This thesis is concerned with the generation of picosecond pulses and their propagation through both resonant and non-resonant media. This was achieved by constructing a passively modelocked Erbium doped fibre laser (EDFL) which was used to study pulse propagation through sections of standard communications grade optical fibre, dispersion shifted optical fibre, and also through an Erbium doped fibre amplifier (EDFA) module. The EDFL produced a train of ˜2 psec pulses at 4 MHz, tunable over the erbium gain band ( 1520 - 1570 nm). The laser was constructed from commercially available components and had the property of stability combined with low pump power requirements to produce ˜50 Watt peak power pulses. The laser cavity geometry included a nonlinear optical loop mirror, which has the property of efficiently switching high peak power pulses, and allowed pulsed operation without the aid of any high-speed electronics. An EDFA module of identical geometry to that used in the laser was also constructed, and this was probed using the pulses from the EDFL. The traditional temporal and spectral measurements were found to be inadequate to allow a complete description of the pulse amplification process to be developed. To overcome this problem the technique of frequency resolved optical gating (FROG) was applied for the first time to optical fibre research, and allowed an indirect measurement of the electric field of the pulse. This complete description of the pulse was used in a numerical model to describe pulse propagation in an optical fibre. Fundamental propagation terms in the model were treated as free parameters in a minimisation scheme, which could be determined for a fibre under examination. This technique was shown to be accurate when used to examine pulse propagation through both standard and dispersion shifted optical fibre. A comprehensive numerical model was developed for the EDFA, and it was apparent from this model that a pulse propagating through an optimised EDFA encounters an atomic inversion distribution which is a strong function of distance along the amplifying fibre. It was also shown from the experimental results that the EDFA exhibited resonant dispersion, which is characteristic for propagation through an atomic medium on resonance.

An investigation has been carried out into the operation of a mode-locked argon laser at a wavelength of 514.5 nm and a repetition rate of 76.8 MHz. The characteristics of the pulses from the laser have been found to depend on the intra-cavity power level in the laser, and at average intra-cavity power levels exceeding 4 W, the pulses from the laser have a duration of typically 35 psec accompanied by a characteristic double-peaked spectrum with a spectral width of 13 GHz These pulse durations are approximately three times shorter than expected based on the inhomogeneously broadened transition bandwidth of 4 GHz. The dependence of the pulse characteristics on the intra-cavity power level has been explained by a model of pulse propagation where the coherent coupling between the pulse and the atomic polarisation in the laser gain medium is included. Detailed examination of the pulse structure reveals the presence of afterpulses that have a typical intensity two orders of magnitude lower than the main pulse. These afterpulses are interpreted as ringing arising from coherent Rabi-type oscillation of the atomic polarisation associated with the circulating pulse in the laser. Numerical simulations of the mode-locked laser have also been developed based on the fully coherent Maxwell-Bloch equations, and the results from the simulations reproduce well the experimentally observed variation in pulse characteristics. Based on results from the simulations, the short pulses observed at high intra-cavity powers are interpreted as the superfluorescent π –pulse solutions predicted in the long distant limit of pulse propagation in a swept-gain amplifying medium. The laser operation has also been studied in a mode-locked cavity dumped configuration. In this case the combination of the coherent mode-locking processes described above with the cavity dumped operation at a repetition rate of 3.8 MHz results in the observation of stable pulses with peak power of 1.6 kW.

The development of a mobility spectrometer which yields a complete spectrum every 25 seconds is described. The spectrometer uses a Differential Gerdien Chamber of the First Order with 8 collecting electrodes, giving a set of 8 "electrode charges" corresponding to each observed spectrum. A Least Squares iteration is used to fit the parameters of a function which describes the complete mobility spectrum. The spectrum function's parameters can be fitted reliably with standard deviations of 3% or better. The spectrometer was used for observations of temporal variations in the atmospheric ion mobility spectrum at Auckland. These show a clear diurnal variation, and demonstrate that the spectrum can change significantly in an interval of 100 seconds. Atmospheric polar conductivities calculated from the mobility spectrum (which agreed well with independently measured conductivities) indicate that large ions can contribute as much as 40% of the total conductivity.

This thesis deals with the electromagnetic radiation that is reflected from and emitted by a clear or cloudy terrestrial atmosphere. Calculations are performed in order to estimate the radiation exitant to space in the visible, the near visible, and the two infrared windows at approximately 4 μm and 11 μm. The NOAA-9 satellite Advanced Very High Resolution Radiometer (AVHRR/2) is used as the data acquisition system. The satellite AVHRR system is described, as well as future TIROSN satellites. The pre-launch calibration of the radiometer is described in some detail. Post-launch calibration is examined using a reflectance difference method. The radiometer gains are shown to have changed by more than 20% from their pre-launch values. A LOWTRAN based atmospheric model suitable for New Zealand is developed to provide cloud free transmission values. Atmospheric scattering is shown to contribute negligibly to visible cloud measurements (for 8-bit radiometer resolution). The Channels 1 to 3 terrestrial reflectance is averaged from more than two years of satellite data. Sea-surface state is examined as a function of windspeed. A method of extracting surface temperature in rugged terrain is developed. Dual temperature measurements at 10.8 and 11.8 μm are used to estimate surface air temperature. The transfer of radiation in a single layer cloud is calculated as a function of cloud thickness, observation/illumination geometry and the effective radius of the cloud drop-distribution. The variation in drop-size is contained in an ensemble of modified-Gamma distributions whose effective radius varies from 1 to 30 μm with a fixed dispersion of 0.25. A technique is established which allows effective cloud radius to be estimated from 3.7 μm AVHRR/2 imagery. The radiation values at 0.65 μm and 11.8 μm are used to correct the 3.7 μm reflectance for variation in cloud thickness and temperature respectively. The procedure is limited to warm clouds of moderate optical depth. In addition, the inversion process generates values of cloud top height and temperature, cloud depth and column liquid water content. The method is used to observe the development of orographic cloud formed over the southern half of the North Island of New Zealand. The inversion results for this type of cloud are in agreement with aircraft measurements carried out by the New Zealand Meteorological Service and are well fitted by a simple drop-growth model.

The results of observational studies of very high energy (VHE) and ultra high energy (UHE) gamma ray emission, by various astrophysical objects, made from 1988 to 1990 with the JANZOS Cerenkov facility are presented. The active galaxy Cen A and the Galactic X-ray binary systems Vela X-1, Cen X-3, and Cir X-1 were monitored for VHE gamma ray emission above 1 TeV. No evidence was found for persistent or episodic emission from any of these objects. Upper limits on the VHE fluxes above 1 TeV of 2.1×10-11, 2.7×10-11, 3.6×10-11 and 3.5×10-11 cm-2 s-1 respectively were obtained for these objects. These limits are consistent with previous observations made by other groups. Various objects in the Magellanic Clouds were monitored for UHE gamma ray emission using the Cerenkov technique at large zenith angles. This technique has been found to be more sensitive at UHE energies -100 TeV than the conventional air shower technique. During 1990 the equipment was modified to allow a sky coverage of 7°×23° at large zenith angles. This enabled most of the extent of the Large Magellanic Cloud to be surveyed at UHE energies. An examination of the UHE database yielded no evidence for persistent emission from SN1987A and selected X-ray pulsars in the Magellanic Clouds. Upper limits on the UHE fluxes (in cm-2s-1) above the given threshold energies were obtained as follows: SMC X-1, 2.4×10-13 (>30 TeV); SN1987A, 2.3×10-13 (>65 TeV); PSR0540-693, 2.3×10-13(>65 TeV); LMC Trans, 1.5×10-13(>130 TeV); LMC X-4, 1.3×l0-l3 (>140 TeV). Evidence was found for two episodes of pulsed gamma ray emission above 65 TeV from SN1987A. On the nights of May 28 and June 23, 1990, a periodicity search using the Rayleigh test showed significant power at 18.356 ms. This is near a periodicity reported at optical wavelengths in September, 1990. The chance probability for the individual period was about 10-6 on each of the two nights. Excess events of the order of 1σ were also seen on these nights. The overall chance probability, allowing for all degrees of freedom, was found to be 0.1%. Evidence was also found for a single episode of gamma ray emission above 140 TeV from LMC X-4 on May 27, 1990. Excess events were seen with a statistical significance of 2.4 σ. A periodicity test showed maximum Rayleigh power at 13.488 s on this night. The chance probability for the individual period was 8×10-3 and the overall chance probability was found to be 0.5%. Adopting a recent estimate of the pulsar population in the Large Magellanic Cloud, an upper bound of 3×1034 erg s-1 on the luminosity of an average pulsar, at energies >65 TeV, was obtained.

Building on Kibblewhite's long term investigations of the nonlinear wave-wave interactions and the infrasonic ocean noise and the microseisms these induce, this thesis further explores the physical nature of these processes. The classical description of this interaction, which takes into account only the homogeneous component of the induced field, has been extended to include the inhomogeneous component. A complete expression for the wave induced noise spectrum is established following a geometrical analysis of the dispersion relations among interacting waves. The relative importance of these two components and their directivity properties are also calculated and discussed. It is shown that while at observation points deeper than 500 meters the effects of the inhomogeneous component can be regarded as negligible, it can cause an increase of noise level of up to 40 dB in the region near the surface of the sea. Furthermore, in contrast to the nearly omni-directional distribution of the homogeneous component of the induced acoustic field, there is a tendency for the energy associated with the inhomogeneous component to focus in the wind direction. Based upon a multilayer analysis of a visco-elastic geoacoustic model, Green's functions and the spectral transfer functions relating the surface source pressure field to the underwater noise and microseism fields are derived for both near and far field cases. A 3-dimensional presentation defined on the dispersion plane (frequency and horizontal wave number) is introduced to describe the sea bottom reflection-loss and, Green's functions, and is extended to include the inhomogeneous region for the first time. The characteristics of this 3-D presentation are explained in terms of the geoacoustic parameters. The influence of the interaction of multiple seas (and swell) on the induced acoustic field are also discussed in this thesis. All these effects are considered in the calculation of the synthetic spectra of both the noise and microseism field. When compared with measured data excellent agreement is found between the theoretical and experimental results, which provides further confirmation that the nonlinear interaction is the most important source of the infrasonic ocean noise, as well as confirming the basic validity of the procedure introduced by Kibblewhite and Ewans to derive the ocean noise spectra from microseism records.

Content removed due to copyright restriction: Nash, W., Pinder, D.N., Hemar, Y., Singh, H. Dynamic light scattering investigation of sodium caseinate and xanthan mixtures (2002) International Journal of Biological Macromolecules, 30 (5), pp. 269-271. Pinder, D.N., Nash, W., Hemar, Y., Singh, H. Dynamic light scattering investigation of guar/dextran mixtures in aqueous solutions (2003) Food Hydrocolloids, 17 (4), pp. 463-468. / Dynamic light scattering has been used to investigate three different non ideal ternary polymer systems. The three systems investigated were sodium caseinate and xanthan aqueous solutions, guar and dextran aqueous solutions and dextran and pullulan aqueous solutions. All solutions have been investigated at a temperature of 25°C. Sodium caseinate and xanthan aqueous solutions with total polymer concentrations ranging from 0.01% w/w to 0.15% w/w and ratios of sodium caseinate of x = 1:3, 1:2, 2:1, and 3:l have been investigated. Guar and dextran aqueous solutions with total polymer concentrations ranging from 0.01% w/w to 0.06% w/w at a ratio of guar to dextran of 1:6 have been investigated. Dextran and pullulan aqueous solutions with total polymer concentrations ranging from 1% w/w to 10% w/w with ratios of dextran to pullulan of 1:3, 1:1, and 3:l have been investigated. The solution concentrations have been chosen to fall in the semi-dilute range, while still being miscible. Three different methods of analysis have been applied to resolve the field autocorrelations function into a sum of decaying exponentials; CONTIN, CONTIN multiq, and a Kohlrausch-Williams-Watts fit. These resulting decay rates have been interpreted using the theoretical method outlined by Sun and Wang. The CONTIN multiq method provided the best overall fit to the data. The Sun and Wang method has not provided results which are consistent with those reported elsewhere in the literature. Additional theoretical effort must be applied to interpret the results from dynamic light scattering on these novel non-ideal systems.

Optical pulse compression using similariton propagation in an optical fibre with decreasing dispersion has been demonstrated for the first time. This compression scheme is a practical application of the sech-similariton solution to the generalized nonlinear Schrodinger equation (NLSE) with distributed coefficients recently found using the self-similarity technique. The sech-similariton solution exhibits a characteristic positive linear frequency chirp, which increases in slope as the pulse compresses. The solution does not develop any side pedestals or deformation in pulse shape as it propagates, making it a promising candidate for a new compression technique. Unlike the adiabatic compression technique, rapid compression can be achieved in a fibre with a specifically designed decreasing group velocity dispersion profile since the sech-similariton is an exact solution to the NLSE. A cost-effective and efficient method of realising decreasing dispersion in a fibre has been developed using a comb-like dispersion profiling technique and its feasibility has been experimentally demonstrated. An optimised experimental similariton compressor system has been built using two industry standard fibres which compressed an 11 ps linearly chirped sech input pulse to 400 fs, yielding a compression factor of greater than 25.

The cross-entropy method is an established technique for solving difficult estimation, simulation, and optimisation problems. The method has its origins in an adaptive importance sampling procedure for rare-event estimation published by R. Y. Rubinstein in 1997. In that publication, the adaptive procedure produces a parametric probability density function whose parameters minimise the variance of the associated likelihood ratio estimator. This variance minimisation can also be viewed as minimising a measure of divergence to the minimum-variance importance sampling density over all members of the parametric family in question. Soon thereafter it was realised that the same adaptive importance sampling procedure could be used to solve combinatorial optimisation problems by viewing the set of solutions to the optimisation problem as a rare-event. This realisation led to the debut of the cross-entropy method in 1999, where it was introduced as a modification to the existing adaptive importance sampling procedure, with a different choice of directed divergence measure, in particular, the Kullback-Leibler cross-entropy. The contributions of this thesis are threefold. Firstly, in a review capacity, it provides an up-to-date consolidation of material on the cross-entropy method and its generalisations, as well as a collation of background material on importance sampling and Monte Carlo methods. The reviews are elucidated with original commentary and examples. Secondly, two new major applications of the cross-entropy methodology to optimisation problems are presented, advancing the boundary of knowledge on cross-entropy in the applied arena. Thirdly, two contributions to the methodological front are (a) an original extension of the generalised cross-entropy framework which enables one to construct state- and time-dependent importance sampling algorithms, and (b) a new algorithm for counting solutions to difficult binary-encoded problems.

Optical pulse compression using similariton propagation in an optical fibre with decreasing dispersion has been demonstrated for the first time. This compression scheme is a practical application of the sech-similariton solution to the generalized nonlinear Schrodinger equation (NLSE) with distributed coefficients recently found using the self-similarity technique. The sech-similariton solution exhibits a characteristic positive linear frequency chirp, which increases in slope as the pulse compresses. The solution does not develop any side pedestals or deformation in pulse shape as it propagates, making it a promising candidate for a new compression technique. Unlike the adiabatic compression technique, rapid compression can be achieved in a fibre with a specifically designed decreasing group velocity dispersion profile since the sech-similariton is an exact solution to the NLSE. A cost-effective and efficient method of realising decreasing dispersion in a fibre has been developed using a comb-like dispersion profiling technique and its feasibility has been experimentally demonstrated. An optimised experimental similariton compressor system has been built using two industry standard fibres which compressed an 11 ps linearly chirped sech input pulse to 400 fs, yielding a compression factor of greater than 25.