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 LiNbO$_3$ 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 LiNbO$_3$ 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 LiNbO$_3$ 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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