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Open AccessArticle

Combination of an Axicon Fiber Tip and a Camera Device into a Sensitive Refractive Index Sensor

1
Department of Mechanical Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
2
Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan
3
Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan
4
Institute of Biophotonics, National Yang-Ming University, Taipei 11221, Taiwan
5
Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan
*
Author to whom correspondence should be addressed.
Sensors 2019, 19(22), 4911; https://doi.org/10.3390/s19224911
Received: 7 October 2019 / Revised: 1 November 2019 / Accepted: 6 November 2019 / Published: 11 November 2019
(This article belongs to the Special Issue Optical Fiber Sensors and Photonic Devices)
An axicon fiber tip combined with a camera device is developed to sensitively detect refractive indexes in solutions. The transparent axicon tips were made by etching optical fibers through a wet end-etching method at room temperature. When the axicon fiber tip was immersed in various refractive index media, the angular spectrum of the emitted light from the axicon fiber tip was changed. Using a low numerical aperture lens to collect the directly transmitted light, a high intensity sensitivity was achieved when the tip cone angle was about 35 to 40 degrees. We combined the axicon fiber tip with a laser diode and a smartphone into a portable refractometer. The front camera of the smartphone was used to collect the light emitted from the axicon fiber tip. By analyzing the selected area of the captured images, the refractive index can be distinguished for various solutions. The refractive index sensitivity was up to 56,000%/RIU, and the detection limit was 1.79 × 10−5 RIU. By measuring the refractive index change via the axicon fiber tip, the concentration of different mediums can be sensitively detected. The detection limits of the measurement for sucrose solutions, saline solutions, and diluted wine were 8.86 × 10−3 °Bx, 0.12‰, and 0.35%, respectively. View Full-Text
Keywords: axicon optical fiber; refractometer; smartphone; intensity sensitivity axicon optical fiber; refractometer; smartphone; intensity sensitivity
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Tai, Y.-H.; Tsai, P.-C.; Ho, Y.-L.; Delaunay, J.-J.; Wei, P.-K. Combination of an Axicon Fiber Tip and a Camera Device into a Sensitive Refractive Index Sensor. Sensors 2019, 19, 4911.

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