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17 October 2006 Development of dual-LED fiber optic surface plasmon sensor for liquid refractive index detection
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In this paper, we present a novel fiber optic SP (surface plasmon) sensor for measuring liquid refractive index. The proposed system consists of a pulse circuit for alternatively flashing dual light emitting diodes (LEDs), the sensor with two optical couplers, and a detection circuit with a photo diode (PD). The wavelengths of dual LEDs are 612nm and 680nm. The used optical fiber is multimode, whose core diameter is 400 micrometer. The fiber is cut and the cladding of 1cm from the end surface is removed. Thick silver layer is evaporated onto the end surface as a mirror and then gold film of 60nm is evaporated around the core surface. The used wavelengths and thickness are determined by considering the optimum condition. The reflected light is detected by the PD. Compared the developed system with a conventional optical fiber SP sensor, which needs a white light and a spectrograph, the system is compact and low cost. Moreover, high sensitive detection system is achieved by monitoring the differential signal between signals corresponding to the wavelengths. Several solutions are measured. The results indicate that the sensor responses only depend on the refractive indices and measurable refractive index range is from 1.3293 to 1.3616.
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Hideki Shibata, Hitoshi Suzuki, Mitunori Sugimoto, Yoshikazu Matsui, and Jun Kondoh "Development of dual-LED fiber optic surface plasmon sensor for liquid refractive index detection", Proc. SPIE 6377, Advanced Environmental, Chemical, and Biological Sensing Technologies IV, 63770G (17 October 2006);

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