Controlling nonlinear optics with dispersion in photonic crystal fibres

Abstract

Nonlinear optics enables the manipulation of the spectral and temporal features of light. We used the tailorable guidance properties of photonic crystal fibres to control and enhance nonlinear processeswith the aim of improving nonlinearity based optical sources. We utilised modern, high power, Ytterbium fibre lasers to pump either single photonic crystal fibres or a cascade of fibres with differing properties. Further extension of our control was realised with specifically tapered photonic crystal fibres which allowed for a continuous change in the fibre characteristics along their length. The majority of our work was concerned with supercontinuum generation. For continuous wave pumping we developed a statistical model of the distribution of soliton energies arising from modulational instability and used it to understand the optimum dispersion for efficient continuum expansion. A two-fold increase in spectral width was demonstrated, along with studies of the noise properties and pump bandwidth dependence of the continuum. For picosecond pumping we found that the supercontinuum bandwidth was limited by the four wave mixing phase-matching available in a single fibre. A technique to overcome this by using a cascade of fibres with different dispersion profiles was developed. Further improvement was achieved by using novel tapered PCFs to continuously extend the phase-matching. Analysis of this case showed that a key role was played by soliton trapping of dispersive waves and that our tapers strongly enhanced this effect. We demonstrated supercontinua spanning 0.34-2.4 ¹mwith an unprecedented spectral power; up to 5 mW/nm. The use of long, dispersion decreasing photonic crystal fibres enabled us to demonstrate adiabatic soliton compression at 1.06 ¹m. From a survey of fibre structures we found that working around the second zero dispersion wavelength was optimal as this allows for decreasing dispersion without decreasing the nonlinearity. We achieved compression ratios of over 15.