0 8 0 0.9 0.8 0.7 0.6 Side-by-side combining of diode bars for power (not brightness) scaling, and WBC of elements for brightness scaling Spatial brightness is 160 mw/mm 2 /sr improvement of ~27 in WBC dimension Highest brightness fiber-coupled diode at >100 W at the time, but issues with smile and AR coatings 4 0 2 0 3 0 4 0 5 0 6 0 C u rr e n t (A ) 7 0 8 0 B. Chann et al., Opt. Lett., vol. 31, p. 1253 (2006) 9 0 0.5 IEEE 12-12.ppt
1-D WBC of Diode Bars Bar#1 Bar#3 WBC dimension Horizontal diode stack (3-bar) Transform lens grating Front View Output coupler Each bar operates at a single wavelength Wavelength fill factor can be higher than for WBC of broad-stripe diode elements for more efficient use of spectrum diffraction-limited slit Insensitive to smile and pointing error enabling use of low-cost stacks Ideally, output beam quality is that of a single bar degraded by smile and collimation error in practice Enables record high brightness fiber-coupled diodes IEEE 12-13.ppt
Étendue of a Diode Bar and Fiber Assume conventional diode bar (broad-area elements) with slowaxis collimation A bar = π/4 x 1 µm x 1 cm = 7.8 x 10-5 cm 2 W bar = π x 0.3 x 0.03 = 2.8 x 10-2 sr A bar W bar = 2.2 x 10-6 cm 2 sr For 50 µm diameter, 0.22 NA fiber Can be reduced in half by polarization combining A fiber W fiber = π 2 /4 x (50 µm) 2 x 0.22 2 = 3 x 10-6 cm 2 sr A diode bar can be efficiently coupled into 50 µm diameter fiber with appropriate optics high power enabled using WBC of bars IEEE 12-14.ppt
TeraDiode s 2-kW fiber-coupled diode laser Fiber-coupled direct-diode laser > 2000 W in 50 m/0.15 NA fiber (95% power content) 967 nm center wavelength Huang et al., Proc. SPIE 8241, paper 824102 (2012) IEEE 12-15.ppt Courtesy of Teradiode 27
Outline Introduction Wavelength (Spectral) Beam Combining Coherent (Phased Array) Beam Combining Background Array-element properties, particularly phase noise Master-oscillator power-amplifier (MOPA) architecture Implementations Active phase-control approaches Demonstrations Oscillator architecture IEEE 12-16.ppt
Historical Comments on Coherent Combining Limited success over decades Many approaches tried Lack of critical analysis Scalability to large arrays Robustness when perturbed Effect of noise Ability to be successful is highly implementation dependent and needs to consider the properties of the array elements Beam quality Polarization controllable Phase (path length) noise Coherence preservation and effect of nonlinearities End Mirrors Common-Resonator Approach Gain Media Lens Binary Grating Common Cavity Mirror J. R. Leger, M. Hotz, G J.. Swanson, W. Veldkamp, Coherent beam addition: An application of binary optics, Lincoln Lab. J. 1(2), 225 (1988) IEEE 12-17.ppt
Approaches for Coherent Combining (not exhaustive) (1 of 2) Master-Oscillator Power Amplifier (MOPA) Phase Conjugation Phase Modulator Amplifiers Reference Phase High-Power Output Master Oscillator Phase Control Wavefront Sensor Low-Power Input Phase Conjugator Phase errors among elements are detected and actively corrected Attributes/Issues/Questions Control systems implementation, particularly when scaling to large N Complexity Phase errors among elements are reversed by phase conjugation Attributes/Issues/Questions Scalability to large N? Required speed of nonlinearity set by noise properties of gain elements Availability of nonlinear materials and efficiency IEEE 12-18.ppt
Approaches for Coherent Combining (not exhaustive) (2 of 2) Common Resonator Evanescent-Wave Coupling Nx1 Grating Splitter Field Profile Gain Elements Output Coupler Gain Elements External resonator used to couple elements (sampled bulk resonator) Attributes/Issues/Questions Scalability to large N? Ability to handle path-length differences among elements (either large or small differences) Active phase control or not? Overlap in evanescent waves between adjacent elements causes phase synchronization Attributes/Issues/Questions Adjacent elements often antiphased Hard to scale to large N Phase is nearest neighbor correlated The common characteristic of recent successful CBC demonstrations is active control of phase errors IEEE 12-19.ppt
Diodes and Fibers as Array Elements Diodes Power Watt class kw class Fibers Beam Quality Can be diffraction limited Can be diffraction limited Polarization control Waveguide structure leads to linear polarization Can be PM and non-pm fiber can use polarization controllers Phase (optical path length) noise Nonlinearities, coherence Low bandwidth (few Hz), low dynamic range (few waves) Low intensity (~10 8 W/cm 2 ), short path lengths (mm-scale), but high material nonlinearity Nonlinear effects are relatively unimportant Bandwidth of 100 s Hz, 100 s to 1000 s of waves High intensity (~10 9 10 10 W/cm 2 ), long path lengths (10 m scale), but low material nonlinearity Nonlinearities (stimulated Brillouin and Raman scattering) need to be managed Array element properties are the key drivers in the design of CBC systems IEEE 12-20.ppt
Phase-Noise Measurement Phase noise affects optical path length and drives required control bandwidth for coherent combining Heterodyne detection used to measure phase noise Phase (rad) Multiple sources contribute to phase noise Thermally induced drift Acoustics and vibration Seed Laser PM Splitter Measurement schematic AO frequency shifter 100 MHz LO DUT Beam Splitter 90º 0º I Q Phase (rad) 50 40 30 20 10 Temporal Waveform Nufern Yb Fiber Amp at 0.5 kw Reference Beam Beam Splitter RF PD 0 0 50 100 150 200 250 Time (msec) Time (ms) IEEE 12-21.ppt
PSD Fiber-Amplifier Phase Noise Phase Noise for a 0.5-kW amp Phase Noise for 3 amps at 0.5 kw Fiber Amp on l/30 residual Fiber amp off -- Pre-amp only -- 0.5 kw -- Amp 1 -- Amp 2 -- Amp 3 Integrated phase noise given by f int 2 ( f ) = ò f psd ( f ) d f Phase noise dominated by environment noise All phase-noise power spectra drop quickly >khz f IEEE 12-22.ppt
Phase Characteristics of Semiconductor Amplifiers Relative Phase Response Optical Frequency (GHz) Phase noise in semiconductor elements is benign Needed servo loop bandwidth <1 Hz Can actuate phase using current Tune 1 wave with 10 s of ma current, small fraction of operating power Modulation bandwidth using drive current to 10 s of khz Phase Sensitivity 1 wave round-trip in 40 ma Phase-Noise Spectrum Phase Transfer Function Modulation Frequency (Hz) IEEE 12-23.ppt
Master-Oscillator Power-Amplifier (MOPA) Architecture with Active Control Phase Modulators Amplifiers Reference Master Oscillator Phase Control Sensor for Phase Control Most recent successful CBC demonstrations have used MOPA Key design considerations Type of amplifier element drives number of elements, control bandwidth, and dynamic range Tiled vs filled aperture Phase control approach Phase actuation device IEEE 12-24.ppt
Example: CBC of 48 Passive Fibers (Tiled) Output Far-Field Spot Strehl ~ 0.8 Fiber Array 48 Piezo Fiber Stretchers Image Relay Reference Beam Photodiode Aperture DBR Laser Source 1x8 Splitters Control Computer Near-Field CCD Phase control by maximizing on-axis far-field intensity using hill-climbing algorithm Phase actuation using fiber stretchers passive fiber has slow length fluctuations 2D tiled output using microlens array for aperture filling to reduce sidelobes IEEE 12-25.ppt Kansky et al., Proc. SPIE, Vol. 6708, 67080F (2007)
Tiled vs. Filled-aperture CBC Implementations Tiled aperture (far-field interference) Side lobes in the far field Errors in amplitude and far-field overlap reduce Strehl ratio Tiled array can be used for wavefront correction, similar to the function of a deformable mirror Filled aperture (near-field interference) Ideally, no sidelobes (highest possible efficiency) Errors in amplitude, near- and far-field overlaps will reduce efficiency of main beam No beam steering or adaptive optics capability Tiled-aperture CBC Filled-aperture CBC Lens array Gain-element array Power in Central Lobe Fill Factor 1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97-0.5 Output 0 0.5 1 1.5 4 3.5 3 2.5 2 Lens array Gain-element array Transform optic DOE Combining loss Output IEEE 12-26.ppt Series1
Active Phase-Control Approaches Highest on-axis intensity in tiled architectures (i. e., best constructive interference) for 0 phase (mod 2π) among elements Metric Based Maximize far-field on-axis intensity (i. e., the metric) Phase-Measurement Based Detect relative phases of the elements and then set the differences to 0 Hill-climbing algorithms (stochastic parallel gradient descent) Optical heterodyning Wavefront reconstruction using focal plane array (FPA) RF heterodyning (subcarrier modulation) IEEE 12-27.ppt
Strehl Stochastic Parallel Gradient Descent (SPGD) for Metric-Based Control Small perturbations (dithers) provide estimate of local slope SPGD is a hill climbing algorithm that maximizes on-axis intensity (Strehl) Does not require a reference beam or phase knowledge Different small perturbations applied to all elements simultaneously (dither) Correction applied proportional to normalized slope Control servo equation: A k+1 = g 1 A k + g 2 (S + -S - )/(S + +S - )*(D k /2σ 2 ) D k : Dither vector (Length equal to number of elements) A k and A k+1 : Present and updated element vectors (Same length as dither vector) σ : RMS amplitude of dither vector S + and S - : On-axis intensity measurements when applying A k +D k and A k -D k g 1, g 2 : Gain (typically = 1) IEEE 12-28.ppt D - x o D + 0 Phase Error Vorontsov et al. JOSA A., 15, 2745 (1998) Standard SPGD convergence time t ct» N elements f dither
Phase Control Using Optical Heterodyning Individual array channels are heterodyned against a common reference to derive error signal Error signal used to drive feedback loop and phase actuators Master Oscillator Seed Experimental Implementation Distribution Phase Actuators Feedback Control Amps Reference RF PD Single-Channel Detail Phase Modulator Frequency Shifter, Dn RF PD n opt n opt + df Beam Sampler Seed Input I, Q Dn Oscillator Loop Filter RF Photodetector Output IEEE 12-29.ppt
Phase Control Using Image Processing Reference wave is interfered with the near field of the array on a FPA Fringes are image processed to derive an error signal to drive feedback loop and phase actuators Implementation Phase Actuators Amps Array Holder and Microlens Array Beam Sampler 2D 48-Element Image Distribution.. Output Seed Master Oscillator Reference Image Processing and Feedback Control 1:1 image relay FPA IEEE 12-30.ppt
Phase Actuation Devices Phase Actuation Device Acousto-optic modulator Electro-optic modulator Piezoelectric fiber stretcher Spatial light modulator Bandwidth (khz) Throw (waves at 1 µm) 100 s infinite Comments >1000 Few Fast 2π snapback increases effective throw Few 10 s Few ~1 Fast 2π snapback increases effective throw Address multiple channels with single device Diode current ~100 ~ 1 Power and phase are coupled, limiting throw Pump power modulation Fast 2π snapback increases effective throw ~1 Few Power and phase are coupled, limiting throw Bandwidth and throw highly dependent on gain element material and design IEEE 12-31.ppt
8-Fiber Coherent Combining Schematic We have coherently combined eight 0.5-kW IPG PM fiber amplifiers Beam combining uses tiled configuration via 1x8 fiber/mlens array Phase control achieved via single detector and SPGD Master Oscillator 1x8 Splitter Df = 10 GHz Phase Modulators Delay Lines Fiber Amps Fiber/mlens array Slit Detector Far-field on-axis intensity COTS EO Phase Modulators +/-30 V, 0.2 ma, 20-MHz BW FPGA-based Phase Controller All eight 0.5-kW IPG PM fiber amplifiers are coherently combined to achieve 4-kW with 70% fill-factor mlens IEEE 12-32.ppt
Fraction encircled energy Intensity, a. u. 8-Fiber Coherent Combining Results We measured far-field profile at full power to characterize combining performance 6 x 104 Far-field Cross-section 5 4 3 2 1 50x 8 amps combined 1 amp only 0 300 350 400 450 500 550 600 650 700 750 1 0.8 0.6 0.4 Tophat Farfield Angle (a.u.) a b c Ideal 70%-fill Array Measured Array 0.2 PITB BQ= (a/c)=1.25 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Bucket Radius (l/d) Achieved highest combined power with good BQ from fibers IEEE 12-33.ppt Yu et al. (2011)
Slab-Coupled Optical Waveguide Laser and Amplifier SCOWLs have large mode areas with a single mode output ~ 4 x 7 mm mode field diameter Demonstrated >1 W CW laser power High density amplifiers arrays were developed: Individually addressable 21 amplifiers with 200 µm spacing Precise position tolerances Back facet HR-coated and front facet AR-coated Array collimated with a microlens array Connector Flex print cable Photographs of SCOWA Array Flex cable traces Coolant lines Array on cooler Single bar cooler Inlet plenum SCOWL array CuW bus IEEE 12-34.ppt
Beam Combination Conceptual Layout Standard MOPA configuration using diffractive optical elements to split and seed individual SCOWAs The far-field of the SCOWA array is sampled and fed into Stochastic Parallel Gradient Descent (SPGD) controller for coherent combination Seed Laser DOE Isolator Double-Pass SCOWA ARRAY SCOWA Current Drivers SPGD Controller Power Detector Near-Field Camera Far-Field Camera IEEE 12-35.ppt
Eleven 21-Amplifier SCOWA Combination Results Measured Far-field Measured Near-field IEEE 12-36.ppt 38.5 W Output Power 218 SCOWAs combined with 500 ma average drive current 400 600 800 1000 1200 Redmond et al., Opt. Lett. Vol. 36, 999 (2011)
IEEE 12-37.ppt Phase-Locking of 218 SCOWAs
1-D Diode Amplifier Array Architecture: Single Output Beam 1-D IA SCOWA array Faraday rotator Polarizing beamsplitter SPGD controller Detector Transform lens 1-D DOE Master Oscillator Output beam Input beam (master oscillator) is split into multiple beams by diffractive optical element (DOE) With proper phases DOE combines beams from amplifiers into a single output beam Placement of Faraday rotator and polarizing beam splitter is different from tiled-output configuration IEEE 12-38.ppt
Coherently Combined 1-D Array: Single Output Beam Single Output Beam 1500 Single-Element Input Element Beam Quality M 2 Beam Width x Beam Width y 1000 2w 0 (mm) 500 1800 1600 1400 M 2 x = 1.1 M 2 y = 1.0 M 2 x 1.1 M 2 y = 1 0 0 10 20 30 40 50 z (mm) Combined-Output Beam Quality Single Beam M 2 Beam Width x Beam Width y 6.5 W single output beam from an array of amplifiers 10.5 W of coherent power Near diffraction-limited output beam IEEE 12-39.ppt 2w 0 (mm) 1200 1000 800 600 400 200 M 2 M 2 x x = 1.4 M 2 y = 1 M y 2 = 1.0 0 5 10 15 20 25 30 35 40 45 z (mm)
Power-Oscillator Configuration A power-oscillator is simpler and more compact than a MOPA implementation Laser Array Transform Lens DOE Output Coupler Output Beam Losses IEEE 12-40.ppt
Combining Efficiency Number Scaling of Passive CBC Cavities CBC in passive cavities (no phase control of individual elements) has been tried many times with poor results Recent analyses show poor combining efficiency if path lengths are unequal among elements BC Efficiency for Random Path Length Relationships Experiment Theory Kouznetsov, et al., Opt. Rev. 12, 445 (2005) Number of Elements IEEE 12-41.ppt
Active Phase-Control Power Oscillator Active phase control of individual elements allows for scaling beyond passive limits Laser Array Transform Lens DOE Output Coupler Beam Sampler Output Beam Losses Phase controller Detector IEEE 12-42.ppt
Intensity (a.u.) 21-Element Diode Array in Power Oscillator Intensity (a.u.) -3200-2800 -2400-2000 -1600-1200 -800-400 0 400 800 1200 1600 2000 2400 2800 3200-3200 -2800-2400 -2000-1600 -1200-800 -400 0 400 800 1200 1600 2000 2400 2800 3200 0.9 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 Combining efficiency = 5% Combining efficiency = 5% -1400-1200 -1000-800 -600-400 -200 0 200 400 600 800 1000 1200 1400 0.9 7 Combining efficiency = 81% Combining efficiency = 81% 7 x position (μm) x position (μm) Random-phase combining efficiency is ~ 5%, consistent with incoherent beam combining of 21 beams Active phase-control enables scaling to large number of elements IEEE 12-43.ppt Montoya et al. (2012)
Summary Laser beam combining is a viable way to scale power and brightness beyond the limits of a single laser beam 100s of elements have been both wavelength and coherently combined Scaling to 1000s of combined elements still poses many challenges and is an active area of research High-brightness products using beam combining are entering marketplace Still plenty of opportunities for innovation IEEE 12-44.ppt