Novel aspects of pulse propagation in photonic crystal fibers

Abstract

The study of nonlinear pulse propagation in photonic-crystal fibers (PCFs) produced new opportunities in fiber-optical research, mainly due to the unique guiding properties of the PCFs. The light is strongly confined to a small area of glass, thus enhancing the interaction of the light with the silica molecules. This caused a widespread distribution of the PCFs for various applications. Still, the demand for fiber-based light sources, having tailored light properties, grows steadily. This gives the motivation for thesis: The study of light traversing PCFs with engineered and unconventional dispersion landscapes. For this work, a large range of different shapes was realized by postprocessing several stock PCFs. Additionally, numerical modeling of light propagation in the devices assisted to understand the governing physical mechanisms.One part of this thesis shows that the influence of the core and cladding geometry on the modal properties can be adjusted to achieve efficient supercontinuum generation, novel pulse shaping mechanisms or nonlinear wavelength conversion. At the beginning we report on novel temporal and spectral modulations in short pieces of highly nonlinear PCFs. In optical fibers the evolution of the propagating light is usually governed by the interplay between the nonlinearity and dispersion of the fiber. In highly nonlinear PCFs this balance can be strongly biased in favor of the nonlinearity, and soliton formation is initially suppressed. Instead, a train of ultrashort pulses develops, having repetition rates in the THz region. We demonstrate the generation of a train of 50 fs pulses with a repetition rate as high as 14 THz; the length of the PCF was only a few centimeters.We discuss the tunability of the pulse-train parameters by the laser and fiber parameters.PCFs can be fabricated to have a submicron core region, which maximizes the nonlinear coefficient. Short pieces of these devices can be used to control the spectrum of laser pulses, for low threshold pump powers. The spectral brightness can be adjusted by the pump wavelength. These fibers are promising candidates for white-light sources with repetition rates in the GHz range.Additionally, the unique dispersion shape and the pronounced frequency dependence of the non-IV linearity of submicron-core PCFs result in unexpected soliton dynamics. Light launched at the maximum dispersion point leads to surprisingly strong and multiple soliton interactions. The proximity of a zero-dispersion point increases the energy transfer into linear radiation by more than 25%, destroying afterwards the existence of one of the solitons.In the next step, the influence of the microstructured cladding on the overall dispersion is balanced to create three zero-dispersion points close to each other, while still being endlessly single-mode.This scenario results in unexpected behavior -both for quasi-CW light and ultrashort pulses.It is shown numerically that fibers of this type offer highly unconventional phase-matching landscapes for four-wave mixing (FW...