Bewersdorf, J., Pick, R., and Hell, S. W.
Multifocal multiphoton microscopy.  Optics Letters 23: 655-657 (1998).

We present a real-time, direct-view multiphoton excitation fluorescence microscope that provides three dimensional imaging at high resolution. Using a rotating microlens disk, we split the near-infrared light of a mode-locked titanium:sapphire laser into an array of beams that are transformed into an array of high aperture foci at the object. We typically scan at 225 frames per second and image the fluorescence with a camera that reads out the images at video rate. For 1.4 aperture oil and 1.2 water immersion lenses at 780-nm excitation we obtained axial resolutions of 0.84 and 1.4 mm, respectively, which are similar to that of a single-beam two-photon microscope. Compared with the latter setup, our system represents a 40—100-fold increase in efficiency, or imaging speed. Moreover, it permits the observation with the eye of high-resolution two-photon images of (live) samples.