Abstract
In an optical fiber based SPR sensor, a segment of metal clad silica fiber is used as the sensing element and resonant coupling occurs to the surface plasmon mode excited at the interface of the metal and ambient which surrounds the sensing element. When polychromatic light is transmitted through the sensing element, the spectrum of light detected at the other end shows minima at a resonance wavelength. In wavelength interrogation, the shift in the resonance wavelength is a measure of change in the refractive index of the ambient. Alternatively, intensity interrogation can be used in which the change in transmitted power at a given wavelength is a measure of change in refractive index of the ambient. The sensitivity and detection accuracy of the sensor depend on the numerical aperture (NA) of the multimode optical fiber used to fabricate the sensor. In the present work, we have carried out a study of the dependence of the sensitivity and detection accuracy on numerical aperture to obtain the optimal value for a multimode optical fiber which is used to fabricate the sensing element under both wavelength and intensity interrogation. For a gold clad sensing element with l/a = 33.33 for the refractive index of the ambient in the range 1.33 to 1.38, the optimized numerical aperture is obtained as \(NA\)= 0.2 for spectral interrogation mode. At this optimal value, the sensitivity is estimated to be 4040 nm/RIU and detection accuracy lies between 20 and 48 μm−1. In single wavelength intensity interrogation, at the He–Ne laser wavelength (633 nm), the optimized numerical aperture is obtained as \(NA\)= 0.42. Experimental results on a sensing element fabricated with gold coating on the core of a plastic clad silica fiber are also reported.
Highlights
A metal dielectric interface supports a TM surface wave, known as surface plasmon mode with the propagation constant a sensitive function of the refractive index of the dielectric layer
When monochromatic TM polarized light is incident through the prism on the metal surface and the reflected light measured as a function of angle of incidence, an surface plasmon resonance (SPR) curve of the type shown in Fig. 1(b) is obtained; the angle at which the minima occurs is the measure of the refractive index of the sensor layer and corresponds to the resonance angle at which there is maximum coupling to the metal-dielectric surface plasmon mode
In conclusion, we have presented an analysis for the optimization of numerical aperture of a plastic clad multimode fiber to be used for an SPR based sensor in both spectral interrogation and single wavelength intensity interrogation mode
Summary
A metal dielectric interface supports a TM surface wave, known as surface plasmon mode with the propagation constant a sensitive function of the refractive index of the dielectric layer. When monochromatic TM polarized light is incident through the prism on the metal surface and the reflected light measured as a function of angle of incidence, an SPR curve of the type shown in Fig. 1(b) is obtained; the angle at which the minima occurs is the measure of the refractive index (nd) of the sensor layer and corresponds to the resonance angle at which there is maximum coupling to the metal-dielectric surface plasmon mode. Such sensors are light in weight, rugged and allow remote sensing. A satisfactory comparison of shift in resonance wavelength with theoretical calculations is obtained for appropriate choice of the sensor parameters
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