Research On Supercontinuum Generation In Normal Dispersion Fibers

ABSTRACT:The photonic crystal fibers with small-core diameters are very important in the research of supercontinuum generation because of their many advantages, such as high nonlinearity, no cut-off single-mode, controllable dispersion and so on. So far, the supercontinuum generation based on the fibers have become an important branch of nonlinear fiber optics. Since the flat-top supercontinuum generation with a broad wavelength range exhibit enormous applications, it is very important to design the photonic crystal fiber with a flat dispersion curve by optimizing the geometric structure of the fibers. In this thesis, we focus on the supercontinuum generation in all-normal dispersion photonic crystal fiber by pumping ultra-short pulses. The contents of my thesis are arranged as follows:Firstly, an single-mode step index fiber, whose zero dispersion wavelength is1298nm, is proposed to compare the results of the supercontinuum generation between the cases of pumping in the normal and anomalous dispersion regimes. Supercontinuum generations in this fiber are analyzed theoretically when the center wavelength of the pump pulse is close to the zero dispersion wavelength in the normal or anomalous dispersion regime. When the pump wavelength is in the anomalous dispersion regime and close to the zero dispersion wavelength, the wavelength bandwidth of the output spectrum is1418nm (at20dB level) with a flatness of42dB.Secondly, the influences of the structure parameters on the dispersion characteristics in the photonic crystal fibers are analyzed.two ways for adjusting the parameters of the photonic crystal fiber are presented. The first on is to change hole diameters with keeping the ratio of hole diameter and the hole pitch unchanged; Another way is adjusting the ratio of the hole diameter and the hole pitch with the constant hole pitch. By an appropriate choice of the geometric parameters of the fibers, an all-normal dispersion property in the photonic crystal fiber can be obtained. Furthermore, the supercontinuum generation in this fiber by pumping femtosecond pulses is analyzed numerically. The wavelength bandwidth of the output spectrum is1131nm with good flatness. Moreover, we also discussed the influence of the pump pulse parameters on the property of the generated supercontinuum.An all-normal photonic crystal fiber with an inner ring of the air holes filled with liquid oil, is presented. The simulation results showed that the obtained optical fiber has a flat dispersion curve ranging from1000nm to2000nm. By adjusting the parameters of the designed photonic crystal fiber, the supercontinuum generated in the fiber can span from918nm to2295nm, and the spectrum has a good flatness over the entire bandwidth.