LAUSR

In this letter, we presented a monolithic high-power random fiber laser (RFL) with a tandem-pumping scheme. The RFL seed centered at 1064 nm was pumped by a 1018-nm fiber laser and reached an output power of 8.66 W. After two amplifiers, the seed was amplified to 4020 W, and the 3-dB width of laser spectrum was broadened from 0.78 nm to 0.99 nm. To increase the efficiency of the 4-kW laser, a specially designed high absorption coefficient (0.6 dB/m @ 1018 nm) 30/250- $\mu \text{m}$ Yb-doped fiber and high-brightness 1018-nm pump laser were adopted. The optical-to-optical efficiency could reach as high as 88.5%. No obvious mode instability was found during the experiment. Furthermore, with the same amplifier stage, we have also built another 4-kW level fiber laser seeded by an oscillator consisting of a pair of fiber Bragg gratings. The comparison of the experimental results showed that the high-power laser based on the RFL seed would have a better control on spectral broadening.