On the Design of Video-Bandwidth Electric Field Sensing Systems Using Dielectric ${\rm LiNbO}_{3}$ Electro-Optic Sensors and Optical Delays as Signal Carriers

As the measurement of wide-band (multi-megahertz) electric fields is often of practical interest in industrial and commercial environments, in this paper, a methodology of design and implementation of a wide-band electric field sensing scheme, using optical delays as information carriers is described. The scheme is based on a lithium niobate (LiNbO₃) birefringent optical waveguide that performs simultaneously as an optical retarder and as a dielectric (electrode-less) sensor. In this scheme, the LiNbO₃ sensor introduces an optical delay and simultaneously senses an on-air electric field and imprints it around an optical delay. At the receiver, the sensed electric field is detected when the sensor and demodulator are optically matched, i.e., when both introduce identical optical delays. An important aspect, when sensing electric fields using LiNbO₃ dielectric sensors, is that the optically sensed field is weaker than the external field by a factor given by the ratio of the permittivity of the surrounding dielectric media over the LiNbO₃ permittivity (boundary condition for normal electric fields). When the surrounding media is air, the optically sensed electric field is 35 times weaker than the external field, as described in this paper. Another practical issue is that a birefringent optical waveguide is highly sensitive to optical polarization variations. Such a sensitivity implies that the output dc component of the received light changes with time (dc-drift). The dc-drift performance of the proposed electric field sensing scheme is measured and reported in this paper. The dc-drift can be minimized using polarization-insensitive LiNbO₃ unbalanced Mach-Zehnder interferometers.