Light propagation in multimoded square hollow waveguides

The irradiance–distance relation for light propagation in multimoded hollow waveguides is calculated from theory and compared with measurement for visible light. These are the first measurements of this relation for square waveguides and the first for round waveguides for visible light. The square waveguides had either silvered glass surfaces or uncoated PMMA surfaces. A geometric optics method based on image formation in a kaleidoscope is used for calculations. The measurements agreed with the predictions of theory in all cases. The loss is not described by a fixed value of an attenuation coefficient as the angular divergence of the light changes with distance. At larger distances the logarithm of the irradiance is linearly dependent on the logarithm of distance, regardless of material type, surface properties and waveguide cross section. The silvered square waveguide shows the lowest measured attenuation while PMMA square waveguides constructed without any special surface preparation performed almost as well as silvered round tubes sourced commercially. The surface roughness present in the silver coating of commercially sourced round tubes compromised their performance. An array of square PMMA waveguides was used to demonstrate an application for signal transport in megavoltage radiation fields for profiling a medical radiation beam and measuring the variation of dose rate with depth in water. (paper)