Abstract. We present an imaging technique to visualize instantaneous intensity distributions of propagating femtosecond optical pulses by making use of the optical Kerr effect (OKE), which is called femtosecond time-resolved optical polarigraphy (FTOP). By modifying the apparatus from that previously reported, we have succeeded in taking instantaneous profiles of femtosecond optical pulses under loosely focused propagation with a satisfactorily large dynamic range, even in a single-shot observation. In particular, we show pulse propagation behaviours in 2, 3 and 4 atm nitrogen gas when pulses with a 5.0 mm diameter are focused by an f = 350 mm lens. FTOP images reveal the dependence of the propagation behaviour on the gas pressure. They also show alterations in the distribution widths and the pulse energy as the pulse progresses, which is considered to result from nonlinear interactions. Shot-to-shot fluctuations of pulse propagations are directly observed from the FTOP images taken under the same experimental parameters. We also discuss the possibility of applying the OKE echo to at-a-stretch measurements at several temporal points.

Keywords: instantaneous intensity distribution, femtosecond, imaging, optical Kerr effect, propagation monitoring