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  • December 2013

Optics InfoBase > Spotlight on Optics > Surface plasmon resonance sensor interrogation with a double-clad fiber coupler and cladding modes excited by a tilted fiber Bragg grating


Surface plasmon resonance sensor interrogation with a double-clad fiber coupler and cladding modes excited by a tilted fiber Bragg grating

Published in Optics Letters, Vol. 38 Issue 22, pp.4911-4914 (2013)
by Mohamad Diaa Baiad, Mathieu Gagné, Wendy-Julie Madore, Etienne De Montigny, Nicolas Godbout, Caroline Boudoux, and Raman Kashyap

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Spotlight summary: When measuring with light extremely small properties, short wavelengths are needed, and when they are not available in free space, Surface Plasmon Resonances (SPR) come to the aid. One example is that of measuring minute refractive index changes in liquid or air. The fifty-year-old Otto’s and Kretschmann’s configurations for exciting SPR requires a prism for total internal reflection, a grating and a sophisticated apparatus to control the input and output angles. Hence, it is not a simple thing to generate, pack and use SPR for remote sensing. Luckily there is an apparatus that includes total internal reflection, a grating, and can effectively have an incident angle change: a fiber with a tilt grating, where the core of the fiber is coated with metal.

In this paper, Baiad and co-authors take this method a step forward, and not only use the internal modes of the core, but harness also guided cladding modes. This is done by incorporating a double-clad fiber. Using the cladding modes is a novel and simple solution for enhancing sensitivity, mainly for two reasons. First, it does not rely on the same channel for exciting and reading the SPR. The gold, used to excite the SPR, is coated only on the cladding which does not hinder the fiber core. Second, it is possible to measure the cladding modes with reflection, which greatly enhances the signal-to-noise ratio, in comparison to measuring the transmission of the core.

What can be measured with such a small prop as a SPR? In this paper, the authors show that by measuring the cladding modes’ reflection it is possible to achieve a sensitivity of 508 nm/RIU, a number that stands for the response of the SPR wavelength to a change of the surrounding refractive index. Though others have reported higher sensitivity, this method can do this for a much larger range of refractive indices, from 1.335 to 1.432, due to its high signal-to-noise ratio. This method is a leap forward since it paves the path for multiple channel sensing, where some channels are used for launching and others for sensing, enabling to measure different properties at once. The possibility of creating a multiclad tilted fiber grating coated with gold shows that technology is thoroughly keeping pace with the speed of material science.

--Nadav Gutman



Technical Division: Optoelectronics
ToC Category: Fiber Optics and Optical Communications
OCIS Codes: (060.2370) Fiber optics and optical communications : Fiber optics sensors
(060.3735) Fiber optics and optical communications : Fiber Bragg gratings


Posted on December 05, 2013

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