FREQUENCY-RESOLVED OPTICAL GATING USING CASCADED 2ND-ORDER NONLINEARITIES

We demonstrate frequency-resolved optical gating (FROG) using cascaded second-order nonlinearities (up-conversion followed by down-conversio n). We describe two different cascaded second-order beam geometries-se lf-diffraction and polarization-gate-which are identical to their thir d-order nonlinear-optical cousins, except that they use second-harmoni c-generation crystals instead of (weaker) third-order materials. Like the corresponding third-order processes, these new versions of FROG yi eld the same intuitive traces, uniquely determine the pulse intensity and phase (without direction-of-time ambiguity), and yield signal ligh t at the input-pulse wavelength (which simplifies the required spectra l measurements). Most importantly, however, we show that these techniq ues are significantly more sensitive than the corresponding third-orde r FROG methods, conveniently allowing, for the first time, the unambig uous measurement of ultrashort similar to 1-nJ pulses, that is, unampl ified Ti:sapphire oscillator pulses.