A polarization-insensitive wide-band electric field sensing scheme using coherence modulation of light

Electro-optic LiNbO3 optical waveguides can be used as optical modulators and also as electric field sensors. In this second application, the optical waveguide senses an electric field, which modulates the intensity of light traveling through the waveguide. In an alternative approach, the electric field can modulate an optical delay, instead of the optical intensity. The fact of modulating an optical delay is also known as optical coherence modulation. An optical delay can be generated when low-coherence light is injected in either a birefringent LiNbO3 optical waveguide or in an unbalanced Mach-Zehnder interferometer. As the modulated optical delay can be transmitted through an optical channel, at the receiver, the sensed electric field can be recuperated by introducing a second optical delay which is matched to the one on the electro-optic sensor. When using birefringent optical waveguides, the sensing-detection process is highly sensitive to optical polarization variations. To minimize such a polarization sensitivity, in this work, a polarization insensitive electric field sensing scheme, based on an unbalanced LiNbO3 Mach-Zehnder interferometer is described. This scheme has been tested and shows a much lower polarization sensitivity when compared to birefringent sensors. The experimental scheme has been tested by sensing wide-band electric fields, in a 0–5 MHz frequency range.