INDEX wave field

The fiber optic evanescent wave sensor (FO-EWS) belongs to a sensor in which the fiber core interacts with the analyte. This interaction occurs through the attenuated total reflection (ATR) and the evanescent wave excitation in a dielectric medium of smaller refractive index in the vicinity of fiber core. If the surrounding medium is fluorescent, then the fluorescence signal in the reaction region of evanescent wave field is excited and detected. This is illustrated in Figure 8.2. 

Consider a medium whose continuous refractive-index profile n(x) varies slowly over a distance equal to a wavelength. A ray path in the region x < 0 of Fig. 35-4(a) touches the turning-point caustic at x = x,p. When there is no path in the region beyond the caustic, the ray is totally reflected from the caustic and no power is lost, i.e. T=0. However, if a transmitted ray originates at the radiation caustic x = x j, then optical tunneling occurs, as described in Chapter 7, and power is lost from the path at x,p to the path at x. If the local plane-wave fields have propagation constant P, then the values of x,p and x are determined by Eq. (35-31). 

Up to this point, we have calculated the linear response of the medium, a polarization oscillating at the frequency m of the applied field. This polarization produces its own radiation field that interferes with the applied optical field. Two familiar effects result a change in tlie speed of the light wave and its attenuation as it propagates. These properties may be related directly to the linear susceptibility The index of... 

Thus, as fi is not in general equal to zero, k 8 = 0, which describes a transverse wave, with the electric field perpendicular to the direction of propagation. The comolex refractive index is then given by... 

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