Polarization-dependent Loss Characteristics Of Long-period Fiber Gratings And Their Applications

Long period fiber grating (LPFG) is a widely used fiber grating, which take advantages of low insertion loss, wide bandwidth, weak reflection, high reaction sensitivity to the environment change, simple craftsmanship, and low cost, etc. Thus it draws much attention from researchers domestic and overseas. During the technological development in recent years, LPFG shows itself extremely vast application prospect. With the deepening of the research, people constantly raise the demand standard to the LPFG function. Therefore, the research to LPFG of its new structure, new feature and new applications become inevitable development tendency.In this thesis, we around LPFG polarization dependent loss (PDL), which mainly includes two aspects, one is that the PDL characteristics of LPFG and its application in torsion sensor were studied. The second is that phase shift long period fiber gratings (PS-LPFG) and cascaded long period fiber gratings (C-LPFG) spectrum and PDL characteristics were discussed.Firstly, LPFG coupled-mode theory and transfer matrix method were expounded. The transmission spectrum of LPFG were analyzed in detail and simulation steps were given. Periodic numbers and the average refractive index modulation were discussed on its transmission spectrum.Starting from the sensing model of LPFG, this part analyzes the transmission spectrum which is formed after LPFG twists, and carries on the experimental research and simulation for PDL features of LPFQ and makes a corresponding theoretical analysis of contrast. According to the Jones matrix and transfer matrix method, the theoretical analysis and numerical simulation, the relationship between PDL response of LPFG with respect to the twist and by measuring the amplitude change of PDL, can be realized torsion sensor measurement without the interference by strain and temperature. In addition, this sensor can measure twist angle and determine twist direction simultaneously. The experimental results and numerical simulation have good match, twist sensitivity is 2.7 dB/rad.Theoretical analysis and numerical simulation of the phase-shift spectral characteristics of PS-LPFG were discussed. This part analyzes the influence from the length, the phase shift location, and the phase shift size of PS-LPFG to the PDL peak of PS-LPFG, and contrastively analyzes the sens ing property of the C-LPFG This part also carries on detailed theoretical research and the simulation to the PDL features of PS-LPFG and C-LPFG A small spectral shift does not result in a big fluctuation in loss in a wavelength, and discusses the influence from its physical parameters to sensing as well.