Enhanced second-harmonic generation with four-wave-mixing broadened fundamental laser

Single-pass second-harmonic generation (SHG) of a phase-modulated fiber laser is an effective scheme for high-power, continuous-wave, narrow-linewidth visible laser generation. However, this approach encounters difficulties in improving conversion efficiency. In this paper, we demonstrate enhanced single-pass SHG conversion efficiency via four-wave-mixing (FWM) broadened fundamental lasers. By co-amplifying two phase-modulated seeds in an ytterbium-doped fiber amplifier, the cascaded FWM redistributes power into multimode spectra, enabling 1.52 times higher efficiency than that of conventional phase-modulated laser schemes in sum-frequency generation (SFG). A green laser up to 76.9 W at 531.5 nm is achieved with a conversion efficiency of 24.2% while maintaining relative intensity noise (RIN) below -120 dBc/Hz above 2 kHz. This approach provides a robust route to high-power and low-noise visible lasers with straightforward phase modulation.