Two-photon-absorption FROG: measuring white-light continuum pulses

Summary form only given. Frequency-resolved optical gating (FROG) is currently the most commonly used technique to fully characterize an ultrashort optical pulse. Various species of FROG have been demonstrated which utilize various optical nonlinearities, such as second harmonic generation (/spl chi//sup (2)/), polarization gate, transient grating, third harmonic generation and self diffraction (all /spl chi//sup (3)/). We devised a new FROG technique based on two-photon absorption (TPA) which is ideally suited for characterizing white-light continuum pulses. Two-photon absorption in various materials is routinely used for autocorrelation measurements of ultrashort laser pulses. Due to the resonant enhancement at the material´s band gap and the lack of phasematching requirements for TPA, this /spl chi//sup (3)/ nonlinearity is an excellent candidate for ultrashort pulse characterization. To produce a FROG trace based on TPA, two pulses are spatially overlapped in the TPA crystal and the spectrum of one pulse is measured as a function of the time delay between the two pulses. The TPA FROG trace is equivalent to the trace generated by polarization gate FROG. As a test of the new technique we have measured the second harmonic generation (SHG) and TPA FROG traces of pulses generated by a standard multipass Ti:sapphire amplifier.