Research On Performance Improvement Method Of φ-OTDR System

Distributed optical fiber sensing technology is a sensing technology that uses optical fibers as sensing units and sensitive media.Distributed optical fiber sensing has the characteristics of strong corrosion resistance,flexible and variable structure,high sensitivity,and distributed measurement.It can monitor physical quantities such as temperature,stress,frequency response,and humidity.Distributed optical fiber sensing technology has been widely used in perimeter intrusion monitoring of key places such as airports and nuclear power plants,railway operation status monitoring,infrastructure structural health monitoring,long-distance pipeline monitoring,transmission line monitoring,etc.,and has important application value.It provides a new solution for the monitoring field and has become a rising star.φ-OTDR is an important branch of distributed optical fiber sensing technology,which has attracted more and more attention due to its characteristics of long monitoring distance,large frequency response range and high sensitivity.It has become a hot research direction at present.φ-OTDR systems are mainly divided into two categories:Distributed Vibration Sensing(DVS)and Distributed Acoustic Sensing(DAS).At present,on the one hand,the φ-OTDR system is limited by the Nyquist sampling theorem,and its maximum detection frequency is only half of the repetition frequency.The repetition frequency is related to the sensing distance.With the increase of the sensing distance,the system detection frequency gradually decreases,which limits the application of φ-OTDR system in long distance.On the other hand,since theφ-OTDR system uses a narrow linewidth light source with high coherence,the backward Rayleigh scattered light will interfere within the pulse,which will cause the backward Rayleigh scattered light to be backward Rayleigh during the interference stacking process.Therefore,the intensity of backscattering Rayleigh light is close to zero in the process of interference superposition,resulting in coherent fading,which makes φ-OTDR system unable to demodulate signal phase information correctly in the monitoring process,resulting in false alarm.On the basis of predecessors,this thesis studies these two problems,and proposes two φ-OTDR system structures based on time division multiplexing technology and frequency division multiplexing technology to improve the applicability of the φ-OTDR system.The main work of this thesis is as follows:(1)This thesis introduces the research background and significance of distributed optical fiber sensing,and summarizes the main classification of distributed optical fiber sensing.Summarize the current research progress of φ-OTDR system.The principle and main performance parameters of the cp-OTDR system are comprehensively introduced,which lays the foundation of this thesis.(2)In this thesis,a long-distance φ-OTDR system with flexible frequency response based on time division multiplexing is proposed and experimentally verified.By reconfiguring the system layout in a time-division multiplexed manner,the back Rayleigh scattered signal is processed in a segmented manner,adding only additional erbium-doped fiber amplifiers,rather than any other complex signal amplification or pulsing,long-term Vibration sensing modulation mechanism for distance,high frequency response.Through time-division multiplexing reconstruction,the trade-off between sensing distance and vibration frequency response in the φ-OTDR system is greatly alleviated.Compared to conventional system layouts,this system allows flexible frequency response of each sensing fiber segment without any crosstalk.In experiments,the system achieved a frequency response of up to 4.5kHz over a sensing distance of 60km.In addition,the frequency response flexibility of the time-division multiplexing system is further verified by successfully identifying vibration events with frequencies up to 20 kHz at the end of the 52 km long fiber.(3)This thesis proposes a φ-OTDR system based on frequency division multiplexing,and actually builds a prototype of the system.The I/Q quadrature phase demodulation method of φ-OTDR system is designed,and the influence of the back Rayleigh scattering light intensity on the phase demodulation is analyzed.The system uses an acousto-optic modulator to generate two detection frequencies(130MHz and 150MHz),and the system has different back Rayleigh scattering intensity curves at different frequencies,and the phase demodulation is designed according to this characteristic.By setting the back Rayleigh scattering amplitude threshold,the back Rayleigh scattering intensity curve can be flexibly selected for signal demodulation.The experimental results show that the system can effectively reduce the probability of coherent fading,and the system does not change the structure of the traditional φ-OTDR system,the cost is low,the measurement bandwidth is not affected,and it has certain application potential.φ-OTDR system based on time division multiplexing and φ-OTDR system based on frequency division multiplexing proposed in this thesis have simple structure and flexibility,which is beneficial to the practical application of φ-OTDR system,and lays an important foundation for the future application of long distance and high frequency distributed optical fiber sensor system.