| The visible supercontinuum (SC), which is characteristic of good spatial coherence and high brightness, have (has) important applications in biophotonics, etc.. By exhibiting an enhanced effective nonlinearity and promoting parametric processes, photonic crystal fibers (PCFs) thus appear to be the ideal media for SC generation. In this thesis, we study the visible SC generation in the highly nonlinear PCF for expanding the blue edge, improving the conversion efficiency and the special mode output.(1) The nonlinear process that is involved in the SC generation is introduced. And the frequency domain generalized nonlinear Schrodinger equation is described in detail. Combining the frequency-domain finite difference method, a SC generation in PCF is given.(2) A PCF with nanosize air-holes (NAHs) in the solid core for the blue extension of SC generation is investigated. The basic concept of the design is to enhance the evanescent wave in the IR part of the SC. On the premise of fixing the zero dispersion wavelength around1μm, the group-velocity of the proposed fiber can match the infrared wavelength of2.5μm with short-wavelength of403nm, which is about60nm shorter than that of conventional high-△PCF. Simultaneously, the nonlinearity is enhanced about three times. The dependence of the PCF characteristics on the NAHs is also discussed. The simulated results confirm the possibility of increasing the blue-shift of the generated SC in the designed PCFs.(3) We demonstrate the generation of highly efficient Cherenkov radiation (CR) in the fundamental mode of a GeO2-doped two zero dispersion wavelengths (ZDWs) PCF. Using a high power femtosecond Yb-doped PCF laser emitting100fs pulses as the pump source, CR with an efficiency of>40%and a bandwidth of38nm at410nm is obtained in the visible-wavelength range when the average power of the pump light is1.27W. It is that injecting the pump light in deep anomalous dispersion regime contributes to such an efficient spectral-isolated CR. The mechanism during the formation of CR is discussed and the experimental results are in good agreement with the calculation.(4) Two special multi-core photonic crystal fiber are studied:the dual-concentric-core PCF and GeO2-doped triangular-core PCF. For the former, when pumping in the inner core and the visible SC is obtain in the outer core. Since the group velocity matching is absent, the spectra in short-wavelength side can be controlled by phase-matching and in long-wavelength side the spectra is determined by the second ZDW. As a result, the output spectrum shape can be well controlled. For the latter, a GeO2doped triangular-core PCF is designed and fabricated to allow the generation of a hollow beam through a nonlinear-optical transformation by femtosecond pulses at1040nm from a high power Yb-doped PCF laser oscillator. The hollow beam SC is obtained at far field by adjusting incident light polarization to excite the high order supermode, behaving as a mode convertor. The SC ranging from540to1540nm is achieved with an average pump power of1.04W. |
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