Real time femtosecond optical pulse measurement using a video-rate frequency-resolved-optical-gating system

Abstract

Second-harmonic frequency resolved optical gating (SHG-FROG) has become a common and reliable technique for the measurement of the amplitude and phase of ultrafast optical pulses of the kind typically produced by mode-locked femtosecond laser. Commonly SHG-FROG measurement is carried out over a period of several seconds to many minutes by using a slowly scanning delay-line coupled to a single-shot spectrometer and other techniques to speed up the acquisition rate.'.' In this work we demonstrate a practical system based on synchronising a fast scanning galvanometer mirror and delay line in such a way that the SHG-FROG trace is viewed directly on a video-monitor and can be acquired and processed using in image acquisition board interfaced to a personal computer. Compared to alternative approaches for rapidly-acquiring a SHGFROG trace, the system provides a more direct output than other mechanically-scanning implementations and is more versatile than a single-shot device because of the ability to control the temporal width of the measured trace. The system is presented in Figure 1 and comprises a dispersion-balanced scanning interferometer that includes silver-coated hollow retro-reflectors and a thin beamsplitter (RI, R2 and BS). One of the retro-reflectors is static and the other is scanned at around 40 Hz using a piezo-electric translalor. The retro-reflectors are adjusted.so the pulses emerge from the interferometer as two parallel but non-collinear beams and these are incident centrally on a concave gold mirror (MZ). The focal path after the gold mirror is folded using a near-normal incidence reflection from a plane gold mirror which steers the beams past the curved mirror and into a Type I phasematched 200pmthick beta-barium borate (BBO) crystal where they combine with a small crossing angle. - aI