The Study Of Characteristics And Application Of The Radiation Mode Of 45 Degrees Tilted Fiber Grating

As an ideal in-fiber device,fiber grating has the advantages of small size,light weight and stable structure.It could achieve the functions of transmitting light and controlling optical signals without destroying the fiber structure,and it could avoid the flaws introduced by the collimating modules.The applications of fiber gratings would be conducive to improving the system integration and structural stability.In addition to having the advantages of traditional fiber grating,45° tilted fiber grating(45° TFG)could couple the S-polarized core mode into radiation mode due to its special tilted grating structure,and different wavelength components of the radiation mode would have different radiated angles,which means that 45° TFG is an ideal polarization-dependent diffraction device.Recently,the polarization characteristics of 45° TFG have been widely investigated.In this thesis,we have theoretically and experimentally investigated the characteristics of radiation mode of45° TFG,and explored its application in polarization spectrum analysis system.We have achieved some innovative results in different aspects.The main work includes the following aspects:(1)The theoretical and experimental research on the spatial distributions of the radiation mode.According to the volume current method(VCM),we have established a three-dimension theoretical model to analyze the distributions of the radiation mode of TFGs.By building up the radiation mode testing platform,we have experimentally investigated the characteristics of the radiation mode of 45° TFG.The experimental results show that the radiation mode of 45° TFG exhibits a quasi-Gaussian profile in azimuthal direction,and has a decaying exponential distribution along fiber axis.The characteristics of radiation modes with different polarization states are quite different,which shows a strong polarization dependency.(2)The theoretical and experimental investigations on the diffraction characteristics of the radiation mode.Based on the wave vectors analysis method,we have presented the theoretical model for analyzing the diffraction characteristics of TFGs.The angular dispersion of TFGs has been numerically characterized in terms of tilt angle,period and wavelength.Besides,we have built up the angular dispersion measuring system to experimentally investigate the diffraction characteristics of TFGs.The measurement results show that the angular dispersion of 45° TFG with 748 nm period is around 0.054°/nm.And the angular dispersion would become larger with the decreasing of tilt angle.(3)The application of 45° TFG in polarization spectrum analysis system.To explore the application of the polarization-dependent diffraction characteristics of 45° TFG,we have proposed a linear polarization spectrometer based on 45° TFG and experimentally evaluated its performance.According to the experimental results,the proposed spectrometer could achieve 0.057 nm spectral resolution and 117 nm wavelength responding range covering from 1495 nm to 1612 nm.Compared with the measuring system based on commercial optical spectrum analyze(OSA)for measuring the polarization-related spectra of excessively tilted fiber grating(Ex-TFG),the proposed spectrometer could avoid the adverse impacts introduced by the performance of polarizers on the measurement results.