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

Article 2 Accepted by Sensor & Actuator A ( on 5 Nov., 1999, Reg. No. 99433)
Earlier studies[3] have concluded that the light energy transmitted out of the fiber-optic is
not affected by the polarization of the incident light. This phenomenon can be explained by
mode mixing in multimode optical fiber. The modeling based on geometric optics was then
developed[4].
We show in this paper the existence of the influences of the polarization of the incident
light. Moreover, in certain conditions, these influences are important. We begin with the
experimental results that demonstrate the apparent differences of the normalized light power
transmitted out of the fiber between the illuminations of p- and s- polarized light. The
measured fiber is only 10cm long that is applicable to the future portable device. A 3D model
that resolves the electric field into p- and s- components during the light propagation has been
developed. The computations agree well with the experiments. The next part of this paper is
devoted to investigate the influences of the p- and s- polarized incident light to the transmitted
light power and the sensitivity of the sensor. The conditions at which this 3D model has to be
used are concluded. The validity and limit of the earlier conclusions[3 are clarified.
2. Experiments
Fig,1, Schematic representation of the fiber-optic sensor
The fiber optic used in our experiments was the multimode step-index silica/silicone
optical fiber (Quartz et Silice PCS 600). A total length of 100mm fiber was cut and its two
ends were carefully polished to minimize the scattering of light. A section of cladding from
4mm to 19mm was removed by mechanical and chemical methods[1-3]. Then a gold film of
55nm thick, monitored by a quartz-crystal detector, was deposited on this section via thermal
evaporation. After 2 days for stabilization of the metal film exposed to air at room
temperature, the fiber (see figure 1) was mounted through a small cell which contained the
tested solutions with certain indices monitored by an Abbe refractometer. The mixture of
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