Optimization of nonlinear effect suppression in gain-switched Raman fiber lasers for enhanced SRS conversion efficiency

This work establishes a comprehensive framework for nonlinear suppression in gain-switched Raman fiber lasers (RFLs) through dispersion-engineered fibers and spectral control strategies. Combining numerical modeling with experimental validation, we decode the triadic interplay among second-order dispersion (15 ps²/km≤<i>β</i>₂ ≤ 30 ps²/km), nonlinear coefficients (4 W^-1km^-1≤<i>γ</i>≤6 W^-1km^-1), and fiber length (<i>L</i> = 30 m) in governing nonlinear phase accumulation. Key breakthroughs include: 1) identification of the <i>β</i>₂-<i>γ</i> equilibrium window that simultaneously suppresses modulation instability (MI), self-phase modulation (SPM) and four wave mixing (FWM) and enhances second-order Stokes conversion efficiency. 2) Experimental realization using UHNA7 fiber (<i>γ</i>=5.27 W^-1km^-1@1660 nm, <i>β₂</i> = 30 ps²/km, <i>L</i> = 30 m) achieving 60 dB radio frequency (RF) signal-to-noise ratio (SNR), 4.6 ns pulse duration, and 99.2% spectral purity. 3) FBG-based spectral purification that compresses 3 dB bandwidth from 13.0 nm to 1.0 nm. The proposed methodology bridges fundamental nonlinear optics with practical laser engineering, offering: 1) a universal design metric for Raman gain optimization, 2) experimentally validated guidelines for MI/SPM/FWM suppression, and 3) spectral-temporal control architecture extendable to multistage RFL systems. This paradigm shift enables high performance gain switched RFLs, paving the way for precision nonlinear photonic systems.