Using Digital Micromirror Devices for focusing light through turbid media Sri Nivas Chandrasekaran, Hans Ligtenberg, Wiendelt Steenbergen, Ivo M. Vellekoop Biomedical Photonic Imaging Group, MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands ABSTRACT The holy grail of biomedical optical imaging is to perform microscopy deep inside living tissue. Unfortunately, biological tissue scatters light, which prevents the formation of a sharp focus. However, recently it was shown that wavefront shaping can be used to focus light through and inside turbid materials. So far, most experiments used liquid crystal devices, which are too slow to match the dynamics of perfused tissue. Since DMD technology is approximately 1000 times faster, it may bring wavefront shaping to in-vivo applications. We will compare analytically the performance of different methods for focusing light through scattering media with an intensity-only light modulator. Keywords: Keywords: DLP, DMD, Wavefront Shaping, Turbidity Suppression 1. INTRODUCTION Optical methods for biomedical imaging can roughly be divided into two categories. Methods in the first category use ballistic (‘unscattered’) light for imaging and/or manipulation. This category includes, a. o., conventional microscopy, fluorescence microscopy, two photon microscopy, and optical coherence tomography. Although these methods have a very high resolution (<ͳ ߤ), the penetration depth in biological tissue is generally limited to about one millimeter because the ballistic component of the incident light decreases exponentially with depth. The second category is that of methods using diffuse light, such as diffuse optical tomography. These methods generally penetrate up to ten centimeters in tissue, at the expense of a severely compromised resolution: typically the resolution is about 1/3 of the penetration depth 1 . A third category of methods is currently under rapid development. These methods shape the wavefront of the incident beam to control the propagation of light inside scattering materials 2 . Light was focused through 3 and inside 4 scattering materials and subsequently used for high-resolution imaging 5-8 and manipulation 9 . The focus is a result of multi-path interference of scattered waves, and it can be as sharp as the Rayleigh diffraction limit, regardless of how many times the light was scattered 10 . The biomedical potential for wavefront shaping is enormous as it combines microscopic resolution with macroscopic imaging depths 11, 12 . Digital wavefront-shaping makes use of a spatial light modulator to shape the incident wave and use iterative algorithms or phase conjugation to construct the optimal wave for focusing at a desired target. Although several analog wavefront shaping techniques exist 13-15 , digital wavefront shaping has several advantages over conventional methods, such as the ability to have an arbitrarily high gain in phase conjugation experiments, and the ability to freely freeze and modify the generated wavefront, for instance to raster scan the generated focus. The main drawback of digital methods is their relative lack of speed. The wavefront shaping system needs to be fast enough to construct a wavefront before Brownian motion or other movement decorrelates the sample. Typically, for perfused tissue the typical time scale is around 1 ms or less. At the moment of writing, commercially available digital micromirror device (DMD) intensity-only light modulators achieve frame rates as high as 32kfps 16 . For iterative wavefront shaping methods 5, 17, 18 , tens to hundreds of measurements are needed to find the optimal wavefront, so it is important to use these measurements as efficiently as possible. Also, for digital optical phase conjugation the question Emerging Digital Micromirror Device Based Systems and Applications VI, edited by Michael R. Douglass, Philip S. King, Benjamin L. Lee, Proc. of SPIE Vol. 8979, 897905 · © 2014 SPIE CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2038893 Proc. of SPIE Vol. 8979 897905-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/26/2014 Terms of Use: http://spiedl.org/terms