Optical characteristics of sapphire laser scalpels analysed by ray-tracing

For laser surgery, high-power densities of light may be obtained by guiding a beam through the tapered end of a fiber or through a tapered rod. Due to the decreasing cross-sectional area of the taper, the power density increases until the beam refracts out of the taper. Ray-tracing was used to determine the irradiance distribution and power density within and at the tip of laser scalpels made of sapphire in relation to their geometry in air and in water. Computer beam profiles were compared to photographed profiles. The beams were emitted in cones with discrete angles which were related to the number of reflections within the scalpel. For taper angles as small as 5 degrees, the increase in power density exceeded 500x. Assuming the scalpel tip to be hemispherical rather than pointed or flat, the increase was 10-30% smaller due to internal reflection losses. The design of laser scalpels may be adapted to obtain adequate power density for effective tissue cutting together with radial energy leaking to promote coagulation and hemostasis.