Research On Sensing And Demodulation Of Long Period Fiber Grating

Fiber grating is very important in the field of the optical fiber communication, optical fiber sensing and optical information processing application.According to the period of the refractive index modulation can be divided into long period fiber grating and fiber Bragg grating, due to the long period, long period fiber grating is a synthetic transmission coupling between fiber core mode and cladding modes, so the resonant wavelength and loss of peak amplitude is very sensitive of external factors such as temperature, stress, refractive index change. Compared with the Bragg grating,Long period fiber grating has a higher sensitivity and higher measurement precision;As a kind of transmission type fiber grating,long period fiber grating has no reverse transmission, don’t need to use isolator in the sensing measuring system.But in reality long period fiber grating sensor in the field of application is far inferior to Bragg grating, one of the major reason is that there is no suitable demodulation technology, and the loss of long period fiber grating has a larger peak bandwidth, at present the study of long period fiber grating is mainly in the laboratory, developed a low cost, high performance of demodulation instrument is the main task of the long-period grating sensor research.This article has a general summary for the development history and research of long period fiber grating, based on maxwell wave equations, this paper analyzed long period fiber grating through coupled-mode theory, the long period fiber grating is coupling between synthetic transmission mode, fiber core mode and cladding mode is derived respectively in the process of transmission dispersion equation, discuss the coupling between different modes, the resonance wavelength of fiber grating, using Matlab simulate the transmission spectrum of long period fiber grating. Long period fiber grating is very sensitive to the environment, such as temperature, stress, and the change of the refractive index factors can cause the change of effective refractive index of fiber core modes or cladding modes, or cause the change of the fiber grating period, thus will change transmission spectrum, the resonant wavelength shift, also loss of peak amplitude etc., by analyzing the changes of the transmission spectra can be obtained outside the change of the parameters, such as temperature, stress, so that they can achieve the goal of demodulation. In view of long period fiber grating sensing characteristic, the article puts forward a demodulation method based on tunable F-P interferometer, the transmission light intensity of sensing grating can be converted to the size of the applied voltage of piezoelectric ceramic, by measuring the change of the voltage to undo the change of light intensity, the spectral range of the demodulation of F-P interferometer is wide, the precision is higher, it is a feasible long period fiber grating demodulation method. Long period fiber optic fiber has unique advantages,the main factor of restricting its development is no good demodulation method,so find a reasonable demodulation method, long period fiber grating will play a more important role in the sensing field.