ITh3C.1.pdf Imaging and Applied Optics © 2014 OSA High-Speed Phase Modulation for Multimode Fiber Endoscope Antonio M. Caravaca-Aguirre 1 , Eyal Niv 1 , and Rafael Piestun 1 1 Department of Electrical, Computer, and Energy Engineering, University of Colorado, Boulder, Colorado, 80309, USA antonio.caravaca@colorado.edu Abstract: We present a reconfigurable high-speed phase modulation system based on a digital light projector for application in endoscopic multimode fibers. A custom driver and FPGA reduce data transfer enabling adaptive wavefront shaping at kilohertz rates. OCIS codes: (110.1080) Active or adaptive optics; (110.7050) Turbid media; (060.2310) Fiber optics 1. Introduction The ability to modulate wavefronts in real time is critical in adaptive imaging and specifically in biomedical imaging. Liquid-crystal spatial light modulators are commonly used in wavefront shaping [1]. However, the slow refreshing rate of these devices (~100Hz) makes them inappropriate for in-vivo applications. Deformable mirrors (DM) [2] can modulate the phase at a few kHz but they present a limited number of controllable segments. Digital micromirror devices (DMD’s) are an inexpensive option and can modulate the wavefront up to 24 kHz [3,4]. However, when processing of feedback data is required, the data transfer between the computer and the device (both DM and DMD) reduces the speed of the system considerably. Taking advantage of the FPGA embedded on the DMD board we have designed and encoded algorithms capable of overcoming these limitations. In particular, we have created a computer generated hologram (CGH) patch for each phase value required. This approach reduces drastically the amount of information transfer to the DMD. We use this high-speed phase modulation method to calibrate a multimode fiber (MMF) for scanning fluorescence microscopy [5]. 2. High speed phase modulation using a DMD A laser beam is phase modulated using a binary-amplitude Lee hologram [3] encoding displayed on a DMD. The DMD pixels are binned to square patches (Figure 1), where each patch corresponds to a controllable spatial input mode. The size of the patches is configurable and determines the number of input modes and the maximum speed achievable. Eight different patches per configuration size are used, which correspond to eight phase values between 0 and 2π. Fig. 1. Lee hologram patches corresponding to a) different configuration size and b) different phase value An FPGA based accelerator construct the 1024x768 Lee hologram from a vector of N input modes using a look- up-table. Such approach reduces the data transfer to the DLP allowing for higher switching speed than previous techniques [3]. An important benefit of this system is that it allows for easy implementation of wavefront shaping algorithms at full speed even when the DLP is controlled from an external computer. Table 1 summarizes the maximum switching speed between different phase masks depending on the configuration chosen. We distinguish