Effect of Temperature on Transmission in Lightguides

Lightguides may be used for transmission in a variety of environments. The temperature of these environments may vary as a function of time. It is anticipated that optical waveguides will have an operating temperature range specification similar to that of cables currently being employed for telephony, which is --40 to +85°C. Variation within this temperature range causes changes in the optical properties of materials comprising the lightguide. To maximize the signal-carrying capacity of lightguides, the refractive-index profile of multimode lightguides is carefully engineered to a specific shape. This is done to minimize mode dispersion. The refractive-index difference between core and cladding of a single-mode fiber must be controlled to provide a fiber which supports only the fundamental mode. It is known that refractive-index dispersion is a function of temperature. In glasses such as those from which optical waveguides are currently being fabricated, two opposing factors regulate the direction and magnitude of the refractive-index variation with temperature, Sn. As the material temperature increases, the positive coefficient of thermal expansion in glasses causes a decrease in density which decreases the refractive index. However, with an increase in the temperature, the other factor, electron polarizability, is increased and the ultraviolet absorption bands are shifted to a longer wavelength. This contributes to an increase in refractive index. 1-3 945