Frequency up-conversion and pulse compression mediated by soliton plasma interactions in gas-filled photonic crystal fiber

Gas-filled hollow-core photonic crystal fiber (HC-PCF) is an ideal vehicle for studying nonlinear fiber optics in gaseous media [1]. It combines the merits of conventional fibers (tight single-mode confinement over long distances) with the advantages of gases: pressure-controlled dispersion, absence of optical damage and transparency in extreme wavelength ranges. In the case of kagomé-style HC-PCF, these features have permitted observation of highly efficient tunable deep-UV generation [2] and ionization-based nonlinear fiber optics [3,4]. In the latter case soliton self-compression produces intensities sufficient to partially ionize the filling gas (~10¹⁵ W/cm²), resulting in plasma-induced phase-modulation and a unique soliton self-frequency blue-shift. The initial dynamics of these phenomena are dominated by higher order soliton propagation and compression, followed by fission and the emission of multiple blue-shifting solitons. It has been predicted using perturbation theory, however, that fundamental solitons will self-frequency blue-shift in the absence of any higher order nonlinear effects [4]. In this paper we show numerically that a fundamental soliton can indeed cleanly blue-shift, and we go on to suggest how an all-fiber integrated device may be designed that allows tunable frequency up-conversion over an octave, combined with pulse compression.