Research On Distributed Polarization And Phase Demodulation Based On Ultra-weak Fiber Bragg Grating

Distributed optical fiber sensors(DOFSs)use optical fiber as the signal transmission medium and a sensitive unit.DOFSs have many significant advantages over traditional sensors,such as small size,low loss,continuous distributed sensing,and so on.Phase-sensitive optical time domain reflectometer(?-OTDR)is a technology developed on the basis of distributed optical fiber sensing technology.It has high sensitivity and fast response speed,so is widely used in the field of dynamic monitoring and micro-disturbance measurement.In recent years,combining ?-OTDR with Ultra-Weak Fiber Bragg Grating(UWFBG)Array has become a new research hotspot.It is expected to greatly improve the performance of frequency response range and signal-to-noise ratio.The combination system of ?-OTDR and UWFBG Arrays treats the UWFBG embedded in the optical fiber as a set of "weak mirrors",which can provide stable,intensity-controllable reflected light signal at a specified position in the optical fiber.Compared with the traditional optical fiber sensing technology,the combination system has better SNR and higher sensitivity.However,In the general structure,due to the polarization state of the light changes randomly along the fiber,the sensitivity of the system may be deteriorated severely when the polarization states of the interference signals are mismatch,which is called polarization fading.In this thesis,a composite-double-probe-pulse(CDPP)technique is proposed,which can eliminate the polarization fading in the combination system.Firstly,the thesis analyzes the characteristics of polarization fading phenomenon by using Jones matrix,and then proposes the CDPP method,aiming to eliminate the effects of polarization fading.The CDPP consists of two pulsed lights with a spatial separation equaling to twice the spatial separation of adjacent UWFBGs.The first detection pulse has a long duration and the second detection pulse consists of two consecutive short pulses of orthogonal polarization states,the pulse width of the first detection is twice that of a short pulse.Subsequently,the CDPP scheme can completely eliminate the polarization fading phenomenon and realize the highly sensitive distributed quantitative measurement.When the position of polarization fading changes,we need to change the position of signal to be demodulated to follow the signal with good visibility.So an "interactive demodulation" method is proposed which can resolve the phase discontinuity caused by the position change of the signal.In the traditional ?-OTDR system,due to factors such as sampling rate,the phase demodulation method is difficult to achieve accurate recovery of large strain signals.Based on the above research on polarization fading,the O-OTDR system with UWFBG is combined with the double-pulse structure to study the accurate recovery of large strain signals.In the paper,the amplitude-phase information fusion demodulation technology is proposed.By comparing the monotonicity of the amplitude signal and the local position of the phase signal,the phase-expanded signal is calibrated by the amplitude signal obtained by demodulation,and finally the large strain signal is accurately restored.In this paper,from the two aspects of CDPP pulse structure and fusion demodulation algorithm,the ?-OTDR system with UWFBG is improved.The CDPP structure pulse fusion system completely solves the polarization fading problem,and the system has higher sensitivity.At the same time,the polarization-phase information fusion demodulation algorithm can also accurately recover large strain signals.In summary,the performance of the ?-OTDR system with UWFBG has been significantly improved by the improvement of the pulse structure and the optimization of the algorithm,which is more suitable for various engineering applications.