Abstract

Subject of study. The spectral characteristics of an interference signal were studied using the method of direct phase demodulation on the interrogator of a fiber-optic sensor for high-temperature measurements. The aim of the study is to develop a method for measuring the absolute temperature of a Fabry–Pérot interferometer, based on phase demodulation using frequency scanning. Method. A mathematical model for direct phase demodulation was implemented in MATLAB. This model allows us to evaluate the change in the optical length of the interferometer under the influence of temperature by performing frequency modulation of the wavelength of the optical source according to the periodic sawtooth law. Owing to the multipath interference in the resonator of the Fabry–Pérot interferometer, the photodetector registers a reflected interference response with a complex spectral composition. The current value of the phase difference in the interferometer is restored by the estimation of the local minimum position of the interference signal, and the absolute temperature of the interferometer is thus determined.Main results. The mathematical model was applied to a wavelength range of 1308–1310 nm of the optical source. The length of the interferometer resonator was matched to the allowable range of the vertical-cavity surface-emitting laser wavelength tuning to achieve the maximum phase sensitivity. A digital filter for the phase demodulation method of the Fabry–Pérot interferometer was selected, and the potential accuracy of the temperature determination method was calculated considering the presence of noise in the measuring system. Practical significance. The proposed method can be used in the interrogation system of fiber-optic interferometric Fabry–Pérot sensors, as well as for high-precision temperature (above 300°C) monitoring systems.

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