| Phase Sensitive Optical Time Domain Reflectometric(φ-OTDR)can quantitatively measure the position,amplitude and frequency of external vibrations.It has the advantages of distributed sensing,high detection sensitivity,fast response speed,anti-electromagnetic interference and other detection advantages,and can work for a long time under harsh conditions such as high temperature and high pressure,and is widely used in some fields such as safety monitoring,Marine acoustic detection,oil and gas pipeline and geological structure detection and so on.However,with the continuous expansion of application fields,the distributed optical fiber vibration sensing system based on φ-OTDR technology has some problems,such as poor vibration positioning ability,low phase deployment accuracy and small dynamic range,which is difficult to meet the detection requirements,restricting its practical process.In the process of φ-OTDR system working for a long time,the temperature fluctuation leads to the frequency jitter of the acousto-optic modulator,the instability of the reference signal during orthogonal demodulation and the frequency drift of the laser,which reduces the overall demodulation characteristics of the system.Because of the weak backrayleigh scattering signal,the vibration location efficiency ofφ-OTDR system is reduced in strong noise environment,especially in low frequency vibration.The traditional inverse tangent phase unwrapping algorithm is limited by the phase constraint condition,and the phase is prone to winding and even distortion.In addition,in most applications,the frequency band generated by vibration is wide,and the dynamic range of the system can only be extended at the expense of other performance parameters,which greatly affects the measurement effect of φ-OTDR system.Improving the demodulation characteristics such as vibration identification efficiency and phase unfolding accuracy is an effective solution to extend the dynamic range and frequency response range of φ-OTDR system.Therefore,in this paper,the traditional heterodyne detection structure is optimized from the optical structure of the system,and an optical synchronous heterodyne structure is proposed to effectively solve the problems of frequency jitter and frequency drift of the modulation signal.At the same time,the orthogonal unbalance compensation method based on least squares algorithm is used to correct the orthogonal unbalance introduced by the optical synchronous heterodyne structure.In addition,the noise suppression method for strong background noise is studied to improve the vibration recognition efficiency of the system.A two-dimensional phase unwrapping algorithm based on recursive branch algorithm is developed to improve the phase unwrapping accuracy of the system.On this basis,a prototype of φ-OTDR based on optical synchronous heterodyne detection method is developed,and its feasibility in artificial source data acquisition and effectiveness in pipeline safety monitoring applications are verified.The main research contents of this paper are as follows:(1)Starting from the back-Rayleigh scattering theory in optical fiber,the related physical mechanism is studied,and the sensing principle of φ-OTDR technology is expounded by establishing a discrete mathematical model.On this basis,the optical path structure,demodulation mode,vibration location and phase expansion principle of traditional heterodyne detection method are introduced.Combined with the main performance indexes of φ-OTDR system,the causes and influencing factors affecting the quality of demodulation signal are analyzed,which lays a theoretical foundation for the follow-up research.(2)Aiming at the frequency drift of key components in the φ-OTDR system,clock asynchronism caused by inconsistent initial phase of laser and pulse modulator,and the mutual restriction between efficiency and accuracy of reference signal acquisition method,a φ-OTDR system structure based on optical synchronous heterodyne detection method is proposed.The orthogonal demodulation method of this structure is discussed in detail.Through the optical synchronous heterodyne,the frequency of the beat signal is followed,and the real frequency of the orthogonal signal is obtained in real time,and the frequency shift fluctuation noise is effectively eliminated.The experimental results of vibration positioning and phase unrolling show that the SNR of vibration positioning obtained by this method on a 10km sensing fiber is>31.4dB,the error coefficient between the phase amplitude and the actual signal amplitude is<1%,and the amplitude of the maximum harmonic term is less than the actual signal amplitude about 28.3dB,which improves the real-time performance and greatly improves the demodulation characteristics of the system.(3)Aiming at the orthogonal unbalance problem introduced by optical synchronous heterodyne structure,the main causes and influencing factors are analyzed,and an orthogonal unbalance compensation method based on least squares algorithm is studied and proposed,and the optical synchronous heterodyne method and its orthogonal demodulation mode are further optimized.By calculating the mean value of the envelope,the DC residual term of the orthogonal unbalance is eliminated.Further,the cyclic symmetry of AC orthogonal unbalance is tracked to complete parameter estimation,and then the orthogonal unbalance correction is completed by adaptive filtering method.The experimental results show that the compensation method can suppress the phenomenon of orthogonal unbalance,increase the amplitude of the maximum harmonic term of the demodulation phase by about 20dB,and reduce the waveform distortion by about 3%.(4)Aiming at the problem that φ-OTDR system has insufficient ability to recognize low frequency vibration under strong background conditions,the main causes and effects are analyzed,and according to the analysis results,a noise suppression method based on empirical mode decomposition and time-frequency peak filtering algorithm is studied.Based on the decomposition characteristics of the empirical mode decomposition algorithm,different types of modal functions are selected self-adaptively by using the threshold screening method of sample entropy,and the time-frequency peak filtering of different modes with different window lengths is carried out.Finally,the vibration location signal after noise reduction is generated by superposition.The experimental results show that this method broadens the lower limit of the system response range to 10～5 Hz,and improves the low frequency vibration location efficiency.(5)As the inverse tangent phase unwrapping algorithm is restricted by π phase constraints,system undersampling,orthogonal unbalance and environmental noise,the unwrapping results appear to be twisted or even distorted,a two-dimensional phase unwrapping method based on recursive branch algorithm is studied and proposed to improve the phase unwrapping accuracy.The phase near the vibration position is extended to a two-dimensional enveloping phase diagram along the time direction.According to the abnormal phase distribution,the two dimensional enveloping phase diagram is divided into sliding Windows of different lengths.Under the constraint of global continuous phase,the local phase is optimized by selecting a suitable integral path to minimize the error,and the global propagation of abnormal noise is suppressed.The experimental results show that the system has a good linear strain response ability,and the upper limit of the dynamic range of the system is steadily increased by 3.21dB in the range of 1-80Hz.Finally,based on the four key technologies of the above research,a prototype based on the optical synchronous heterodyne detection method is developed,and the connection and control relationship between the transmitting unit and the receiving unit are introduced,and the performance parameters of the prototype are tested in the laboratory.The experimental results show that compared with the traditional digital coherent demodulation instrument,the average sensitivity of the prototype is 6.97dB re 1rad/με·m,the dynamic range is 121.5dB@100Hz,the frequency response range is 5Hz-2499Hz,and the vibration demodulation performance is significantly improved.Based on the developed prototype,the artificial source test is carried out with the conventional moving-coil detector.The results show a good consistency in synchronous response to the source excitation signal,which preliminarily validates the feasibility of the prototype in seismic data acquisition.In addition,a preliminary experiment of pipeline safety monitoring was carried out by simulating third-party damage events by tapping.The test results show that the response time of a single vibration event is about 3.2072s,and the average frequency is 19.5503Hz,which verifies that the prototype can effectively monitor the location and situation of vibration events.The four key technologies proposed in this paper effectively extend the application range of φ-OTDR system,and have great significance for the subsequent practical process of distributed fiber optic vibration sensing system,and provide technical support for China’s resource reconnaissance and national safety production construction. |
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