Characteristic Analysis For Ultra Long Period Fiber Grating

Ultra long period fiber grating is the new type of rejection type optical fiberdevices,which is input forward in recent years on the basis of long period fiber grating(LPFG),and is the key element of optical fiber communication and optical fiber sensingsystem.But ULPFG no matter from line shape or the perceptual, presents differentcharacteristics from the long period fiber grating compared to the long period fibergrating.The resonance peak wavelength and amplitude of ultra long period fibergrating are very sensitive with the outside environment changing, and these resonancepeaks have different sensitivity in the characteristics of temperature, strain, bendingand refractive index.These special properties for researching the field of optical fibersensing has a wide range of potential applications and important academic value.This paper bases on long period fiber grating as the research object, using thecoupled mode theory, the F-P theory and transfer matrix method to analyze it in theory.Polarization and test system of the fiber optic components are used for studing thetransmission spectrum of ultra long period fiber grating.First of all, basing on the common fiber grating, this paper promotes to thecoupled-mode theory of the long period fiber grating and deduces coupled-mode theoryof the ultra long period fiber grating.at the same time, the characteristics of ultra longperiod fiber grating is analyzed systematically.Secondly, this paper deeply studies the transmission properties of ultra long periodfiber grating and analyzes wavelength how impact on the transmission spectra of ultralong period fiber grating. Through the different sizes of axial tension to change thewavelength of ultra long period fiber grating period, author studies the influence onmove of transmission spectra, and then uses the transfer matrix analysis of multiplecascade to simulate numerically the transmission spectra of F-P cavity is not onlyaffected by the cavity length, also affected by the reflectivity of the cavity.Finally, this paper compares the transmission spectra of ultra long period fibergrating and the interference spectrum of simulation F-P cavity. Through comparisonsincreasing the cavity length of F-P and the wavelength of ultra long period fibergrating,we can discover that spacing of the transmission peaks of ultra long period fiber grating and the axial tension of transmission peak transmission peak driftdirection after period increasing are consistent with F-P theory, and indicate ultra longperiod fiber grating spectral features largely in line with theory of F-P, it providesanother method to analyse ULPFG different from the coupling theory.