Research On Wavelength Conversion Based On Nonlinear Effects In Photonic Crystal Fiber And Its Application

Photonic crystal fiber has many obvious advantages such as high nonlinearity,controble dispersion, which make it very suitable for research on nonlinear effects and wavelength conversion. The laser spectrum can be broadened by nonlinear effects such as four-wave mixing, stimulated Raman scattering, stimulated Brillouin scattering and so on. Supercontinuum, generated based on nonlinear effects in photonic crystal fiber,has found many applications in fields such as biophotonics, optical sensing, nonlinear spectroscopy, photo-communication and so on. In this dissertation, we studied supercontinuum generation in different photonic crystal fiber to construct a compact supercontinuum source. We studied the nonlinear effects in photonic crystal fibers and devoted to building a simple low-cost all-fiber supercontinuum source. The main content is as follows:The full vector frequency domain finite difference (FDFD) method is introduced. The virtues of FDFD is analysed, and the introduced of a kind of equalization of the medium boundary, wich is benefit for improving the computing accuracy. And then, the deduction of the generalized nonlinear Schrodinger equation is introduced and the split-step Fourier method is used to numerical model the supercontinuum generation phenomenon. We analyzed the characteristics of split-step Fourier method and matters need attention when using this method.We studied the supercontinuum generation in a seven-core photonic crystal fiber experimentally and simulatelly. We propose that introducing a piece of tapered section could change the group velocity curve and may lead to the compression of the pump pulses.In the compression process, the spectrum in the long wavelength region would speed up and overplap with other spectrum in the time domain and new spectrum would generate by nonlinear effects such as self-phase modulation, four-wave mixing and so on. The experiment results agree well with the simulated ones, and the supercontinuum generated in tapered seven-core photonic crystal fiber is flatter than in the uniform one. The short wavelength region of the supercontinuum was enhanced visibly as well.We fabricated a photonic crystal fiber with large air holes. The silicon bridge forms a series of fiber cores. The dispersion and nonlinear characteristics of a rectangle core were disgussed in details. We pumped the rectangle core with a femtosecond mode locked Yb-doped fiber laser with central wavelength of 1040 nm and realized high efficiency Cerenkov radiation generation.The convertion efficiency reached ?43% at best. Experiemt results demonstrated the dispersion wave was linear polarizated. We simulated the losses of the two nondegenerate fundamental modes and fond they differed much especially in the long wavelength region. So the rectangle core acted as single-mode single-polarization and leading to linear polarized Cerenkov radiation generation.A simple open cavity laser is proposed for supercontinuum generation. Broadband supercontinuum with the wavelength range of ?630-1700 nm with low multimode LD pump power (?1.7W) is obtained. Giant nanosecond pulses generation can be realized by simply pumping a piece of Yb-doped double cladding fiber combined with two pieces of long passive fibers without any reflectors or modulators in this laser. It is confirmed that the establishment of the passive self-Q-switch is mainly based on the stimulated Brillouin scattering effect. The peak power of the giant nanosecond pulses is high enough to generate over 1000 nm supercontinuum even in standard single-mode fiber, which makes this supercontinuum laser source low-cost and compact. Replacing the standard single-mode fiber with a piece of high nonlinear photonic crystal fiber, broader supercontinuum can be obtained attributing to the high nonlinear coefficient and the blue-shift of the zero-dispersion wavelength.A very simple figure 9 mode locked all fiber laser was built based on nonlinear sagnac loop mirror. It used double cladding Yb-doped fiber as gain fiber and could deliver picsecond pulse with average output power as high as 1W. with only one amplifier stage,the output power can exceed 10W. We realized high power supercontinuum generation with the help of low-loss splicing technology of PCF and single mode fiber.