Brillouin Optical Correlation Domain Analysis With 2 Mm Resolution Based On Intensity Modulation

In recent years,with the rapid development of civil engineering construction and infrastructure construction in China,the demand for structural health monitoring of railways,bridges,tunnels,buildings,etc.has been increasing.The fiber optic sensor has been favored by people because of its small size,light weight,resistance to harsh environmental erosion,and resistance to electromagnetic interference.Among them,the spatial resolution of Brillouin Optical Correlation Domain Analysis(BOCDA)technology can be protected from the phonon lifetime,so it has a high spatial resolution,and the spatial resolution of phase modulation BOCDA technology can reach the order of centimeters.However,the phase modulation BOCDA technology is limited by the bandwidth and amplification capability of the microwave amplifier,and its spatial resolution is difficult to further improve.To solve this problem,this paper proposes a BOCDA technology based on intensity modulation phase coding,which can reduce the spatial resolution of microwave amplifiers,thus achieving higher spatial resolution sensing.The main research contents of this paper are as follows:(1)Based on the coupled wave equation,the theoretical simulation model of phaseencoded BOCDA is established,and the discretization method of high computational efficiency of moving coordinate method is proposed.It is compared with the time required for the common time implicit format and spatial backward difference method simulation.Then the theoretical simulation model of BOCDA is discretized by moving coordinate method.(2)The effects of the bandwidth and modulation depth of the modulated signal on the suppression of the sound field at the incoherent peak are analyzed by the phase modulation BOCDA system and the intensity modulated BOCDA system.The simulation results show that the intensity-modulated BOCDA system can better suppress the establishment of the acoustic field at the incoherent peak,thus suppressing the SBS effect at the incoherent peak and making the detected optical signal has higher signal to noise ratio.(3)The influence of the bandwidth and amplification capability of the microwave amplifier on the phase modulation BOCDA system and the intensity modulated BOCDA system is analyzed.The experimental results show that the signal-to-noise ratio of the phasemodulated BOCDA system is very low when the bandwidth of the microwave amplifier is insufficient or the amplification capability is insufficient.It is even difficult to extract the effective signal and the purpose of sensing is not achieved.The intensity-modulated BOCDA system has low sensitivity to the bandwidth and amplification capability of the microwave amplifier.Even when the bandwidth and amplification capability of the microwave amplifier are low,the signal-to-noise ratio is high,and an effective signal can be extracted.Finally,a high spatial resolution BOCDA sensing experiment based on intensity modulation was performed with a spatial resolution of about 2 mm.