Resonant frequency conversion in photonic crystal fibres

Abstract

This thesis is concerned with the frequency conversion process of resonant radiation, whereby a pulse of light in the anomalous dispersion regime of a nonlinear optical fibre sheds light to a resonant dispersive wave in the normal dispersion regime. Using pulses from a Ti:sapphire laser and a photonic crystal fibre (PCF), this mechanism provides efficient, tunable conversion from the near infra-red to visible femtosecond pulses. This promising technology has improved noise properties over the more conventional PCF supercontinuum source, greater tunability than frequency doubling techniques, and a highly portable design requiring only millimetres of fibre. Progress from the literature towards the development of resonant radiation as a practical light source is reviewed, with areas for improvement identified.

A systematic experimental investigation of the effect of incoupled pulse energy, input pulse wavelength and input pulse chirp is performed, expanding the parameter-space from previous works. Particular attention is paid to input pulse chirp, the importance of which has become clear in recent simulations. Using a selection of fused silica solid-core PCFs, extraordinary tunability of the resonant radiation over all visible wavelengths is demonstrated, rivalling the supercontinuum source for visible applications. Unusual regimes of conversion are presented, including generation of multi-peak, very broad, or very narrow spectra. Resonant radiation on the negative branch of the PCF dispersion relation is also demonstrated, in a regime not previously explored.

As a primary novel result, it is shown that optimising input pulse chirp when using ~1nJ incoupled pulse energies boosts the conversion efficiency above the current record for visible resonant radiation. The interpretation and implications of this behaviour are discussed. Temporal measurements of resonant radiation using cross-correlation frequency-resolved optical gating (XFROG) demonstrate a visible pulse duration of 59fs, potentially compressible to even shorter durations.