The Study On The Generation Of Cascaded Four-wave Mixing In Photonic Crystal Fibers

The source of cascaded four-wave mixing(CFWM) is generated from a cascade of four-wave mixing(FWM) processes. It owns a number of applications ranging from multi-wavelength source for optical sensing, dense wavelength-division multiplexing system to high repetition rate pulse source. CFWM with good coherence can also be initiated to generate optical frequency combs. Recent years, the development of highly nonlinear fibers and photonic crystal fibers(PCFs) has opened new horizons in the generation of broadband CFWM products. However, most of the CFWM products centered wavelength at 1.5μm. Other spectral regions are also of great interest—especially generating a broadband cascade centered at 1μm is indispensable for a frequency comb that covers the visible region and the telecommunication window simultaneously. On the other hand, it was difficult to obtain high CFWM efficiencies at smaller spacing and with broader wavelength band. In this dissertation, We have proposed a CFWM source generated in PCFs, mainly pumped by a high peak power pulse and seeded by low power continuous waves. By adopting the pumping schemes of two-wavelength, three-wavelength and more-wavelength, we theoretically and experimentally investigate the generation of CFWM centered at 1μm in PCFs. The primary contents are presented as follows:1. Embarking from the theory of nonlinear optics, phase matching conditions and the gain of FWM have been numerically studied. After analyze the influence factors of FWM, CFWM with two-wavelength pumping in different dispersion regions is also investigated. And we conclude that in the two-wavelength pumping scheme, wavelengths of the continuous waves should be selected according to the different nonlinear gains.2. Based on the theoretical and numerical analysis, two-wavelength pump sources for the CFWM products are set up. And the generations of CFWM products are investigated in three different kinds of PCFs. When the zero dispersion wavelength(ZDW) of PCF is longer than the pulse wavelength, stimulated Raman scattering(SRS) is the main nonlinear effects for the spectral spanning. Taking advantages of SRS and FWM, CFWM products spanning for 300 nm with more stokes peaks and frequency spacing smaller than 13.2THz are achieved. As for the all-normal dispersion PCF, the unique dispersion characteristics makes it suited to the generation of coherent supercontinuum. But the phase matching condition is difficult to be satisfied, so idea CFWM cannot be achieved in this kind of PCF. When ZDW is shorter than and close to the pulse pump wavelength, modulation instability(MI) domains. So CFWM with different frequency spacing can be easily obtained in this kind of PCF. In the experiment, we realized the CFWM with wavelength spanning for 700 nm, with is the broadest band in fibers.3. Influence of the CFWM to the spectral spanning are numerically and experimentally investigated. After the investigation with different continuous wavelengths we find that if the CFWM triggered by continuous wave is only generated in the anomalous dispersion region, spectrum will be broadened. Otherwise, the spectrum will be narrowed after the continuous wave seeding. Then the numerical results give the relation between continuous wavelength and its influence(narrow or broaden) to the spectrum.4. we proposed the scheme for CFWM products with a pulse and two continuous waves pumping. Using this three-wavelength pumping mechanism, equally frequency spaced and unequally frequency spaced CFWM can be obtained. The equally frequency spaced CFWM owns the advantage of smaller frequency spacing. In the experiment, the frequency spacing is as small as 477 GHz. A new kind of equally frequency spaced CFWM are generated, whose frequency spacing is not equal to but 1/N of the pumping spacing, where N is a positive integer. We first achieve the unequally spaced CFWM with spectral envelopes. After numerical research on the CFWM generation with more wavelength pumping we find that CFWM with narrower frequency spacing can be generated with more continuous wavelengths seeded, if one the of continuous waves is seeded near the MI gain peaks and all frequency spacings are as small as possible.