Abstract

We report and demonstrate the first technique for measuring the full, time-dependent intensity and phase, E(t), of an individual femtosecond pulse. This new technique, which we call “frequency-resolved optical gating” (FROG), uses the polarization-spectroscopy optical-gate arrangement with an instantaneously responding χ⁽³⁾ sample medium, such as glass. Here, however, the pulse is split and one version of the pulse gates the other (see Figs. 1 and 2). We then measure the signal spectrum as a function of the delay between the two input pulses. Because the signal pulse is shorter than the input pulses (by a factor of √3 for Gaussian pulses), the signal pulse reveals, for a given delay, the frequency of a particular temporal component of the ultrashort pulse (See Fig. 2). For reasonably well-behaved pulses, the output plot of signal intensity vs. frequency and delay graphically displays the pulse instantaneous frequency vs. time (See Fig. 3). More importantly, inversion of ω(t) to obtain t(ω), followed by integration of this result, yields the phase vs. frequency, φ(ω). In conjunction with the pulse spectrum, I(ω), which is also naturally obtained in FROG, this result yields the full amplitude and phase of the pulse field in the frequency domain, E(ω). Simple Fourier transformation yields E(t). We have demonstrated the method using a microscope slide as a nonlinear medium and ~620-nm, ~200- μJ, nearly transform-limited ~100-fsec pulses and ~200-fsec chirped pulses.

© 1992 Optical Society of America