All-optical electric field sensor in domain inverted LiNbO3 for harsh environment

Optical high-voltage sensors have outstanding advantages in terms of isolation and immunity to electromagnetic interference. So far, several configurations have been proposed, mostly based on integrated Mach-Zehnder interferometers [1] or polarization/phase rotation in piezo-electric crystal [2]. While the first scheme requires initial electrical bias to compensate for the phase mismatch between the two arms, the second one requires interrogation or phase noise reduction systems that are expensive to implement. Near cut-off optical waveguide devices have been already reported in LiNbO3. In particular, the use of waveguides at cut-off was proposed for modulation in the field of optical communications [3,4] and sensing [5]. We present a novel integrated optical high voltage sensor based on a Z-cut LiNbO3 which operates without any metallic parts. The proposed device is sketched in Fig. 1 (left). An annealed proton exchange (APE) waveguide near cut-off is fabricated in Z-cut LiNbO3 and centered in a domain inverted region. The application of an external electric field parallel to the z axis of the device produces a refractive index change Δn± between positive and negative domains given by ne3·r33·E, where E is the intensity of the external electric field along the z-axis, ne=2.14 and r33=30.8 pm/V are the refractive index and the electro-optic coefficient along the z-axis, respectively. As a consequence the optical mode will broaden so that, after a sufficient propagation length, a loss is produced due to a mode-profile mismatch of the guided modes between active and passive regions.