Bend Distortion Effect on Transverse Mode Instability

Dan Cheng,Yujun Feng,Ting Feng,Fengping Yan

doi:10.1109/jphot.2021.3140088

Abstract

Transverse mode instability (TMI) limits the power scaling of high-power fiber lasers and amplifiers. One method of mitigating TMI is to strip unwanted modes by bending the fiber. However, the bending-induced mode distortion will change the TMI coefficient and influences the TMI threshold. In this work, we analyzed the bending effects on the mode profiles by inducing different refractive index tilts corresponding to different bending radii. With these mode profiles and with different bending conditions, a modified TMI coefficient was calculated to analyze the bending effects on TMI for different fibers. The simulation results show that tighter bending can lead to a somewhat lower TMI coefficient and slightly improve the TMI threshold.

Highlights

H IGH-POWER fiber lasers (HPFLs) with near-diffractionlimited beam quality are required in numerous industrial and scientific applications [1], which has led to surging demand for power scaling of HPFLs
The output power of fiber lasers has been greatly scaled up due to the rapid development of the high-brightness pump source and the double-cladding fiber fabrication technology, further power scaling in single-mode HPFLs is still hindered by optical nonlinear effects, which are highly dependent on the optical power density
We introduced a perfectly matched layer (PML) outside the fiber cladding to absorb leakage of the mode; this results in an imaginary part in the solved effective refractive index

Summary

INTRODUCTION

H IGH-POWER fiber lasers (HPFLs) with near-diffractionlimited beam quality are required in numerous industrial and scientific applications [1], which has led to surging demand for power scaling of HPFLs. Besides the bend-induced mode loss, bending a fiber leads to mode distortion, which will change the value of the TMI coefficient, influencing the TMI threshold of a fiber laser or amplifier. We numerically investigated the influence of the bending induced-mode distortion on the TMI coefficient in conventional LMA step-index fibers with fiber sizes of 20/400 μm and 25/400 μm. Distortion decreases the TMI coefficient to some extent and improves the TMI threshold, the improvement is small compared with the improvement caused by other parameters such as fiber core size, NA, V-parameter, and bending loss [13], [21].

THEORETICAL STRATEGIES

SIMULATION RESULTS AND DISCUSSION

CONCLUSION