Mapping femtosecond pulse front distortion and group velocity dispersion in multiphoton microscopy

Abstract

Group velocity dispersion (GVD) and pulse front distortion of ultrashort pulses are of critical importance in efficient multiphoton excitation microscopy. Since measurement of the pulse front distortion due to a lens is not trivial we have developed an imaging interferometric cross-correlator which allows us to measure temporal delays and pulse-widths across the spatial profile of the beam. The instrument consists of a modified Michelson interferometer with a reference arm containing a voice-coil delay stage and an arm which contains the optics under test. The pulse replicas are recombined and incident on a 22×22 lenslet array. The beamlets are focused in a 0.5 mm thick BBO crystal (cut for Type I second harmonic generation), filtered to remove the IR component of the beam and imaged using a 500 fps camera. The GVD and pulse front distortion are extracted from the temporal stack of beamlet images to produce a low resolution spatio-temporal map.