Hollow Dielectric Waveguides for Carbon Dioxide Laser Power Delivery

The present work is an investigation of the optical and mechanical properties of waveguides for the delivery of CO_2 laser radiation with particular emphasis being placed on hollow waveguides. The attenuation, near field output intensity distribution, polarization maintaining ability, and power threshold of several types of hollow waveguides were measured. Theoretical approximations of the attenuation in hollow waveguides are compared with numerical solutions. The use of these waveguides in neurosurgical applications was investigated. The limits of the validity of an approximation of the characteristic equation were investigated by comparing them to a numerical solution of the equation. The approximation of the attenuation of the HE_{11} mode was found to be accurate for low index hollow waveguides. The optical properties of five types of round hollow waveguide were measured. The lowest attenuations of PbF_2 coated aluminum and AgI coated silver waveguides were 0.6 and 0.3 dB/m respectively when straight. These waveguides were highly multi-mode and unable to preserve the polarization of the light propagating within them. The lowest attenuations of Ge coated silver and hollow sapphire waveguides were about 1 and 0.6 dB/m respectively for a 1 mm bore size. The output intensity distributions of these waveguides were close to single mode and they were capable of preserving the polarization of the propagating light. The measured losses of the sapphire are more than twice the predicted losses. The excess attenuation of the sapphire waveguides correlates exactly with losses expected from the measured roughness of the inside surface of the sapphire proving surface quality is the limiting factor in present waveguide transmission. The high power capabilities of the sapphire waveguides were found to be in excess of 1500 W. The power limit of the sapphire was determined by flaws in the material and a thermal expansion mismatch between the sapphire and the positioning mount. The effect of laser light delivered to human disc and rat brain tissue through a sapphire waveguide was found to be equivalent to laser light delivered by conventional means.