title: Laser beam shaping: properties and applications
Creators: name: Mitchell, Kevin J.
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: vector beam, vector beam shaping, diffractive optics, spatial light modulation, group delay, polarisation, digital micro mirror device, DMD, structured light, wavefront correction, adaptive optics, beam shaping.

description: The field of laser beam shaping advances the study of optics by pushing the limits of structured light and its applications. This thesis aims to exploit classical optical theory using state of the art structuring techniques to introduce two novel demonstrations: (1) high-speed digital micro-mirror device (DMD) light shaping and (2) broadband dual spatial light modulator (SLM) arbitrary vector beam shaping. These shaping techniques are also employed to demonstrate the fundamental group delay of structured light in free space, for Bessel and focused Gaussian beams. Motivation for the structuring of light in both scalar and vector regimes is explained for a wide range of applications, from optical fibre communications and microscopy, through to computational imaging and micro-manipulation. The thesis continues by providing an in-depth background to the relevant principles of optical wave theory, and provides an overview of holographic beam modulating techniques to be used thereafter. The first key result in this thesis is the high-speed (4kHz) generation of arbitrary vector beams using a DMD, by way of tuning the intensity, phase and polarisation of the light. Widely used vector profiles such as radial, azimuthal, uniformly circular polarisations and Poincare beams are characterised using spatially-resolved Stokes parameters. The intention is to promote the DMD as a cost-effective SLM which provides a switching rate that is up to ~2 orders of magnitude faster than competing liquid crystal (LC-)SLMs. Extended details into the practical considerations of using a DMD as a diffractive optical element are discussed. The second result in this thesis is the broadband (100nm) generation of arbitrary vector beams using a pair of LC-SLMs in tandem. The wavelength-dependent dispersion of diffractive optics is a major issue in the beam shaping of broadband light; the system presented in this thesis employs a second dispersion-correcting SLM within a Sagnac interferometer. The vector beam profiles from the previous result above are reproduced across four wavebands within the 100nm range. Furthermore, the effects of chromatic dispersion in both DMDs and LC-SLMs are investigated, as is the use of in-situ wavefront correction in beam shaping systems. In the final investigation reported in this thesis, the fundamental group velocity delay of structured light in free space is demonstrated in a classical interferometer. An elegant and novel approach is presented, which interferes one Gaussian beam with another which has, at some point in its propagation, been structured and then destructured by two LC-SLMs in tandem. The associated reduction of group velocity arises from the free space boundary conditions associated with structured beams, which in turn affects their wavevectors; this effect is demonstrated for Bessel beams and focused Gaussian beams using a simple interferometric approach. The delays detected are on the order of 1 micron over 10cm. Furthermore, the structuring and destructuring of beams with many constituent wavevectors (c.f. optical speckle) is presented, and its delay is theorised in anticipation of future studies. It is intended that this thesis serves as a comprehensive account of the practicalities of vector beam shaping in different regimes for potential applications, bolstered by meaningful investigations to unearth the true nature of structured light and its properties.