KRAMERS-KRONIG ANALYSIS OF MOLECULAR EVANESCENT-WAVE ABSORPTION-SPECTRA OBTAINED BY MULTIMODE STEP-INDEX OPTICAL FIBERS

Spectral distortions that arise in evanescent-wave absorption spectra obtained with multimode step-index optical fibers are analyzed both th eoretically and experimentally. Theoretical analysis is performed by t he application of Kramers-Kronig relations to the real and the imagina ry parts of the complex refractive index of an absorbing external medi um. It is demonstrated that even when the extinction coefficient of th e external medium is small, anomalous dispersion of that medium in the vicinity of an absorption band must be considered. Deviations from Be er's law, band distortions, and shifts in peak position are quantified theoretically as a function of the refractive index and the extinctio n coefficient of the external medium; the effect of bandwidth for both Lorentzian and Gaussian bands is also evaluated. Numerical simulation s are performed for two types of sensing sections in commonly used pla stic-clad silica optical fibers. These sensors include an unclad fiber in contact with a lower-index absorbing liquid and a fiber with the o riginal cladding modified with an absorbing species. The numerical res ults compare favorably with those found experimentally with these type s of sensing sections. (C) 1996 Optical Society of America