The possibility of tailoring the ne vs cLi 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 ⁶Li(n,)³H reaction induced by thermal neutrons, allowed easy monitoring of c_Li profiles in a large number of samples fabricated under various fabrication conditions. Our systematic study revealed that, though every particular waveguide could be characterised by very similar mirror-shaped extraordinary refractive index (n_e) as well as c_Li depth profiles, in contrast with up to now generally accepted opinion, there was no linear relationship which unambiguously attributed Δn_e to Δc_Li. The most important fabrication step appeared to be the post-exchange annealing, during which the lithium atoms were transported towards the sample surfaces. The annealing regime pre-destined not only the depth distribution of the lithium atoms but, as a consequence of it, also other properties of the waveguiding region. That knowledge allows us to fabricate the APE waveguides with a priori given properties for a wide range of special applications. We have also formulated the n_e vs c_Li semi-empirical relationship, which was proved to fit all our fabricated APE waveguides.