In this thesis investigations of short pulse XeCl, KrF and ArF excimer lasers and HF laser transmission in fused silica and fluoride glass fibres respectively are reported, together with studies of polymer and soft tissue ablation in air and saline. Etch rate and photoacoustic measurements have been used to study the ablation process for the tissue and polymer samples with excimer lasers. The main emphasis is on the HF laser where a UV preionized transverse discharge in SF6 - C3H8 mixtures was used to generate ≃400ns (FWHM) pulses of energy up to 380mJ. Transmission measurements made on a 500μm core diameter fluoride glass fibre gave a distributed loss coefficient of −3 1.5x10 cm−1, and a maximum useful input fluence of ≃15Jcm−2 set by non-linear loss at the fibre input surface. For bovine cornea in air the onset of ablation occured at a fluence of ≃0.5Jcm−2, removal rates increased slowly up to ≃3Jcm−2 but above this increased sharply, reaching ≃17μm per pulse at 8Jcm−2. In saline the interaction becomes considerably more complicated because strong-heating of water at the fibre tip leads to the formation of a hot, high pressure vapour cavity (optical cavitation). Under these conditions the damage range may extend well beyond the beam penetration depth as a result of flow effects (eg. jetting) and intense acoustic emission associated with the 'bubble' growth and collapse. The dynamics of cavitation 'bubbles' have been investigated using pulsed dye laser shadowgraphy for various fibre-tissue geometries and results in free liquid modelled using the Rayleigh-Plesset theory. Time resolved photoacoustic measurements have also been made and revealed that very large transient pressures are generated in tissue near the fibre tip when ablation occurs in liquid; for example, at 8Jcm−2, B peak pressures reached about 1.5x108Pa. When the fibre-to- sample spacing was varied physical surface damage was evident out to distances of ≃250μm for cornea and ≃2mm for retina. As these are much greater than the characteristic beam absorption length in water (≃1.6μn), the main damage mechanism is then not through photoablation but jetting or acoustic emission associated with optical cavitation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:755101 |
| Date | January 1993 |
| Creators | Khosroshahi, Mohammed Etrati |
| Contributors | Dyer, Peter E. |
| Publisher | University of Hull |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hydra.hull.ac.uk/resources/hull:16870 |
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