Kilohertz binary phase modulator for pulsed laser sources using a digital micromirror device

Abstract

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.Controlling the amplitude and phase of an optical field, a process known as wavefront shaping, is desirable in many applications. In biomedical imaging it enables deeper imaging into tissue by compensating for scattering or aberrations introduced by refractive index inhomogeneities [8,12]. In addition, several applications of wavefront shaping, such as holographic optogenetic stimulation, dynamic aberration correction during scanning microscopy and fast optical phase conjugation, benefit from precise and fast dynamic control of the amplitude and phase of light. Because of their high switching rates (in the kHz range) and their high pixel count, digital micromirror devices (DMD) can be an attractive and cost-effective solution [2]. Consisting of an array of micron-sized mirrors, each of which can be tilted by ±12• (mechanical angle) depending on an applied ON or OFF state voltage, DMDs are conventionally used as binary amplitude-only modulators. Since phase modulation results in higher focus enhancement [14] in digital optical phase conjugation and generally lower background in holographic applications, a variety of techniques have been explored to turn amplitude modulators, into both binary [15] and multilevel phase-only modulators [3,6,9]. While these previous techniques usually trade off the number of the effective degrees of freedom or diffraction efficiency to achieve phase modulation, they have been successfully applied to wavefront shaping of monochromatic light. In some applications, such as nonlinear microscopy and optogenetic stimulation, which use ultra-short laser pulses as the excitation, broadband laser light (typical bandwidth: 10 nm) will undergo www

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