Effects Of Nonlinearity, Apodization And Phase-shift On The Reflectivity Spectrum Of Fiber Bragg Graings

Fiber grating is a recently developed optical device. Its advantages overcompeting technologies include high wavelength selection, all-fiber geometry,low insertion loss, simple structure and small volume. All these characterspresent a broad application field for fiber grating in the field of optical fibercommunications. The optical properties of the optical fiber core would be changed eternallywhen it is exposed to UV light. Using this photosensitive property, opticalfiber grating is made to have a periodical index change along the fiber core.When light is transmitted through such kind of waveguide, it experiencesperiodical phase changes. Coupled-mode theory is a most important theory in the study of thetransmission characters of fiber gratings. This theory deals the forward andbackward light separately. Gratings are the medium that couples the forwardand backward light. Based on this theory, this thesis studied the effects of - 81 -吉林大学硕士学位论文nonlinearity, apodization and phase-shift on the reflective spectrum of FBGs. A brand new method--zero figure method is used to study of nonlinearity ofthe reflectivity spectrum. Study results show that:1．As the light intensity increases, the stop band moves to higher wavelength.2．When the light intensity reaches a certain value, the right edge of the stop band will move across the original central of the stop band. This means that, when the light intensity reaches a certain value, the light with a Bragg wavelength will not be reflected anymore, it will be transmitted. Based on this phenomenon, fiber grating can be used as a switch for theBragg wavelength. The merits of Zero Figure method include simple calculation, easyapplication and vivid demonstration of nonlinear effect. Undoubtedly, ZeroFigure method will have important applications in the study of nonlinearity ofthe FBGs. The sidelobes in the reflectivity spectrum of common fiber Bragg gratingwill greatly deteriorate the filtering properties. Optical apodization canremove these sidelobes. In this paper, transmission matrix is introduced tocalculate the reflectivity spectrum of apodized fiber gratings. Numericalresults show that apodization can effectively suppress the sidelobes in the - 82 -吉林大学硕士学位论文reflectivity spectrum. The reflectivity in the stop band can reach 100%, andthe light frequencies outside the stop band will not be affected by the stopband and will be transmitted out the fiber gratings. By analyzing andcontrasting the reflectivity spectrum obtained under different apodizationfunctions, we find these laws that guide the characters of apodization:1. For a certain apodization function, the band width, steepness of the band edges and the extent of the sidelobe suppression are all related to the parameters of the apodization function. As a matter of fact, sidelobe suppression will deteriorate the band width and the steepness of the band edges. So the parameters of the apodization functions have to be carefully selected before an apodized FBG is fabricated.2. Partially apodized FBGs have a wider bandwidth and steeper band edges than the fully apodized FBGs.3. The reflectivity spectrum apodized with Sup-Gauss function and Tanh function have a more complicated variation with the parameters. By selecting appropriate parameters, the sidelobes adjacent to the stop band and the sidelobes far away from the stop band can be suppressed selectively.4. Tanh function, super-Gauss function, Cauchy function, Gauss function and cosine function can effectively suppress the sidelobes adjacent to the stop band.5. Raised-cosine function, Tanh function and super-Gauss function can effectively suppress the sidelobes faraway from the stop band.6. The more slowly the index modulation reaches its maximum value, the...