This thesis reports on an investigation into Cerenkov Free Electron Lasers. These devices are basically travelling wave tubes but having a dielectrically lined cylinder as the slow wave structure rather than a helix. If an electron beam is injected into the centre of this structure, an interaction between the electrons and the electromagnetic (e-m) TMo I mode can occur which can lead to amplification of the e-m wave. Two different systems have been constructed. The first one was designed to operate as an oscillator at 12.4GHz and used a rectangular X-band waveguide microwave coupler. It was thought that the non-operation of this device could have been due to a lack of net gain, and so a second system was designed having a smaller diameter dielectric liner in order to achieve higher gain but at a slightly higher frequency of operation (l6.9GHz). In both systems, the interception of the electron beam with the dielectric liner was small. Unfortunately, even though a maximum electron beam current of 120mA was achieved, leading to an expected small signal gain of 1200%, no microwave output was observed either. At this stage it was considered that there must he something more fundamental at fault with these systems. After a thorough investigation. it was discovered that the small gap which always exists between the dielectric liner and the waveguide affected the dispersion relation of a Cerenkov system. Theoretically, gaps as small as 1 % of the diameter of the waveguide were found to have a serious effect, and although these gaps would not stop the operation of the Cerenkov device, microwave output would only be expected at a voltage far from that expected. It was found that the problem could be overcome by coating the outer surface of the dielectric tube with a layer of conducting material, such as silver paint, which effectively removes the gap. Further tests of a Cerenkov free electron laser with this improvement are in progress.

Laser induced breakdown spectroscopy (LIBS) is a technique that can determine the elemental composition and quantities of a sample by the spectral analysis of a laser induced plume. This study was undertaken to develop, characterise and assess the use of the LIBS technique on geological samples in different pressure and gaseous environments. The experimental range chosen was dictated by the planetary conditions on Titan and other extra-terrestrial bodies with the samples analysed chosen to complement a range of rock types. A LIBS system was developed, together with associated experimental apparatus able to acquire results in varying pressure and gaseous environments. The capability of LIBS to analyse weathered rock samples was investigated under various ambient conditions; pressures of 160x103 Pa to 0.4x10-3 Pa and ambient gaseous mixtures of air, nitrogen and methane. Particular attention was paid to temporal and power considerations under such regimes. As was expected, the chosen delay time to optimise the emission signals needed to be increased with increasing ambient pressure. At power values as low as 28.5 mJ/pulse (using a 6 ns pulse from a doubled Nd:YAG laser at 532 nm) a valid emission signal could be obtained. Increasing the laser power resulted in a reduction in the overall signal to noise ratio. It was observed that ambient methane quenches the optical emission signal due to non-radiative transitions. In spite of this, valid qualitative data are obtainable, even when emissions due to carbon transitions from both the sample and the gaseous environment, are present. Results are presented which support the premise that the LIBS technique can be used to investigate both the surface and depth compositions of geological samples under extra-terrestrial conditions.

The work documented in this thesis is based on the production, manipulation and control of ultrashort laser pulses in the near infra-red region of the electromagnetic spectrum. Pulses were created using Cr⁴⁺:Forsterite as a gain crystal with the SESAM mode-locking technique. The aim of the work presented was to use the SESAM as a control device within the laser in addition to its function as the mode-locking element. In this thesis two methods of SESAM based control were investigated. The first technique, optical switching, used an auxiliary diode laser operating with a wavelength of 640 nm, to pump the SESAM in addition to the intra-cavity field of the Cr⁴⁺:Forsterite laser. The localised heating effect induced a change in the absorption spectrum of the SESAM, in turn changing the operating regime of the laser. Using this technique it was possible to switch between pulses of duration 34.5 ps to 263 fs in timescales of 1-3 ms. The return switching time was 0.5 ms. The effect of diode pump power on switching times and stability was also investigated, showing the possibility of controlling these aspects by monitoring the applied power. The second technique was an all-electrical control technique and in this case SESAM control was achieved by reverse biasing the device to induce the quantum confined Stark effect. This also changed the absorption spectrum of the SESAM and switching was achieved. In this work three SESAM designs were used, each with a different QW shape making up the absorber layer. With a square well device, switching was achieved between 6.42 ps to 2.85 fs and 116 ps to CW operation with switching time ~20ms. A device with a pair of QWs (with separation 2nm,facilitating quantum tunnelling) was then used. With this device switching was achieved from 188 fs to a regime which appeared to be producing Q-switched pulses.A stepped geometry QW device was also tested, but had insufficient absorption to induce mode-locking. The results obtained show that both switching techniques are viable control methods for the Cr⁴⁺:Forsterite laser, and indicate that there is also potential to use SESAM control in other solid-state lasers.

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