3.5 kW bidirectionally pumped narrow-linewidth fiber amplifier seeded by white-noise-source phase-modulated laser

Abstract

We demonstrate an all-fiber narrow-linewidth amplifier employing a bidirectional pump scheme and cascaded white-noise-source phase-modulated seed laser. The stimulated Raman scattering effect in the amplifier is investigated by substituting different types of seed lasers. The influence of pump distributions and seed injection power on mode instability (MI) in the amplifier is also experimentally investigated. As a result, a 3 dB linewidth of 0.175 nm and a beam quality of M 2 ≈ 1.5 are obtained at the output of ~3 kW, without observation of MI and stimulated Brillouin scattering effect. With the further increase of pump power, MI occurs as the output exceeds 3.17 kW, along with beam quality degradation. Optical efficiency decreases to 71.5% at the ultimate output of 3.5 kW. Therefore MI becomes the main limitation to further power scaling.

All-fiberized and narrow-linewidth 5 kW power-level fiber amplifier based on a bidirectional pumping configuration

“…High-power narrow-linewidth fiber lasers with excellent beam quality have been highly desired for spectral and coherent beam combinations [1][2][3]. The power scaling of single-mode narrow-linewidth fiber lasers has been under intense investigations [4][5][6][7][8][9][10][11][12][13][14][15]. This mainly involves comprehensive suppressions of the transverse mode instabilities (TMI) and nonlinear inelastic scattering effects, such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) [16][17][18].…”

“…It should be noted that those two fiber laser systems are established in free-space structure, which is not suitable for compact assembling and maintenance. Therefore, most of the reported all-fiberized narrow-linewidth fiber amplifiers are based on conventional active fiber and require comprehensive optimization of structure and parameters of the fiber amplifiers [7][8][9][10][11][12][13][14][15]. As for all-fiberized format, 3.7 kW output power had been achieved with near diffraction limited beam quality and spectral linewidth of ~ 0.3 nm [15].…”

Seeding High Brightness Fiber Amplifiers With Multi-Phase Coded Signal Modulation for SBS Effect Management

“…One conventional route seems to be feasible is to compare the SBS threshold at identical "spectral linewidth". In previous studies, different definitions of "spectral linewidth" of narrow-linewidth fiber amplifier, such as full-width at half-maximum (FWHM) [31], 85% fraction of total power [33] and root-meansquare (RMS) [34], have been proposed. However, different spectral distributions or spectral envelopes could be produced by using different modulation signals.…”

“…As for SBS effect, several suppressing techniques have been proposed, such as selecting large mode area, short active fibers [18,19], applying thermal or stress gradients [20][21][22][23], acoustic tailoring techniques [24,25], multilongitudinal-mode (MLM) fiber oscillator [26], multi-tone injection [27,28] and phase modulation [29][30][31][32][33][34]. Among all of them, phase modulation technique is a simple and robust approach to mitigate SBS effect and control the linewidth of fiber amplifiers to promise an excellent temporal coherence [35].…”

“…Among all of them, phase modulation technique is a simple and robust approach to mitigate SBS effect and control the linewidth of fiber amplifiers to promise an excellent temporal coherence [35]. Aiming on this approach, some typical modulation signals have been proposed and applied, which mainly includes sine-wave signal modulation [9,29], white noise signal modulation [30,31], pseudo-random bit sequence (PRBS) modulation [10,32], and piecewise parabolic signal (PPS) modulation [33] and multi-objective nonlinear optimized signal (MONOS) modulation [34]. Within these modulation signals, WNS and PRBS are the two prevail phase modulation techniques in multi-kilowatt level, narrowlinewidth fiber amplifiers [10,[30][31][32].…”