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