High-Δn optical waveguides in LiNbO3: Thallium-lithium ion exchange

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Abstract
Waveguides with large Δne have been fabricated in x‐cut LiNbO3 using ion exchange in a TlNO3 melt. The exchange was carried out at ∼240 °C, producing surface index increases of Δne ≅0.12–0.13. No increase in Δn0 was seen. A simple index profile resulting in the best fit to the observed mode structure is a step‐index profile. Analysis of the diffusion process shows that such a profile can be produced by ion exchange in which self‐diffusion coefficients of the two counterdiffusing species are concentration dependent. This provides the first direct evidence that ion exchange occurs in LiNbO3.