Abstract
If the measurand changes during the spectrum acquisition process, it easily leads to the failure of the classic demodulation algorithms of low-finesse optical fiber Fabry-Perot (FP) sensors. To address this problem, a novel demodulation model is proposed based on the definition of the instantaneous frequency. The proposed model establishes the relationship between the optical path length (OPL) of the FP cavity and the instantaneous frequency distribution of the FP interference spectrum. The link between the classic FFT algorithm and this model is discussed, and it is found that this model can be viewed as a generalized form of the FFT algorithm. Based on this model, the Doppler-induced demodulation error is analyzed. The analysis uncovers that the average frequency of the FP interference spectrum should be used for the evaluation of the error, and the error is proportional to the variation of OPL during the spectrum acquisition period. Further, numerical simulation and an experiment were carried out to verify the proposed model, and results show that the proposed model is effective for the dynamic low-finesse FP cavity. It is the first time that the idea of instantaneous frequency is introduced for the FP demodulation, and this model provides us a new way to cope with the FP sensing signal.
Highlights
Low-finesse optical fiber Fabry-Perot (FP) sensors are versatile, and they have been widely used for detecting various physical quantities, such as strain, temperature, pressure, vibration, and magnetic field [1]–[8]
When utilized for measurement, the optical path length (OPL) of the FP cavity changes with the measurand, so demodulating the OPL from the interference spectrum plays an important role in FP sensing applications [6]
RELATIONSHIP WITH THE FFT ALGORITHM As mentioned above, the FFT demodulation algorithm estimates the L according to the frequency of the spectrum, and the proposed demodulation model is established on the instantaneous frequency of the spectrum
Summary
Low-finesse optical fiber Fabry-Perot (FP) sensors are versatile, and they have been widely used for detecting various physical quantities, such as strain, temperature, pressure, vibration, and magnetic field [1]–[8]. W. Zhang et al.: Demodulation Model of Dynamic Low-Finesse FP Cavity be quick enough to keep the OPL of the FP cavity constant during the acquisition process. We propose a demodulation model for dynamic low-finesse FP cavity based on the definition of the instantaneous frequency. Using this model, the Dopplerinduced demodulation error is analyzed. Because the f (i)(kstart) can be calculated through the i-thorder derivative of the f (k), the dynamic FP cavity could be demodulated if the f (k) (or normalized fnorm(k) = 2f (k)/fs, fs is the sampling frequency of the spectrum) is known. Because the model is derived from (1) which is an expression for two-beam interference, theoretically, it is effective for the low-finesse FP interferometer and the Mach-Zenhder and Michelson interferometer
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