Bend-loss-dependent efficiency and thermal-induced beam quality evolution of narrow-linewidth fiber amplifiers assisted by a low-NA-active fiber

Due to the inherent advantage of balancing large mode area and single-mode operation, low-NA fiber has the potential for power scaling of high-power fiber lasers. In this contribution, lasing characteristics of low-NA large-mode-area fiber have been investigated in detail by laser efficiency, TMI threshold, temporal signal, and beam quality measurements. It is found that TMI threshold in low-NA fiber does not monotonically increase as the bending diameter decreases with the existence of an optimal region. Laser efficiency is drastically increased as launched pump power scales, and the phenomenon could be attributed to reduction of mode bending loss induced by fiber core NA increase with thermal load. Besides, the beam quality factor (<i>M</i>²) is synchronously increasing as launched pump power scales, which coincides with the laser efficiency increase with launched pump power. Moreover, the beam profile is relatively stable below ∼1.1<i>k</i><i>W</i> and fluctuates in a quasi-static state on a time scale of seconds before TMI occurs. The origin of mode evolution relies on the thermally induced waveguide structure change of the low-NA fiber due to its vulnerability to thermal perturbation, which provides new insights for mode evolution of the low-NA fiber.