Universal Sensor Using Electro-Optic Sensing Principles

Abstract

Techniques like high power microwaves (HPM), pulse scattering, EMP, ESD and transient radar still lack reliable electric field monitors. One of the main obstacles to develop such field sensors are the enormous difficulties to miniaturize the sensor head providing simultaneously sufficient sensitivity, dynamic range and electrical isolation. This paper will present a recent sensor development of a universal photonic field probe for transient and continuous wave electric field measurements. The probe head incorporates three electrooptic Lithium Niobate (LiNbO₃) modulators packaged in purely dielectric material. The modulators cover a frequency range from dc to 3.5 GHz, where in the range dc to 1 GHz an amplitude flatness of ± 0.5 dB can be specified. The modulator input impedance has been optimized to provide maximum electric field sensitivity in conjunction with a broad band dipole antenna.

The sensor system (see example fig. 1) including an 240 mW low noise, single frequency laser and 18 GHz bandwidth GaInP photo detectors reaches a pulse sensitivity of 4.5 V/m. For narrow band measurements a sensitivity limit of about 100 μ V/m/√Hz can be extrapolated. The 1 dB compression point has been set to 12 kV/m peak to peak and could be increased to about 50 kV/m loosing sensitivity correspondingly. The sensor operates passively and is inherently potential-free by means of optical fibers with a length of up to 2 km. The modulators have been hermetically sealed and allow measurements in liquids in a depth of up to 4 meters.