Effects of four-wave-mixing in high-power Raman fiber amplifiers

TL;DR

This paper develops coupled amplitude equations to analyze four-wave-mixing effects in high-power Raman fiber amplifiers, revealing how these nonlinear interactions influence the Raman threshold and stability, with implications for optimizing amplifier performance.

Contribution

It introduces a detailed theoretical model for FWM effects in RFAs and demonstrates how pump laser stability can suppress these effects and enhance the Raman threshold.

Findings

FM FWM can reduce the Raman threshold by over 50%.

IM FWM causes additional Raman Stokes generation in higher modes.

Stable pump lasers significantly suppress FWM effects and double the Raman threshold.

Abstract

In this work, we derive and present the coupled amplitude equations to describe the evolutions of different spectral components in different transverse modes for Raman fiber amplifiers (RFAs). Both the effects of the four-wave-mixing in the fundamental mode (FM FWM) and the inter-modal four-wave-mixing (IM FWM) on high-power RFAs are demonstrated through numerical simulations. Specifically, effective FM FWM interactions could occur and lead to a drop of the Raman threshold for RFAs by over 50%, despite that the corresponding wave-vector mismatch is rather big. In addition, the IM FWM could also impact the Raman threshold for RFAs with additional generation of the first order Raman Stokes light in the higher-order mode. We also investigate the effects of the intensity fluctuations in the initial inserted pump and seed lasers on high-power RFAs. It reveals that the temporal stability of the initial inserted pump laser have much more significant impacts on high-power RFAs than that of the initial inserted seed laser. Notably, through applying temporal stable laser as the initial inserted pump laser, both the FM FWM and IM FWM effects could be effectively suppressed, and the Raman threshold for high-power RFAs could be increased by over twice.