Lithium Niobate (LiNbO3) optical retarders used as electric field sensors

Electric field sensing using LiNbO₃ polarization interferometers, acting as optical retarders, has been demonstrated in previous works. A polarization interferometer is implemented by an birefringent optical waveguide, which is constructed on a Z-cut-Y propagating LiNbO₃ crystal. However polarization interferometers are quite sensitive to optical polarization, which results in DC-drift of the sensed signal at the output. To minimize the DC-drift of the electric field sensing, unbalanced LiNbO₃ Mach-Zehnder interferometers can also be used. Such a device is inherently polarization insensitive when constructed on an X-cut Z-propagating LiNbO₃ crystal. Either the polarization or the unbalanced interferometers introduce optical delays, which can be modulated by the sensed electric field. At the output of the optical retarders the variations of the electric field are imprinted on the optical delay variations. To recuperate the electric field information, optical demodulation is achieved by introducing a second optical delay, which is matched to the sensor´s optical delay. In this paper a description of the two types of optical retarders, when used as electric field sensors is presented.