636 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 10, NO. 3, MAY/JUNE 2004 A Two-Axis Electrothermal Micromirror for Endoscopic Optical Coherence Tomography Ankur Jain, Student Member, IEEE, Anthony Kopa, Student Member, IEEE, Yingtian Pan, Gary K. Fedder, Senior Member, IEEE, and Huikai Xie, Member, IEEE Abstract—This paper reports a 1-mm , two-axis, single-crys- talline-silicon (SCS)-based aluminum-coated scanning mi- cromirror with large rotation angle (up to 40 ), which can be used in an endoscopic optical coherence tomography imaging system. The micromirror is fabricated using a deep reactive ion etch post-CMOS micromachining process. The static response, frequency response, resonance frequency shift, and thermal imaging of the device are presented. A 4 4 pixel display using this two-dimensional micromirror device has been demonstrated. Index Terms—Electrothermal actuation, microactuators, op- tical coherence tomography, optical scanners, two-dimensional (2-D) mirrors. I. INTRODUCTION O PTICAL coherence tomography (OCT) is an emerging medical imaging technology that produces high-res- olution cross-sectional images of biological samples [1]. OCT exploits the short temporal coherence of a near-infrared broad-band light source and can image the cellular structure of tissues at depths greater than conventional microscopes. It is noninvasive and has the potential to reduce or guide invasive and time-consuming biopsy procedures. Another very attractive feature of OCT imaging is the high resolution. An OCT system with 1- m axial resolution has been demonstrated [2], which is about two orders of magnitude higher than that of ultrasound imaging. Infrared light is also much safer than X-rays. OCT has been proved to be clinically useful in the field of ophthalmology, and has great potential for use in cardiovascular, gastrointestinal, and pulmonary imaging through the use of en- doscopes and catheters [3]. Endoscopic OCT systems have been used to detect cancers at a very early stage [4], [5]. For these in- ternal organ applications, the imaging probe must be small, and fast image scanning is required. Various methodologies have been proposed to transversely scan the optical beam across the internal tissue surface. Some endoscopic OCT devices use a rotating hollow cable that carries a single-mode optical fiber, Manuscript received October 17, 2003; revised March 23, 2004. This work was supported in part by the NASA University of Central Florida/University of Florida Space Research Initiative and in part by the Florida Photonics Center of Excellence. A. Jain, A. Kopa, and H. Xie are with the Department of Electrical and Com- puter Engineering, University of Florida, Gainesville, FL 32611 USA (e-mail: ajain@ufl.edu; hkxie@ece.ufl.edu). Y. Pan is with the Department of Biomedical Engineering, State University of New York, Stony Brook, NY 11794 USA. G. K. Fedder is with the Department of Electrical and Computer Engineering and the Institute of Robotics, Carnegie Mellon University, Pittsburgh, PA 15213 USA. Digital Object Identifier 10.1109/JSTQE.2004.829194 Fig. 1. Schematic of a MEMS-based endoscopic OCT system. CM: collimator. MM: micromirror. while others use a galvanometric plate or piezoelectric trans- ducer that swings the distal fiber tip to perform in vivo trans- verse scanning of tissue [5]–[7]. Microsystem technology (MST) [also called microelectrome- chanical systems (MEMS)] is another emerging technology that makes miniature sensors and actuators. MEMS mirrors have been widely used for optical displays and optical switching. The small size, fast speed, and low power consumption of MEMS mirrors make them ideal for use in an endoscopic OCT imaging probe. In fact, researchers have started to use MEMS mirrors for the transverse scanning of endoscopic OCT systems [8], [9]. The authors have previously demonstrated a 5-mm-diameter MEMS-based OCT endoscope that used a one-dimensional (1-D) electrothermal mirror to scan the light beam onto the biological tissue [8]. By performing 1-D transverse scans of the tissue, high-resolution cross-sectional two-dimensional (2-D) images were obtained. Fig. 1 shows a schematic of a MEMS-based OCT setup. The collimated light in the sample arm of the Michelson’s interferometer is reflected off the beam steering micromirror and focused into the tissue. The same mirror collects the backscattered light from the tissue, and the tissue microstructure is determined by low-coherence interferometry when scanning tissue in-depth and laterally. Zara et al.also reported a MEMS-based OCT probe [9]. However, the 1-D transverse scanning along with the axial scanning can only generate 2-D images. Therefore, the whole imaging probe has to be moved to scan an area of an internal organ, so the imaging efficiency is low. Some other drawbacks of these single-axis transverse-scanning probes include the complexity and inaccuracies involved in repositioning the 1077-260X/04$20.00 © 2004 IEEE