Research On Distributed Optical Fiber Temperature Sensing System Based On A Home-made Laser Source

With the advancement of technology,long-distance and large-scale temperature measurement plays an important role in military industry,transportation,agriculture and other research fields.Optic fiber temperature sensing systems have attracted much attention due to their advantages of anti-electromagnetic interference,fire and explosion protection,small size and light weight.At present,optic fiber temperature sensing systems mainly include: point fiber temperature sensing system,quasi-distributed fiber temperature sensing system(Q-DTS)and distributed temperature sensing system(DTS).DTS technology was proposed in 1970 s.Since the distributed temperature sensing technology was proposed,it has been widely used in fire detection,oil well temperature detection,power cable and other fields due to its features such as simple structure,high spatial resolution and suitable for long-distance sensing compared with Q-DTS.DTS system measures the temperature information carried by the fiber based on the temperature sensitive effect of backward Raman scattering generated in the fiber,and then determines the temperature situation at each point in the entire fiber through optical time-domain reflection(OTDR)technology,so as to conduct the temperature information along the fiber in the form of continuous function.Since DTS systems are expensive on the market,this thesis proposes a method to reduce the cost of DTS systems by using a home-made laser source.The purpose of the thesis is to develop a low-cost,long-distance sensing,high-precision DTS system.The specific research contents of this thesis include:1.We describe the Raman scattering theory and the OTDR technique in detail.Based on the temperature sensitive effect of Raman scattering,we discussed three typical demodulation methods of the DTS system.After compared the advantages and disadvantages of these three methods,we determined a two-way demodulate temperature method by using Stokes light as the reference light and Anti-Stokes light as the signal light.2.In order to set up a low-cost temperature measurement system,we chose to build a pulsed fiber laser with a pulse width of 12 ns,peak power of 60.7 W and repeatable frequency of 100 Hz to 100 KHz which used in the DTS.The method is: we used current pulse modulation laser diode to generate pulsed laser,and used a master oscillator power-amplifier(MOPA)to amplify the weak seed source signal power.The home-made pulsed fiber laser is then used as a light source in a DTS system.3.The devices needed in the system are analyzed and selected.A DTS is built after a home-made pulsed fiber laser is selected as the light source.In this experiment,the signal-to-noise ratio of the system is improved by means of cumulative averaging.In the process of signal demodulation,the fiber with a front-end length of 150 m is taken as the calibration fiber,and we demodulated the system by the demodulation method of Stokes light and Anti-Stokes light to measure the temperature.4.The important criteria of a DTS are sensing distance,spatial resolution,temperature resolution and temperature precision.After testing and analysis,the sensing distance of the system can reach to 10 Km,the spatial resolution is 1.2m,the temperature resolution is 1?,and the temperature precision up to ?0.8?.Finally,the stability of the system is tested for 180 minutes in the laboratory,and the stability of the system meets the application requirements.