Fiber-optic Fabry-Perot temperature sensor system based on low-coherence interferometry.

摘要

A fiberoptic Fabry-Perot interferometer (FFPI) temperature sensor system based on low coherence interferometry using a fiber Mach-Zehnder interferometer (FMZI) as a phase modulator is implemented and tested. Two fiber arms of the FMZI are wound on piezoelectric tubes and a ramp modulation signal is applied to one of the piezoelectric tubes to interrogate the optical phase within the FFPI which serves as a temperature sensing element. The temperature of the fiber modulator is precisely stabilized using a double copper chamber with an active feedback circuit to prevent environmental temperature drift from affecting measurements by changing the state of polarization of light propagating in the FMZI.; Two length matched FFPIs are used in the system. One is the sensor FFPI which is used for temperature measurement and the other is the reference FFPI whose temperature is fixed. The displacement in relative position of the two central fringes from the FFPI's is a nearly linear function of the temperature difference between sensor and reference FFPI's. A unique software algorithm which performs correlation between the fringe data calculates the fringe displacement precisely.; For error free, high precision temperature measurement, it was found that it is necessary to thermally anneal the sensor FFPI to relieve residual stress, which apparently introduced during fabrication of internal mirrors using fusion splicing. An FFPI sensor which is annealed at 1100{dollar}spcirc{dollar}C for 30 minute and suffered additional cyclic heating and cooling up to 1000{dollar}spcirc{dollar}C for 5 times showed error free performance.; For an FFPI sensor of 1mm cavity length, the system resolution is measured to be 0.025{dollar}spcirc{dollar}C. When a longer cavity is used for the sensor, a higher resolution is predicted at a cost of reduced dynamic range. Repeatability of the sensor was also tested from room temperature up to 800{dollar}spcirc{dollar}C. Accuracy of the FFPI sensor is better than 0.1{dollar}spcirc{dollar}C, as determined by comparison with a thermocouple which serves as a reference. Long term stability test for a period of 5 days shows a maximum drift of 0.065{dollar}spcirc{dollar}C which is believed mainly attributable to temperature drift in the fiber Mach-Zehnder modulator. This could be improved by enhancing temperature stabilization of the chamber for the modulator.