Research On OFDR Distributed Fiber Sensing Technique Based On DBR Laser

Linear tunable laser is used as light source in the distributed fiber sensing system based on the optical frequency domain reflectometry.The optical frequency domain reflectometry adopts the heterodyne coherent detection technology to extract the Rayleigh scattering signal in the optical fiber.Then the frequencied of beat signals are mapped to corresponding position of the fiber through frequency spectrum analysis.Optical frequency domain reflectometry sensing system has the advantages of high spatial resolution,which has important applications in structural health monitoring of spacecraft,flight vehicles,and precise instrument.As a result of many years of study,basic system scheme and signal processing flow of optical frequency domain reflectometry sensing technique are achieved.However,its light source is usually a tunable external cavity diode laser.This laser has problems such as large volume,large weight,poor enviromnebtal adaptability,high cost and dependence on imports.These problems have become one of the main bottlenecks restricting the popularization of this technology.Tunable distributed Bragg reflector lasers are widely used in optical communication.It is a relatively mature tunable laser source.However,due to the limited tuning range of single longitudinal mode,only segmented wavelength tuning can be realized.In view of the above problems,the optical frequency domain reflectometry sensing demodulation method based on tunable distributed Bragg reflector laser and spectral splicing method is proposed in this paper.In addition,some improvement measures of demodulation algorithm are proposed to solve the outlier problem with high spatial resolution and large temperature or strain range.The research works carried out in this paper is as follows.(1)In view of the existing optical frequency domain reflectometry system large,high power consumption,dependence on imports and other problems,the driving module of tunable distributed Bragg reflector laser is designed,which can realized with the wavelength range of1.2nm without mode-hopping and 40nm with the wavelength range of multi-segment splicing.Using Michelson interferometer,the laser module can achieve interferometric measurement of arm length difference of 50m or more.Compared with the laser module of the current commercial optical frequency domain reflection system,the volume of this laser module has been reduced from 1269cm~3 to 143cm~3,and the power consumption has been reduced from20W to 3W.(2)Aiming at the problem that tunable distributed Bragg reflector laser can only achieve segmented continuous scanning,and existing demodulation algorithm cannot be applied to distributed fiber sensing system based on optical frequency domain reflectometry with segmented continuous scanning as the light source,a Rayleigh scattering signal splicing method baed on thermal stability etalon is proposed.The feasibility is confirmed by theoretical and simulation analysis.The optical frequency domain reflectometry sensing system based on tunable distributed Bragg reflector laser is set up.Experiments show that the system can realize the recognition of optical link reflection signal with 1mm spatial resolution and distributed optical fiber sensing with mm-level high spatial resolution.(3)In response to the problem of measurement outliers in cross correlation demodulation caused by location-deviation of fiber at high spatial resolution and long measurement distance,a location-deviation compensation algorithm based on maximum cross-correlation is proposed.This algorithm compensates the location-deviation caused by the temperature or strain change of the sensing fiber and the auxiliary interferometer effectively through finding the maximum cross-correlation of adjacent measuring points.Eliminated measurement outliers caused by location-deviation of fiber.(4)In response to the problems of prone to demodulation errors and computationally heavy at large temperature or strain ranges in common cross-correlation algorithm,the cross-correlation algorithm based on adapted temperature or strain range is proposed.This algorithm is associated with the temperature or strain range.Large temperature or strain range demodulation can be achieved by eliminating the irrelevant sprctral components.Meanwhile,by reducing the number of shifts in the cross-correlation,the operation times of the inner product are effectively reduced and the efficiency of modulation is improved.(5)According to the above research,principled prototype of optical frequency domain reflectometry sensing system based on tunable distributed Bragg reflector laser is set up.The temperature and strain sensing performances are tested.The experiment results show that 5mm spatial resolution and more than 12m measure length are achieved.The maximum temperature measurement error of the system is 0.67℃in the temperature range of-40℃to 80℃.And the maximum temperature measurement error of the system is 2.52℃in the temperature range of-40℃to 80℃.The maximum strain measurement error of the system is 4.3μεin the temperature range of 0 to 100με.And the maximum strain measurement error of the system is10μεin the temperature range of-5000μεto 5000με.