Application of micro-attenuated total reflectance infrared spectroscopy to quantitative analysis of optical fiber coatings: effects of optical contact.

Micro-attenuated total reflectance (ATR) infrared spectroscopy is one of the few methods applicable for an in situ analysis of polymer coatings. In this method, the information is collected using an internal reflection element (IRE), which is brought into contact with the coated substrate. Perfect optical contact is hardly achievable for non-flat substrates and/or for those samples that cannot be well aligned with respect to the IRE. Consequently, the infrared peak intensities, their ratios, and all quantities calculated from the spectra become dependent upon the optical contact quality. In this work, we suggest a model that describes the peak intensities in terms of the optical contact. As an illustration, we apply this model to polymer-coated optical fibers. Relatively small diameters of tested fibers and their cylindrical shape result in imperfect optical contact between the sample and the IRE. Spectroscopic approaches of determining the degree of cure of polymer coatings are analyzed in view of the obtained results. Ways of minimizing the error induced by imperfect optical contact are suggested.