Analysis of instantaneous profiles of intense femtosecond optical pulses propagating in helium gas measured by using femtosecond time-resolved optical polarigraphy - art. no. 033813

We analyze instantaneous profiles of intense femtosecond optical pulses pro pagating in helium gas measured using femtosecond time-resolved optical pol arigraphy (FTOP). Considering the characteristics of the optical Kerr effec t, relations linking images obtained by FTOP with intensity distributions o f optical pulses are derived under approximations applicable to a relativel y wide range of experimental conditions. By inversely applying the derived equations, images proportional to instantaneous intensity distributions of propagating light pulses are constructed from snapshot images taken in the experiment. From an analysis of the images obtained, we discuss the tempora l changes in the optical-pulse profiles. We can directly observe the energy reduction of the optical pulses after they pass through the simultaneously generated laser plasma. It is confirmed that each filament can be separate ly investigated by this method even under multifilament conditions. Further more, the images clearly reveal intensity distributions along the propagati on axis as well as cross-section distributions of femtosecond-pulse filamen ts, and show temporal shape modulations shorter than the incident pulse wid th in the middle of the propagation. We specify the volume where the tight energy concentrates at a particular instant of time, and directly observe t he cross-section expansion of the volume at the focal point brought about d ue to nonlinear effects.