Frequency-resolved optical gating measurement of ultrashort pulses passingthrough a high numerical aperture objective

We investigate using collinear Type II second-harmonic generation frequency -resolved optical gating (SHG FROG) to measure the pulse intensity and phas e at the focus of high numerical aperture (NA) oil objectives. Because of t he strong focusing for such objectives, it is not clear theoretically that such a measurement should work, Such objectives can produce severe distorti ons of the pulse as a function of radius in the objective. In addition, the standard SHG FROG algorithms are based on the assumption that the fundamen tal and second-harmonic fields are plane waves that can be described by the paraxial approximation, and for high NA objectives, such assumptions are s uspect, We show that such measurements work remarkably well. The tight focu s, while a theoretical difficulty, eliminates many of the problems traditio nally associated with SHG FROG including the difficulty of phase matching a nd walkoff of different polarizations in the crystal. Specifically, we use collinear Type II SHG FROG to measure the intensity and phase at the focus of a Zeiss CP-Achromat 100x, 1.25 NA, infinity-corrected oil objective, and accurately retrieve 20 fs pulses.