Harnessing Longitudinal Modes of a Fiber Laser for Nonlinear Effects Enhancement

The manipulation of spectral-temporal characteristics continues to be a fundamental research focus in laser systems. In this study, leveraging the Vernier effect between two sets of longitudinal modes and a gain-mediated mode-selection mechanism in a fiber laser, we demonstrate controllable spectral transitions from single-longitudinal-mode (SLM) to dual-longitudinal-mode (DLM) and, ultimately, multi-longitudinal-mode (MLM) operation. Concurrently, the temporal profiles evolved from constant-intensity outputs to sinusoidal waveforms and finally noise-like pulses. To evaluate how these spectral-temporal transitions affect nonlinear phenomena, we employed second-harmonic generation (SHG) as a representative nonlinear process and systematically analyzed the output performance of the SHG wave through experimental measurements and theoretical modeling. Our findings reveal that increasing the number of longitudinal modes in a fiber laser substantially enhances the SHG effect, with MLM operation yielding nearly double the output power and conversion efficiency compared to SLM operation. This work advances the understanding of spectral-temporal dynamics in fiber lasers and their interplay with nonlinear optical effects, while also providing insights for applications in spectroscopy, quantum information processing, underwater optical detection, and related fields.