Short-pulse four-wave mixing enhanced in hollow fibers as a method of highly sensitive gas-phase analysis

The possibilities of using four-wave mixing enhanced in hollow fibers for i mproving the sensitivity of gas-phase analysis are explored by studying thi rd-harmonic, difference-frequency, and sum-frequency generation processes. The use of 30-ps pulses with an energy of several millijoules in these expe riments allowed pump power densities up to 10(12) W/cm(2) to be coupled int o a hollow fiber, thus increasing the intensity of signals generated throug h four-wave mixing as compared with the nanosecond regime. Hollow fibers ar e shown to expand the possibilities of nonlinear-optical analysis of gases by allowing the generation of third-harmonic and sum-frequency signals, whi ch vanish in the regime of tight focusing in a medium with normal dispersio n.