Miniaturization of fiber optic surface plasmon resonance sensor

doi:10.1016/S0925-4005(98)00205-6

Sensors and Actuators B: Chemical

Volume 51, Issues 1–3, 31 August 1998, Pages 311-315

https://doi.org/10.1016/S0925-4005(98)00205-6 Get rights and content

Abstract

A novel design of surface plasmon resonance (SPR) sensor is reported which leads to a highly miniaturized optical fiber sensing element with high sensitivity. A surface plasmon wave is excited on a thin metal film on a side-polished single-mode optical fiber and variations in the refractive index of analyte are detected by measuring changes in the intensity of the light back-reflected from a mirrored end face of the fiber. The operation range of the sensor is tuned toward aqueous media by using a thin tantalum pentoxide overlayer. It is demonstrated that the sensor is capable of detecting changes in the refractive index below 4×10⁻⁵.

Introduction

Within the last decade the surface plasmon resonance (SPR) sensing technique has received a great deal of attention and has become a leading technology for affinity-based biosensing. The SPR technology has been exploited by Pharmacia Biosensor for the development of BIAcore and BIAlite systems and another commercial SPR sensor system has been recently developed by Texas Instruments. However, all the commercially available SPR sensor systems rely on bulk-optic implementation of the attenuated total reflection method which limits the degree of miniaturization of SPR sensing elements. In order to overcome this limitation, SPR sensors based on optical waveguides have been widely studied and within the last few years, various approaches to miniaturization of SPR sensing elements using optical fibers 1, 2, 3and integrated optical waveguides 4, 5have been proposed.

In this paper we report a novel design of SPR fiber optic sensing elements which allows developing highly miniaturized SPR probes. Based on theoretical analysis, the sensor structure is designed for operation in aqueous environment and optimized in order to achieve the maximum sensitivity.

Section snippets

Sensor structure and principle of operation

The reported sensor is based on a standard single-mode optical fiber. Its cladding is locally removed to get access to the evanescent field of the guided mode and this area is covered by a thin gold film which may support a surface plasmon wave (SPW), Fig. 1. The guided mode propagates in the fiber and excites the SPW at the outer interface of the gold film if the two modes are closely phase-matched. The propagation constant of SPW depends dramatically on the refractive index of the medium

Theory

For theoretical analysis of the proposed sensing structure, a single-mode optical fiber was substituted by an equivalent three-layer dielectric planar waveguide. The transfer of optical power through the structure has been calculated by using the mode expansion and propagation method [7]. Based on the carried out theoretical analysis, parameters of the sensing structure have been chosen which result in the sensor operating in aqueous environment with optimized sensitivity and resolution. The

Experimental

A short piece of conventional PCS fiber core is used to support the sensing area of a standard single-mode optical fiber to provide the sensor with desired mechanical stability. The silica core of a PCS fiber with the diameter of 560 μm was glued by low-melting-point glue into a slot engraved into a silica block. Then a curved slot of the radius of 25 cm was made into the PCS fiber. A single-mode fiber with stripped off fiber jacket in the length of about 2 cm was then glued into the curved

Discussion

There is a relatively good agreement between the theory and experimental results regarding the refractive index range at which the resonant excitation of SPW occurs. An increase in the sensor sensitivity may be achieved by extending the dynamic range of the sensor and setting the operation point of the sensor closer to the bottom of the SPR dip. The dynamic range of the sensor can be substantially increased (by up to 20 dB) by reducing the optical power loss in the fiber to fiber coupler. This

Conclusions

A laboratory prototype of the miniaturized SPR single-mode optical fiber probe operating in aqueous environment has been developed and its ability to measure variations in the refractive index has been studied. The average sensitivity of the developed SPR fiber probe was determined to be 1200 dB/RIU within the region of refractive indices 1.328–1.338. In conjunction with an optoelectronic system which can measure changes in the optical power of 1%, the sensor is capable of resolving variations

Acknowledgements

This research has been supported by the Grant Agency of the Czech Republic under contracts 102/96/1561 and 303/96/1358.

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Miniaturization of fiber optic surface plasmon resonance sensor

Abstract

Introduction

Section snippets

Sensor structure and principle of operation

Theory

Experimental

Discussion

Conclusions

Acknowledgements

Sensors and Actuators B

Sensors and Actuators B

Sensors and Actuators B

Fibre-optic sensor based on surface plasmon resonance

Electron. Lett.

Chemical sensing by surface plasmon resonance in a multimode optical fibre

Pure Appl. Opt.