CHARACTERIZATION OF ARBITRARY FEMTOSECOND PULSES USING FREQUENCY-RESOLVED OPTICAL GATING

We introduce a new technique, which we call frequency-resolved optical gating (FROG), for characterizing and displaying arbitrary femtosecond pulses. The method is simple, general, broad-band, and does not require a reference pulse. Using virtually any instantaneous nonlinear-optical effect, FROG involves measuring the spectrum of the signal pulse as a function of the delay between two input pulses. The resulting trace of intensity versus frequency and delay is related to the pulse's spectrogram, a visually intuitive transform containing both time and frequency information. We prove, using phase retrieval concepts, that the FROG trace yields the full intensity I(t) and phase phi(t) of an arbitrary ultrashort pulse with no physically significant ambiguities. We argue, in analogy with acoustics problems, that the FROG trace is in many ways as useful a representation of the pulse as the field itself. FROG appears to have temporal resolution limited only by the response of the nonlinear medium. We demonstrate the method using self-diffraction via the electronic Kerr effect in BK-7 glass and few muJ, 620 nm, linearly chirped, approximately 200 fs pulses.