Multiplexed fiber-optic Bragg stack sensors (FOBSS) for elevated temperatures

Abstract

A novel fiber-optic technique for the measurement of temperature is demonstrated. By depositing alternating quarter-wave layers of silicon nitrite and silicon-rich silicon nitrite at the end of an optical fiber, we can fabricate the equivalent of a Bragg grating of a high-temperature material to form a fiber Bragg stack sensor. When heated, the Bragg stack expands and a change in wavelength of the reflective peak is registered. Thus, the wavelength of peak reflectivity is a function of temperature. The sensors, which have previously been shown to be capable of surviving temperatures in excess of 900/spl deg/C, can be wavelength division multiplexed. Using a CCD solid-state spectrometer, we demonstrate the multiplexing of eight temperature sensors. Crosstalk effects, arising from the side lobes observed in the wave stack spectra, limit the performance of the multiplexed sensors. We show that this problem can be minimized by apodizing the Bragg stacks during fabrication. Currently, the sensor system is estimated to have a temperature resolution of 2/spl deg/C using a CCD spectrometer with a spectral resolution of 2.1 nm.