Adam Sharp

Continuous-wave Raman lasers are emerging as promising light sources that combine their characteristic wavelength conversion with low quantum-limited frequency noise, making them attractive for quantum and sensing applications. Linewidth studies so far have been based on free-space Raman crystal lasers, with linewidth limited by technical noise due to vibrations and air currents. Here, we demonstrate a monolithic crystal Raman laser, based on a 15 mm-long biconvex coated YVO ₄ crystal with a 14 mm end-face radius of curvature. A 1064 nm pump is actively locked to the cavity using the Pound–Drever–Hall technique, generating Stokes emission at 1174 nm with a threshold pump power of 260 mW. The integrated linewidth of 190 kHz over 1 ms integration time was approximately 53 times narrower than the pump. The high-offset-frequency noise at 50MHz is suppressed by more than a factor of 130,000 relative to the pump, resulting in a measurement-limited upper bound of 20 Hz for the Lorentzian linewidth. The laser operated in a single-longitudinal-mode over 7 h and showed strong resistance to environmentally induced mode hopping, highlighting the potential of monolithic Raman lasers for robust precision optical sources.

Raman lasers are emerging as promising sources of optical frequencies with low phase noise. Studies so far have centered on diamond as the Raman gain medium, a material well suited to stable single longitudinal mode operation. Here we report a single longitudinal mode GdVO ₄ Raman laser based on a doubly resonant cavity stabilized to the pump frequency using Pound–Drever–Hall locking and providing output wavelengths at 1164 nm and 1174 nm. Frequency noise measurements at high Fourier frequencies indicate a Lorentzian linewidth below 3 Hz, with the observed value limited by technical noise in the laser cavity and the linewidth measurement system. To our knowledge, this represents the narrowest linewidth reported for any Raman laser and demonstrates that non-diamond Raman materials can support narrow linewidth operation and a greater diversity of output wavelengths.

We investigate key factors affecting the power output of a diamond Raman laser with double-pass intracavity second harmonic generation. Using experiment and model, we investigate laser power and efficiency as the temperature of the frequency doubling crystal is scanned through its phase matching condition. It is found that the inclusion of the Gouy phase shift in the calculation of round-trip phase offset for the second-harmonic, making its second pass through the frequency doubling material, is required to adequately account for the observed power behaviour. The experiments also reveal “drop-outs” in laser power at regular temperature intervals, which we attribute to flipping of the mode polarisation state around null points in the net retardance of the diamond and doubling crystal. Temperature scanning provides a simple technique for determining cavity conditions that optimise phase offset, avoid polarisation “drop-outs”, and lead to optimal mode sizes.

Raman lasers are well known for wavelength shifting and beam clean up, but less so for linewidth narrowing. Recent advances in design and material choice have enabled stable single frequency operation and the opportunity to investigate their narrow linewidth properties. Here, we use modeling and experiments to investigate linewidth and frequency noise as a function of pump noise. The model reveals that the picosecond-scale phonon dephasing time, typical of most Raman materials, suppresses pump frequency noise by factors up to 108 at Fourier frequencies up to 1011 Hz. To support the model, frequency noise spectra were measured for the input and output beams of a singly-resonant Raman laser operating at 1178 nm, revealing noise suppression by more than 104 at offset frequencies above 1 MHz. The results show that Raman lasers offer orders-of-magnitude greater linewidth-narrowing than Brillouin lasers, even for cavities of moderate Q factor. This makes them a promising route toward ultra-narrow linewidth lasers across the spectrum with adaptability to a wide range of pumps.

We report an intra-cavity frequency doubled diamond Raman laser operating at 607 nm. A z-fold cavity design was configured to prevent back reflections into the fiber amplifier, which avoided the use of isolators in the pump beam path. A maximum output power of 60 W was generated in two output beams at an optical conversion efficiency of 28% from the 1045 nm pump. A maximum single-beam output power of 40 W was obtained using a highly reflecting end mirror for the visible. Output power and single frequency stability are negatively impacted by an increasing role of stimulated Brillouin scattering at output powers above 10 W.