Modeling of the thermal-lens induced mode coupling in fiber amplifiers

Abstract

In this paper, a novel model for describing the thermal-lens induced mode coupling (TLMC) effect in the high power fiber amplifier is proposed firstly, to the best of our knowledge. Based on the rate equation and couple-mode equation, the model is built with both consideration of the thermal-lens induced mode field variation and the gain effect. With this model, the mode coupling behaviors with various pump schemes are numerically investigated and the impact of the TLMC is quantitatively discussed in a finite-length single-frequency fiber amplifier for paradigm. It is found that the higher-order mode (HOM) can be induced by the TLMC even with pure single-mode input signal and this effect is most severe with the co-pumped scheme while weakest with the counter-pumped scheme. It is also revealed that the TLMC will enhance the average HOM content and will keep predominant until the incident HOM is larger than a certain content. Such an enhancement of HOM content can play an important role as the seed triggering the transverse mode instability (TMI) and the pertinent study is helpful on investigating the relationship between the thermal-lens effect and the TMI.