Research On Distributed FBG Demodulation With High Speed And High Spatial Resolution

With the advancement of science and technology,structural health monitoring has become a research hotspot.In the field of precision machinery and equipment,higher technical requirements are imposed on the response speed and spatial resolution of the sensing system.In recent years,the sensing technology based on the fiber grating has been rapidly developed,but it is difficult for the existing fiber grating sensing technology to have both a high response speed and a spatial resolution.In response to the technical deficiencies of optical fiber sensing,the distributed fiber grating demodulation technology based on optical frequency domain reflectometry(OFDR)is mainly studied,the light source with high-speed frequency sweeping is realized by employing a distributed feedback laser(DFB),and a distributed fiber grating interrogator with high-speed and high spatial resolution is achieved.The main research contents are as follows:(1)Theoretical research on FBG demodulation with High Speed and High Spatial Resolution.Analyze the sensing mechanism of Fiber Bragg Grating(FBG),simulate and analyze the spectral characteristics under non-uniform strain and the segmented demodulation method of long grating.Study the demodulation principle of OFDR,analyze the key characteristics of OFDR system such as spatial resolution and dynamic range,and analyze the demand of system swept light source,study the mechanism of high-speed tuning of DFB,analyze the applicability and existing problems of DFB swept-frequency light source in OFDR system,test DFB beat frequency capability,and verify the dynamic detection range of OFDR system based on DFB.(2)Design and implementation of the high-speed swept light source based on DFB.The DFB current modulation driving circuit is designed and some tests are made to show the performance of the light source.the frequency spectrum of DFB output light,the linearity of DFB current-wavelength modulation and dynamic line width are analyzed to verify that the DFB high-speed sweep light is suitable for the OFDR system.For the interference problem caused by the amplitude modulation of the DFB light source,simple compensation methods are proposed in hardware and software,respectively,to eliminate the amplitude modulation interference of the light source.(3)Design and implementation of the fiber grating demodulation system with high speed and high spatial resolution.The optical construct based on OFDR is constructed employing a DFB swept light source,and design and implement a demodulation algorithm based on OFDR-DFB,analyze the nonlinear effect of the system and propose a modified algorithm based on Hilbert transform algorithm to improve the spatial resolution of the system.(4)Application research of FBG Demodulation with high speed and high spatial resolution.A distributed FBG temperature test is performed on the demodulation system to show the highest spatial resolution of the system,the high-speed vibration test using an acceleration FBG sensor is performed to show the frequency range of the vibration signal that can be recognized by the system,and the system repeatability experiment is performed to verify the stability of the system.Explore the practical application of the demodulation system on the gear platform.By arranging a long fiber grating at the tooth root of the gear,a dynamic distributed detection of the tooth root stress of the gear is performed to verify the applicability of the demodulation system in the structural health monitoring.