The Material Dispersion Zero in Infrared Optical Waveguide Materials

Operation of optical waveguides at wavelengths up to 1.3 jum and 1.6 jum is being actively investigated and losses of a few tenths dB/km appear to be feasible. Operation at a longer wavelength has the attraction that much lower losses should be possible since the A-4 relationship controls the scattering loss, subject of course to the achievement of adequately low intrinsic and impurity absorptions. This should permit large distances between repeaters. The utilization of very long lengths of low-loss fibers is critically dependent on the material dispersion parameter (A/c)(d2n/dA2), where n is the refractive index at wavelength A and c is the velocity of light. At the wavelength A0 where the material dispersion is zero, the delay distortion in multimode fibers is minimized and maximum bandwidth can be achieved. In single-mode fibers, zero total dispersion requires balancing the waveguide dispersion with the material dispersion; the maximum bandwidth then occurs at a wavelength longer than A0. Several reports have discussed possible longer wavelength fiber materials, including the extended discussion of Goodman1 for the 4-jum band, the polycrystalline materials Tl(Br,I) of Pinnow et al.2 and Ag(Cl,Br) of Garfunkel et al.,3 and ZnCl2 glass of Van Uitert and 327