To improve the accuracy of vibration location information of the Φ-OTDR system, a method of optical fiber sensing signal processing based on Compressed Sensing (CS) is proposed in this paper. First, a sensing matrix is constructed by sparse array and observation array, and the mathematical modeling of CS is established. Then the reconstruction algorithm is designed to complete the accurate reconstruction of the vibration signal with a small number of sampling values. Second, the time domain signal composed of all scattered light at the vibration source is analyzed to complete the measurement of vibration frequency. Finally, a heterodyne Phi-OTDR system is built, using PZT to simulate the vibration source. And 36 scattered light information is continuously collected at the frequency of 100 Hz and 500 Hz. The experimental verification of the CS method in this paper is carried out, besides, the signal-to-noise ratio (SNR) is compared with the traditional difference method, Empirical Mode Decomposition (EMD) method, and Variational Mode Decomposition (VMD) method. The experimental results show that the SNR of this method is improved to 40.41 dB and 30.62 dB, the optimal spatial resolution is 9m, and the maximum relative error of frequency measurement is 8.1%. Compared with the results of traditional I/Q demodulation, the frequency information obtained by the two methods is consistent, and there is no obvious deviation from the actual frequency of PZT. While improving the SNR of the system, this method can greatly simplify the signal processing process and improve the positioning accuracy of the system, which has a certain practical application value in the fields of infrastructure monitoring, resource exploration, and vibration detection.

A novel axial strain sensor based on an optical microfiber coupler (OMC) operating at the dispersion turning point (DTP) was designed to realize high-sensitivity measurements of axial strain. First, we explain the measurement principle of axial strain sensors based on microfiber couplers. Then, we calculate the measurements, with the theoretical calculations showing that the sensor has high sensitivity when the effective refractive index (ERI) difference between the odd and even modes in the coupling region of the OMC is equal to zero. Finally, we carried out axial tensile experiments on the OMC. The results show that the OMC has the highest axial strain sensitivity when it operates at the DTP. The maximum sensitivity is 166.9 pm/ $\mu \varepsilon $ , with a linear range of 0- $400 ~\mu \varepsilon $ . The sensor is highly sensitive to axial strains and has a simple structure. It has broad application prospects in the measurement of mechanical quantities such as strain, acceleration and vibration.

To improve the signal-to-noise ratio (SNR) of vibration location information in Φ-OTDR system, a variable step size normalized least mean square (VSS-NLMS) denoising method is proposed in the article. First, the mathematical principle of VSS-NLMS method is established. The method can adaptively filter the input signal and adjust the parameters. It can also update the step factor adaptively. The simulation results show that the SNR of this method is improved from -1.16 dB to 12.01 dB. Second, a heterodyne coherent optical system is built. After the detected beat signal is demodulated by I/Q quadrature demodulation, the amplitude signal and phase signal of the Rayleigh backscattered light are acquired. The amplitude signal determines the location information and the phase signal determines the vibration signal. Finally, the wavelet denoising method, the empirical mode decomposition soft threshold (EMD-soft) denoising method and the variational mode decomposition nonlinear wavelet threshold (VMD-NWT) denoising method are compared with the VSS-NLMS denoising method in the experiment. The experimental results show that the location information SNR of the VSS-NLMS denoising method is improved to 59.31 dB, 46.81 dB, 50.14 dB, 34.00 dB and 67.09 dB for 5 groups of 100 Hz - 500 Hz vibration events at the location of the 10.14 km sensing fiber. At the same time, the vibration signals of 5 groups are demodulated. This method has laid a foundation for the application of oil and gas pipelines.

Herein, a broadband ultrasensitive acoustic sensor based on an optical nanofiber coupler (ONC) attached to a diaphragm is designed and experimentally demonstrated. The ONC is sensitive to axial strain and works as the core transducing element to monitor the deformation of the diaphragm driven by acoustic waves. We first theoretically studied the sensing property of the ONC to axial strain and the deformation of the diaphragm. The results reveal that ONC working at the dispersion turning point (DTP) shows improved ultra-sensitivity towards axial strain, and the largest deformation of the circular diaphragm occurs at the center. Guided by the theoretical results, we fabricated an ONC with a DPT at 1550 nm, and we fixed one end of the ONC to the center of the diaphragm and the other end to the edge to construct the acoustic sensor. Finally, the experimental results show that the sensor can achieve accurate measurement in the broadband acoustic wave range of 30~20,000 Hz with good linearity. Specifically, when the input acoustic wave frequency is 120 Hz, the sensitivity reaches 1923 mV/Pa, the signal-to-noise ratio is 42.45 dB, and the minimum detectable sound pressure is 330 μPa/Hz^1/2. The sensor has the merits of simple structure, low cost, and high performance, and it provides a new method for acoustic wave detection.

Aiming at the frequency drift of the laser and acousto-optic modulator in the traditional phase-sensitive optical time domain reflectometric (Φ-OTDR), a Φ-OTDR system based on optical synchronous heterodyne is proposed and demonstrated. The method of optical synchronization heterodyne is used to follow the beat frequency signal to eliminate the interference of frequency shift fluctuation in the phase information and the ubiquitous residual frequency before the optical path heterodyne. The signal-to-noise ratio (SNR) is more than 31.4 dB on a 10 km sensing optical fiber with a probe light pulses of 5 kHz repetition rate and 100 ns pulse width. The error between the demodulated signal amplitude and the actual signal amplitude is less than 1%, and the maximum harmonic amplitude is about 28.3 dB less than the actual signal amplitude. While improving the real-time performance, this research greatly improves the demodulation characteristics of the system, which is of great significance to the advancement of the practical process of the phase-sensitive optical time domain reflectometric system.

The problems of the traditional type grating angular displacement encoder make it difficult to balance the measurement performance of miniaturization, high precision and high resolution, and the ability to resist vibration and shock is poor. In this article, a multivector variable-area capacitive angular displacement encoder based on a rotating electric field is proposed. Based on the principle of constructing a rotating electric field, a three-layer transmissive sensing structure with upper and lower layers of electrode plates and a modulation rotor as the middle layer is proposed and designed. A single-channel signal is used as the input excitation signal of the capacitive sensor to avoid the problem of mismatching multiple input signals. A pair of quadrature signals reflecting the spatial angle information is obtained through electric field demodulation and differential processing. The phase demodulation method based on the time scale is further adopted to improve the precision of demodulation. Simulations and experiments show that the sensor has a diameter of 50 mm, a thickness of 5 mm, a resolution of $0.81''$ , an accuracy of $2.5''$ , and a maximum nonlinear error of $40''$ . The design is verified to be feasible, which provides a new solution for capacitive angular position sensor design.