Jones, Phillip S; (1993) Development and characterisation of holmium and erbium lasers for the ablation of biological tissue. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The development of pulsed laser systems operating in the infrared at 2.1 µm and 2.94µm, based on Cr:Tm:Ho:YAG and Er:YAG laser crystals, and the ability of these lasers to ablate biological tissue is reported. Thermal lensing in the laser crystals has been investigated and found to be the main factor restricting the operating ranges of these lasers. Additionally, increases in the threshold of holmium lasers due to thermal population of the lower laser level increases the amount of heat dissipated in the crystal lattice, leading to increased thermal lensing. Thus, the divergence properties of resonators containing these crystals depends, additionally, on the operating temperature. Modelling of the divergence behaviour of resonators based around Cr:Tm:Ho:YAG and Er:YAG laser crystals in simple resonator geometries is demonstrated using computer based ray tracing algorithms. The temporal behaviour of these lasers has been experimentally assessed and compared to a 'rectangular pump pulse' theory. Using this theory it is possible to predict the delay between the start of the excitation pulse and the start of the laser pulse but not the duration of the output pulse. The reasons for this are discussed. Pulses of 2.94µm radiation ablate soft tissue more efficiently than similar pulses of 2.1µm. Mass loss due to laser radiation is shown to be linear with dose for the 2.1µm radiation. However, at 2.94µm mass removal is impeded at high doses by the extension of a charred zone into the ablation crater. Operation at high fluences is required to overcome this problem. However, there is an increase in mechanical damage to surrounding tissue and a change in crater shape at fluences greater than 0.085 J mm-2 coinciding with a significant impulse being imparted to the tissue. The maximum mass loss per unit of delivered energy at 2.1µm and 2.94µm are approximately 48% and 60% of that expected for ablation of a pure water target. Routes for the energy loss are discussed. The energy lost in the form of kinetic energy is determined experimentally to be less than 1% of the total energy delivered. A linear model was found to best described the ablation performance at both wavelengths. The implications of these findings are discussed.

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