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Acousto-optic modulator system for femtosecond laser pulses.

Femtosecond laser pulses have made a revolution in multiphoton excitation microscopy, micromachining, and optical storage for their unprecedented high peak power. However, modulation of their intensity with acousto-optic modulator (AOM) is frustrated by dispersion which results in a significant stretch in pulse width. Here we report a scheme composed of two acousto-optic deflectors (AODs) to modulate the intensity of the femtosecond laser pulses with simultaneous compensation for the temporal dispersion. With commercial AODs, we demonstrated such an AOM system for the femtosecond laser pulses with overall transmission efficiency of around 80%. The pulse width of the exit beam is 115-177 fs for an input pulse of 110 fs, across the wavelength range of 720-920 nm when the temporal dispersion compensation is optimally tuned at 800 nm. The fluorescence intensity in a two-photon microscopy experiment performed using this system increased 5.5-fold over that of the uncompensated AOM.