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

The full characterization of the electric field of ultrashort laser pulses is important in multiphoton microscopy. This must be achieved at the focal plane of the microscope objective (in situ), requiring a collinear geometry. A well-known option for collinear pulse characterization is the second-harmonic generation collinear Frequency Resolved Optical Gating (SHG-cFROG) technique. Previous implementations of SHG-cFROG have involved Type I/Type II phase matching with Fourier filtering to remove fringes, novel algorithms for cFROG interferograms, and pulse shaper generation of collinear phase locked pairs. However, these approaches are potentially challenged by aspects of: 1) interferometric drift, of particular concern for longer pulses; 2) technical and/or algorithmic complexity; and 3) long acquisition times (high density sampling). To address these issues, we introduce a variation of SHG-cFROG named Fringe-averaged collinear FROG (FRACOL-FROG), which relies on using a kHz piezo-driven retroreflector to dither the path length of one arm of the collinear FROG interferometer. By averaging over interference fringes during the integration time of the spectrometer, we demonstrate: 1) robustness with respect to interferometric drift; 2) the ability to use standard FROG retrieval algorithms; 3) dramatic reduction in the density of sampling points without loss of resolution (⪆200-fold speedup); and 4) quantitative tabletop and in situ characterization of highly chirped pulses (~100 fs-3 ps). FRACOL-FROG is a rapid and robust method well suited to the complete in situ characterization of shaped ultrashort pulses in multiphoton microscopy.