Temperature-Induced Changes of Sensitivity in the Unbalanced Hi-Bi Fiber Sagnac Interferometer

A temperature sensor based on the unbalanced Hi-Bi fiber Sagnac interferometer is analyzed in the temperature interval 15 250 C. It is shown that the sensitivity of such a sensor exhibits irreversible changes at temperatures higher than 200 C. To achieve this, it is proposed to anneal the fiber at 200 C for 1.5 h to avoid hysteresis and extend the measured temperature interval. Measurements were done for the polymer-coating-induced birefringence and the residual stresses causing hysteresis. It is also shown that the sensor sensitivity is reduced by 27 % when stresses are removed.