Optical-fiber temperature sensor based on differential spectral reflectivity

A novel interference film-based optical-fiber temperature sensor is proposed and demonstrated in this paper. Two wavelengths, lambda 1 and lambda 2, are selected so they are located in the sloped and flat regions of the transmission spectrum of a dielectric multi-layer filter, and the entire transmission function of the filter varies as a function of temperature. The intensity of the light at lambda 1 is thus sensitive to changes in temperature, while the intensity of the light at lambda 2 is immune to those changes. A measure of temperature is then extracted from the ratio of the detected optical power at lambda 1 to the detected optical power at lambda 2. Experimental results indicate a resolution of better than +or- 0.2 degrees C within a total measurement range of 30-120 degrees C. Furthermore, these results indicate that the sensor output is immune to losses caused by both fiber bending and fluctuations in the output power of the light source. Thus, applications in practical and low-cost structural material systems are suggested.