Spectral broadening of femtosecond laser pulse through ionizing-gas-filled capillary

This paper presents experimental results of spectral broadening of femtosecond laser pulses induced by self phase modulation from plasma nonlinearity while propagating through ionizing-gas-filled capillary. Spectral broadening of the output laser pulses from capillary is in excess of 50 nm under experimental conditions of 10 mm long capillary and with a back pressure of 50 Torr nitrogen. The input and output laser pulse energy is 27 and 5.4 mJ, respectively. Both the maximal input and output energy of laser pulses under this scheme are much higher than those employing Kerr-effect exclusively with laser intensity lower than multi-photon or self-focusing threshold of the gas. Phase of the spectrally broadened laser pulses is measured to be quadratic at exit of the capillary. This novel plasma nonlinear source has the potential application in the direct generation of few-optical-cycle pulses with energies of at least several mJ.