3 kW narrow-linewidth linearly polarized fiber laser with high-purity single-mode output and high PER enabled by suppressing mode and polarization coupling

Abstract

Laser beam combining techniques, such as coherent and spectral beam combination, are effective strategies for the further power scaling of fiber lasers. In practical applications, beam quality and polarization extinction ratio (PER) of the narrow-linewidth fiber laser sources can significantly affect the performance of the combining system, including the combining efficiency, beam quality, and thermal regulation. In this paper, a narrow-linewidth linearly polarized fiber laser with high-purity single-mode output and high PER is demonstrated. A theoretical model for the optimization of our amplifier is firstly constructed, considering mode coupling and thermal-induced polarization coupling. Expression of the birefringence employed here is the recently corrected stress-induced birefringence. With this model, we optimize the numerical aperture, bending radius and coiling configurations of gain fiber for suppressing mode coupling and polarization coupling. In experiment, we achieve a 3.116 kW linearly polarized narrow-linewidth fiber laser with the beam quality of M_x² = 1.09/M_y² = 1.06 and a PER of 22.1 dB at maximum power. The thresholds for transverse mode instability, stimulated Brillouin scattering, and self-pulsing are not reached. This work provides an effective design method for high-performance narrow-linewidth linearly polarized fiber lasers. To the best of our knowledge, our results represent the highest level of both beam quality and PER achieved for narrow-linewidth linearly polarized fiber lasers at this power level.