Research On The Measurement Rate Improvement Of Quasi-distributed Sensing Based On ?-OTDR

Distrubuted acoustic sensing(DAS)can be used to measure many physical quantities around the sensing fiber.Phase-sensitive optical time-domain reflectometry(?-OTDR),as a main technology of DAS,has attracted the attention of scholars,because of its long sensing distance,high sensitivity and good capability of dynamic detection.Recently,?-OTDR system based on ultra weak fiber Bragg grating(UWFBG)array has been proposed,such systems can be called quasi-distributed acoustic sensing(Q-DAS)systems,which has higher sensitivity and signal-to-noise ratio(SNR)than the ordinary ?-OTDR using single-mode fiber(SMF)as the sensing medium.Therefore,Q-DAS has become one of the promising technologies in optical fiber sensing.In the Q-DAS system,due to the limitation of the round-trip time of the probe light in the fiber,the frequency response of the system will be insufficient.The restriction of measurement rate and sensing distance makes Q-DAS limited in underwater ultrasonic sensing,high-pressure pipeline leakage and other fields that require high-frequency sensing.Therefore,it is very meaningful to improve the measurement rate of Q-DAS system.At present,the research on improving the measurement rate still has limitations such as low spectrum utilization and complicated modulation.This thesis proposes a new method to improve the measurement rate of the Q-DAS system,which uses the interval between UWFBGs to multiplex the probe pulse,thereby increasing the measurement bandwidth of the system and further improving the measurement slew rate(SR).The main work of this thesis is summarized as follows:1.The principles of DAS and Q-DAS are introduced,and the mathematical model of phase demodulation Q-DAS is derived.Firstly,several kinds of scattering in the fiber are introduced.Then the sensing mechanism of SMF-based ?-OTDR is explained in detail.Finally,the sensing principle and the multiplexing model of FBG are introduced,and for the first time,the linear relationship between the change of external environment and the phase of reflected signal in the phase-demodulated Q-DAS system is derived,which lays the foundation of this thesis.2.A technique to improve the measurement rate based on interleaved chirped pulses(ICP)in Q-DAS system is proposed.Firstly,the principle of the ICP scheme is introduced:a series of positive and negative chirped interleaved pulses are injected into the fiber,and the interval between UWFBGs is used to multiplex probe pulses and enhance the measurement rate.Then the spectrum utilization of the frequency division multiplexing(FDM)scheme and the ICP scheme is compared in the experiment: the spectrum utilization of IICP scheme is 1.5 times to FDM scheme.Eventually in a sensing distance of 860 m,a high-bandwidth,high-precision sensing system with a frequency response bandwidth of166.7 kHz and a strain resolution of 1.8p?/(?) is realized.3.A technique to improve the measurement rate/slew rate based on interleaved identical chirped pulses(IICP)in Q-DAS is proposed.Firstly,the meaning of SR is introduced,and the limitations of FDM scheme in improving SR are analyzed: due to the different phase offsets between the different frequency channels,the phase unwrapping algorithm has errors,which cannot improve the SR.Then,the principle of the IICP method to improve the SR of Q-DAS is introduced.Finally,in the experiment,the comparison with the FDM scheme highlighted the superiority of the IICP scheme: it can accurately demodulate large strain signals.Compared with the Q-DAS using the traditional single pulse scheme,the Q-DAS based on the IICP scheme can enlarge the system bandwidth and SR by 5times at the same time,achieving a Q-DAS with high spectrum utilization,high sensing bandwidth and high SR.The innovative method proposed in this thesis to improve the measurement rate of the Q-DAS breaks the inherent restriction between the measurement rate and the sensing distance,and greatly enlarge the SR of Q-DAS for the first time.It opens up a new direction for the study of phase-demodulated Q-DAS systems,and provides the possibility for the application of Q-DAS in high-frequency and large-strain environments.