We propose an -cut LiNbO3 optical modulator using gap-embedded patch-antennas for wireless-over-fiber systems. The proposed device is composed of an array of narrow-gap-embedded patch-antennas and an optical waveguide located at the center of the gap without a buffer layer. The modulation efficiency of the proposed -cut LiNbO3 optical modulators was enhanced by 6？dB compared to the -cut LiTaO3-based devices. 1. Introduction Wireless-over-fiber technology suitably supports wireless communication systems in microwave/millimeter-wave bands by compensating for large transmission losses of metallic cables [1]. In wireless-over-fiber technology for communication systems, microwave/millimeter-wave signals are converted to lightwave signals and transferred through optical fibers with low transmission loss. Large transmission bandwidth and no induction are also advantages of the optical fibers [2]. The wireless-over-fiber systems are composed of two domains: one is a microwave/millimeter-wave domain and the other is an optical domain. Therefore, conversion devices between the microwave/millimeter-wave and optical signals are required to realize the wireless-over-fiber systems. A microwave/millimeter-wave generation for downlink in wireless-over-fiber systems can be achieved by a high-speed photodiode [3]. On the contrary for uplink conversion, the microwave/millimeter-wave signals can be directly converted into lightwave signals by use of high-speed optical modulation technology [4]. A conversion device from wireless microwave to lightwave signals can be composed of wireless microwave antennas and electrooptic (EO) modulators [5]. Wireless microwave signals can be received by the antennas. The received signals are transferred to the EO modulators by a connection line such as coaxial cables. However, microwave signal distortion and decay might occur in the coaxial cables due to high-frequency operation. In order to reduce the microwave signal distortion and realize a simple compact device, wireless microwave-lightwave signal converters using integration of wireless microwave antennas and optical modulators have been developed. Several EO modulators using antenna-coupled resonant modulation electrodes were reported [6–10]. They were composed of planar antennas for wireless microwave signal receiving, resonant electrodes for optical modulation, and their connection lines on an EO crystal as a substrate. The antennas, resonant electrodes, and connection lines should be tuned precisely to obtain good resonance and impedance matching conditions for effective
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