Advanced controllable solid-state Raman lasers

This thesis presents work on developing advanced controllable continuous-wave (CW) solid-state Raman lasers operating at near-infrared and visible wavelengths for astronomy and medical applications. An adaptive-optics (AO) technique, which is commonly applied in astronomy, was for the first time integrated within self-Raman lasers for improving the Raman laser output performances and wavelengths control. Over the last decade, intracavity solid-state Raman lasers have been widely utilised to extend the spectral coverage of common crystalline laser materials, especially when they are combined with second harmonic generation or sum frequency generation. However, due to the non-elastic nature of stimulated Raman scattering, a significant thermal lensing is generated within the Raman gain medium, which results in parasitic thermo-optical distortions inside the cavity. The magnitude of this effect scales directly with the Raman laser output power and has been identified as the main limitation in power-scaling crystalline Raman lasers. In this thesis, an AO-based feedback control loop system, consisting of an intracavity bimorph deformable mirror, a photodiode sensor and a PC-based control program using a random-search algorithm, has been implemented inside several Raman laser platforms demonstrating its potential for Raman laser output power-scaling. A power improvement of up to ~ 45 % is reported for a Nd:YVO4 self-Raman laser at λ = 1176 nm using an intracavity AO optimisation. Moreover, a frequency-doubled Nd:GdVO4 self-Raman laser in the yellow waveband (λ = 586.5 nm) has also been power-scaled using the intracavity AO technique, achieving a total power enhancement of ~ 41 %. These represent the potential to significantly alleviate the detrimental thermal lens effect and open avenues which will enable solid-state Raman lasers to reach new output power levels. In addition, several CW Nd:YVO4 self-Raman lasers based on the primary and secondary Raman shifts of YVO4 ( 893 cmˉ¹ and 379 cmˉ¹ respectively) were characterised and reported in this thesis. For the first time an on demand wavelengths selection between laser output at λ = 1109 nm and λ = 1176 nm has been achieved with hundreds of milliwatts output power using an intracavity AO control technique. This represents an important step on the way towards automatic wavelength selectable high power Raman lasers.