Temperature-induced changes of sensitivity in the unbalanced Hi-Bi fiber Sagnac interferometer

A temperature sensor based on the unbalanced Hi-Bi fiber Sagnac interferome ter is analyzed in the temperature interval 15-250 degrees C. It is shown t hat the sensitivity of such a sensor exhibits irreversible changes at tempe ratures higher than 200 degrees C. To achieve this, it is proposed to annea l the fiber at 200 degrees C for 1.5 h to avoid hysteresis and extend the m easured temperature interval. Measurements were done for the polymer-coatin g-induced birefringence and the residual stresses causing hysteresis. It is also shown that the sensor sensitivity is reduced by 27% when stresses are removed.