Phase-sensitive Optical Time Domain Reflectometry Based On Ultra-weak Fiber Bragg Grating And Its Quantitative Demodulation

The distributed optical fiber sensing(DOFS)technology is a new sensing technology that has been gradually developed in the past several decades,using optical fiber as transmission and sensing unit simultaneously.Compared with traditional electrical sensor,distributed optical fiber sensing technology has many prominent advantages such as light weight,anti-electromagnetic interference and so on.Phase-sensitive Optical Time Domain Reflectometry(?-OTDR)is a rising star in all the optical fiber sensors,which has not only the advantages of optical fiber sensor but also the characteristic of high sensitivity and response speed.Based on these features,?-OTDR can be applied to feeble perturbation measurement and dynamic measurement.However,there exists some shortcomings of traditional ?-OTDR yet.Firstly,?-OTDR implements its sensing ability using RBS,but RBS is too weak to detect in direct detection configuration unless using avalanche photodiode(APD).So is necessary to average ?-OTDR curves to improve the signal to noise ratio(SNR)in many circumstances,which will decrease the maximum detectable frequency of?-OTDR as result.Then,due to the randomness of the coherent superposition for the scattering signal,there is no constant relationship between the perturbation and the change of the jagged appearing.,thus the strain on the fiber cannot be demodulated directly from the power of the ?-OTDR signal.Aiming at the problem of maximum detectable frequency is limited by SNR caused by the weak RBS,a vibration sensing technique using double optical pulses based on ?-OTDR and ultra-weak FBG(UWFBG)array is proposed.In this system,UWFBGs with the similar reflectivity are integrated with fiber to generate higher reflect signal to take the place of RBS,which will improve the SNR as result.By using double probe pulse,very simple detection setup is enough for receiving the optical signal and locating the events.Benefited from the high SNR,this system can achieve the measurement of ceiling frequency limited by the fiber length.Double pulses ?-OTDR can get the location and frequency of the perturbation along the fiber,but it cannot restore other more complete information of the perturbation.For the intensity signal,it is easy to appear frequency multiplication,in this case,it is difficult to recognize whether it is the feature of the perturbation itself or frequency multiplication from the spectrum.To solve the quantitative measurement problem,the double pulses ?-OTDR system is adjusted by adding a local reference signal and changing the detect structure to heterodyne detection.Single short probe pulse combined with heterodyne coherent detection cannot locate the perturbation directly from the intensity variation of the signal,because it does not change with the perturbation in this circumstance.However,using double pulses can solve this problem easily,which will reduce the work of data process and improve the response speed certainly.The experiment results indicate that this method can demodulate different signal waveforms effectively and accurately.Furthermore,the frequency as low as 0.2Hz can be demodulated in this configuration by using twice-differential method.