Comparison of broadband and ultrabroadband pulses at MHz and GHz pulse-repetition rates for nonlinear femtosecond-laser scanning microscopy

Nonlinear optical imaging of human skin and of polychromatic microspheres was carried out to compare and evaluate the imaging properties of three different excitation femtosecond lasers: a spectrally tunable 80 MHz Ti :sapphire oscillator that produced 100 fs pulses (spectral width similar to 10 nm) and two ultrabroadband Ti: sapphire oscillators with repetition rates of 85 MHz and 1 GHz. The latter of these two and the 100 is laser were combined with a laser scanning microscope (TauMap). The intensities of images of the polychromatic microsphere samples obtained with both lasers are in accordance with the usual dependence of two-photon processes on laser pulse parameters, i.e. the intensity is proportional to the square of the mean laser power and the reciprocal pulse duration. In contrast to that, skin images measured with all three different excitation sources with mean powers of each laser adjusted to the particular pulse length and repetition rate exhibited discrepancies from this relation. For characterization of the ultrabroadband GHz laser, the measurements are supplemented by spectra of second-harmonic-generation signals of urea and collagen.