Amplitude modulation of femtosecond laser pulses in the megahertz range for frequency-multiplexed two-photon imaging.

We present a frequency-multiplexed multi-site two-photon imaging method utilizing amplitude modulation of femtosecond laser pulses in the MHz range to tag each excitation beam and the corresponding fluorescence signals with specific frequencies. The frequency tags are generated with an interferometric scheme employing acousto-optic deflectors (AODs) to achieve precise spatial overlap of femtosecond laser pulses with periodically varying phase shift. Creating matching excitation beam patterns in each interferometer arm using multiple AOD driving frequencies, and subsequently overlapping these matching patterns, results in multiple encoded excitation beams with unique beat frequencies available for scanning. As a proof-of-concept, we demonstrate multiplexed two-photon image acquisition using test samples, and compare the performance of this approach to conventional two-photon laser scanning microscopy.