A low-perturbation near-field imager equipped with optical MST probes

In this paper, the design and implementation of a near-field microwave imager based on a probe consisting of an optically modulated scatterer (OMS) is described and discussed. The probe uses a commercial-off-the-shelf (COTS) photodiode chip and a short dipole and it fulfils the requirements for a minimum scattering antenna. Measurement using this OMS probe is nearly perturbation-free due to the low scattering of the measured radio-frequency electric field by the optical fiber used to carry the modulating light signal. An array of OMS probes is then developed to eliminate or reduce the delays associated with a mechanical positioning system in a planar near-field setup. Optical power switching between the array elements is done with a switched laser diode array using a number of fiber-pigtailed laser diodes. The performance assessment of the near-field imager, including its linearity and accuracy, is made by comparisons with a known field distribution.