According to the direction of scattered light propagation, currently common distributed fiber optic sensing technologies are divided into two categories: backscattering distributed fiber optic sensing technology and interference distributed fiber optic sensing technology.
Backscattering distributed optical fiber sensing technology
The backscattered light in the optical fiber mainly includes three types: Rayleigh scattering, Brillouin scattering and Raman scattering. According to the different signal measurement methods, DOFS technology is divided into two categories: optical frequency domain technology and optical time domain technology. Optical frequency domain technology generally has high spatial resolution, but the measurement process is complicated and the sensing distance is limited; while optical time domain technology is simple to implement and has the characteristics of long distance and high precision. DOFS based on RBS mainly includes: optical time domain reflectometer (OTDR), coherent optical time domain reflectometer (C-OTDR), phase sensitive optical time domain reflectometer (Φ-OTDR) and optical frequency domain reflectometer (OFDR) based on Rayleigh scattering; Raman optical time domain reflectometer (ROTDR) based on Raman scattering; Brillouin optical time domain reflectometer (BOTDR) based on Brillouin scattering and Brillouin optical time domain analyzer (BOTDA) based on stimulated Brillouin scattering.
Optical Time Domain Reflectometer (OTDR) In 1976, the light pulse time-of-flight method was introduced into optical fiber, which can be regarded as the predecessor of OTDR technology; in 1977, the concept of OTDR was formally proposed. Once OTDR technology was launched, it quickly became the standard means for diagnosing and locating optical fiber link faults. Today, OTDR technology is very mature. The main research institutions include University of Electronic Science and Technology of China, Tianjin University, Taiyuan University of Technology, Chinese Academy of Sciences, Nanjing University, etc. The main production companies include Canada's EXFO, Japan's ANRITSU, the United States' VIAVI and China's Jilong Company, the 34th Institute of China Electronics Technology Group Corporation, etc.
Compared with OTDR, which uses a wide-spectrum light source and direct detection, C-OTDR uses a narrow-linewidth laser with high coherence and a heterodyne coherent detection optical path structure, which can effectively improve the system's noise resistance. Its principle is shown in Figure 1. In 1982, the OTDR system began to use coherent detection technology and achieved a detection distance of 30 km, but the concept of C-OTDR was not formally proposed until 1984. Since the 1990s, C-OTDR has gradually developed into a mainstream device for long-distance optical communication systems, especially online monitoring systems for transoceanic optical communication systems.