1. Background Introduction

　　Distributed Temperature Sensing system (DTS) uses light as the carrier of temperature information and optical fiber as the medium for transmitting temperature information to obtain the temperature distribution of each point on the optical fiber; in addition, it uses optical time domain reflectometry (Optical Time Domain Reflectance Measurement Technology). -Domain Reflectometry, OTDR) to obtain spatial temperature distribution information.

　　In a distributed system, the optical fiber is both the sensing medium and the transmission medium to be measured, so the sensing part has a simple structure; compared with point sensors, the information acquisition cost per unit length is greatly reduced, and the cost performance is high; and its outstanding advantages are Continuous sensing can be carried out in a large space. Combined with the inherent characteristics of optical fiber, such as high temperature and high pressure resistance, corrosion resistance, lightning resistance, resistance to complex and harsh conditions, resistance to radio frequency and electromagnetic interference, small size, light weight, etc., the distributed optical fiber temperature measurement system is very applicable and widely used .

Typical applications of distributed optical fiber temperature measurement system include：

1. Temperature detection of oil field natural gas shaft
   

2. High-voltage cable temperature monitoring

3. Temperature monitoring of large transformers

4. Leakage monitoring of various pipelines (natural gas, oil, etc.)
   

5. Building fire monitoring

6. Temperature monitoring of key equipment and production process in large factories

7. Tunnel fire monitoring

8. Dam leakage and temperature monitoring
   

2. Working Principle

　　The distributed optical fiber temperature sensor is a kind of scattering optical fiber sensor. The backscattered light received by the light pulse includes Rayleigh scattering, Raman scattering and Brillouin scattered light. Therefore, the scattering optical fiber sensor includes OTDR using Rayleigh scattering. B-OTDR using Brillouin scattering, B-OTDA (Brillouin Optical Time-Domain Analysis, B-OTDA) and R-OTDR using Raman scattering.

　　The usual DTS temperature measurement principle and positioning principle is R-OTDR, which uses Raman scattering. Its working principle is that the light pulse will undergo Raman scattering in the optical fiber, and the back scattered light propagates back to the starting end of the optical fiber. Each backscattered light corresponds to a scattering point on the optical fiber, and the corresponding position and temperature information can be obtained by collecting and analyzing the time and intensity information of the backscattered light.

　　The distributed optical fiber temperature measurement system is mainly composed of three parts: temperature measurement host, temperature sensing fiber and temperature demodulation software, as shown in the figure below, where the temperature measurement host is the core equipment of the system. The temperature measurement host mainly includes a laser, 1*3 wavelength division multiplexer, avalanche photodiode module, amplifying module, data acquisition module and constant temperature device. Its working principle is that the pulsed semiconductor laser emits laser pulses, and the back Raman scattering occurs in the optical fiber to obtain Stocks and Anti-Stocks Raman scattered light, which are respectively converted into voltage signals by avalanche photodiodes, and then amplified by high-speed AD converters. The intensity information is obtained by sampling, and the temperature information of the entire section of optical fiber is demodulated by the temperature demodulation software.

 
Figure 1: Schematic diagram of the distributed optical fiber temperature measurement system.

Instruments and components required by the system:

1. Distributed optical fiber temperature measurement host: (including components)

    Pulsed semiconductor laser

    1*3 wavelength division multiplexer 

    Avalanche photodiode

    Amplify module 

2. Temperature sensing fiber