Optically Modulated Probe for Precision Near-Field Measurements

In this paper, the design and implementation of an accurate, sensitive, and cost-effective near-field (NF) probe are discussed. The probe is based on the modulated scatterer technique (MST), providing very low perturbation on the field to be measured. It consists of a commercial off-the-shelf (COTS) photodiode chip and an antenna acting as a scatterer. The optically modulated scatterer (OMS) essentially makes the NF measurements perturbation free. A matching network is added to the probe structure to increase its sensitivity. The radiation characteristics of the probe, including cross-polarization response and omnidirectional sensitivity, are both theoretically and experimentally investigated. Finally, the performance and reliability of the probe are studied by comparing the measured NF distributions to the simulated distributions.