External cavity diamond Raman lasers for high-power continuous-wave beam conversion

Solid-state Raman lasers represent a convenient way of shifting the output of lasers to longer wavelengths with enhanced brightness. In the continuous-wave (CW) regime, the conversion is challenging due to the inherently low gain of the third-order nonlinear process. High pump intensities are needed to reach threshold, and thus many CW Raman lasers utilize cavity architectures in which the pump and Stokes beams are resonated. However, the power-scaling of such devices becomes challenging due to thermal effects in the Raman medium and potentially other intra-cavity elements. This thesis presents a novel approach to the power-scaling problem. It is shown that a combination of optical quality diamond with an external-cavity Raman oscillator design enables efficient CW Raman conversion and high-power output. Diamond Raman lasers (DRLs) were studied under continuous and quasi-continuous-wave high-power pumping using conventional Nd and Yb laser technologies. A fourfold increase in output power of crystalline Raman lasers (up to 22W) was achieved with a good conversion efficiency (45% optical-to-optical), and the first demonstration of a fibre laser pumped DRL is also presented. Theoretical models were derived in order to describe the DRL performance. The models were used to determine optimization procedures to maximise power and efficiency as a function of design variables of the laser cavity, pump laser, and diamond size and quality. Diamond polarization properties, in particular the birefringence, were found to be crucial to achieving efficient and predictable performance. The models were also used to predict design when scaling output power and to determine performance limiting factors. The investigated DRL architecture was found to be well suited for the examination of the fundamental aspects of frequency broadening in Raman lasers. A DRL was investigated using a single-frequency fibre laser system as the pump laser. The observed laser dynamics, which include some unexpected observations of amplitude and frequency instabilities, highlight some key areas for future development of stimulated Raman scattering theory. This study of power-scaling of Raman lasers shows that the developed DRL systems have the potential to operate efficiently with any high-power high-spectral-density laser technology. The superior thermal properties of diamond make the design constraints more flexible and help to mitigate loss mechanisms. This thesis shows that there are excellent prospects for highly efficient operation when power scaling well-beyond the 100 W level, and greatly narrowing the power gap between Raman lasers and other high-power laser technologies.