I. Introduction
High-speed optical modulation technology has been the fundamental basis of research in the field of microwave photonics since the concept of the microwave photonics was proposed by pioneering researchers [1]–[5]. A lot of optical modulators/modulation technologies in the microwave (MW) and millimeter-wave (MMW) bands have been developed and implemented for many application fields combined with radio-over-fiber (RoF) techniques. However, MMW-band optical modulators are still developing in contrast to the exceptional performance of the MW-band LiNbO3 electro-optic modulator (EOM), which is a crucial device for long-haul optical fiber communication systems and optical signal processing systems [6]–[8].