Abstracts Category : Other

Photonic crystal fibers: characterization andsupercontinuum generation

by Zhaoming (1973 Zhu

Photonic crystals are periodic dielectricstructures that exhibit photonic bandgaps (PBGs), spectral regionswithin which light is forbidden to propagate inside the medium. Asan exciting area in photonic crystals and fiber optics, photoniccrystal fibers (PCFs, both index-guiding and PBG-guiding) havebrought us many new opportunities by enabling new photonic devicesfor a range of applications. In this thesis, I investigatedindex-guiding PCFs in two respects: modeling their opticalproperties and studying supercontinuum (SC) generation in them. </br>As the first part of my research, I developedand/or applied several efficient methods in modeling andcharacterization of index-guiding PCFs, some of which can be easilyapplied in studying PBG-guiding PCFs. Based on a plane-waveexpansion approach, I proposed a method by which one can obtain theeffective cladding indices by solving the full-vector wave equationfor a periodic cladding structure. This work improves the effectiveindex model for the characterization of PCFs, which can provideinsights into some of the interesting properties of PCFs, including'sendlessly single mode operation" and novel dispersion properties.In order to rigorously model PCFs, however, one needs to solve thefull-vector wave equation because the complex structures andhigh-index contrast make the scalar wave equation insufficient. Tothis end, I investigated two methods: multipole scattering methodand full-vector finitedifference frequency-domain (FDFD) method.Both methods were demonstrated to be efficient and can accuratelycharacterize linear optical properties of PCFs, includingpropagation constants, modal fields, dispersion, birefringence andmodal area. I also studied birefringent properties of PCFs undersqueezing or twisting by using a finite element method. This studyshows that stress-induced linear birefringence in a PCF is reducedwhen compared to a standard fiber, and that twist-induced circularbirefringence is enhanced in a PCF having small air-fillingfraction in the cladding. </br>In the second partof the thesis research, I studied SC generation in PCFs. The higheffective nonlinearity of a PCF due to the small core size can becombined with a shifted zero-dispersion wavelength (shifted to awavelength region where high-power laser sources - Ti:sapphirefemtosecond lasers - are available). This combination facilitatesthe generation of broadband SC spanning more than 2 octaves. Usinga generalized scalar nonlinear Schrdinger equation, I firstnumerically studied the effect of input pulse frequency chirping onSC generation in PCFs. The simulations show that the SC bandwidthincreases with linear chirping, and that the coherence of SCimproves as linear chirping increases. Most interestingly, anoptimal positive linear chirp is identified that maximizes the SCbandwidth, corresponding to the formation of only one red-shiftingRaman soliton. Next I numerically investigated the polarizationproperties of SC spectra generated in birefringent PCFs, by solving