  The controlled modulation of an optical wavefront is required for aberration
correction, digital phase conjugation or patterned photostimulation. For most
of these applications it is desirable to control the wavefront modulation at
the highest rates possible. The digital micromirror device (DMD) presents a
cost-effective solution to achieve high-speed modulation and often exceeds the
speed of the more conventional liquid crystal spatial light modulator, but is
inherently an amplitude modulator. Furthermore, spatial dispersion caused by
DMD diffraction complicates its use with pulsed laser sources, such as those
used in nonlinear microscopy. Here we introduce a DMD-based optical design that
overcomes these limitations and achieves dispersion-free high-speed binary
phase modulation. We show that this phase modulation can be used to switch
through binary phase patterns at the rate of 20 kHz in two-photon excitation
fluorescence applications.
