Study on thermal-lens induced mode coupling in step-index large mode area fiber lasers

In this paper, the thermal-lens induced mode coupling in step-index large mode area fiber laser is systematically investigated. The pertinent mode coupling coefficient is studied firstly, to the best of our knowledge. It is demonstrated that the mode coupling can be induced by the thermal-lens induced waveguide changing along the active fiber. It is found that the mode coupling can be enhanced mainly by the thermally-induced mode distortion and refractive index variation, both of which will become severe with the large thermal load. The impacts of fiber configuration parameters on the mode coupling are discussed. It is found that in the straight fiber, the mode coupling in a larger-core fiber can be weakened when the thermal load is low, but it will become stronger when thermal-lens effect is severe enough. However, in the bent fiber, enlarging core size, reducing core numerical aperture (NA), or decreasing bend radius will all aggravate the mode coupling. Especially when NA is excessively reduced, the mode coupling will be dramatically raised even with a small thermal load. The pertinent study is significant for understanding the mode coupling phenomenon in high-power fiber lasers.