Thermally induced optical phase effects in fiber optic sensors

Induced optical phase effects attributed to time varying temperatures at the fiber surface in single-mode fiber optic interferometric sensors are calculated. A Green's function approach is used for predicting temperature distributions within fibers for general time-dependent external temperatures. Analytic expressions for temperature distributions within fibers exposed to external linear, step, and sinusoidal temperature variations are derived using standard differential equation methods, assuming heat conduction to be the only heat transfer mechanism. With these expressions, thermally induced optical phase effects are predicted for an infinitely long uniform-index fiber. Plots are given that show the significance of fiber diameter and thermal properties on the time-dependent behavior of thermally induced phase effects for specific external temperature variations. It is shown that thermally induced phase effects can be reduced by jacketing fibers with elastomers having certain thermal properties.