Photonic crystal fiber half-taper probe based refractometer

Abstract

A compact singlemode -photonic crystal fiber -singlemode fiber tip (SPST) refractive index sensor is demonstrated in this paper. A CO2 laser cleaving technique is utilsed to provide a clean-cut fiber tip which is then coated by a layer of gold to increase reflection. An average sensitivity of 39.1 nm/RIU and a resolvable index change of 2.56×10 -4 are obtained experimentally with a ~3.2 µm diameter SPST. The temperature dependence of this fiber optic sensor probe is presented. The proposed SPST refractometer is also significantly less sensitive to temperature and an experimental demonstration of this reduced sensitivity is presented in the paper. Because of its compactness, ease of fabrication, linear response, low temperature dependency, easy connectivity to other fiberized optical components and low cost, this refractometer could find various applications in chemical and biological sensing. © 2013 Optical Society of America Optical fiber based photonic devices have been successfully used in a range of sensing applications. To date they have offered numerous advantages over conventional electrical sensors due to their immunity to electromagnetic interference, resistance to erosion, small size, high sensitivity and capability of remote sensing. In the past few years, several types of refractive index (RI) optical fiber sensors have been developed. The most common approaches rely on fiber Bragg gratings (FBGs) [1,2], long period gratings (LPGs) [3,4], microbending [5], Fabry-Perot interferometers [6,7], and microfiber coil resonators [8]. However, most of them require expensive fibers or equipment to produce.An inexpensive singlemode-multimode-singlemode (SMS) fiber based sensor utilizing multimode interference in the multimode fiber (MMF) core section has been proposed [9][10][11][12]. This SMS fiber based refractive index sensor reported in [12] allows for the measurement of the external refractive index with a maximum sensitivity of 1815 nm/RIU (refractive index unit) and a dynamic range of 0.095 from 1.342 to 1.437. However given the fact that a long portion of an MMF needs to be etched chemically and also must be exposed to the surrounding medium in order to achieve a large refractive index change, the length of the MMF in the SMS structure is required to be sufficiently long, which makes it difficult to implement a compact probe type sensor with good spatial resolution. For example, in order to obtain a maximum sensitivity of 1815 nm/RIU, experimentally the diameter of the MMF part needs to be etched down to 80 µm and the length of the MMF must to be controlled accurately to 42 mm, but this increases the risk of breakage and difficulty. On the other hand, the high temperature dependence of the SMS structure significantly influences the refractive index measurement and additional temperature compensation is essentially needed.Recently significant effort has been devoted to develop fiber devices with a range of advantages such as a large evanescent field, strong confinement, smart footprint, compact physical ...

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