| Phase-sensitive optical time-domain reflectometer(Φ-OTDR)is an important branch of distributed optical fiber sensing(DOFS),which has the advantages of easy laying,high response speed,etc.In Φ-OTDR,the quasi-distributed acoustic sensing(QDAS)system is one of the research focuses in recent years,in which the common singlemode optical fiber(SMF)inscribed with fiber Bragg grating(FBG)array/scattering enhancement point(SEP).The high-reflection points in Q-DAS provide stronger and more stable reflection,thus,providing better sensitivity and signal-to-noise ratio;In addition,the interference fading phenomenon is avoided in Q-DAS system.Q-DAS has a good development prospect and research significance,and how to improve the technical index of Q-DAS is a research hotspot in recent years.Sensing distance and sensing bandwidth are the two key indexes in Q-DAS system.However,there is a trade-off between them.All existing methods to improve the sensing bandwidth of Q-DAS system have the limitation of consuming extra spectrum resources.How to effectively improve the sensing bandwidth of the system has become an increasingly concerned problem.In this thesis,a quasi-distributed acoustic sensing technology based on orthogonal polyphase code is proposed.By introducing multiple-inmultiple-out(MIMO)technology into Q-DAS,the orthogonal codes on the same carrier(OCSC)scheme is demonstrated to realize long-distance sensing,and the sensing bandwidth of the system can be improved by orders of magnitude without consuming extra spectrum resources.The main work of this thesis is summarized as follows:1.The basic sensing principle of Φ-OTDR system based on Rayleigh scattering and Q-DAS sensing principle based on FBG are expounded,and the linear relationship between the change of external environment and the change of reflected signal phase in phase demodulation Q-DAS system is expounded.An OCSC scheme is demonstrated,and the principle of how to use the orthogonality of coded digital domain to achieve the increasement of scan rate is derived.2.Based on the sensing principle of OCSC scheme,the generation principle of orthogonal polyphase code is demonstrated.Then the influence of Doppler effect on the OCSC scheme is analyzed and the constraint conditions of the optimization problem of the generated code pattern are modified.Furthermore,the principle and the steps of generating high-quality orthogonal polyphase codes based on SQP algorithm are expounded.What’s more,the advantages of the algorithm are described by comparing the generated codes with the existing ones.3.On the basis of the generated orthogonal polyphase codes with high quality,a QDAS system based on OCSC is realized,and the three factors which may cause the degradation of coding rejection ratio are analyzed respectively.Besides,the feasibility of this system is verified theoretically.Subsequently,the experimental results show that the sensing bandwidth of 49 kHz is successfully achieved on a 5.19 km sensor fiber;the noise level of 10 pε/(?)is achieved on the 5 m spatial resolution,and the system has good linear response.The Q-DAS system based on OCSC shows that the scan rate of the system can be enlarged without consuming extra spectrum resources,which greatly improves the spectrum efficiency.4.A long-distance and wide-bandwidth Q-DAS system based on OCSC is demonstrated.Twenty orthogonal multiphase codes are used to carry out sensing experiments on hundred-kilometer-level sensing fibers.In addition,the effect of OPC technology to break the inherent trade-off of spatial resolution,SNR and sensing distance is demonstrated.The frequency drift phenomenon of laser in fiber sensing is analyzed,and the selection of pulse-width of codes is described in detail under the influence of frequency drift.The experimental results show the sensing bandwidth is enlarged by 20 times compared with traditional Q-DAS system.Furthermore,the advantages of real-time processing of the system is illustrated,showing the superiority of the system performance. |
PDF Full Text Request