Phase offset and polarisation flipping in diamond Raman lasers with intracavity second harmonic generation

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.