Characterization of the near-surface region in ion-exchanged glass waveguides

In the characterization of graded-index glass waveguides, use of the standard m-line method by several laboratories has produced increasing discrepancies in the refractive index profile with decreasing film depth. We have addressed this very critical problem by a direct near-surface (DNS) approach, where the polarimetric Abelès-Hacskaylo method was extended as an admittance-matching condition for inhomogeneous films. In this paper, we review the measurement procedures of the same Ag+-exchanged waveguides by the DNS approach and by the m-line method, whose results showed significant disagreement at the film-air interface. We search the underlying reasons for this disagreement and test them against non- optical measurements (RBS and EDS) of the ion-concentration profile, to reach a better understanding of the near-surface region, as well as of the distinct probing range of the m-line (micro) and DNS (nano) techniques.