Novel spectral fiber optic sensor based on surface plasmon resonance

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Abstract

A novel fiber optic surface plasmon resonance (SPR) sensing device based on spectral interrogation of SPR in a miniature fiber optic sensing element using depolarized light is reported. Optimization analysis of the sensor based on the equivalent planar waveguide approach and the mode expansion and propagation method is presented. A laboratory prototype of the sensor has been proved to be able to measure refractive index variations as small as 5×10−7. Suitability of the sensor for biosensing has been demonstrated by detecting IgG via respective monoclonal antibodies immobilized on the SPR sensor surface.

Introduction

Sensors capable of on site (bio)chemical analysis are needed in many areas including industrial process control, medical analysis, and environmental monitoring. Surface plasmon resonance (SPR) sensors holds potential for applications in these areas. Miniaturization of SPR sensing devices, as a prerequisite for the development of portable SPR sensor systems, has been receiving growing attention. In particular, waveguide-based SPR sensing structures has been widely studied, as the utilization of optical waveguides offers numerous advantageous features such as small size, robustness, and potential for remote sensing. Various SPR sensors based on integrated optical waveguides [1] and optical fibers [1], [2], [3], [4] have been developed.

In SPR sensors based on multimode fibers [2], [3], the sensing element encompasses a multimode optical fiber with an exposed core coated around with a thin metal layer supporting surface plasma waves (SPW), [2]. These sensors exhibit a rather limited resolution mainly due to the modal noise presented in multimode fibers causing the strength of the interaction between the fiber-guided light wave and the SPW to fluctuate. To overcome this inherent limitation of SPR sensing devices based on multimode fibers, SPR sensors based on a single-mode optical fiber were proposed [4], [5]. They include SPR sensors based on tapered [5] and side-polished [4] single-mode optical fibers. The SPR sensors using tapered fibers rely either on spectral interrogation at rather short wavelengths (and therefore exhibit rather low sensitivity) or on amplitude interrogation (exhibiting also low sensitivity due to rather broad SPR dips caused by variations in the SPR condition along the sensing region). Amplitude SPR sensors based on side-polished single-mode optical fibers offer superior sensitivity [4], although suffer from adverse sensitivity to fiber deformations, as any fiber deformations change the state of polarization of the fiber mode and consequently also the strength of its interaction with SPW.

In this paper, we propose a novel approach to the development of fiber optic SPR sensing devices based on spectral interrogation of SPR in the side-polished fiber optic sensing element using depolarized radiation. This approach allows construction of highly sensitive all-fiber optic sensors with reduced adverse sensitivity to fiber deformations. We demonstrate applicability of the sensor to refractometry and affinity biosensing.

Section snippets

Sensor structure and principle of operation

The proposed SPR sensing element, in detail described in [6], consists of a standard single-mode optical fiber with locally removed cladding and a thin gold film supporting SPW, Fig. 1. The SPW may be excited by a guided mode propagating in the fiber if the two waves are closely phase-matched. As the propagation constant of the SPW depends dramatically on the refractive index of the medium adjacent to the gold film (sample), the strength of the interaction between the SPW and the fiber mode,

Theory

As the geometry of the considered SPR sensor element structure is very complex, rigorous modeling of its waveguiding properties is very difficult. Therefore, in our simulations the optical fiber was substituted by an equivalent planar waveguide. The resulting planar SPR sensing structure was then analyzed using the mode expansion and propagation (MEP) method [8]. The effect of fiber curvature was accounted for by substituting the bent waveguide by 21 sections of a straight waveguide of

Experimental

The fiber optic SPR sensing element was fabricated using a single-mode optical fiber with the cut-off wavelength of 724 nm (SM800, Fibercore, Ltd., UK). The fiber was side-polished to the proximity of the fiber core in order to attain d0 ≈500 nm. Thickness of the residual cladding layer was measured using the liquid drop method [11]. Then, the fiber was coated by a gold film (thickness of 65 nm) and a thin tantalum pentoxide overlayer (thickness of 19 nm) by vacuum evaporation. The configuration of

Discussion

Observed sensor performance agrees well with the performed simulations in terms of position, depth, and width of SPR dips (Fig. 3, Fig. 4, Fig. 5, Fig. 7, Fig. 8, Fig. 9). The residual polarization of the fiber mode (below 1%) was found to cause the sensor resolution to drop by almost two orders of magnitude — from 5×10−7 to 3×10−5 RIU. Lyot depolarizers with slightly better performance, which are now becoming available, are expected to improve resolution of the sensor operating in the dynamic

Conclusions

We have reported a new approach to the development of fiber optic SPR sensing devices based on spectral interrogation of SPR in a fiber optic sensing element using depolarized light. Sensor sensitivity of 3100 nm/RIU and resolution as high as 5×10−7 RIU were demonstrated. The attained resolution is comparable with the resolution of best bulk-optic table-top SPR devices. We have demonstrated suitability of the developed SPR fiber optic sensor for biosensing. In model biodetection experiment, the

Acknowledgements

This work has been supported by the Grant Agency of the Czech Republic under the contracts 102/99/0549 and 102/00/1536. Authors acknowledge Dr. V. Malina (IREE, Prague, Czech Republic) for thin film deposition, and Dr. W. Ecke and Mrs. Kerstin Schroeder (IPHT, Jena, FRG) for the fabrication of Lyot depolarizer and useful discussions.

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