| Phase-sensitive optical time-domain reflectometry(?-OTDR)as a distributed optical fiber sensing system that can measure weak vibration signals in real time,has extremely broad application prospects in the fields of structural health monitoring,geological hazard monitoring and intrusion detection.However,while ?-OTDR uses an ultra-narrow linewidth laser as the system light source to improve the sensitivity of the system,it also inevitably introduces a new problem called fading noise.Since the ultra-narrow linewidth laser is used,the coherence length of the light source is much longer than the measurement length of the system.Therefore,the Rayleigh backscattered(RBS)light generated by multiple scattering positions within the pulse width of the inject light in the optical fiber will interfere,thus the RBS trace is jagged.This phenomenon of signal amplitude fading caused by interference,even submerged in noise,is called interference fading.In addition,in the coherent detection ?-OTDR,polarization fading will also be introduced by the inconsistency of the polarization states of the optical local oscillator and the signal light.Both interference fading and polarization fading will cause the signal-to-noise ratio(SNR)of the demodulation phase of the signal to decrease,and demodulation phase distortion will occur.This is very unfavorable for the promotion of the ?-OTDR system in engineering applications.Therefore,it is a very important research work to suppress the fading noise of the ?-OTDR system.At present,in order to eliminate the influence of fading noise in ?-OTDR,the solutions proposed by researchers mainly use multi-frequency technology and polarization diversity technology alone.This method usually requires changing the system hardware structure and the structure is more complicated.In addition,some scholars have proposed to suppress fading noise through the improvement of data processing algorithms,such as moving average method and wavelet denoising method,but these methods have not essentially achieved fading noise suppression.In this article,in order to be able to directly improve the fading suppression capability of the existing commercial ?-OTDR system without changing the structure of the system,and enhance the performance of the system.We propose a ?-OTDR structure based on space division multiplexing(SDM),which realizes SDM by using multi-core fiber(MCF).And for the obtained multiple sets of independent measurement signals,this paper proposed a new optimization algorithm(MSSS)based on the highest spectral similarity besides inheriting the existing optimization algorithm based on the maximum amplitude(MAS),and compared the effects of the two algorithms.By replacing the sensing fiber in the system and optimizing the data processing algorithm,the purpose of suppressing the fading noise of the system can be improved without changing the hardware structure of the system.In the confirmatory experiment,this method reduces the distortion rate of the demodulation result from an average of 9.4%to less than 2% under the condition of continuous operation.In addition,the fading noise suppression methods in the current research are mostly only for suppressing coherent fading or polarization fading.In this article,in order to obtain a better fading noise suppression effect,we proposed a ?-OTDR system that combines frequency division multiplexing(FDM)and polarization diversity reception(PDR)technologies.In the confirmatory experiment,we realized FDM by using two acousto-optic modulators(AOM)with different frequency shifts,and realized PDR by using polarization diversity double-balanced photodetector(PBPD).In the verification experiment,this method reduces the average distortion rate of the demodulated phase of the signal of the system in a short time from about 41.19% to1.89%.And compared with the reconstructed signal obtained by using only FDM and only using PDR method,the distortion rate is reduced by about 16% and 24%.Using this method,the ?-OTDR system can obtain a better fading noise suppression effect in a short time measurement.The two fading noise suppression structures proposed in this article are based on the basic principle of using multiplexing to obtain multiple sets of independent measurement signals to avoid fading noise.In this paper,through multiplexing from the three angles of space domain,frequency domain,and polarization state,combined with the proposed optimization algorithm,the improvement of the system's fading noise suppression capability is achieved.Among them,the ?-OTDR based on the use of MCF to achieve SDM realizes the fading noise suppression under long-term monitoring without changing the system structure,and the ?-OTDR based on the fusion of FDM and PDR technology achieves better fading noise suppression effect in short-term monitoring than when FDM or PDR technology is used alone.The two methods proposed in this article have good fading noise suppression capabilities,and it can provide reliable anti-noise means for the engineering application and promotion of ?-OTDR under field conditions. |
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