Not available / text

Not available / text

This thesis provides a unique method of producing an Isotropic dosimetry probe for monitoring light in highly scattering media, such as tissue. The requirement of such a probe is made apparent through a detailed review of current mathematical methods used to predict light distribution based on Radiative transfer theory and its subsequent solution under slab geometry. Initial methods of probe manufacture are discussed with the eventual technique involving a light cured polymer material providing the final method of construction. The resulting probe Is used to assess response uniformity and shadow effects. Mathematical modelling of the probe under spherical geometry is developed with experimental verification of boundary effects caused by refractive index mismatch presented for both internal and external Illumination. Typical application of the probe Is demonstrated resulting In direct measurement of backscattered reflectance and penetration depth. Indirect methods of measurement involving absorber modified penetration depths, reduced scattering co-efficients and an added absorber experiment are compared with theory. Further examination of the internal illumination of a probe In a scattering medium is related through theory and experiment to the optical properties of the surrounding medium. Continuing work on tissue . classification, fluorescence measurement including photodegradation, and a hyperthermia dosimetry probe is introduced.

This thesis presents an investigation of the modal gain characteristics of Vertical Cavity Surface Emitting Lasers (VCSELs). This is motivated by the experimental observations which tend to indicate fundamental mode operation at just above lasing threshold and multi-transverse mode operation at higher injection current levels. The complete mode spectrum of the cylindrical cavity is first analysed to illustrate that the original expectations that single (wavelength) mode operation solely due to the large wavelength separation created by short device lengths may not be realisable. The modal gains of different modes are calculated to demonstrate that mainly the difference between the modal gains, and not the separation between the resonances, provides a more satisfactory explanation of the fundamental mode operation at just above lasing threshold. At higher injection current levels, the increase in the modal gains of the higher order transverse modes (due to spatial hole burning) explains the excitation of higher order transverse lasing modes. The model relies on calculating the modes of a cylindrical dielectric resonator and the the corresponding modal gains are obtained from a perturbation analysis which takes into account the gain profile due to the injected inversion population distribution. A self consistent evaluation of the inversion population distribution (which provides the required local gain profile) is derived from the corresponding diffusion equation for the injected carriers in the active layer of the device. The development for obtaining the above lasing threshold inversion population distribution (which includes radial and azimuthal variation) has been done hierarchically such that the numerical procedures developed in the simpler stages of the model directly apply to solve a part of the next level of sophistication. This hierarchy has helped to provide a very efficient and compact numerical procedure and may be seen as an important aspect of the work done in the thesis. Further refinements include the evaluation of the injection current profile dependence on injection contact geometry and current spreading.

Strained layers are incorporated into many electronic devices and particularly into semiconductor lasers. These strained layers can relax, both elastically and plastically, which often impairs the performance of the device. This thesis presents several methods for calculating elastic strain relaxation: a Fourierseries method for stresses imposed on the surfaces of a rectangular block; a Fourier-integral for stress imposed on the surfaces of an infinite layer; and a Green-function method for the stress field about buried inclusions. The methods are used to calculate the strain distributions in a transmission electron microscopy sample, the relaxation at the end facet of a strained-layer laser, and the strain field about a rectangular buried layer. The effects of the strain relaxation on the optical absorption of the laser facet and the zone-centre band structure of the buried layer are discussed. The equilibrium theory of critical thickness is examined in detail and is shown to make unreasonable predictions for highly strained layers; a modification which corrects this behaviour is suggested. The equilibrium theory equates the line tension of a strain relieving dislocation to the strain energy it relieves in the layer. The additional energy corrections which can be included in the line tension are discussed, together with the failure of the equilibrium theories to reliably predict plastic relaxation in all situations.

Not available / text

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

No description available.

No description available.