Millimeter-wave electro-optic modulator using stacked patch-antennas embedded with micrometer-gap for radio-over-fiber system

We propose a new wireless electro-optic (EO) modulator using multi-layered stacked patch-antennas with a narrow gap for radio-over-fiber systems. This device uses a structure of vertically stacked patch antennas like the Yagi-Uda antenna, with an EO crystal and a low-dielectric-constant material hybrid substrate. The electric field for optical modulation and the patch length are increased by using the multi-layered antenna stacked structure. Therefore, the conversion efficiency from wireless to optical signals can be improved. In the analysis by use of HFSS, the operational frequency was shifted to the higher region by 7 GHz, and the modulation efficiency improved by 3.2 dB with the double stacking structure. In the experiments, the improvement of the modulation efficiency by 3.0 dB was verified, and the measured frequency response and directivity were in good agreement with the designed ones. The conversion of wireless 60 GHz band signals with QPSK modulation and the transfer of the converted signal over 1 km long fiber were also demonstrated.