Frequency dependence of the phase shift in optical-fibre thermal modulators

The optical phase shift as a function of frequency was determined in the 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 fibre surface and core temperatures are determined as a function of the frequency of the heating due to an electric current passing through the coating. The phase modulation arises from a combination of the temperature change at the core (predominant at low frequencies) and strains produced by the thermal expansion of the metal coating (predominant at the high frequencies). Applications include phase and birefringence control in optical-fibre interferometers.