In this Letter we present a high temperature multipoint sensing method using sapphire fiber air gap-based extrinsic Fabry-Perot interferometers. Three sensors are fabricated and tested in a single sensing link. Experimental results show that the air gap-based high temperature sensors have a very high temperature sensitivity (>20 nm/ degrees C) and resolution (<0.3 degrees C) and are capable of operating at temperatures well above 1000 degrees C. The multiplexed sapphire sensors present a significant advancement over traditional single-point sensors for critical high temperature applications.
Fiber optic Fabry-Perot sensors are commonly interrogated by spectral interferometric measurement of optical path difference (OPD). Spurious jumps in sensor output, previously attributed to noise, are often observed in OPD-based measurements. Through analysis and experimentation based on intrinsic Fabry-Perot interferometric (IFPI) sensors, we show that these discontinuities are actually caused by a time-varying interferogram phase term. We identify several physical causes for varying initial phase and derive a threshold value at which it begins to cause errors in the sensor output. Finally, we present a total phase measurement method as an alternative to OPD-based techniques to reduce the occurrence of output signal jumps.
A submicrometer-thick zirconium dioxide film was deposited on the tip of a polished C-plane sapphire fiber to fabricate a temperature sensor that can work to an extended temperature range. Zirconium dioxide was selected as the thin film material to fabricate the temperature sensor because it has relatively close thermal expansion to that of sapphire, but more importantly it does not react appreciably with sapphire up to 1800°C. In order to study the properties of the deposited thin film, ZrO 2 was also deposited on C-plane sapphire substrates and characterized by x-ray diffraction for phase analysis as well as by atomic force microscopy for analysis of surface morphology. Using low-coherence optical interferometry, the fabricated thin-film-based sapphire fiber sensor was tested in the lab up to 1200°C and calibrated from 200°to 1000°C. The temperature resolution is determined to be 5.8°C when using an Ocean Optics USB4000 spectrometer to detect the reflection spectra from the ZrO 2 thin-film temperature sensor.
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