The possibility of tailoring the n(e) vs c(Li) relationship in lithium niobate optical waveguides

We present results of our study of concentration profiles of lithium (c(Li) ) in annealed proton exchanged (APE) waveguiding layers as measured by the neutron depth profiling (NDP) method. This non-destructive method, based on the Li-6(n,alpha)H-3 reaction induced by thermal neutrons, allowed easy mo nitoring of c(Li) profiles in a large number of samples fabricated under va rious fabrication conditions. Our systematic study revealed that, though ev ery particular waveguide could be characterised by very similar mirror-shap ed extraordinary refractive index (n(e)) as well as c(Li) depth profiles, i n contrast with up to now generally accepted opinion, there was no linear r elationship which unambiguously attributed Deltan(c) to Deltac(Li). The mos t important fabrication step appeared to be the post-exchange annealing, du ring which the lithium atoms were transported towards the sample surfaces. The annealing regime pre-destined not only the depth distribution of the li thium atoms but, as a consequence of it, also other properties of the waveg uiding region. That knowledge allows us to fabricate the APE waveguides wit h a priori given properties for a wide range of special applications. We ha ve also formulated the n(e) vs c(Li) semi-empirical relationship, which was proved to fit all our fabricated APE waveguides. (C) 2001 Elsevier Science B.V. All rights reserved.