Research On Optical Cable Condition Monitoring Methods Based On Long Period Fiber Grating

Optical cable is widely used in astronomical telescopes to improve the efficiency of astronomical observation because of excellent optical transmission performance.The serious degradation of the focal ratio,which caused by external deformation,will seriously affect the imaging quality of the astronomical telescope.In this research,core-offset long period fiber grating（CO-LPFG）and intermittent spiral long period fiber grating（IS-LPFG）are designed to monitor the bending and torsional deformation of optical cables.The arc-remodified long period fiber grating（AR-LPFG）is designed to monitor the absolute bending state caused be external bending deformation.The advantages of LPFG for the monitoring of astronomical optical cable state are highlighted by comparing with the optical time domain reflectometry（OTDR）technology.The CO-LPFG was prepared by combining the CO₂ laser polishing and arc discharge technologies.The maximum and minimum bending sensitivity reaches 37.07 nm/m^-1and 5.08 nm/m^-1.The intermittent-spiral long period fiber grating（IS-LPFG）is fabricated by CO₂ laser polishing technology.Because of elliptic birefringence superposition mechanism,the torsion sensitivity can be improved to 0.15 nm/rad/m with a spectrum loss of 0.1 d B.The fiber sensors of CO-LPFG and IS-LPFG are utilized to investigate the response of optical spectrum of optical cable under external bending and twisting.Based on the fiber optic cable structure and experimental data,the spectral response characteristics of the fiber optic sensor in the fiber optic cable bending process are analyzed.IS-LPFG is used to investigate the spectral changes of optical fiber under torsional deformation.The torsion applied on the outer layer of the optical cable introduce a slight effect to the inner fiber,and the torsion-immune mechanism of the cable structure is analyzed by a series of simulation experiments.The sensor monitoring method of absolute bending state is studied by using arc-remodified long period fiber grating（AR-LPFG）.During the discharge process,the refractive index distribution of the long period fiber grating（CO₂-LPFG）prepared by the CO₂ laser is remodulated,resulting in multiple resonant peaks in the original transmission spectrum.The absolute state of bending is measured by the ratio of the moving distance of the central wavelength of the resonant peaks.A series of comparative experiments show that the method has high reliability and stability.This paper presents a method to monitor the bending and torsion of optical cable and corresponding long period fiber grating sensors with different structures.The long period fiber grating sensor written by CO₂ laser and arc discharge technology can still guarantee stable spectrum after the second coating and sealing into the cable,which can maintain excellent stability and mechanical properties in practical application fields.