Suppression of self-excited oscillation for thulium-doped fiber laser with MOPA structure

In high-power thulium-doped fiber laser (TDFL) systems with master oscillator power amplification (MOPA) structure, amplified spontaneous emission (ASE) induced self-excited oscillation significantly effects power amplification. Based on the self-built TDFL system, the relationships between the self-excited oscillation and active fiber length, input signal power, fiber bending radius were investigated experimentally. The experimental results show that when the active fiber length decreases from 3.5m to 3.2m, the threshold of self-excited oscillation increases from 107W to 158W, the output laser power increases from 40W to 63W, the optical conversion efficiency increases from 35.75% to 38.73%; when the input signal power increases from 3.9W to 14.7W, the pump power threshold corresponding to the self-excited oscillation increases from 93W to 218W, the output laser power increases from 30W to 98W, the optical conversion efficiency increases from 37.24% to 41.65%; when the bending radius of active fiber decreases from 110mm to 85mm, the self-excited oscillation threshold increases from 185W to 244W, the output laser power increases from 71W to 112W, the optical conversion efficiency increases from 33.97% to 43.53%. The results in this work provide guidance for suppressing self-excited oscillation to further improve the power of TDFL with MOPA structure.