Abstract
This paper describes the characteristics of optical fiber sensors with surface plasmon resonance (SPR) at 1,310 nm in which the scattering loss of silica optical fiber is low. SPR operation in the infrared wavelength range is achieved by coating a thin tantalum pentaoxide (Ta2O5) film. The novelty of this paper lies in the verification of how the hetero-core scheme could be operated as a commercial base candidate in the sense of easy fabrication, sufficient mechanical strength, and significant sensitivity as a liquid detector under the basis of a low loss transmission network in the near infrared wavelength region. The effect of Ta2O5 layer thickness has been experimentally revealed in the wavelength region extending to 1,800 nm by using the hetero-core structured optical fiber. SPR characterizations have been made in the wavelength region 1,000–1,300 nm, showing the feasible operation at the near infrared wavelength and the possible practical applications. In addition, the technique developed in this work has been interestingly applied to a multi-point water-detection and a water-level gauge in which tandem-connected SPR sensors system using hetero-core structured fibers were incorporated. The detailed performance characteristics are also shown on these applications.
Highlights
In the fields of medicine and biology, optical sensors based on surface plasmon resonance (SPR) have been developed as high-resolution refractive index sensors which are sensitive to small changes in refractive index induced due to events such as the combination of antigens and antibodies
Many optical fiber SPR sensors have been reported [9,10,11] since optical fibers offer us one of the simplest optical waveguides which could be useful tools to achieve remote sensing and sensor miniaturization demanded in the applications operating in outdoor conditions
Optical fiber sensors based on SPR could be operated in the limited wavelength in the range 600–700 nm under a similar environment [9,10,11] as well. This is because that SPR excitation condition is derived from the relationship between the thickness of thin metal film, the angle of incident light and the refractive index of silica. In contrast to those visible range operation, it was demonstrated by Homola et al that the resonance wavelength of SPR excitation could be shifted toward longer wavelengths by coating a high-refractive index thin film such as tantalum pentaoxide (Ta2O5) [12,13] in addition to the conventional SPR metal coating
Summary
In the fields of medicine and biology, optical sensors based on surface plasmon resonance (SPR) have been developed as high-resolution refractive index sensors which are sensitive to small changes in refractive index induced due to events such as the combination of antigens and antibodies. In contrast to those visible range operation, it was demonstrated by Homola et al that the resonance wavelength of SPR excitation could be shifted toward longer wavelengths by coating a high-refractive index thin film such as tantalum pentaoxide (Ta2O5) [12,13] in addition to the conventional SPR metal coating They demonstrated a SPR sensor operating at 825 nm for refractive index measurements of aqueous environments employing a Ta2O5 coated single-mode optical fiber whose cladding layer was removed [14,15]. Favorable characteristics of the hetero-core SPR sensor include its simple structure with easy fabrication which has no need of cladding removal and its simple interrogation method based on the intensity mode operation owing to a broader SPR spectrum of multi-mode transmission fibers (MMF) Another advantage of this SPR sensor can be found in the flexibility in the change of the insertion loss, the sensitivity by adjusting the length of sensor portion. Tuning an Operation Wavelength of a Hetero-Core Optical Fiber Based on SPR Using
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