Influence of core NA on Thermal-Induced Mode Instabilities in High Power Fiber Amplifiers

TL;DR

This paper investigates how the core numerical aperture (NA) affects thermal-induced mode instabilities in high power fiber amplifiers, revealing dependencies on fiber parameters and pump wavelength through numerical and experimental analysis.

Contribution

It provides a comprehensive numerical and experimental study on the influence of core NA and V-parameter on mode instability thresholds in high power fiber amplifiers.

Findings

Core NA significantly affects MI for fibers with smaller core-cladding ratios.

Threshold increases linearly with decreasing V-parameter above 3.5, exponentially below.

Linewidth effects are negligible for linewidths under 1nm with specific fiber parameters.

Abstract

We report on the influence of core NA on thermal-induced mode instabilities (MI) in high power fiber amplifiers. Influence of core NA and V-parameter on MI has been investigated numerically. It shows that core NA has larger influence on MI for fibers with smaller core-cladding-ratio, and the influence of core NA on threshold is more obvious when the amplifiers are pumped at 915nm. The dependence of threshold on V-parameter revealed that the threshold increases linearly as V-parameter decreases when V-parameter is larger than 3.5, and the threshold shows exponentially increase as V-parameter decreases when V-parameter is less than 3.5. We also discussed the effect of linewidth on MI, which indicates that the influence of linewidth can be neglected for linewidth smaller than 1nm when the fiber core NA is smaller than 0.07 and fiber length is shorter than 20m. Fiber amplifiers with different core NA were experimentally analyzed, which agreed with the theoretical predictions.