Studies On Wavelength Demodulation And Multiplexing Methods Of Fiber Optic Fabry-Perot Sensors

Extrinsic Fabry-Perot interferometric(EFPI) sensors based on white-light interferometry have drawn much attention due to their various advantages, including low phase noise in the low frequency range, simultaneous multi-parameter sensing and absolute measurement ability of the cavity length. Traditional demodulation methods of white-light interferometry require a high wavelength resolution, while limited to detect only the static or quasi-static signals. In this dissertation, two wavelength demodulation methods based on the parameter estimation algorithm were studied in detail, which have the capacity of accurate and absolute measurement with a lower wavelength resolution. The fast fiber Bragg grating analyzer(with a low wavelength resolution but a high sampling frequency) and EFPI sensors were utilized to detect the quasi-static signals of the liquid level and displacement, and the dynamic acoustic signals. The experimental results demonstrated the feasibility of the parameter estimation algorithm based demodulation methods(the variable sampling lengths Fourier Transform method and the cross correlation method). A hybrid multiplexing method of time division multiplexing(TDM) and frequency division multiplexing(FDM) was investigated, which effectively enhanced the multiplexing capacity of the system. A TDM/FDM hybrid system with four diaphragm-based EFPI sensors was built, and dynamic sensing was performed with a high resolution and low phase noise in the low frequency range.The main results and innovations of this dissertation are as follows:(1)The influencing factors of the EFPI sensors demodulation methods were comprehensively analyzed and classified. The broadband source and the detecting system are regarded as the two main error sources. The Hilbert Transform method and the nonlinear energy operator method were used to eliminate the influence of the envelope of the broadband source. A window function was applied to improve the demodulation resolution.(2)The basic theories of the Fourier Transform method and the cross correlation method were deduced and compared in detail. We introduced a Fourier Transform method with variable sampling lengths, and studied the two demodulation methods through intensive theoretical simulation and experimental validation. Experimental results showed that the demodulation resolution of Fourier Transform method with variable sampling lengths was enhanced seventy times compared with the fast Fourier Transform method. With the envelope of the broadband source eliminated, the cross correlation method obtained a resolution of 36.7pm. Dynamic sensing results showed that the noise spectrum was flat below 400Hz(-60 d B re. nm/?Hz).(3)A TDM/FDM hybrid multiplexing system based on white-light interferometry was proposed and set up. The semiconductor optical amplifiers were utilized as both the pulse modulators and the optical amplifiers. A further analysis of the crosstalk in the FDM system was operated, and the Kaiser window function was used to suppress the crosstalk. Experimental results showed that the crosstalk was suppressed from-37 d B to-48 d B.(4)The sensitivity of the EFPI sensors was calibrated with the liquid level measurement. The sensitivity of an EFPI sensor was calculated to be about 3nm/Pa. As an absolute measurement system, it has the capacity to simultaneously detect dual-parameter(the absolute measurement of the static sensing and the relative measurement of the dynamic sensing) with high resolution.