Scattering effects in crystalline infrared fibers

Abstract

The scattering-loss mechanisms in crystalline infrared fibers are examined by using scattering theory. Three limiting cases of scattering processes are discussed: Rayleigh scattering for scatterers with dimensions small compared with the wavelength, scattering in the Rayleigh–Gans diffraction limit for large scatterers with small changes in the optical phase of the scattered radiation, and anomalous diffraction for large scatterers with large optical phase differences. In all limits the field distribution along the fiber and the total loss are calculated by using the ray-optics approximation. We found that small changes in the field distribution and an approximately constant loss varying as λ⁻⁴ characterize Rayleigh scattering, whereas diffusion processes resulting in varied distribution and losses until steady state is reached characterize the diffraction-limited cases. In steady state this loss is proportional to λ⁻¹ for the Rayleigh–Gans diffraction limit and to λ for anomalous diffraction, whereas for short fiber lengths the loss is proportional to λ⁻² and λ² in the Rayleigh–Gans diffraction limit and anomalous diffraction, respectively. These losses are added to a surface-scattering loss from similar scatterers, which is proportional to λ⁻² in the Rayleigh–Gans diffraction limit and is approximately constant for the anomalous-diffraction case. Spectral measurements on silver halide fibers showed that the behavior from 3 to 14 μm is similar to that predicted by the Rayleigh–Gans diffraction-limited case. CO₂ laser measurements at 10.6 μm of the far-field distribution and the integrated-light scattering are in good agreement with our model for this limiting case, with an additional loss by hot spots that is probably related to cracks and large-scale defects in the fiber.