Distributed fiber-optic vibration sensors receive extensive investigation and play a significant role in the sensor panorama. Optical parameters such as light intensity, phase, polarization state, or light frequency will change when external vibration is applied on the sensing fiber. In this paper, various technologies of distributed fiber-optic vibration sensing are reviewed, from interferometric sensing technology, such as Sagnac, Mach–Zehnder, and Michelson, to backscattering-based sensing technology, such as phase-sensitive optical time domain reflectometer, polarization-optical time domain reflectometer, optical frequency domain reflectometer, as well as some combinations of interferometric and backscattering-based techniques. Their operation principles are presented and recent research efforts are also included. Finally, the applications of distributed fiber-optic vibration sensors are summarized, which mainly include structural health monitoring and perimeter security, etc. Overall, distributed fiber-optic vibration sensors possess the advantages of large-scale monitoring, good concealment, excellent flexibility, and immunity to electromagnetic interference, and thus show considerable potential for a variety of practical applications.

In the past two decades Brillouin-based sensors have emerged as a newly-developed optical fiber sensing technology for distributed temperature and strain measurements. Among these, the Brillouin optical time domain reflectometer (BOTDR) has attracted more and more research attention, because of its exclusive advantages, including single-end access, simple system architecture, easy implementation and widespread field applications. It is realized mainly by injecting optical pulses into the fiber and detecting the Brillouin frequency shift (BFS), which is linearly related to the change of ambient temperature and axial strain of the sensing fiber. In this paper, the authors provide a review of new progress on performance improvement and applications of BOTDR in the last decade. Firstly, the recent advances in improving the performance of BOTDRs are summarized, such as spatial resolution, signal-to-noise ratio and measurement accuracy, measurement speed, cross sensitivity and other properties. Moreover, novel-type optical fibers bring new characteristics to optic fiber sensors, hence we introduce the different Brillouin sensing features of special fibers, mainly covering the plastic optical fiber, photonic crystal fiber, few-mode fiber and other special fibers. Additionally, we present a brief overview of BOTDR application scenarios in many industrial fields and intelligent perception, including structural health monitoring of large-range infrastructure, geological disaster prewarning and other applications. To conclude, we discuss several challenges and prospects in the future development of BOTDRs.

In recent years, pattern recognition technologies for distributed optical fiber vibration sensing have attracted more and more attention, aiming to intelligently recognize vibration events along with the optical fiber. Firstly, distributed optical fiber sensors for vibration detection are introduced. Secondly, this paper presents the state of the art of pattern recognition models used in distributed optical fiber vibration sensing systems. The feature extraction method, the model structure, and the processing performance are reported. As the results of the comparison, the support vector machine is a small sample learning method with a solid theoretical foundation and it has excellent generalization ability. The artificial neural network is suitable for massive data learning and multi-classification problems. Also, deep learning can learn more features information by a deep nonlinear network structure in an automated way, and thus has better accuracy and robustness. Furthermore, different applications of pattern recognition for distributed optical fiber vibration sensing are provided, including perimeter security, pipeline monitoring, and railway safety monitoring. Finally, the prospects of pattern recognition for distributed optical fiber vibration sensing are discussed.

Optical frequency domain reflectometry (OFDR) is a kind of distributed optical fiber sensors (DOFS) which has attracted an increasing amount of research attention. The OFDR sensor has a high spatial resolution and large dynamic range, which makes it useful in a large amount of sensing scenarios. In this paper, we will provide a review of the recent advances in OFDR. We introduce the sensing principles and progress for performance improvement of the system. We also cover the applications of OFDR-based sensing system, including sensing of temperature, strain, vibration, pressure, magnetic field, refractive index, radiation, 3D-shape, gas, gas flow rate, and so on.

In recent years, phase demodulation methods for optical fiber vibration sensors (OFVS) have attracted more and more attention, aiming to accurately detect vibration signals. The principles of phase modulation and demodulation in OFVS are firstly introduced. Then, three classical phase demodulation methods are described, including phase generated carrier (PGC) demodulation method, ${3}\,\,\times \,\,{3}$ coupler demodulation method, and in-phase and quadrature (IQ) demodulation method. Furthermore, the state of art of phase demodulation methods in various OFVS are reviewed. As the results of analysis, different structures of OFVS need appropriate demodulation methods for vibration detection. The Fourier transform method, fringe counting method and PGC demodulation method have strong applicability for Fabry-Perot interference OFVS. The scalability of interferometers provides convenience for PGC demodulation method and ${3}\,\,\times \,\,{3}$ coupler demodulation method in the OFVS with composite structure. PGC demodulation method, ${3}\,\,\times \,\,{3}$ coupler demodulation method and quadrature demodulation method are good choices for dynamic sensing in FBG-OFVS. The IQ demodulation method is a popular choice for phase sensitive optical time domain reflectometry at present. This paper provides a reference for the OFVS to realize vibration detection with appropriate phase demodulation method.

Optical fiber sensors are polarization sensitive and generally affected by polarization fading. This paper contributes to the optimal choice of polarization fading suppression methods for different optical fiber sensors, by means of comparative analysis of the mechanism of polarization fading, the strategy of polarization state control, and the applicability of suppression methods. The comparison results show that the polarization diversity receiving method is suitable for remote multi-channels optical fiber sensing system. The polarization orthogonal pulse pair method is only suitable for optical fiber sensing systems based on the backward scattering light. Meanwhile, the Faraday rotating mirror method is only suitable for optical fiber sensing systems based on the forward light interference. The polarization pulse coding method is suitable for sensing systems with high sensitivity and stability requirements. The polarization control method may increase the control complexity of sensing system for simultaneous variation of multiple polarization states. The full and partial polarization-maintaining fiber methods can be used to keep the polarization state of sensing light along the optical fiber, but the additional system cost limits the application. The polarization scrambler method is currently only available for sensing systems based on Brillouin scattering light. This paper provides a reference for the feasible selection of polarization fading suppression schemes for different optical fiber sensing systems.