STh4N.3.pdf
CLEO:2014 © 2014 OSA
Diffractive Coherent Combining of >kW Fibers Gregory D. Goodno, Stuart J. McNaught, Joshua E. Rothenberg, Peter A. Thielen, and Martin P. Wacks Northrop Grumman Aerospace Systems, Redondo Beach, CA 9027, USA
[email protected]
Abstract: Three non-PM, kW-class fiber amplifiers were coherently combined into a 2.4-kW, M2=1.2 beam with 80% efficiency. No power-limiting effects were observed, which anchored scaling predictions to higher powers and efficiencies with larger fiber count arrays. OCIS codes: (140.3298) Laser beam combining; (140.3290) Laser arrays; (140.3510) Lasers, fiber.
1. Introduction Coherent beam combining (CBC) of fiber amplifiers in an actively phase-locked architecture allows power scaling without sacrificing the outstanding beam quality and efficiency that are the hallmarks of Yb-doped single-mode fibers [1]. To date, demonstrations of this approach have been limited to fiber power levels ≤ 500 W where nonlinear and thermal effects are relatively modest, and where polarization-maintaining (PM) fibers may be used [2]. For systems intended for use outside lab environments, use of higher power fibers would minimize channel counts, minimize size and weight, and enable further power scaling. However, fiber coherence has been found to degrade above 1 kW due to self phase modulation [3], thus limiting the expected combining efficiency. In this work, we report for the first time to our knowledge CBC of multiple fiber amplifiers operating above the 1-kW level. The system architecture integrates non-PM, >kW fibers with active controls and a minimalist free space optical system for diffractive coherent combining in a packageable configuration. A 3-fiber prototype generated a 2.4-kW, near diffraction-limited output beam with no measurable power-dependent losses, thus anchoring performance extrapolations to higher channel counts. 2. Experimental System The experimental system layout is shown in Fig 1. A single-frequency 1064-nm master oscillator (MO) was preamplified and phase-modulated to 18 GHz linewidth to suppress stimulated Brillouin scattering (SBS) before being split to seed each of the 3 fiber amplifier chains. An electro-optic phase modulator and a polarization controller were inserted in each seed fiber path prior to injection into the amplifiers. Each amplifier chain comprised 3 serially isolated large mode area Yb-doped gain stages, spliced to ~2-m long passive delivery fibers. These splices were responsible for some amount of power coupling (typically a few %) from the fundamental LP 01 mode to higher order modes that manifested as polarization extinction ratio (PER) and wavefront errors, contributing to CBC losses. Maximum output powers from the 3 chains were 1.3 kW, 1.1 kW, and 0.6 kW. Pol MO
Yb fiber amplifiers
Splices
Fiber Tip Array
Primary Mirror
f DOE Multichannel phase & polarization controller
SPC Beam sampler
PM Fiber Non-PM Fiber Free-space optical Electrical
2.4 kW output
Fig 1. Experimental layout of the 3-fiber diffractive CBC system. f, frequency broadening;
phase modulator; Pol, polarization controller.
The passive delivery fibers from each amplifier were spliced to AR-coated endcap carriers that were assembled into an array on a common athermal baseplate, enabling replacement of any emitter without affecting its neighbors. The beams from each fiber emitter freely expanded before being collimated by a spherical primary mirror that also served to direct each beam onto a diffractive optical element (DOE) at the proper angles matching the diffractive orders of the DOE structure. In contrast with prior high power DOE experiments [4], [5], no individual collimating lenses were required, which minimized optical loss and reduced the propensity for thermal aberrations or misalignment from high power operation. The HR-coated, continuous surface-relief DOE was designed to split a single, near-normal input beam into N = 3 output beams at equal angles. Spherical aberration (SA) imposed by the primary mirror was removed downstream of the DOE using a magneto-rheologically finished (MRF) static phase corrector (SPC) mirror that imposed the conjugate SA, resulting in a near-planar wavefront output beam. A single, ~10 mW sample of the combined output beam was coupled back into single-mode fiber to feed sensors for active phase and polarization-locking. Phase-locking was based on the LOCSET multi-frequency dither
STh4N.3.pdf
CLEO:2014 © 2014 OSA
HPI Combined Output Power
3000
Calculated Loss contributions
Combined Power (W)
2500
Wavefront mismatch
2.6%
0.8% 2000
PER
0.6% 7.7%
1500
1.0% NF
1000
Group delay mismatch
1.2%
500
Piston phase error
1.7%
0
ASE 2.4%
0
500
1000
1500
2000
2500
3000
3500
Total Input Power (W)
Fig 2. Coherently combined output power with a fit to 80% slope efficiency. Inset: Near field and far field beam profiles at 2.4 kW.
Fiber mode mismatch Power mismatch
0.7%
FF
Intrinsic DOE loss
2.6%
Self phase modulation Spatial misalignment
Fig 3. CBC losses calculated from the measured individual fiber amplifier characteristics and a perturbative loss model [8].
technique, which entails applying a small radio frequency (RF) phase dither at a unique frequency to “tag” each laser channel [6]. These phase dither tags were also leveraged to de-multiplex the polarized field strength of each channel, which allowed each controller to lock the amplifier outputs to a common linear state of polarization [7]. 3. Results and Discussion Fig 2 shows the combined output power and near-Gaussian beam profiles at 2.4 kW. The combined beam had 80% combining efficiency, 16 dB PER, and M2 = 1.2, all of which were independent of power. Thermal imaging of the (uncooled) free-space optical components showed 1 kW fibers without significant thermal or nonlinear losses clarifies a scaling path to substantially higher powers than demonstrated here. By utilizing a single common focusing optic, an athermal fiber tip array with replaceable elements, and a single beam sample for phase- and polarization-locking of the entire array, the system was designed to be operable and maintainable outside lab environments. Increasing the fiber count and using matched fibers is projected to increase combining efficiency for higher power systems. This work was sponsored by the High Energy Laser Joint Technology Office and the Air Force Research Laboratory under contract W9113M-10-C-0022 for the Robust Electric Laser Initiative (RELI) program. At least a portion of the technology which is discussed in this paper is the subject of one or more pending patent applications. [1] G. D. Goodno and J. E. Rothenberg, “Engineering of coherently combined, high-power laser systems,” pp. 3-44 in Coherent Laser Beam Combining, A. Brignon, ed. (Wiley-VCH Verlag GmbH & Co., 2013). [2] C. X. Yu et al, “Coherent combining of a 4 kW, eight-element fiber amplifier array,” Opt Lett. 36, 2686 (2011). [3] G. D. Goodno et al, “Active phase and polarization locking of a 1.4-kW fiber amplifier,” Opt. Lett. 35, 1542 (2010). [4] S. Redmond et al, “Diffractive coherent combining of a 2.5 kW fiber laser array into a 1.9 kW Gaussian beam,” Opt. Lett. 37, 2832 (2012). [5] P. A. Thielen et al, “Two-dimensional diffractive coherent combining of 15 fiber amplifiers into a 600-W beam,” Opt. Lett. 37, 3741 (2012). [6] T. M. Shay et al, “Self-Synchronous and Self-Referenced Coherent Beam Combination for Large Optical Arrays,” IEEE J. Selected Topics Quantum Electron. 13, 480 (2007). [7] G. D. Goodno et al, "Multichannel polarization stabilization for coherently combined fiber laser arrays," Opt. Lett. 37, 4272 (2012). [8] G. D. Goodno, C. C. Shih, and J. E. Rothenberg, “Perturbative analysis of coherent combining efficiency with mismatched lasers,” Opt. Express 18, 25403 (2010). [9] K. J. Creedon et al, "High efficiency coherent beam combining of semiconductor optical amplifiers," Opt. Lett. 37, 5006 (2012).
