1.3kW monolithic linearly-polarized single-mode MOPA and strategies for mitigating mode instabilities

TL;DR

This paper demonstrates a 1.3kW linearly-polarized fiber laser with high beam quality and explores strategies, including tight coiling of gain fiber, to mitigate mode instabilities and enable power scaling.

Contribution

It introduces a semi-analytical model for mode instability suppression and experimentally verifies that tight coiling of gain fiber effectively increases MI threshold.

Findings

Achieved 1.3kW output power with beam quality M2 < 1.2.

Tight coiling of gain fiber raises MI threshold by up to three times.

Semi-analytical model aligns well with experimental results.

Abstract

We report on the high power amplification of 1064nm linearly-polarized laser in all-fiber polarization-maintained MOPA, which can operate at output power level of 1.3kW. The main amplifier was pumped with six 915nm laser diodes, and the slope efficiency is 65.3%. The beam quality (M2) was measured to be <1.2 at full power operation. The polarization extinction rate of the fiber amplifier was measured to be above 94% before mode instabilities (MI) sets in, which reduced to about 90% after the onset of MI. Power scaling capability of strategies for suppressing MI is analyzed based on a novel semi-analytical model, the theoretical results of which agree with the experimental results. It shows that mitigating MI by coiling the gain fiber is an effective and practical way in standard double-cladding large mode area fiber, and, by tight coiling of the gain fiber to the radius of 5.5cm, the MI threshold can be increased to 3 times higher than that without coiling or loose coiling. Experimental study has been carried out to verify the idea, which has proved that MI was suppressed successfully in the amplifier by tight coiling.