Study On Strain Demodulation Methods Of Distributed Optical Fiber Based On OFDR

Structural health monitoring systels have been successfully used for durability and safety assessment of infrastructure such as bridges,buildings,offshore platforms and aerospace vehicles.By using various sensors to collect,transmit.monitor and diagnose the state information of key parts of the structure in real time,the damage and health status of the structure can be discovered in time,so as to predict the safety of the structure and provide early warning of possible disasters..Among them,strain is an important parameter reflecting structural state information.and obtaining accurate and detailed strain data is the technical basis for structural health assessment.Optical frequency domain reflectometry(OFDR)is an emerging development direction in distributed optical fiber measurement and sensing technology.OFDR-based distributed optical fiber strain measurement technology has high signal-to-noise ratio,high spatial resolution,high sensitivity,etc.The advantage is that it is easy to obtain the strain information of the structure in a large area.At present,the distributed optical fiber strain measurement technology based on OFDR has become an important reason for limiting the wide application of distributed optical fiber sensing systems due to its limited strain measurement range,low reliability of strain measurement and sensitivity to measurement environment.This paper focuses on the OFDR-based distributed fiber strain demodulation method.The main contents and results are summarized as follows:(1)Based on the establishment of the strain calculation accuracy model,a simple and effective quality factor parameter for evaluating the Rayleigh backscattering spectrum signal quality is proposed,which can quantify the overall or local Rayleigh backscattering spectrum signal quality of the fiber under test.The evaluation verified the validity and accuracy of the proposed quality factor parameters for evaluating the quality of Rayleigh backscattering spectrum signals.The results show that the standard deviation error of Rayleigh backscattering spectrum shift calculation is closely related to the quality factor parameter.The Rayleigh backscattering spectrum with larger quality factor has smaller standard deviation error,so Rayleigh backscattering spectrum signals with higher quality factor are considered to have higher signal quality.(2)Based on the quality factor parameters of Rayleigh backscattering spectrum,a method and algorithm flow for determining the length of fixed subset window and adaptive subset window are proposed.According to the Rayleigh backscattering spectrum quality factor,combined with the required strain calculation accuracy and spatial resolution requirements,the optimal subset window length can be automatically determined.The experiment verifies the effectiveness of the fixed subset window or the adaptive subset window calculated by the quality factor to improve the accuracy of strain calculation.Compared with the traditional method,the method of determining the length of the subset window based on the measurement experience or spatial resolution.The proposed subset window determination method can significantly improve the measurement accuracy of the strain and reduce the technical requirements for the operator.(3)Aiming at the causes of strain demodulation errors caused by large strains,a strain demodulation method based on Zero-normalized cross-correlation(ZNCC)reliability guidance is proposed.In this method,the Rayleigh backscattering spectrum database of the fiber under test in the initial state is established,and the spectral range of the reference Rayleigh backscattering spectrum can be flexibly intercepted according to the requirements of the strain or temperature measurement range,even if the Rayleigh backscattering spectrum offset is outside the measurement range and the registration segment can also be found in the intercepted reference Rayleigh backscattering spectral subset.The ZNCC cross-correlation evaluation criterion is used to evaluate the similarity between the reference Rayleigh backscattering spectrum subset and the measured Rayleigh backscattering spectrum subset.The Newton iterative method is used to quickly search for the path to the best registration segment,and the calculated Rayleigh backscattering spectrum shift can be evaluated by the ZNCC coefficient distribution for confidence.The experiment verifies the accuracy of the method for measuring strain and the advantage of measuring large strain.For the measurement of large strain(5000??),accurate and reliable demodulation can be achieved with an error range of 1%.(4)A strain step-by-step demodulation method is proposed for the attenuation of Rayleigh backscattering spectrum intensity caused by bending or torsion in the measurement process.The method uses the dynamic reference Rayleigh backscattering spectrum to replace the fixed reference Rayleigh backscattering spectrum in the traditional demodulation method.Through the step-demodulation strategy,the relative strain of each step is demodulated first,and finally the absolute strain value is obtained.By using the strain step-by-step demodulation method,stable and accurate measurement is achieved while measuring the Rayleigh backscattering spectrum intensity attenuation by 15 dB.