Acta Photonica Sinica, Volume. 52, Issue 8, 0806002(2023)
A Modified Three-wavelength Demodulation Method for Small Signals of EFPI
Fiber optic Extrinsic Fabry-Perot Interferometers (EFPI) are frequently utilized in many acoustic sensing scenarios due to their simple structure, ease of fabrication, high sensitivity, high phase consistency, and strong resistance to electromagnetic interference. However, the cavity length of the EFPI sensor is susceptible to environmental variables such as temperature and air pressure, and the drifting of the orthogonal working point caused by the change of cavity length will lead to signal fading and distortion. Nevertheless, several demodulation methods are less practical or even ineffective when dealing with small signals: Dual-wavelength method for Mach-Zehnder interferometer is inconvenient to apply to the EFPI demodulation; the Ellipse-Fitting Algorithm's (EFA) Lissajous figure will degenerate into a straight line for small signals, and there are also the disadvantages of poor real-time performance and slow demodulation speed; the second-order Differentiate-and-Cross-Multiply (DCM) operation has wide applicability, but the Direct Current (DC) term must be accurately removed, for small signals, the removal of DC term is difficult; the Bessel method has the same difficulties as DCM, and it can only demodulate single-frequency signals; The method of using tunable laser feedback to control the orthogonal working point has the drawback of high cost, and lasers with wavelength scanning function have high requirements for hardware reliability; Phase Generated Carrier (PGC) technology requires a complex carrier modulation system with a limited frequency response range, and the system is complex and large when PGC uses piezoelectric transducer to generate phase carriers. In contrast, intensity demodulation has the advantages of a linear transfer function, simple signal processing, high sensitivity and is suitable for the detection of high-speed and small signal. JIANG Yi et al. proposed a Symmetrical Demodulation Method (SDM) suitable for unstable cavity length and unknown cavity length, which constructs two equal interference phase differences between the three output signals by selecting specific three wavelengths, and then recovers the phase of the signal through mathematical operations. This method has the advantages of large dynamic range and simple operation, and it is more suitable for the detection of large signals. However, in the case of small signals, the SDM algorithm may lead to increased noise and error in the demodulation result.If the wavelength of the light source is 1 550 nm, the interference phase of EFPI changes 0~2π, corresponding to the cavity length variation range of 775 nm. When the cavity length change caused by vibration is less than 30.11 nm, that is, the radian value in the interference phase is less than 0.22 rad, the approximate error of
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Ruimu YANG, Xiaopeng DONG. A Modified Three-wavelength Demodulation Method for Small Signals of EFPI[J]. Acta Photonica Sinica, 2023, 52(8): 0806002
Category: Fiber Optics and Optical Communications
Received: Feb. 28, 2023
Accepted: Apr. 4, 2023
Published Online: Sep. 26, 2023
The Author Email: DONG Xiaopeng (xpd@xmu.edu.cn)