Significance Internet of Things(IoT)technology is an important part of the new generation of information technology.The IoT is a huge network formed by combining various information devices and sensors.This huge network is based on various types of sensors,and the sensor,as a bridge between the physical world and the digital world,is an integral part of the IoT architecture.Distributed fiber-optic sensing(DFOS)technology is widely used in many fields because of its long-distance,large-range,high-precision,and multi-point measurement capabilities. However,most of the DFOS sensing systems use a single scattering mechanism.The parameters measured by a single scattering mechanism are limited,so it fails to fully and accurately reflect the real state of the measured object,and it is difficult to fully and effectively meet the needs of health monitoring or automatic control.In order to solve the problem that the conventional DFOS system only uses a single scattering mechanism,researchers have proposed a series of DFOS sensing systems with multi-mechanism in recent years,using the same system to measure multiple parameters.In this paper,the multi-mechanism DFOS technology developed in recent years is reviewed,and the different multi-mechanism DFOS systems and their performances are classified from the perspective of different scattering mechanisms(Table 1). Progress There are three kinds of scattering in optical fibers,namely Rayleigh scattering,Brillouin scattering,and Raman scattering.DFOS systems that use only one of these scattering mechanisms measure limited parameters.The multi-mechanism DFOS systems can measure more parameters by using multiple scattering mechanisms in one system,so as to reflect the state of the measured object more comprehensively. The multi-mechanism DFOS systems are divided into five categories according to the sensing mechanism used.The system combining Rayleigh scattering with Brillouin scattering can be used not only to measure temperature,strain,and vibration but also to separate the response of temperature and strain.Systems that combine Rayleigh scattering with Raman scattering can be used for sensing temperature and vibration events.Systems that combine Brillouin scattering with Raman scattering are generally used to separate system responses due to temperature and strain. The methods of combining scattering mechanisms in the systems are different.In this paper,these combination methods are divided into two categories:the combination based on multiplexing(wavelength division multiplexing,space division multiplexing,and time division multiplexing)and the combination of different scattered light generated by the same probe light.Multiplexing-based combination methods are straightforward in principle,but complex systems often require special sensing fibers or sacrifice measurement speed.The system using different scattered light of the same probe light has a simple setup,but special modulation and demodulation schemes are required.In addition,there may be an influence between different scattering mechanisms when the different scattering of the same probe light is used in the same system. In addition to the combination of different kinds of scattered light in optical fibers,we also enumerate distributed sensing systems using scattered light and single point optical fiber sensing systems using interference structures or gratings.Compared with distributed sensing systems,single-point optical fiber sensing systems have the advantages of high precision and large measurement range,but the number of measured points is limited,and special optical fiber structures(such as fiber grating)are required.In practical applications,distributed fiber optic sensors or single-point fiber optic sensors can be flexibly selected according to different scenario requirements. Conclusions and Prospects Finally,the prospect of multi-mechanism DFOS technology is provided.With the increasing demand for large-scale sensing and monitoring,researchers have proposed more and more multi-mechanism DFOS systems to measure more parameters and improve sensing performance.The future development of multi-mechanism DFOS systems should focus on the aspects of system complexity,sensing performance,data processing,and practical applications. One of the natural advantages of multi-mechanism DFOS systems is the multiplexing of components in a sensor system with different mechanisms,which can significantly reduce the cost of the system when multiple sensing functions are implemented.The data processing method expands the application scenarios,increases the functions of the system,and improves the performance of the system without increasing the complexity and cost of the system hardware.Finally,how to deeply integrate the multi-mechanism DFOS systems with practical applications is an important direction.In order to achieve this direction,the design of the sensor system,the layout of the sensor cable,and the use of multi-sensor information should be considered and designed.
