Optical cooling of solids and Laguerre-Gaussian mode generation

Abstract

This thesis covers two areas of laser physics: optical cooling of rare-earth-doped solids by anti-Stokes fluorescence and the generation of pure Laguerre-Gaussian laser modes using a ring-shaped pump beam. A novel laser-based approach to determining local variations in the temperature of transparent samples is developed. This technique is based on monitoring the frequency-shift of the axial modes of a simple, diode-pumped solid-state laser resonator in which the sample is placed. A theoretical resolution of <4.5mK is calculated for a perfectly isolated probe resonator. The technique is validated by comparison to thermocouple measurements of a control sample. The advantages of this diagnostic technique have been demonstrated for the discrimination of local optical cooling from parasitic heating effects. Local cooling of 1.4±0.1K has for the first time been observed in Yb3+:CaF2 using this measurement technique.

During the course of the optical cooling experiments regular pulse packets have been observed for sustained self-pulsing in a fibre laser operating significantly above the lasing threshold. This regular pulsing behaviour is observed to break down to irregular pulsing behaviour close to threshold. Extending the resonator length has been demonstrated as a technique for the suppression of self-pulsing. A hollow-core-fibre beam-shaping technique has been developed to selectively generate pure Laguerre-Gaussian laser modes. An astigmatic mode-converter analysis of these modes has proven that an axial mode cannot simultaneously possess both senses of azimuthal phase. The sense of the azimuthal phase has been observed to flip around the peak of the gain spectrum.

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