Photothermal refraction and focusing.
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Date
1997
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Abstract
This thesis begins with an introduction to the interaction and refraction of light
in continuous media. It is shown how these properties can be exploited to achieve
focusing of parallel light rays in such a medium. Past work on Gas Lenses is reviewed,
highlighting the progress in design of gas lenses, leading to a justification
for the research described in the rest of the chapter. Original work by the author
on the subject of continuous gas lenses at low and high pressure is then presented.
Experiments show that gas lenses at low pressure have stable foci, but long focal
lengths, while at high pressure two foci are produced, both of unstable character.
These results are explained by a simple theory, and future applications of such lensing
properties are presented.
Chapter two introduces the concept of the Colliding Shock Lens (CSL), and presents
shallow water wave simulations, conducted by the author, as a useful analogy to the
interaction of shocks in the CSL. All the properties of the CSL lensing action are
reproduced in the water simulations, yielding useful insight, by means of a simple
experiment, into the physics of interacting shock waves.
Chapter three presents original work by the author on the subject of multiple pulse
thermal lensing. A theory is developed which predicts the behaviour of thermal
lenses seen in an industrial laser chain. Experiments on thermallensing, as well as
some solutions, are presented and discussed.
Chapter four revises the theory of Zernike Polynomials and their application to the
study of aberrations. Thermal aberrations are studied, including the aberrations
introduced by thermal lensing and thermal blooming. The relationship between
aberrations and subsequent beam quality and beam propagation is explored.
Chapter five looks at the use of adaptive mirrors for mode matching. Although the
theory of adaptive systems is well known, no-one has as yet tackled the problem of
correcting for mode matching changes. A new way of thinking about mode matching
is proposed, and the merits of this system, called characterisation space, are
explained.
Chapter six comprises the theory and design of a novel vacuum chamber which has
applications in gas lens designs. All the gas lenses used in pressure experiments were
housed in compressional vacuum chambers. The idea of a Tensional Vacuum Vessel
(TVV) is introduced, and experiments show that such chambers are very successful
low vacuum chambers. The advantages and applications of TVVs are discussed,
specifically those relating to gas lens applications.
At the end of this thesis it was apparent that more questions had been generated
than answers. This is probably true of any study. Chapter seven therefore outlines
some as yet unanswered questions, and gives some suggestions for starting points.
Some of this work is presently being undertaken by the author.
Description
Thesis (Ph.D.)-University of Natal, 1997.
Keywords
Gas lenses., Refraction., Laser beams--Scattering., Theses--Physics.