Distributed fiber acoustic sensing (DAS) technology can continuously spatially detect disturbances along the sensing fiber over long distance in real time. It has many unique advantages, including, large coverage, high time-and-space resolution, convenient implementation, strong environment adaptability, etc. Nowadays, DAS becomes a versatile technology in many fields, such as, intrusion detection, railway transportation, seismology, structure health monitoring, etc. In this paper, the sensing principle and some common performance indexes are introduced, and a brief overview of recent DAS researches in Shanghai Institute of Optics and Fine Mechanics (SIOM) is presented. Some representative research advances are explained, including, quantitative demodulation, interference fading suppression, frequency response boost, high spatial resolution, and distributed multi-dimension localization. The engineering applications of DAS, carried out by SIOM and other groups, are summarized and reviewed. Finally, possible future directions are discussed and concluded. It is believed that, DAS has great development potential and application prospect.

Phase-demodulated phase-sensitive time domain reflectometer (-OTDR) has attracted much attention because of its linear relationship between the phase variation and the amplitude of the vibration. However, it suffers from limited signal-to-noise ratio (SNR) and signal fading, due to the inherent low Rayleigh scattering coefficient and randomly spatial non-uniform distribution of the refractive index. Here, a -OTDR system based on wavelength diversity is demonstrated, which can enhance the SNR and reduce signal fading simultaneously by using differential vector aggregation method. A theoretical model is built, and the linear relationship between SNR improvement and diversity scale is analyzed. The verification experiment is conducted, and in the further experiments, the frequency and wavelength diversity technology are cascaded to achieve a diversity scale of 20. After differential vector aggregation method, the external disturbance signal is well reconstructed and the noise floor is reduced by 13.4dB, which is basically consistent with the theoretical analysis.

It is a basic task in Brillouin distributed fiber sensors to extract the peak frequency of the scattering spectrum, since the peak frequency shift gives information on the fiber temperature and strain changes. Because of high-level noise, quadratic fitting is often used in the data processing. Formulas of the dependence of the minimum detectable Brillouin frequency shift (BFS) on the signal-to-noise ratio (SNR) and frequency step have been presented in publications, but in different expressions. A detailed deduction of new formulas of BFS variance and its average is given in this paper, showing especially their dependences on the data range used in fitting, including its length and its center respective to the real spectral peak. The theoretical analyses are experimentally verified. It is shown that the center of the data range has a direct impact on the accuracy of the extracted BFS. We propose and demonstrate an iterative fitting method to mitigate such effects and improve the accuracy of BFS measurement. The different expressions of BFS variances presented in previous papers are explained and discussed.

We demonstrate a compact fiber optic accelerometer (FOA) utilizing a push-pull spring-mass structure with a size of 25 mm × 25 mm × 25 mm, showcasing the excellent performance of strong sensitivity, minimal self-noise, and wide dynamic range. The presented sensing probe is a Michelson interferometer (MI) that employs the 3 × 3 coupler demodulation method to extract the information of vibration. Both arms of the MI are fiber coils wrapped around a movable push-pull folding spring structure, serving as the sensing arm for detection, which results in twice the sensitivity compared to a single-sensing arm accelerometer. The theoretical design and finite element analysis are validated experimentally, confirming that the proposed FOA demonstrates a mean sensitivity of 48.73 dB re rad/g over the frequency band of 5 Hz-1 kHz, closely matching the simulated results. It also achieves a large dynamic of 161.12 dB at 50 Hz. Noise evaluation experiment results reveal a low average noise level of 15.85 ng/Hz1/2 of the proposed sensor, indicating that it is one of the fiber optic interferometric accelerometers with the lowest noise reported to date.

A novel photonic method for low power radio-frequency (RF) signal detection by a tunable optoelectronic oscillator (OEO) with high gain based on equivalent phase modulation (EPM) is proposed and experimentally demonstrated. A dual-parallel Mach-Zehnder modulator is utilized to generate the EPM signal with a controllable optical carrier to sideband ratio, by which the gain to the RF signal by the OEO loop can be enhanced. A phase-shifted fiber Bragg grating (PS-FBG) with a narrow-band notch in its reflection band implements the phase modulation to intensity modulation conversion by removing one sideband of the EPM signal. The gain and noise performance of the detection system are theoretically analyzed and simulated. In the experiment, the maximum gain of 10.4 dB is obtained at 8 GHz, and the detection sensitivity for RF signals from 2 to 18 GHz are within the range of -87.7 to -84.3 dBm by using the developed detection system. Compared to normal phase modulation based scheme, the EPM-based system offers 9.1 dB higher gain. Utility of the proposed approach for detecting modulated RF signal is also investigated and an optimization scheme is discussed.