Comparative study on transverse mode instability of few-mode fiber laser amplifier under different bending diameters

Abstract

Transverse mode instability (TMI) has been one of the major factors limiting the power scaling of fiber lasers in the past decade. To achieve a high-power output of fiber laser, there are multiple ways to suppress TMI, such as changing pump wavelength, fiber parameters (such as core diameter and numerical aperture (NA)), pump scheme, and bending radius. Changing the bending diameter is an effective method to suppress TMI. However, the relationship between bending diameter and TMI threshold has not been studied systematically. We built a few-mode fiber laser amplifier with a master oscillator power amplification (MOPA) structure employing 976 nm laser diodes as the pump source. The TMI characteristics of the amplifier were studied by changing the bending diameter. The gain fiber is coiled circularly, and the bending diameter of the gain fiber changes from 9 cm, 10 cm, 11 cm, and 12 cm. Generally, it is believed that the smaller the bending diameter, the larger the TMI threshold due to a higher bending loss of modes. In this experiment, the TMI threshold and beam quality have been demonstrated. The experimental results show an anomaly characteristic of TMI. The TMI threshold improved from 734W, 835W, 976W, and 1006W when the bending diameter of YDF increased from 9 cm, 10 cm, and 11 cm to 12 cm. At the same time, the beam quality at the TMI threshold is degraded from 1.26, 1.4, and 1.72 to 1.8, corresponding to the changing bending diameter. This experiment revealed TMI characteristics in a few-mode fiber laser amplifier that differed from a quasi-single-mode fiber laser amplifier. Although the phenomenon is not consistent with common TMI with near single-mode fiber lasers, the result will be helpful for the design and power scaling of few-mode fiber lasers.