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This thesis describes work performed at the National Physical Laboratory to improve the electrical risetime calibration of instruments such as fast sampling oscilloscopes. The majority of the work can be divided into four sections: development of an ultrafast optoelectronic pulse generator; measurement of fast electrical pulses with an electrooptic sampling system; de-embedding of transmission line and transition effects as measured at different calibration reference planes; and calibration of an oscilloscope. The pulse generator is a photoconductive switch based on low-temperature Gallium Arsenide, which has a very fast carrier recombination time. Sub-picosecond electrical pulses are produced by illuminated a planar switch with 200 fs optical pulses from a Ti: sapphire laser system. The pulses are measured using a sampling system with an external electro-optic probe in close proximity to the switch. The electro-optic sampling system, with a temporal resolution better than 500 fs, is used to measure the electrical pulses shape at various positions along the planar transmission line. The results are compared to a pulse propagation model for the line. The effects of different switch geometries are examined. Although the pulse generator produces sub-picosecond pulses near to the point of generation, the pulse is shown to broaden to 7 ps after passing along a length of transmission line and a coplanar-coaxial transition. For a sampling oscilloscope with a coaxial input connector, this effect is significant. Frequency-domain measurements with a network analyser, further electro-optic sampling measurements, and the transmission line model are combined to find the network transfer function of the transition. Using the pulse generator, the electro-optic sampling system and the transition knowledge, a 50 GHz sampling oscilloscope is calibrated. The determination of the instrument step response(nominal risetime 7 ps) is improved from an earlier value of 8.5 -3.5 / +2.9 ps to a new value of 7.4 -2.1 / +1.7 ps with the calibration techniques described.

Previous work has shown that the risk of hearing loss is related to the cumulative sound energy received by the ear. The instrument which computes a measure of this parameter is an integrating sound level meter (ISLM). Optimum design requirements for ISLMs which meet a variety of demanding specifications have been determined. Evaluation procedures have been proposed and the necessary specialized tone burst signal sources have been designed. An early survey of instruments which were available revealed serious shortcomings in their ability tointegrate accurately a typical impulsive signal. The aim of this work was to determine and critically evaluate the limiting mechanism, and to subsequently investigate design strategies leading to high performance instruments. Techniques for extending the performance of several standard circuits have been developed. In addition, a novel method for storing data in analogue and digital form and subsequently recovering a continuous output has been developed and is fully described. Combinations of analogue, random logic and microprocessor-based techniques have been investigated. Complete instruments which utilize different combinations have been designed and evaluated. The optimum performance for an instrument which meets the highest specification is shown to be achieved by the use of analogue techniques for the majority of the signal processing, with a combination of random logic and microprocessor-based techniques for control and data storage.

The aim of this work is to increase the applicability of Coherent Anti-Stokes Raman Spectroscopy (CARS) to temperature measurement in practical devices. Particular emphasis is placed on combustion thermometry and high pressure steam systems are also considered. A study is made of the temperature measurement accuracy attainable in the range 290 to 1050 K, using broadband CARS. Accuracies of 1 - 2% are attained, and laser cross coherence effects are found to be important. The determination of temperature probability density functions is of great importance to combustion science. Their measurement using single shot CARS requires the analysis of very large numbers of spectra. A study is made of fast methods of data analysis and the temperature measurement precision attainable using them. A very rapid data analysis method suitable for use in fluctuating temperature, pressure and concentration environments is developed. The temperature precision attainable using CARS is limited by CARS signal noise. For systems with high temperature fluctuations, detector counting statistics are found to make a dominant contribution to this. The spread in measured temperature probability density function width due to signal noise is characterised for the CARS system used, as a function of CARS signal strength for the temperature range 290 to 1050 K. A fast CARS signal analysis method is applied to map temperatures and temperature fluctuations in the flame zone of a turbulent oil spray furnace. The temperature measurements are compared with Discrete Droplet and Continuous Droplet oil spray model predictions (Stopford, 1984) with good agreement, particularly in the former case. In the post flame region, where turbulent fluctuations are less severe, averaged measurements of H[2]O concentrations were made.

The main goal of the project was to develop new experimental techniques to further investigate the phase diagram of the ferromagnetic superconductor UGe2 at high pressures. Of particular interest is the metamagnetic transition (FM1-FM2) which is thought to play the key role in the unconventional superconductivity in UGe2. The project attempted to detect experimentally the critical ferromagnetic fluctuations associated with the FM1-FM2 transition. The development of the experimental equipment constitutes a substantial part of the project and includes: (1) a 2-axis Rotation Stage for High Pressures, (2) an Ultrasonic Sapphire Anvil Cell and (3) a High Pressure Pulse Echo Setup. (1) allows transport measurement on single crystal samples with simultaneous two-axis rotation at temperatures down to 2 K, magnetic fields up to 9T and pressures up to 5GPa to be made. (2) employs Resonant Ultrasound Spectroscopy on a sample in the anvil type cell with an optical access at cryogenic temperatures. (3) uses the Pulse Echo Technique inside a cylinder type cell allowing for ultrasonic measurements on samples with a well-defined orientation. The design, technical challenges and tests results are presented and discussed. The existing techniques, used in the project, included Inelastic Neutron Scattering. A single crystal of UGe2 was studied at 12 kbar with a large volume two-layered piston-cylinder cell. No critical fluctuations were detected at Tx associated with the FM1-FM2 transition. Instead, a small inelastic signal below this transition temperature was observed at the threshold of the instrument sensitivity. The fluctuations at TC associated with the paramagnetic-to-ferromagnetic transition (PM-FM) were detected and the energy scale of these fluctuations was found to have decreased, as compared to the ambient pressure case, in proportion to the resistivity - a result supporting recent theory. The elastic constant c11 of a single crystal of UGe2 was measured as a function of temperature using (3) at ambient pressure as well as pressures up to 11 kbar. The elastic constant exhibits clear features associated with the two transitions, PM-FM and FM1-FM2. The temperature dependence of the elastic constant at ambient pressure was modelled with two main contributions (Schottky-like and Wiess-like) to the magnetic heat capacity coming from the two energy scales, Tx and TC, as well as two corresponding Gruneisen parameters, Ωx and ΩC. The values of Gruneisen parameters obtained from the data fitting were in good agreement with the values determined from the slope of the phase transition lines on the phase diagram. The temperature dependence of the elastic constant at high pressures was modelled according to the two-component system with coupled moments. The behaviour of the fitting parameters (including the Ωx and ΩT ) was consistent with the phase diagram of UGe2 and captured important features of c11 temperature evolution. The way forward would be to continue the measurements of c11 at higher pressures but also to repeat the measurements at different ultrasonic frequencies in order to obtain the temperature dependence of the ultrasonic attenuation which will provide more insight on the FM1-FM2 transition.

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