Research On Distributed Optical Fiber Sensing System Based On Stable-State Brillouin Fiber Ring

The distributed optical fiber sensing system based on Brillouin scattering follows the linear relationship between Brillouin frequency shift and environmental variables such as temperature and strain to realize the sensing of the state of the target area.Typical technical solutions include Brillouin Optical Time Domain Analysis(BOTDA)and Brillouin Optical Time Domain Reflection(BOTDR).Brillouin scattering based distributed optical fiber sensing technology has a wide range of application prospects in the fields of bridges,tunnels,dams,earthquake prediction,temperature monitoring and so on.It is an important research branch in the current sensor field,and has a close relationship with industrial production related to structural health and real-time state monitoring.In the traditional distributed optical fiber sensing system based on Brillouin scattering,the Stokes optical power generated is very low,which makes the signal-to-noise ratio of the sensing signal low,and makes it difficult to receive the signal.Most of the light source energy is contained in the transmitted light of the sensing fiber.This part of the energy in the transmitted light is not used by the distributed optical fiber sensor system,resulting in serious energy waste.In addition,in the scheme of BOTDA,the input light source should be set at both ends of the sensing fiber,which leads to high system complexity.Aiming at the problems of low sensing signal power,low signal-to-noise ratio and complex system structure existing in the traditional Brillouin scattering based distributed optical fiber sensing system,this paper has done the following research work:1.In this paper,the formation principle of Brillouin scattering and the research results of distributed optical fiber sensing technology based on Brillouin scattering in recent years are investigated and summarized.The principle of Brillouin scattering and the working principle of distributed optical fiber sensing system based on Brillouin scattering are briefly introduced.For the investigation and summary of Brillouin scattering,the types and phenomena of Brillouin scattering are described firstly.And then the formation process of spontaneous Brillouin scattering and stimulated Brillouin scattering is described from the perspective of quantum mechanics theory and wave optics theory.The mathematical model of Brillouin scattering based on wave optics is introduced.And the threshold phenomenon of stimulated Brillouin scattering in optical fiber is analyzed.At the system level,this paper introduces several representative schemes of using Brillouin scattering effect to realize environmental temperature and strain sensing in recent years,and also analyzes and compares the technical characteristics of different schemes.2.In this paper,based on the common single-mode fiber as the sensing medium,a stable-state Brillouin fiber ring structure is proposed.According to the structure,a mathematical model describing the relationship between the output Stokes light power and the incident light power is established.Experiments are designed to prove the high output signal power characteristics of the stable-state Brillouin fiber ring structure through the experimental data.And the stable-state layout is compared.The scattering spectrum of fiber ring structure and common single-mode fiber under the same input power is analyzed,and the influence of stable-state Brillouin fiber ring structure on stimulated Brillouin scattering threshold is analyzed.Compared with the open-loop configuration,the threshold value of the designed stable-state Brillouin fiber ring structure shifts downward,and the Stokes optical power increases.On bundle of 10 km single-mode fiber,the threshold value of the designed stable-state Brillouin fiber ring is reduced by 5.86 dB,and the Stokes optical power is increased by 42.11 dB.The stable-state Brillouin fiber ring structure proposed in this paper provides a feasible idea for making full use of the energy of the pump light under a certain input optical power,improving the conversion ratio of the input and output optical energy,and reducing the energy loss of the system.3.According to the characteristics of high output power of stable-state Brillouin fiber ring,this paper studies the application mode of stable-state Brillouin fiber ring in distributed sensing system,mainly aiming at BOTDR and BOTDA system,puts forward the method of reducing system complexity and improving sensing signal power by using the structure of stable-state Brillouin fiber ring.On the one hand,this paper proposes a standalone mode of stable-state Brillouin fiber ring,that is,in the BOTDR system,the structure of stable-state Brillouin fiber ring is used to replace the sensing fiber to sense the temperature and strain of the target area,so as to make full use of the energy of the input light and convert it into the energy of the output Stokes light.On the other hand,the mirror mode of the stable-state Brillouin fiber ring is proposed in this paper,that is,in the BOTDA system,the stable-state Brillouin fiber ring is connected to the end of the sensing fiber to replace the probe light generation module in the BOTDA,so as to achieve the goal of reducing the complexity of the system.