Abstract
We propose a novel way to enhance the sensitivity of single-mode--multimode--single-mode
(SMS) fiber structure in the measurement of surrounding refractive index (RI),
which is realized by depositing the multimode fiber section with a high RI
overlay. First, as an essential basis of the analysis of the novel structure,
characteristics of mode reorganizations in the deposited multimode fiber are
discussed. Second, based on full-vector mode expansion, a theoretical analysis
of transmission characteristics of the deposited SMS fiber structure is presented.
Third, aimed at RI measurement ranges which are of common interest in liquid
and gas sensing, the transmission spectra of the structure in response to
the surrounding RI are investigated by simulations. To well utilize the collective
effect of multiple interference modes in mode reorganizations, the monitoring
based on wavelength shifts of valleys and the optimization of the overlay
are analyzed for various application requirements. Lastly, largely enhanced
sensitivities for RI measurements are demonstrated with properly designed
overlay. For RI ranges from 1.31 to 1.35 and 1.0 to 1.03, average sensitivities
of 900 and 206 nm/RIU are achieved with overlay thicknesses of 340 and 470
nm, respectively.
© 2012 IEEE
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