Research On High-performance Microwave Photon Modulation And Demodulation Technology In BOTDR

The Brillouin optical time domain reflectometer(BOTDR)based on self-published Brillouin scattering can simultaneously measure the temperature and stress along the fiber,and the measurement distance can reach more than 100 kilometers.It is used in bridge construction,oil and gas pipelines,high-voltage transmission lines,etc.There are significant advantages in the field of large-scale infrastructure health monitoring.In the BOTDR sensing system,the back Brillouin scattered light signal is very weak,which puts high requirements on the signal modulation and demodulation technology.In order to improve the performance of the BOTDR sensing system,in terms of signal modulation,an external modulator is usually used to modulate the continuous light emitted by the laser into high extinction ratio detection pulse light to improve the signal-to-noise ratio of the backscattered signal.In terms of signal demodulation,in order to improve the intensity of the back Brillouin scattered signal,the self-heterodyne coherent detection method is usually used.However,the local reference light is directly coherent with the back Brillouin scattered light,and the center frequency of the signal is up to about 11 GHz,which is not conducive to signal collection and processing.A feasible method is to use the external modulator to change the reference optical frequency and reduce the bandwidth of the coherent signal to reduce the difficulty of data acquisition and processing.Whether it is detection pulse light synthesis or backscattered light demodulation,electro-optic modulator(EOM)is usually selected as the external modulator,and the modulation and demodulation function is realized by the interaction of the microwave signal and the optical signal in the X-band.However,due to its dual-arm interference structure,the initial phase difference is sensitive to ambient temperature and strain.Operating point drift occurs during long-term operation,resulting in a decrease in the extinction ratio of the detected pulse light and a reduction in the frequency shift efficiency of the reference light,and the EOM itself has limited extinction.Seriously affect the measurement accuracy and system stability of the BOTDR sensor system.In view of the above problems,this paper has conducted an in-depth study on the high extinction ratio pulse modulation method and the reference optical frequency shift method in the BOTDR system.On the one hand,a working point locking technology for high stability electro-optic modulator based on lock-in amplifier and synchronous optical switch using a synchronous optical switch to replace the coupler in the existing method eliminates the effect of pulsed light on the locking accuracy,while further improving the extinction ratio of the pulsed light.On the other hand,a phase lock technology based on AC light detection circuit is proposed,which improves the light detection circuit in the traditional EOM operating point locking method based on lock-in amplifier.The AC light detection circuit is used to detect only the disturbance signal used for operating point locking to improve the working point locking accuracy.Based on the above technology,this paper developed a high extinction ratio detection pulse light modulation module and a high efficiency reference optical frequency modulation module,tested the performance of the modulation and demodulation module and compared with the traditional method.After using the high extinction ratio detection pulse light modulation module developed in this paper,the extinction ratio of the detection pulse light can reach 52.7 d B,and the fluctuation within 4 hours is less than ± 1 d B.After using the high-efficiency reference optical frequency modulation module developed in this paper,the reference optical carrier suppression ratio can reach 32 d B,and the fluctuation within 4 hours is less than ± 0.75 d B.Use the modulation and demodulation module developed in this paper to upgrade the existing BOTDR.Therefore,at the end of the 46.81 km sensing fiber,the maximum error of temperature measurement of the BOTDR system at a spatial resolution of 20 m is reduced from 2.9 ? to 0.9 ?.Experiments prove that the high extinction ratio detection pulse light modulation module and high efficiency reference light frequency modulation module developed in this paper are beneficial to the performance improvement of BOTDR system.