FREQUENCY-DEPENDENCE OF THE PHASE-SHIFT IN OPTICAL-FIBER THERMAL MODULATORS

The optical phase shift as a function of frequency was determined in t he range from DC to > 10 kHz for some single-mode optical-fibre phase modulators, formed by depositing a metal layer on the cladding surface by vacuum evaporation. A thermal analysis is presented in which the f ibre surface and core temperatures are determined as a function of the frequency of the heating due to an electric current passing through t he coating. The phase modulation arises from a combination of the temp erature change at the core (predominant at low frequencies) and strain s produced by the thermal expansion of the metal coating (predominant at the high frequencies). Applications include phase and birefringence control in optical-fibre interferometers.