Phase Sensitive Optical Time Domain Reflectometry Based On Frequency Drift Compensation

Phase-sensitive Optical time domain reflectometer,as a relatively new type of fully distributed optical fiber sensing technology,has higher response speed and higher sensitivity than other early optical fiber sensing technologies and can detect even weaker external disturbance events.Not only can locate the position of the disturbance,but also the phase information can be demodulated.?-OTDR uses coherence fading effect of the back-propagation Rayleigh scattered light in the fiber.As long as the external disturbance event causes the length change at the wavelength level on the fiber,this disturbance signal can be captured by the ?-OTDR.Therefore,the ?-OTDR system has a high sensitivity and often used to detect dynamic disturbance events?obtain the position and frequency information.The parameters such as the sensitivity and dynamic range of the ?-OTDR system are not only affected by the pulse,but also closely related to parameters such as line width and frequency stability of the laser.If the frequency shift of the narrow linewidth laser is severe,there will cause a distortion of ?-OTDR curve,which not only reduces the signal-to-noise ratio,but also makes the entire system lose its ability to sense low-frequency and weak disturbancee events.Aiming at the shortcomings of traditional ?-OTDR,this paper proposes two schemes to overcome the above problems.At the same time,the relation between laser frequency drift and the resulting phase shift are explored by simulating.1.We propose a laser frequency drift compensation system in ?-OTDR based on Mach-Zehnder interferometer(MZI).By introducing a connecting fiber and a time-delayed optical fiber with an optical path difference in ?-OTDR to form a MZI,which is formed as a laser frequency monitoring and frequency drift compensation module with a fixed arm length difference.The signal intensity and phase information of the MZI in the system contains the information of the laser frequency drift and drift phase,through demodulate the signal of MZI,the effect of the laser frequency drift on the sensing is eliminated by the phase compensation.This compensation system can help us make a direct judgment of the system's stability and improve the accuracy of recovered vibration signal.2.An algorithm of eliminating frequency drift in ?-OTDR based on the twice difference method is proposed.On the premise of not changing the structure of the existing sensing system,two points having the same interval as the signal to be compensated are selected in the vibration area or the non-vibration area,and the phase shift between the two points is used to characterize the change of the laser frequency drift.Compensated by the phase difference of two selected points,we can eliminate the influence of the frequency drift of the ?-OTDR system,thereby improving the accuracy of recovery the vibration event and improving the reliability of the system.What is more,the algorithm is simple and can realize real-time compensation for laser frequency drift.3.The relationship between the frequency drift and the output phase of the linear laser was explored through simulation.The effect of the above two frequency drift compensation methods was further analyzed by changing the density of the scattering point and the frequency drift rate over a length of 10m.In the experiment,by using the above two methods,the length of the sensing fiber used is 6 km,and the vibration signal is applied to the sensing fiber at 5 km through an arbitrary function generator to drive the piezoelectric ceramic.Through the above-mentioned laser frequency drift compensation scheme,we can accurately recover a vibration signal with the frequency of 0.05 Hz and the amplitude of 5.89n?.This scheme can effectively correct the waveform distortion and improve the overall sensing performance?detection capability and system sensitivity of the ?-OTDR system.