Antenna-Integrated EOM

A high-speed electro-optic (EO) modulator is a key device for the conversion from radio signals to optical signals in modern optical communication networks and microwave photonics systems. Commercially available high-speed EO modulators using traveling-wave electrodes and optical waveguides composed of LiNbO3 crystal or semiconductor (GaAs, InP, Si, etc.) can be operated in the frequency ranges from DC to ~40 GHz and are widely used for long-haul optical fiber communication systems, data centers, and optical signal processing systems. However, higher frequency operations over 40 GHz are still challenging owing to significant loss effects at feeding cables and connectors, and to coupling difficulty of modulation signals to electrodes. One promising solution to overcome these problems is to feed high-frequency (millimeter-/THz-wave) signals to optical modulators via free space with a planar antenna integrated on the modulator substrate surface. Therefore, antenna-integration on the substrate of EO modulators are rather attractive for advanced microwave photonic applications. In particular, signal conversions from wireless millimeter-/THz-wave signals to optical signals are to be indispensable for Beyond-5G/6G mobile wireless communication systems with the radio-over-fiber (RoF) technology. In this chapter, the basis of high-speed EO modulation and the recent study for antenna-integrated EO modulators are discussed.