Output-coupling-dependent linewidth narrowing in resonantly pumped diamond Raman lasers

Narrow-linewidth diamond Raman lasers hold significant potential for applications in special-wavelength laser generation, coherent detection, and precision measurement. Resonant pumping is an effective approach for achieving low-threshold excitation of diamond Raman lasers. In this operating regime, the output-coupler transmittance directly affects the Stokes cavity loss, but its influence on the realized Raman linewidth has not been systematically investigated. Here, a narrow-linewidth continuous-wave pump laser combined with Pound–Drever–Hall locking is used to resonantly pump a diamond Raman oscillator, and the Stokes characteristics are compared for different output-coupler transmittances. The output-coupler transmittance significantly affects the Raman lasing threshold, conversion efficiency, locking robustness, and output linewidth. Increasing the transmittance from 0.6% to 3.0% increases the Stokes output power and conversion efficiency but shortens the stable locking duration. Conversely, reducing the transmittance progressively narrows the Stokes linewidth. At a transmittance of about 0.2%, a Stokes linewidth of 5.70 kHz, slightly narrower than that of the pump laser, is achieved. This result indicates that low output-coupling loss extends the intracavity Stokes photon lifetime and enhances the suppression of pump-to-Stokes frequency-noise transfer, thereby enabling linewidth compression. This study reveals the role of output-coupling loss in controlling the linewidth evolution of resonantly pumped diamond Raman lasers and provides important experimental guidance for the design of low-threshold, highly stable, narrow-linewidth diamond Raman lasers.