Effects of thermal fiber annealing on the temperature compensation of interferometric fiber-optic current sensors

In this paper, we present an experimental and theoretical study on how thermal fiber annealing influences the temperature dependence of the interferometric fiber-optic current sensor. Such sensors measure the current-induced phase shift (Faraday effect) between left and right circularly polarized light waves in a reflective fiber coil around the current conductor. The fiber-optic phase retarder at the coil entrance that generates the (nearly) circular light waves may also serve to compensate for the temperature dependence of the Faraday effect. Thermal annealing serves to remove disturbing bend-induced fiber birefringence in the sensing coil - but it can also affect the retarder parameters. We investigate, to our knowledge for the first time, how thermal annealing affects the optical phase retardation and its temperature dependence of elliptical-core fiber retarders and show how the retarder parameters must be set prior to annealing in order to achieve a temperature-independent sensor after annealing.