FIBEROPTIC TEMPERATURE SENSORS BASED ON DIFFERENTIAL SPECTRAL TRANSMITTANCE REFLECTIVITY AND MULTIPLEXED SENSING SYSTEMS

A concept for optical temperature sensing based on the differential sp ectral reflectivity/transmittance from a multilayer dielectric edge fi lter is described and demonstrated. Two wavelengths, lambda(1) and lam bda(2), from the spectrum of a broadband light source are selected so that they are located on the sloped and flat regions of the reflection or transmission spectrum of the filter, respectively. As temperature variations shift the reflection or transmission spectrum of the filter , they change the output power of the light at lambda(1), but the outp ut power of the light at lambda(2) is insensitive to the shift and the refore to the temperature variation. The temperature information can b e extracted from the ratio of the light powers at lambda(1) to the lig ht at lambda(2). This ratio is immune to changes in the output power o f the light source, fiber losses induced by microbending, and hence mo dal-power distribution fluctuations. The best resolution of 0.2 degree s C has been obtained over a range of 30-120 degrees C. Based on such a basic temperature-sensing concept, a wavelength-division-multiplexed , temperature-sensing system is constructed by cascading three sensing -edge filters that have different cutoff wavelengths along a multimode fiber. The signals from the three sensors are resolved by detecting t he correspondent outputs at different wavelengths.