Tracing the Phase Distortion of a Single Femtosecond Light Pulse

A frequency tracer (FT) based on noncollinear second harmonic generation autocorrelator for two-dimensional time-frequency imaging of a single femtosecond light pulse without using a spectral apparatus [1] had implemented for a first time. An experimental test of frequency tracer has been performed by measuring the instantaneous frequency of femtosecond pulses produced by a chirped pulse amplification Ti:sapphire laser system. In a normal operation the system is able to produce pulses up to 1.5 mJ energy and about 90 fs duration FWHM. The frequency traces were collected for pulses at different compressor grating separations. Experimental data on the linear chirp and pulse duration have been checked against a simple model, where the shape of the gausian pulse changes due to propagation through a dispersive path between pair of diffraction grating in a compressor. Detuning of the grating separation from its optimal value results in quadratic (in terms of frequency) phase shift of spectral components of output pulse. The data on chirp and pulse duration versus grating separation are in a good agreement with the estimations of dispersions at the system output. The registered images shown not only linear chirp of pulses (which was measured unambiguously) but also the fourth order phase distortion (cubic chirp). We prove by numerical calculations that images produced by the frequency tracer have simple intuitive meaning: the frequency trace is formed by maxima of two-dimensional image and does not require any iterative retrieval algorithm for visualising the phase distortions [2]. The calculations show that the second-harmonic frequency tracer allows to measure the even-order phase distortions of femtosecond light pulses: chirp, fourth-order, etc. These distortions include the phase self-modulation (Kerr-nonlinear contribution) of symmetric pulse.