M.D. Fontana

Analysis and Control of the DC Drift in LiNbO

The drift issue induces slow drifting of the optimum operating point for high efficiency or large nonlinearities in analog optical links, and requires complex control of the offset bias voltage for achieving high extinction ratio in digital optical links. We discuss and analyze the different sources of the drift in commercially LiNbO3 Mach-Zehnder modulators. The different extrinsic and intrinsic origins are compared in terms of phase shift and the different corresponding orders of magnitude are given, pointing out the predominant role of the intrinsic (dc) drift. We show the large role played by the electrical inhomogeneities at the surface of the LiNbO3 substrate by highlighting the link between the time dependence of the dc drift and the electrical conductivity measured at the surface and in the volume of the LiNbO3 substrate. This allows to propose a solution to the drift issue which consists in the engineering of the electrical conductivity of the lithium niobate substrate.

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We propose in this paper a fast, sensitive and reliable technique of characterisation based on micro-spectroscopy (Raman and polaron luminescence), which allows at the determination of the chemical reduction profile after titanium diffusion or other high-temperature annealing treatments, and thus to the electrical conductivity, in the section of the waveguides. This profile allows us to predict the DC drift of active components using the electro-optical effect. Moreover, coupled with confocal microscopy, this technique can give 2D and 3D maps of a waveguide at micro scale.

-Based Mach–Zehnder Modulators

Our study is aimed to achieve electro-optic modulators with an enhanced stability even at long time. For this we focused our attention on the non-uniformities in the dielectric permittivity and electrical conductivity which appear in the various successive layers which constitute the optical device. We thus show that polaron luminescence and Raman spectroscopy can be efficiently used to control the uniformity of the electrical conductivity of the wafer prior to the fabrication, and then the Li-out-diffusion and Ti-in-diffusion during the process of the waveguide fabrication.