Four-wave mixing of picosecond pulses in hollow fibers: expanding the possibilities of gas-phase analysis

Third-harmonic, difference-frequency, and sum-frequency generation processe s in hollow fibers are experimentally studied with 30-ps pulses having an e nergy of several millijoules. The experimental dependence of the difference -frequency signal on the pressure of the gas filling the fiber agrees well with the results of calculations when the contribution of higher order wave guide modes is taken into consideration, thus indicating the importance of nonlinear-optical processes involving higher order waveguide modes of a hol low fiber. Hollow fibers are also shown to expand the possibilities of nonl inear-optical analysis of gases by allowing the generation of third-harmoni c and sum-frequency signals, which vanish in the regime of tight focusing i n a medium with normal dispersion.