Surface-plasmon-resonance sensor based on H-shaped optical fibre

ABSTRACT We propose and theoretically study a novel surface-plasmon-resonance sensor based on an H-shaped, elliptical-core optical fibre. The two grooves of the H-fibre are coated with a thin, uniform metal layer that in turn is covered with a high-index dielectric layer to allow broad spectral tunability. The sensor maintains linear polarization and facilitates effortless splicing. Electromagnetic mode analysis indicates a sensitivity of 1800 nm/RIU (refractive-index unit) for aqueous analytes. Keywords: Surface plasmon resonance, transfer matrix me thod, modal analysis, finite element method. 1. INTRODUCTION Recently, there has been increasing interest in developing fibre-based plasmonic sensors by using specialty or microstructured optical fibres as a template [0–3]. These sensors make use of a surface plasmon polariton that is bound to propagate along a metal-dielectric interface with highly dielectric-dependent characteristics. In closed all-in-fibre sensors, the dielectric substance (analyte) is infiltrated into the metalized pores of the fibre, and hence, such structures might be best employed with very small sample volumes or as packaged (end-sealed) sensor heads. Despite recent advancements [4], it is still challenging to fabricate and employ the closed structures, and thus, open metal-coated sensor fibres that are embedded into an ambient analyte have retained their popularity. The microstructured-fibre-based open sensors can be operated in the same way as, e.g., those based on the D-fibre [5] or a tapered circular-core fibre [6]. In this work, we propose a novel surf ace-plasmon-resonance (SPR) sensor struct ure based on a highly birefringent H-shaped optical fibre. Such polarization-maintaining fibres are typically fabricated in two steps. First, an appropriate preform is prepared with the stack-and-draw technique so as to yield a fibre with two large pores running through its length (a.k.a. the side-hole fibre) [7]. If the preform originally embodied a circular up-doped core, it will assume an elliptical shape in the final drawing process. Second, the tw o holes are opened and exposed to the outside environment by chemically etching a segment of the fi bre from opposite sides [8]. For SPR sensor operation, a uniform metal layer is then deposited via the etched openings. Also, in order to flexibly tune the SPR wavelength and the resonance characteristics, an additional high-index dielectric layer is assumed to be deposited on top of the metal layer. The principle of using such a thin auxiliary layer is described in Ref. [9]. With both of these material layers in place, the structure is expected to support a plasmonic (non-degenerate) fundamental mode that would spatially extend all the way to an ambient analyte in the etched openings. Our goal is to find a sensor structure for aqueous analytes that would operate in the O-band near the 1.3-micron wavelength. This paper is organized as follows. In Section 2, we detail the proposed design and use the customary transfer matrix formalism to understand the dependence of the SPR characteristics on the relevant design parameters. Once the right parameters for the operation in the O-band are found, a rigorous electromagnetic mode analysis is performed in Section 3 to assess the sensitivity of an example sensor fibre. The paper is concluded in Section 4.