Développement de capteurs à fibre optique basés sur la résonance ...

Article 2 Accepted by Sensor & Actuator A ( on 5 Nov., 1999, Reg. No. 99433 )
multilayered planar structure. Both of the reflectivity to the p- and s- polarized electric field
components in this configuration can be calculated by matrix method[l 1].
The programs written in Matlab have been developed. The computed results for the
measured fiber are illustrated in figure 6 by utilizing the parameters: the dielectric constant of
55nm gold film is -13.8+i 1.8 at wavelength of 670nm, the refractive index of core and
cladding are 1.457 and l.407+i 54406x11115 respectively at the room temperature. The
imaginary part of the index of the cladding is previously determined by the least-square fit of
the experimental curve measured in air for the same fiber prior to the deposition of gold layer.
The computational curves agree well with the experimental ones (cf. figure 3).
4. Analysis of the fiber-optic sensor
4.1 Sensitivity
This kind of fiber-optic sensor usually works with a fixed incident angle to monitor the
variation of the detected parameter such as the refractive index of the bulk medium by
measuring the change of the transmitted light power. Figure 7 illustrated both the theoretical
and experimental responses of the measured fiber in p- and s- polarized illumination at some
fixed incident angles of 8°, 12° 16° and 200 respectively. The good agreements between the
computations and the experiments prove the validity of this 3D model.
The sensitivity of the sensor in monitoring the refractive indices of the bulk media is
defined as the derivative of the normalized light power transmitted with respect to refractive
index. Correspondingly, the curves of sensitivity for p- and s- polarization of incident light
respectively are calculated and plotted in figure 8. They demonstrate that the p-polarization
illumination has higher sensitivity than s-polarization. It is evident because only p-polarized
component of the electric-field serves to the SFR sensing. However, the sensitivity between
p- and s- polarization illuminations becomes hard to be distinguished as the SPR sensing part
is far away from the input end of the fiber. An example is given in figure 9 fòr ii, the distance
from the input end o[the fiber to the SFR sensing part, is equal to 80mm, as other parameters
in these calculations are kept constant.
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