Group-velocity-matched ultrashort-pulse synthesis in a hollow fibre filled with a Raman-active gas

Group-velocity mismatch between the pump and probe pulses is shown to limit the minimum duration of ultra-short pulses synthesised through the generation of multiple Stokes and anti-Stokes sidebands in a Raman-active medium preliminary excited by a short laser pulse. The group delay of pulses involved in short-pulse synthesis in a Raman-active gas can be reduced by using a hollow fibre with a properly chosen inner diameter, gas pressure, and waveguide modes. The number of Stokes and anti-Stokes sidebands in the nonlinear response of a Raman-active gas can be considerably increased under these conditions, opening the way to substantially reduce the duration of light pulses synthesised using this method.