Heterodyne linewidth measurement of a free-running single frequency diamond Raman laser

Narrow linewidth lasers are important for many applications but are challenging to develop due to various mode stability issues, requiring complex locking mechanisms. Raman lasers offer an alternative and simple means to attain high-power single mode lasers with a large wavelength range. However, their linewidth properties have not been studied in detail. In this thesis, the linewidth of a 1240 nm quasi-continuous wave diamond Raman laser (DRL) was studied using heterodyne beat note detection with a 20 kHz linewidth reference laser. The heterodyne signal was analysed using two methods based on an RF analyser and a Fourier approach, which revealed a large chirp of the DRL frequency. A model was developed which showed the chirp was consistent with the calculated temperature increase in the diamond due to the inelastic stimulated Raman scattering process, which increases the cavity optical path length, down-chirping the DRL. An upper bound linewidth of 768 ± 207 kHz was obtained, resolution limited by the chirp, which represents an order of magnitude improvement on previously reported measurements for a DRL. This work also demonstrates, to the best of our knowledge, the first observation of a passively chirped Raman laser and highlights a new approach to high power, sub-MHz linewidth lasers.