Optical fiber extrinsic Fabry-Perot interferometer (EFPI) sensing technology measures external physical quantities by detecting interference spectrum changes caused by variations in the sensor cavity length or refractive index of the medium inside the cavity. Compared to traditional sensing techniques, the EFPI sensor features high sensitivity, small size, and immunity to electromagnetic interference, and has extensive applications in measuring physical quantities such as pressure, temperature, vibration, displacement, and acceleration. Optical fiber EFPI sensors have been widely applied to aerospace, energy exploration, underwater acoustics, and defense industries, playing an increasingly important role. In complex environments, a single sensor often fails to provide detailed information about the target, which necessitates the integration of multiple sensors into an array for more accurate measurement. However, in optical fiber sensor arrays, crosstalk occurs during the transmission and demodulation of optical signals, and it is the interference among different channel signals. When the crosstalk in a multiplexing system exceeds -40 dB, the multiplexing capacity will be decreased with significant signal detection bias. Therefore, crosstalk has become a challenging problem hindering the development and applications of multiplexing technology in optical fiber sensor arrays. We propose a five-step phase shift demodulation scheme based on multi-wavelength demodulation. Compared to a single-wavelength demodulation scheme, the multi-wavelength demodulation scheme averages the demodulation results of the five-step phase shift signals at multiple consecutive operating points, reducing crosstalk in the sensor array and improving the interference resistance and reliability of the sensing system. Moreover, this demodulation scheme lowers the requirements for the extinction ratio of the optical switches in the sensor array and the cavity length consistency among different elements, thus promoting the development of large-scale multiplexing in optical fiber F-P sensor arrays.

The interference spectrum of the F-P sensor is obtained by utilizing white light interference (WLI) technology. The spectrum is sampled at regular intervals in terms of wavelength, and preliminary spectral data processing is performed by eliminating the envelope and fitting an ellipse. When the reflection spectrum has N wavelengths, N_s groups of five-step phase shift interference signals can be obtained. The phase relationship between each group of five-step phase shift interference signals is adopted to yield two orthogonal signals, and the changes in the beginning phase are derived by an arctangent algorithm. According to the relationship between phase and cavity length, averaging N_s groups of phase changes can be utilized to determine the dynamic cavity length changes of the F-P sensor. The feasibility of the proposed scheme is validated via numerical simulations. Compared to single-wavelength demodulation schemes, the multi-wavelength demodulation scheme reduces the impact of fundamental frequency crosstalk (FFC) and total harmonic crosstalk (THC). This scheme employs spectral information from multiple different wavelength sources and thus reduces crosstalk at different wavelengths, which allows the sensing signal to be transmitted and processed more accurately and stably.

As the extinction ratio increases, both FFC and THC exhibit an approximately linear decreasing trend. Meanwhile, the FFC and THC of the multi-wavelength demodulation scheme are significantly lower than those of the single-wavelength demodulation scheme. When the extinction ratio reaches 25 dB, the FFC of the multi-wavelength demodulation scheme can be reduced to below -50 dB, while the single-wavelength demodulation scheme requires an extinction ratio of over 45 dB to achieve the same crosstalk level (Fig. 2). Furthermore, when the number of average wavelengths Ns=nλ02/2ΔλΔLint, the multi-wavelength demodulation scheme shows the best crosstalk suppression capability (Fig. 3). Additionally, both FFC and THC reach their minimum values when the demodulation parameter is close to π/2 radians, providing solid theoretical basis for the applications of the proposed scheme in optical fiber F-P sensor arrays (Fig. 5). As the cavity length differences increase, FFC and THC gradually exhibit fluctuations. This indicates that the multi-wavelength demodulation scheme can better suppress crosstalk under certain differences in the cavity lengths of elements S1 and S2 (Figs. 6 and 7).

A five-step phase-shift demodulation scheme based on multi-wavelength averaging is proposed to suppress crosstalk in optical fiber F-P sensor arrays. A parallel multiplexing system based on fiber F-P sensors with two elements is established, and the crosstalk in the system is subjected to theoretical analysis and numerical simulations. The results indicate that the crosstalk magnitude among different channels in the sensor array is related to the extinction ratio ε, the average wavelength number N_s , the wavelength interval Δλm, and the cavity length variation among different elements. Numerical simulations conducted with controlled variables show that the FFC and THC of the multi-wavelength demodulation scheme are significantly lower than those of the single-wavelength demodulation scheme. Furthermore, the proposed scheme can meet the crosstalk requirements of the array with a lower extinction ratio, enabling the composition of larger-scale fiber F-P sensor arrays. Additionally, it exhibits better crosstalk suppression among elements with different cavity lengths, addressing the issue of cavity length consistency among different elements in the optical fiber F-P sensor array. This greatly reduces the fabrication complexity of the sensors and improves the scalability of the multiplexing system.