4.2 kW, 0.11 nm, near single-mode isolator-free narrow-linewidth fiber laser of a one-stage amplifier seeded by a taped fiber oscillator

High-power narrow-linewidth fiber amplifiers (NLFAs) have wide applications in beam combining, lidar system, nonlinear frequency conversion, etc. It's a challenging task to simultaneously control beam quality and linewidth of high-power NLFAs. Here, what we believe to be a novel scheme as high-power, high beam quality, narrow-linewidth all-fiber one-stage amplifier is demonstrated. The seed is a tapered fiber oscillator combining a composite cavity and long-length tightly coiled fiber to maintain the narrow linewidth and the high beam quality. In the amplification stage, low-NA confined-doped 30/600 μm fiber is employed to mitigate transverse mode instability (TMI) and nonlinear effects. No isolators or bandpass filters are required in this NLFA, achieving an all-fiber robust amplifier. Then, a 4.17 kW NLFA with 3-dB linewidth of 0.11 nm and beam quality factor M² < 1.5 is achieved, giving a record spectral brightness of 14.55 GW·nm^-1·mm^-2·sr^-1 for this kind of NLFAs. This work provides a robust and low-cost approach for achieving high-power NLFAs.