The Study On Supercontinuum Generation By Pumping Photonic Crystal Fibers With Long-pulses And Continuous-wave

The study on supercontinuum (SC) generation has attracted widespread interest, not only because it provides further understanding into the physical mechanism of nonlinear effects and new insight into the influence of fiber dispersion on pulses'temporal and spectral evolution, but also because SC sources have recently found a wide range of applications such as wavelength-division multiplexing in optical communications, spectroscopy, optical frequency metrology, optical coherence tomography, military field. In this dissertation, we theoretically and experimentally investigate SC generation by pumping photonic crystal fiber (PCF) with single-wavelength and dual-wavelength scheme in the long-pulses and continuous-wave (CW) regime, respectively. The primary contents are presented as follows:1. SC generation by pumping PCF with single-wavelength scheme in long-pulses and CW regime have been numerically investigated by solving the generalized nonlinear schr?dinger equation (GNLSE) with the adaptive split-step Fourier method (ASSFM). In long-pulses regime, we study modulation instability (MI), which plays an important role in the breakup of long-pulses, and fission of higher-order solitons into redshifted fundamental solitons and blueshifted dispersive waves by gradually increasing the peak power of incident pulse, and finally get a flat and broadband SC which have a good agreement with the experimental results. In the CW regime, we investigate the breakup of CW, self-steepening after the formation of ultra-short pulses and soliton trapping that can extend the spetrum into the visible wavelength by gradually increasing the length of PCF, apply the method of averaging the generated spectra in many simulations, and lastly achieve a good agreement between the simulated SC and the experimental observations..2. We have theoretically and experimentally studied four-wave mixing (FWM) in PCF, and proposed the optical wavelength converter. First of all, we deduce the evolutive equations for the signal and idler waves, parametric gain and phase-matching condition from a set of coupled amplitude equations. Secondly, we numerically research several elements'influence on FWM such as the structural parameters of PCF, the peak power and width of incident pulse, and then summarize some useful rules about FWM. Thirdly, FWM (the signal wave at747nm and the idler wave at1848nm) is generated by pumping PCF in its normal dispersion region with a Nd:YAG Q-switched nanosecond microchip laser. Lastly, we propose the optical wavelength converter based on FWM in PCF. It is more compact and flexible compared with the optical wavelength converter based on FWM in laser. It only need one laser that can provide pump source and converted wavelength at the same time, and can realize the wavelength conversion in a larger wavelength range owing to the variable structure of PCF. 3. SC generation by pumping PCF with dual-wavelength scheme in long-pulses and CW regime have been numerically investigated based on the theoretical model of all-fiber dual-wavelength-pumped experimental scheme. In long-pulses regime, we study SC generation in the two different views, on one hand, the sole evolution of the remained pump wave and the signal wave is simulated, respectively, to explore their role in the final spectral formation; on the other hand, their combined evolution are simulated to oberve the interaction caused by cross-phase modulation (XPM), in addition, we also take into account the influence of group-velocity mismatch on SC generation. Moreover, we apply the all-fiber dual-wavelength-pumped scheme to CW regime and investigate CW dual-wavelength-pumped SC generation. Considering the higher output power of CW laser, the numerical simulations indicate that this scheme is promising for the high spectral power densities (SPD) and visible SC generation.4. SC generation by pumping PCF with picosecond, nanosecond pulses and CW have been experimentally studied. Firstly, we propose one method for effectively increasing mode field diameter (MFD) of PCF by heating up to collapse air holes in the cladding. The increase of MFD can not only evidently enhance optical coupling efficiency and surface damage limit, but also apparently reduce splicing loss caused by mismatch of modal field. Secondly, SC generation with a high optical-to-optical conversion efficiency of up to 85%, which spans from below 600nm to beyond 1700nm of 15dB bandwidth, is achieved by pumping PCF with 14ps pulses at 1064 nm. Thirdly, SC spanning from 710nm to1700nm of 10dB bandwidth with average power up to1.2W is generated by pumping PCF with 200ns pulses at 1064 nm. Finally, we try to explore SC generation in CW-pumped regime and analyze some relevant difficulty in the experiment, such as the heat accumulation at the splicing point, to provide the useful references for high SPD and broadband SC.