is also covered. This is a perfectly viable way to generate a supercontinuum. However, it is not possible to generate the same bandwidths by this method. Soliton fission regime In the soliton fission regime a short, high power, femtosecond pulse is launched into the PCF or other highly nonlinear fiber. The femtosecond pulse may be considered as a high order soliton, consequently it rapidly broadens and then fissions into fundamental solitons. During the fission process excess energy is shed as dispersive waves on the short wavelength side. Generally these dispersive waves will undergo no further shifting and thus the extension short of the pump is dependent on how broadly the soliton expands as it breathes. The fundamental solitons then undergo intra-pulse Raman scattering and shift to longer wavelengths (also known as the soliton self-frequency shift), generating the long wavelength side of the continuum. It is possible for the soliton Raman continuum to interact with the dispersive radiation via four-wave mixing and cross-phase modulation. Under certain circumstances, it is possible for these dispersive waves to be coupled with the solitons via the soliton trapping effect. This effect means that as the soliton self-frequency shifts to longer wavelengths, the coupled dispersive wave is shifted to shorter wavelengths as dictated by the group velocity matching conditions. Generally, this soliton trapping mechanism allows for the continuum to extend to shorter wavelengths than is possible via any other mechanism. The first supercontinuum generated in PCF operated in this regime and many of the subsequent experiments also made use of ultra-short pulsed femtosecond systems as a pump source. One of the main advantages of this regime is that the continuum often exhibits a high degree of temporal coherence, in addition it is possible to generate broad supercontinua in very short lengths of PCF. Disadvantages include an inability to scale to very high average powers in the continuum, although the limiting factor here is the available pump sources; and typically the spectrum is not smooth due to the localised nature of the spectral components which generate it. Whether this regime is dominant can be worked out from the pulse and fibre parameters. We can define a soliton fission length, , to estimate the length at which the highest soliton compression is achieved, such that: where is the characteristic dispersion length and is the soliton order. As fission tends to occur at this length then provided that is