Supercontinuum generation in tapered fibres

Summary form only given. The key feature of the microstructured fibre is the large index step between core (silica) and cladding (mostly air). This permits confinement in a very small core and hence (a) high intensity for a given power (or pulse energy), and (b) zero or anomalous dispersion at the pump wavelength, despite the strong normal dispersion of bulk silica. The disadvantage is the need for the special fibre with its tiny core. Having obtained fibre (perhaps at some expense), the user finds input coupling problematic and sensitive to mechanical instabilities, with high intensities that can degrade the endfaces. A large index step is also found in conventional telecoms fibre tapered (heated and stretched) to a narrow uniform waist /spl sim/2 /spl mu/m in diameter and several cm long. Tapering need not increase the loss by more than 0.1 dB. As a waveguide, the waist is like the core of the microstructured fibre - a thread of glass surrounded by air - and we found that such a structure similarly broadened fs pulses from a Ti:sapphire laser to a two-octave supercontinuum. The output was in the fundamental mode even where the fibre itself was multimode.