Fringe-averaged collinear frequency-resolved optical gating: in situ characterization of ultrashort pulses in nonlinear microscopy

In situ characterization of the electric field of ultrafast pulses is critical in multiphoton microscopy. Although second harmonic generation-based collinear Frequency-Resolved Optical Gating (FROG) addresses this need, the interferometric measurement is challenged by interferometric drift instability, the required high sampling density of the acquired data, and the inability to directly use the interferometric data with conventional FROG retrieval algorithms. We address these issues by combining low-pass Fourier filtering with active kHz dithering of the interferometric path length difference using a piezo-driven retroreflector. We demonstrate successful electric field retrieval for pulses of variable duration (∼100 fs-3 ps), in situ characterization of a chirped pulse in a nonlinear microscope, and a significant reduction in acquisition time, without loss of resolution, by undersampling.