Long wavelength extension of diamond Raman lasers
thesisposted on 28.03.2022, 23:49 by Alexander Sabella
Ideal wavelengths for laser applications often do not align with commonly available laser sources, particularly in the mid-infrared, where there are reduced laser options. Raman lasers are a means of simultaneously extending the spectral coverage and enhancing the brightness of existing laser technology. Historically, average powers in crystalline Raman lasers have been restricted by poor thermal handling of the deposited heat,which is intrinsic to the frequency shifting process. In addition, solid-state Raman lasers traditionally are only reported in the visible and near-infrared, with operation atlonger wavelengths limited by material absorption and reduced Raman gain coeﬃcients.Synthetic diamond is a nonlinear frequency conversion material that has the potential to challenge these limitations, with a high Raman gain coeﬃcient, excellent thermal properties and broad transmission. This thesis investigates the performance of pulsed diamond Raman lasers across the near and mid-infrared regions.Conversion of a 1.064 m Nd:YAG laser to ﬁrst (1.240 m) and second (1.485 m)Stokes wavelengths was studied, with the quantum conversion eﬃciencies achieved equivalent to, or exceeding, other Raman or optical-parametric-oscillator materials.Mueller matrix modelling was used to determine the optimal crystal orientation and pump polarisation. In particular, aligning the pump polarisation with a h111i crystal axis produced a 33% enhancement in the Raman gain coeﬃcient, compared topreviously utilised crystal orientations.Knowledge of the Raman gain coeﬃcient of diamond in the mid-infrared is vitalfor designing long wavelength Raman lasers, due to the reduced gap between laser and damage thresholds. Detailed investigations consider two measurement methods. Theﬁrst is a reﬁnement of the pump-probe technique that examines the eﬀect of correlating intensity structure and laser line width. The second is a novel approach based on fourwave-mixing, which removes the need to accurately characterise the spatial, temporal and spectral properties of the lasers involved.Extending the wavelength of diamond Raman lasers to the mid-infrared necessitates overcoming the challenges of a diminished Raman gain coeﬃcient, increased crystal absorption and weaker optical coatings. Demonstrations of tuneable ﬁrst Stokes outputfrom 3.4-3.8 m and second Stokes output at 7.3 m represent the longest wavelengths reported from a solid state Raman laser. Cavity losses restrict the obtainable conversion eﬃciencies, and several methods, including four-wave-mixing and microstructured facets, are considered as means to improve performance.