Visualization of optical pulse filamentation by femtosecond time-resolved optical polarigraphy

We have observed the filamentation of optical pulses in carbon disulfide(CS2) using femtosecond time-resolved optical polarigraphy(FTOP). A pump-probe setup is used to capture the propagation of a 150 femtosecond laser pulse in CS2. The probe pulse propagates in a direction perpendicular to the pump. The high intensity of the pump pulse causes a transient index change in the material through the Kerr effect. The induced birefringence is proportional to the intensity of the pump and can be captured by monitoring the polarization of the probe. The probe pulse is imaged on a CCD camera to recover the intensity profile of the pump pulse. We have used this technique to observe the spatial evolution of the pulse as a function of power and propagation distance. Initially, the pulse propagation causes a coarse redistribution of the intensity. The beam then breaks up into stable light filaments which propagate for several millimeters, and finally the beam profile becomes unstable to small fluctuations in the input power.