Optical Fibers in Integrated Molecular Sensor Systems - More Than Interconnects
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Optical Fibers in Integrated Molecular Sensor Systems – More Than Interconnects Xuan Yang1,2 and Claire Gu1,* 1 Department of Electrical Engineering, University of California, Santa Cruz Santa Cruz, CA 95064, U.S.A. 2 Engineering Technologies Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. * [email protected] ABSTRACT Optical fibers have been successfully used in long-haul communication, endoscopy, and other optical systems to transmit optical power as well as information from one point to another, serving as interconnects at various scales. In integrated sensor systems, optical fibers have been frequently employed to connect the source and the detector, due to their flexibility, compactness, and low loss. However, optical fibers can provide more functions than a simple transmission channel. In this paper, we review our work on optical fibers as a platform for molecular sensors based on Raman spectroscopy (RS) and surface enhanced Raman scattering (SERS). The fibers serve to significantly increase the sensitivity of RS/SERS and to facilitate the integration of a compact sensor system. We will discuss the principles of operation of various building blocks, demonstrate our recent results, and highlight some potential applications. 1. INTRODUCTION Optical fibers have been successfully employed in various systems to transmit information and/or energy. For example, optical fibers form the backbone of our long-distance communication networks, owing primarily to their large bandwidth [1]. In the medical applications, endoscopes have provided doctors invaluable insights into otherwise inaccessible locations, making medical diagnosis and surgery more accurate and safer [2]. Most of these applications rely on the unique properties of optical fibers that can confine optical waves inside the small core, guide the light through the fiber for a relatively long distance with manageable loss or dispersion, and when added with protective layers become flexible and easy to interface with other passive and active devices. Besides being excellent interconnects, optical fibers also offer a unique platform for sensors. Fiber sensors [2, 3] take advantages of characteristics, additional to what make them perfect transmission channels, to detect physical changes in or around the fibers. For examples, fiber optic ring interferometers have been used in gyroscopes (rotation sensors); Faraday rotation inside an optical fiber has been used in magnetic sensors; lead flint glass fibers have been used as radiation sensors; and the list goes on. In this paper, we are particularly interested in the applications of optical fibers in the detection of molecules in both liquid solutions and in gas phase. Specifically, we use the optical fibers as a platform in both normal Raman scattering (RS) and surface enhanced Raman scattering (SERS). The fibers serve to significantly increase the sensitivity of RS/SERS and to facilitate the integration of a compact sensor system. The general demand for sensors to detect chemical and
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