Studies On Optical Properties Of Fiber Gratings And Splice Losses Of Photonic Crystal Fibers

Fiber gratings have the advantages of narrow linewidth, low insertion loss, electromagnetic anti-interference, high sensitivity, light weight, small size and easily realizing WDM et al. So fiber gratings have been widely applied in the all fiber systems, such as fiber lasers, fiber communications and fiber sensors, and it is useful to theoretically investigating fiber grating spectra. In the past two decades, photonic crystal fibers (PCFs) have been developed and have an attractive prospect in the applications of fibers and fiber devices. However, PCFs need to be spliced to other fibers in the application, so the low-loss splice is a very important problem. This dissertation is carried out basing on above questions and contents are as follows:1. The double clad fiber (DCF) is equivalent to a single mode fiber and an annular core fiber, and then theoretically investigated by using coupled mode method with four-order Runge-Kutta-Gill formula. The results show that the modes can significantly couple to the modes with identical symmetry, and the light exhibits oscillating transmission along the DCF. Finally, the influences of the wavelength, radius and refractive index of the inner clad on the average period and average power are numerically discussed.2. The fiber grating spectra are numerically investigated by using coupled mode method with four-order Runge-Kutta-Gill formula. Because of the asymmetric refractive index profile, the fiber gratings have the polarization dependent loss and the difference group delay, and the influences of fiber grating parameters on them are numerically discussed. Then the linear pulse responses of the fiber Bragg gratings are numerically investigated, and the results show that the ultrashort pulse is split into several pulses as soon as it is reflected from the fiber Bragg gratings. The influences of the grating strength, the chirp and spectra of the incident on the reflected pulses are also discussed. Finally, the influence of the mode loss on the fiber grating spectra is numerically investigated for the model of two mode coupling, and the results show that the mode loss can reduce the coupling efficiency and the cross-coupling peak is declined.3. Because of the high splice losses between the PCF (with a small core size) and the DCF (with a large core size), the core size of the PCF is enlarged by using the method of controlled air hole collapse, and the mode filed match between the processed PCF and the DCF. Then the processed PCF is cleaved in the middle of completely collapsed region and fused to the DCF, and the splice loss is significantly declined by using this method.4. The rotation loss is proposed on the base of the asymmetric mode field distribution of the PCFs, and the rotation losses of three types of PCF are numerically investigated by using finite element method. The influences of the PCF's parameters (e.g., rotation angle, wavelength and air hole diameter) on the rotation loss are also numerically discussed. Finally, the two identical PCFs are fused in the experiment, and the maximum of measured rotation loss is consistent with the simulation result. Thus the results are useful for further reduction of splice losses of PCFs.