Multi-kW coherent combining of fiber lasers seeded with pseudo random phase modulated light

Abstract

We report efficient coherent beam combining of five kilowatt-class fiber amplifiers with a diffractive optical element (DOE). Based on a master oscillator power amplifier (MOPA) configuration, the amplifiers were seeded with pseudo random phase modulated light. Each non-polarization maintaining fiber amplifier was optically path length matched and provides approximately 1.2 kW of near diffraction-limited output power (measured M 2 <1.1). Consequently, a low power sample of each laser was utilized for active linear polarization control. A low power sample of the combined beam after the DOE provided an error signal for active phase locking which was performed via Locking of Optical Coherence by Single-Detector Electronic-Frequency Tagging (LOCSET). After phase stabilization, the beams were coherently combined via the 1x5 DOE. A total combined output power of 4.9 kW was achieved with 82% combining efficiency and excellent beam quality (M 2 <1.1). The intrinsic DOE splitter loss was 5%. Similarly, losses due in part to non-ideal polarization, ASE content, uncorrelated wavefront errors, and misalignment errors contributed to the efficiency reduction.

Seeding fiber amplifiers with piecewise parabolic phase modulation for high SBS thresholds and compact spectra

“…Random phase modulation of a narrow band laser has been realized by applying a white noise generator to an external electro-optic phase modulator [3][4][5][6]. More recently, PRBS waveforms with π phase shifts have been extensively investigated for SBS mitigation [7,8]. A comprehensive theoretical study showed that "for a fiber length of length 9 m the patterns at or near 7 n = provide the best mitigation of SBS with suppression factors approaching 17 dB at a modulation frequency of 5 GHz" [9].…”

A novel coherent beam combining system based on active polarization-and-phase control

“…The coherent synthesis efficiency of the experimental system is up to 99%, and the polarization extinction ratio of the output laser more than 29 dB. In 2016, Angel et al of U.S. Air Force Research Laboratory used the five polarization-controlled narrow linewidth based amplifiers achieved 5 kW output power [9]. The amplifier is based on pseudo-random phase modulation and takes a low power sample from the output of each laser and polarizes it with General Photonics polarization controller.…”

Directly Emitting a High-Power Phase-Locked Laser Array by an Internal Phase Sensing System

“…Then, a beam splitter is usually set in the optical path to sample the expanded laser array after they are emitted to the free space, namely the external phase sensing technique [24][25][26]. At last, some mature phase-locking techniques are applied to lock the phase, such as the dithering technique [27][28][29], the stochastic parallel gradient descent (SPGD) algorithm [30][31][32], interference measurement [18,33,34], and machine learning [35][36][37][38][39], etc. Despite that the external phase sensing technique has exhibited impressive performances for phase locking, further power scaling still faces challenges.…”