Research On Phase Noise Suppression Methods Of OFDR Sensing System

With the progressing of intelligentize and robotization level in China,precision manufacturing gain the opportunity of substantial development,which is in a significant need of sensing technology.Optical-fiber-based sensing technologies are widely used due to the low cost with elevated application flexibility and safety.Among the various technologies,quasi-distributed Fiber Bragg Grating sensing systems have elevated measurement accuracy,while low at spatial resolution as the single-point sensing features.BOTDR systems based on analysis of spontaneous Brillouin scattering are capable of long range continuous sensing,while whose spatial resolution is restricted within level of several meters.BOTDA systems based on stimulated Brillouin scattering analysis have higher spatial resolution,while the need of double-ended signal injection causes the limit of application.Nevertheless,OFDR systems based on Rayleigh scattering frequency domain analysis show promise in both spatial resolution and measurement instantaneity.Furthermore,the single-end injection property enhances its engineering applicability.The OFDR systems run with the base of tunable laser as the light source,whose problems of tuning nonlinearity and phase noise have obvious adverse effect on the sensing accuracy and spatial resolution of the system.The solutions to the problems can enhance the fitness in the high precision strain sensing scenarios,which is vital to the OFDR system performance enhancement.Therefore,this article conducted the research of OFDR tuning nonlinearity and phase noise problems in the following sequence:(1)The article firstly studied the strain and temperature sensing mechanism of OFDR.The effects that the strain and temperature causes on Rayleigh backscattering parameters are analyzed,along with the strain measurement theory based on optical heterodyne detection.The problems of tuning nonlinearity and phase noise in the laser source were studied in several aspects which consists of generation mechanism,causing effects,and solutions.Furthermore,the polarization fading problems occurs widely in coherent detection systems was studied along with solution to the problem.(2)The affecting factors of OFDR system spatial resolution and effective measurement range were analyzed.Based on the analysis result,the optical path of OFDR hardware system was designed along with the component selection.The signal processing scheme based on the tuning nonlinearity and phase noise compensation method and the demodulation scheme based on optical heterodyne detection were programmed.(3)The OFDR strain sensing hardware and software system was built and the strain positioning sensing experiment was conducted.The experiment was operate firstly by analysis of signals from the main interferometer and the auxiliary interferometer.The tuning nonlinearity and phase noise compensation method based on interpolation resampling was conducted to test its validity.And a strain positioning experiment was operated by analysis of signals from OFDR system under different strain conditions.The validity of OFDR strain positioning measurement was tested.Furthermore,the demodulation scheme was optimized by adjusting several parameters.As a result,the system achieved a 316 mm spatial resolution on 6 m long sensing fiber.