Real-time intuitive spectrogram measurement of ultrashort optical pulses using two-photon absorption in a...

Real-time acquisition of intuitive spectrograms based on twophoton absorption frequency-resolved optical gating is demonstrated in a wavelength range around 1500 nm using an InP crystal for a two-photon absorption medium. Rapid wavelength-delay scanning, based on a counterrotating spectrometer mirror synchronized with a delay stage, is introduced and incorporated with lock-in detection for the real-time spectrogram acquisition. It is shown that the frequency marginal and average delay time of acquired spectrograms provide the spectral intensity and group delay time of optical pulses under test. This allows the direct and rapid measurement of the magnitude and phase of ultrashort optical pulses in the spectral domain without using pulse retrieval algorithms to reconstruct the pulse shapes.