Broadband Signal Reconstruction Research Based On Multi-coset Sampling In ?-OTDR System

The distributed optical fiber sensing is a type of sensing technology,in which both the sensor and transmission medium are optical fiber for distributed optical fiber sensing technology.Phase-sensitive optical time-domain reflectometer(?-OTDR)is a research focus of distributed optical fiber sensing.The ?-OTDR technology can be divided into two classes,the distributed vibration sensor(DVS)system and the distributed acoustic sensor(DAS)system.Because it owns many advantages such as the capability of real-time measurement and high-level sensitivity,it is widely used in many military and civilian fields such as natural disaster detection,infrastructure health monitoring,border incursion monitoring.Thus,it owns more and more important research and applicatioh value.The sensing fiber length and frequency response range are important parameters in(p-OTDR.In order avoid waveform overlapping of Rayleigh backscattering,the round-trip time of the probe pulse traveling through the entire fiber under test must be shorter than the time interval of the probe pulse in(p-OTDR system.Simultaneously,the frequency response range is a half of the rate of probe pulses according to Nyquist sampling theorem.Usually the pulse repetition rate is set higher due to the noise.Therefore,there are an intrinsic tradeoff in(p-OTDR system between the sensing fiber length and frequency response range.Thus,it is crucial to break through the limitation of Nyquist sampling theorem under a certain sensing distance.Inspired by the serial MCS method,this paper design a sampling sequence with non-uniform time intervals,which can recover broadband vibration signal information using fewer probe pulses than traditional uniform one in cp-OTDR system,and the continuous to finite algorithm is used to recover the spectrum of vibration signal.Different types of signals are reconstructed both in the simulations and the experiments.The main research contents are as follows:(1)This paper first introduces the principle and application of distributed optical fiber sensing technology.The sensing systems are classified according to their structures,and their application scenarios,advantages and disadvantages are described.Then the history of development of distributed sensing system is investigated.Furthermore,the research background of this paper is explained.Then the principle of amplitude detection and phase demodulation in the ?-OTDR system based on Rayleigh backscattering are introduced.The system structure and principle of three popular phase-demodulation systems are introduced.In the end,this paper analyses the main parameters in the cp-OTDR system and the methods to improve these parameters are introduced.(2)In order to break through the inherent tradeoff of sensing length and frequency response range in the ?-OTDR system,this paper apply the multi-coset sampling and the spectrum reconstruction method to the ?-OTDR system.The sub-Nyquist sampling and reconstruction of broadband signal are realized in this paper.The principle of the multi-coset sampling and the spectrum reconstruction method are introduced.Different kinds of signals are used in the simulation.In the experiment,this paper designs a non-uniform sampling sequence with an average sampling rate of 4.8kHz.Furthermore,both the time domain and spectrum of a broadband chirp signal with a center frequency of 4.6kHz are successfully reconstructed in the traditional ?-OTDR system.The innovation of this thesis is mainly as follows:(1)Inspired by multi-coset sampling technology,this paper designs a non-uniform sampling pulses.And the continuous to finite algorithm is used to determine active spectrum ranges.In the experiment,both the time domain and the frequency domain information of a sparse broadband chirp signal have been fully extracted.Thus,this paper break through the limitation of Nyquist sampling theorem.It makes a significant step for broadband signal detection and shows great potential on future investigation and application.(2)This paper proposes a novel method based on the serial multi-coset sampling(MCS)strategy which can recover the sparse broadband vibration signal at a sub-Nyquist sampling rate.The complete signal can be recovered from far less sampling times.The decreasing of the data size,which may improve the signal processing speed and reduce the requirement of data storage equipment.