Pigtailed electro-optic probes for vectorial electric field mapping

摘要

Electro-optic measurement (EO) constitutes an efficient technique to characterize electrical (E) fields : indeed,the Pockel's effect properties (linear modification of refractive indices of some non-centrosymetric crystals induced by the E-field)~1 leads to a vectorial measurement. Thus, it allows to map the E-field vector and its transient evolution, either in free space or inside guiding structures. Pigtailed EO sensors are naturally becoming a reliable and consistent mean of characterization for many applications, e.g. high power microwaves (HPM), electromagnetic interference (EMI), on chip diagnostic, bio-electromagnetism (e.g.influence of mobile phones on the human body). Even if these non-invasive sensors provide a greater temporal and spatial resolution (femtosecond and sub-millimeter, respectively) than commonly used sensors (antennas, bolometers), it remains temperature dependant and quite low sensitive. EO probes are based on the modification of a laser beam (either its polarization, phase or amplitude) crossing an EO crystal. We demonstrate here the last developments and improvements for EO probes as well as for whole EO setups,exploiting polarization state or amplitude modulation. The sensor is constituted by a polarization maintaining (PM) fiber carrying the beam to the crystal and taking it back once modulated, gradient index lense(s) managing the shape of the beam, half or quarter wave plate controlling the input and output polarizations and a crystal (either anisotropic: LiTaO_3, LiNbO_3, DAST, KTP or isotropic : ZnTe, InP) converting the E-field into a modulation. Our probes are fully dielectric and cylindrically shaped (length ～1 cm and diameter ～ 2-3 mm). The setup is made of a 1.5 μm DFB laser, some photodiodes (low and high speed) added with a polarization state analyser arrangement in case of EO probes based on polarization state modulation scheme. The measurement bench is fully automated and compensate/measure the temperature deviation simultaneously. Sensitivity of our EO probe reaches 0.7 V.m~(-1) Hz~(-1/2), the bandwidth covers an ultra wide frequency band (kHz - and more than 20 GHz), the selectivity (orthogonal E-field components rejection) is about 25 dB, and a spatial resolution greater than 100 μm is achieved. Transient and frequency measurements and 2D E-field mapping will be presented during the conference.