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.

The electro-optic (EO) coefficients r 22 of a congruent LiTaO 3 bulk crystal, with dimensions of 6 × 5 × 20 mm 3 ,havebeen measured over a wide frequency range (from dc up to 150 MHz) using the novel and elaborate step-voltage time-response method (TRM). At low frequencies, these results are compared with those obtained with the frequency-doubling EO-modulation (FDEOM) method. From these measurements, the clamped and unclamped EO coefficients, r S ,respectively, have been derived and the acoustic contribution r a 22 to the EO effect has been deduced and compared with theoretically calculated values. Also, using the values of acoustic contribution, r a ,thetrue value of the EO coefficient r 22 has been extracted. Lastly, the above results are compared with those obtained for a pure congruent LiNbO 3 crystal in the framework of a typical intensity-modulation (IM) EO system.

The frequency dependence of electro-optic coefficients in congruent and stoichiometric LiNbO 3 ,L i T a O 3 , β-BaB 2 O 4 and KTiOPO 4 was determined and compared to each other for the first time in a wide frequency range from DC up to 150 MHz. The measurements have been investigated as a function of laser wavelength from 0.5 to 1.3 µm. Clamped and unclamped electro-optic coefficients as well as the acoustic contribution have been established for each crystal.