基于准正交啁啾脉冲调制的 φ-OTDR 干涉衰落抑制研究

In recent years，safety accidents have occurred frequently in our country，including the collapse of some large facilities，the leakage of oil and gas pipeline，the damage of railway track and so on. These occurrences underscore the imperative for enhanced safety monitoring methodologies to mitigate risks associated with major industrial accidents. Particularly for extended infrastructure networks and cross-regional pipeline systems characterized by vast spatial coverage，distributed monitoring solutions are necessary. Distributed optical fiber vibration sensing system has the advantages of simple structure，long detection distance，and electromagnetic interference resistance. Consequently，it has been widely applied in engineering fields such as earthquake monitoring，intrusion detection，pipeline monitoring. As a key technology in distributed optical fiber vibration sensing system，phase-sensitive Optical Time Domain Reflectometry（φ -OTDR）has the advantages of high sensitivity and fast response speed. The predominant phase demodulation technique employed in φ -OTDR systems capitalizes on the linear proportionality between phase variations in Rayleigh backscattering light and external perturbation intensity，enabling quantitative reconstruction of disturbance waveform. Because φ -OTDR uses highly coherent detection light，the Rayleigh backscattering light at different locations will interfere with each other. The photocurrent signal formed by the photodetector will produce strong fluctuation，resulting in an extremely low signal-to-noise ratio at some interference fading points. Therefore，it may produce false phase，which makes the system give false alarms and affects the accurate judgment of the system. As a method to suppress interference fading，the rotated-vector-sum method needs to be provided with vector components of different strength，and the vector signals are rotated in the same direction to sum，so as to maximize the strength of arbitrary vector addition. In past studies，academics have taken different ways to generate or utilize multi-frequency signals. In this paper，a new combination scheme of multi-frequency signals and rotated vector is proposed. It can use quasi-orthogonal chirped pulse and matching filter to provide vector components with different intensity. The components are rotated to the same direction and then added to improve the intensity. We also study the filtering effect of the matched filter，which can be expressed by cross-correlation value. The cross-correlation value of signals with opposite chirp polarity in non-matched filtering are discussed from three aspects：the relationship between band overlap rate and cross-correlation maximum，the relationship between signal bandwidth and cross-correlation maximum，and the overall cross-correlation value. The cross-correlation value of the quasi-orthogonal chirped pulse formed by the selected parameters is lower than that of the orthogonal code and special optical pulse code. It verifies the feasibility of using quasi-orthogonal chirped signal for distributed fiber sensing. Furthermore，we compared and analyzed the intensity fluctuation，interference fading probability and the phase curve. This scheme can reduce the intensity fluctuation of Rayleigh scattering curve by 20 dB，and the interference fading probability is reduced to 0.25%，which greatly reduces the false phase values and successfully inhibits interference fading. At the same time，the signal to noise ratio of demodulated vibration signal can reach 36 dB. The distributed fiber optic vibration sensing scheme can reduce the influence of interference fading，improve the precision of system positioning and the accuracy of disturbed signal recovery，and has high application value.