MODE-INDEPENDENT ATTENUATION IN EVANESCENT-FIELD SENSORS

Generally, the total power attenuation in multimode evanescent-field s ensor waveguides is nonproportional to the bulk absorbance because the modal attenuation constants differ. Hence a direct measurement is dif ficult and is additionally aggravated because the waveguide absorbance is highly sensitive to the specific launching conditions at the waveg uide input. A general asymptotic formula for the modal power attenuati on in strongly asymmetric inhomogeneous planar waveguides with arbitra rily distributed weak absorption in the low-index superstrate is deriv ed. Explicit expressions for typical refractive-index profiles are giv en. Except when very close to the cutoff, the predicted asymptotic att enuation behavior agrees well with exact calculations. The ratio of TM versus TE absorption has been derived to be (2 - n(o)(2)/n(f)(2)) for arbitrary profiles. Waveguides with a linear refractive-index profile show mode-independent attenuation coefficients within each polarizati on. Further, the asymptotic sensitivity is independent of the waveleng th, so that it should be possible to directly measure the spectral var iation of the bulk absorption. The mode independence of the attenuatio n has been verified experimentally for a second-order polynomial profi le, which is close to a linear refractive-index distribution. In contr ast, the attenuation in the step-profile waveguide has been found to d epend strongly on the mode number, as predicted by theory. A strong sp read of the modal attenuation coefficients is also predicted for the p arabolic-profile waveguide sensor.