5.1 kW Tandem-Pumped Fiber Amplifier Seeded by Random Fiber Laser With High Suppression of Stimulated Raman Scattering

In this work, high-power tandem-pumped random fiber lasers are investigated in detail. A 6.5 W random fiber laser seed centered at 1070 nm with a half-open cavity structure was built. Boosted by two amplifier stages, the random fiber laser achieved a power of several kilowatts. Stimulated Raman scattering (SRS) mitigation is challenging in tandem-pumped fiber lasers. To suppress SRS, several methods were applied, including avoiding external feedback in the oscillator stage, incorporating a high core/cladding diameter ratio gain fiber, and using a band pass filter (BPF) to mitigate Raman noise. The latter proved to be the most effective method; therefore, SRS mitigation without and with a BPF was investigated at 3.5 kW, leading to suppression ratios rising from 14 to 30 dB. By increasing the pump power continuously to 5.6 kW, we realized a record output power of 5.1 kW, corresponding to an SRS suppression ratio of 27 dB. Owing to the high-brightness 1018 nm fiber lasers serving as the pump sources and the high gain coefficient in the second amplifier stage, the entire system demonstrated high efficiency and stability. At the maximum power level, the total optical-to-optical efficiency reached 89.0%. This work offers an economic and practical technique for realizing high-power and high-efficiency random fiber lasers with effective SRS mitigation, while the experimental results suggest the potential for further power scaling.

I. Introduction

Since their inception toward the end of the 20th century, random lasers, via multiple scattering in a disordered gain medium, have shown some intriguing properties that distinguish them from traditional fixed cavity structure lasers [1]–[3]. However, limited by low output power and poor directivity, random lasers are very difficult to apply practically. In the past decade, a novel one-dimensional random laser utilizing optical fiber as the medium has been proposed and studied in depth [4]–[7]. The random Raman fiber laser (RRFL) is based on Rayleigh scattering and amplified by stimulated Raman scattering (SRS). The waveguide structure of the fiber restricts scattering in the radial direction, leading to greatly enhanced directivity and a relatively low laser threshold, which makes RRFLs more applicable than other random laser designs.