A spectral Attenuated Total Reflection (ATR-) sensor with variable path lengths is proposed. Based on a new polymer
cladding material, the numerical aperture and the related maximum propagation angle in Polymer Clad Silica Fiber
(PCSF) have been increased significantly. The penetration depth of the evanescent field into the cladding of step-index
fibers depends strongly on the propagation angle. We found that when removing its cladding, the effective path length in
the interaction section can be adjusted, using meridional rays/modes of different propagation angles. Because the fiber
length is short, the mode conversion in the light-transporting section of the PCSF is negligible.
After measuring the optical properties of the PCSF, the ATR with selected liquids surrounding the light-guiding core is
determined in dependence of wavelength and excitation-angle. Especially, the possibilities to change from low to high
propagation angles including the excitation and detection system will be discussed.
High-order skew modes will be excited in multimode step-index fibers using special excitation conditions. As a result, light
with an angle of incidence larger than the maximum angle for meridional modes given by the numerical aperture
of the fiber can be coupled into a fiber. Combining the selective mode-excitation with new powerful broadband light-sources,
the spectral light-guidance of such skew modes in different optical fibers will be described in detail. Results of the proposed
system in context of different light-sources will be discussed. A new evanescent sensor approach based on controlled
coupling of skew modes will be introduced. Finally, first steps to construct such sensors for medical and analytical
applications will be presented.
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