Thesis-1993-Calverley.pdf (7.75 MB)
Download fileOptical studies of laser transmission at 10.6μm
thesis
posted on 2013-05-15, 13:22 authored by Gayle J. CalverleyIn the development of optical components, a thorough knowledge of the
behaviour and performance of materials intended for use is required. This
project arose from some early measurements on the laser initiation of the
semiconductor-to-metal transition in vanadium dioxide thin film coatings. There
are several parameters that can be measured to assess the optical behaviour of a
substance, among which transmission, reflectivity and the refractive index are
included.
The work in this thesis develops and uses a number of techniques for assessing
the high energy laser transmission of several materials at a wavelength of
1O.6μm under different experimental conditions. Spectrophotometer facilities
have also been used to examine low-energy transmission between 2.5 and 26μm.
The first test system was developed for investigations requiring high total
incident energies. This was achieved using focused pulsed radiation from a TEA
CO2 laser and the development work on this system involved pulse energy,
spatial pulse profile and temporal pulse profile measurements. Attenuation was
provided by a combination of calcium fluoride discs with additional polyethylene
tetra phthalate sheets of varying thicknesses for fine control. Testing showed
them to behave linearly over the required incident energy ranges. Pyroelectric
detectors enabled both total energy and temporal observations to be made. In
the case of the temporal observations, both the transmitted and reflected pulses
from the specimen were observed and compared to the incident temporal profile.
Temporal studies of this kind were carried out on thin film vanadium dioxide
coatings on germanium substrates, matching plain germanium substrates, and
indium antimonide (InSb) and cadmium mercury telluride (CMT) wafers.
Incident energy against transmitted energy characteristics were also obtained for
these specimens as well as for an AR coated mounted germanium window. The
initiation of the semiconductor-to-metal transition (SMT) in vanadium dioxide
was achieved and appeared to be power rather than energy-dependent. This was
confirmed by both the time-resolved studies and transmitted energy measurement
techniques. The germanium specimens behaved linearly over the total incident
energy range of 0-600mJ used for testing, whilst optical limiting was observed
in the InSb and CMT wafers. Damage thresholds for all specimens except the
mounted germanium window were also obtained. The optical nature of the SMT
in vanadium dioxide was examined further using multi shot post-sample profiling techniques. This showed the occurrence of diffraction by the laser-induced
metallic state, which appeared to be acting as an optical stop. An experimental
model using a substrate disc with a metal stop attached was successfully
developed to examine this conjecture further.
The second test system developed was based on a 6W continuous wave CO2
waveguide laser. Fixed position pyroelectric detectors were used to give
transmission readings of the chopped beam through a range of low incident
energies. Alternatively, the system could be operated as a scanning
spectrophotometer to produce a spatial transmission profile across the diameter
of a sample. As one of the problems associated with using coherent radiation is
the formation of interference fringes from light reflected from the front and rear
surfaces of a sample, this technique is particularly useful for illustrating fringing
caused by etalon effects or optically uneven sample surfaces. Results obtained
this way have been successfully compared to theoretical computer models of
fringe structures. It was found to be necessary to AR coat some of the samples
to simplify the measurement techniques developed and this system was used to
measure the effectiveness of the AR coatings.
Finally, by combining the pulsed and continuous wave lasers into a probe beam
system, it was possible to observe the recovery of the vanadium dioxide coating
from its laser-induced metallic state to the semiconductor state normally
maintained at room temperature.
History
School
- Science
Department
- Physics
Publisher
© Gayle Jodine CalverleyPublication date
1993Notes
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.EThOS Persistent ID
uk.bl.ethos.572546Language
- en