Abstract

Ever since its discovery spectral continuum generation by ultrashort laser pulses has attracted much interest owing to its significance for ultrafast spectroscopy. In this paper we report, for what is to our knowledge the first time, the generation of nearly diffraction-limited truly white-light femtosecond continuum exhibiting comparable spectral intensities all over the range of 400-1100 nm. This performance could be obtained by propagating very short (≈ 25 fs) pulses through a capillary (or hollow waveguide) filled with some noble gas as a nonlinear medium. The key advantage of waveguiding lies in the fact that energy transfer into higher-order transverse modes and hence associated accumulation of spatial effects (e.g. self focusing/defocusing, filamentation) is confined to a relatively short propagation length over which higher-order modes dephase with respect to the fundamental one. Spatially uniform self-phase modulation (SPM) of the fundamental mode, however, can accumulate over much longer distances limited merely by propagation losses and possible pulse broadening effects. As a result, hollow waveguides offer the potential for a much stronger nonlinear phase accumulation without degradation of beam quality as compared to bulk media.

© 1998 IEEE