Origin of SRS-induced beam quality distortion under TMI threshold.

In high power fiber lasers, the degradation of beam quality caused by Raman effect has attracted more and more attention in recent years, but its physical mechanism is still unclear. We're going to differentiate between heat effect and nonlinear effect by duty cycle operation. The evolution of beam quality at different pump duty cycles has been studied based on a quasi-continuous wave (QCW) fiber laser.

2 × 2 kW near-single-mode bidirectional high-power output from a single-cavity monolithic fiber laser.

Traditional monolithic fiber lasers can only achieve unidirectional high-power laser output. In this Letter, a novel high-power linear cavity fiber laser that can achieve bidirectional high-power output is proposed and demonstrated. In an ordinary laser resonant cavity, we replace the high-reflectivity fiber Bragg grating with a low-reflectivity fiber Bragg grating to realize bidirectional laser output.