Phase Sensitive Optical Time-domain Reflectometry Based On Novel Quadrature Signals

Distributed optical fiber sensing(DOFS)was proposed in the 1970 s and has received extensive attention from all walks of life since its inception.After half a century of development,distributed optical fiber sensing has been applied to important fields such as resource exploration,scientific research,and industrial production,resulting in extremely huge social value.Like the point-based fiber optical sensing,the DOFS has the advantages of high sensitivity,resistance to harsh environments such as high temperature or corrosion,and immune to electromagnetic radiation,etc.More importantly,DOFS has extremely high sensor multiplexing capabilities compared to traditional point-based sensors,which is conducive to large-scale networking for wide-range high-precision measurements.Among all kinds of DOFS technologies,phase-sensitive optical timedomain reflectometry(?-OTDR)has superiourity in high sensitivity and fast response speed,resulting in its populiarity in seismic wave detection,geological resource exploration,intrusion monitoring and other fields.With the proposing of the concept of integrated communication and sensing,using the massive optical fiber communication networks has gradually emerged to realize applications such as vehicle tracking,anthropogenic noise monitoring and seismic wave measurement,which makes ?-OTDR have a broader application prospect.?-OTDR essentially realizes the multiplexing of notional sensors along the optical fiber through time division multiplexing,so there is an inherent contradiction between its sensing bandwidth and sensing distance.At the same time,the existence of fiber attenuation also makes important sensing performances such as noise level,spatial resolution,and anti-fading ability are closely related to the sensing distance.In recent years,the acoustic sensing distance of ?-OTDR has achieved 100 kilometers,and the application scenarios of ?-OTDR has gradually developed to the field of high-frequency acoustic sensing such as hydrophone and predischarge monitoring for high-voltage cables.Consequently,the contradiction between sensing bandwidth and distance has become more and more prominent.As well,the requirements for noise level,spatial resolution,and anti-fading performance are increasing.This thesis aims to achieve broadband high-performance ?-OTDR,takes the orthogonal multiplexing of sensing channels as the main idea,and studies the scheme to overcome the restrictive relationship between the key performance parameters of ?-OTDR.In this way,the measurement repetitation rate can be enlarged,and consequently the key performance such as the sensing bandwidth,spatial resolution,noise level etc.could be improved.The main works are summarized as follows:1.The theoretical basis of single channel receiver for ?-OTDR—the KramersKronig relation in heterodyne detection ?-OTDR is discussed,and an ultra-highbandwidth low-noise ?-OTDR based on continuous chirped-wave is proposed.First,the Kramers-Kronig relation in the Rayleigh scattering signal of heterodyne ?-OTDR is studied through theoretical derivation.The relation ship between the requied channel number for signal retrival of ?-OTDR and the signal characters is revealed,and the roles of signal negative frequency and the signal-signal-beat interference are discussed at the same time.Then a continuous chiped-wave(CCW)?-OTDR is proposed,which can solve the problem that the traditional pulsed ?-OTDR cannot truly achieve continuous collection of disturbance imformantion in both time and spatial domain.The time-and frequencydomain resources can be fully utilized with CCW ?-OTDR,resulting in high spectrumefficiency channel-multiplexing and low noise level.In the demonstration experiment with 1013 m long fiber,the sensing bandwidth as high as 1.042 MHz is achieved,with a spatial resolution of 4.4 m and a strain noise level of 5 p?/?Hz.The measurement speed is increased by more than 20 times compared to conventional single pulse solutions.In addition,CCW ?-OTDR can achieve continuous information acquisition in both time and spatial domain,so that it can realize digital domain reconstruction of performance parameters in post-processing after measurement.This solves the problem that the traditional ?-OTDR cannot optimize the disturbance quality separately for multi-point disturbances with different characteristics.This technology will promote the expansion of ?-OTDR to the field of long-distance,high-frequency sensing,as well as the field of complex signal sources such as anthropogenic noise measurement and urban traffic monitoring.2.A channel multiplexing scheme based on orthogonal codes with the same frequency is proposed for quasi-distributed ?-OTDR,which scheme can multiply the measurement repetition rate of the system without occupying additional receiver bandwidth.The signal character of the proposed system is analyzed through the theoretical derivation based on the basic model of quasi-distributed sneisng.In turn,the characteristic requirements of orthogonal codes for the proposed scheme and its generation scheme is gived.Finally,three sets of orthogonal codes with the same frequency are used in the demo experiment.A sensing bandwidth of 166.7 k Hz is realized on an 860 m fiber,with a spatial resolution of 5 m,and a noise level of 4.6 p?/?Hz.This scheme provides a new direction for improving the spectrum efficiency of ?-OTDR and further improving its sensing performance.3.The noise lower-bound of coherent optical time-domain reflectometry(COTDR)sensing system based on coherent detection is clarified from theory,simulation and experiment.Through rigorous theoretical analysis,it is proved that the coherent detection COTDR has different noise and signal characteristics compared with the traditional time delay estimation problem,so it cannot directly use the Cramér-Rao lower bound of the time delay estimation problem to evaluate its noise lower-bound.Then,starting from the original theory of unbiased estimation,the Cramér-Rao lower bound of coherent detection COTDR is deduced,and finally verified by experiments.This work is an exploration of the performance limit of a distributed sensing system based on coherent detection COTDR,and provides theoretical guidance for the further research of such systems.