Electro-Optic Modulators Based on X-Cut LiNbO_3 Crystals Using Patch-Antennas Embedded with a Narrow-Gap

摘要

Guided-wave electro-optic (EO) modulators based on x-cut LiNbO_3 crystals using patch-antennas embedded with a narrow-gap (〜5μm width) were proposed for high-speed radio-over-fiber systems. A wireless microwave signal can be received by the patch-antennas and directly converted to a lightwave signal through optical modulation by induced microwave electric fields across the gap. By using an x-cut LiNbO_3, which has lower dielectric constants than LiTaO_3, as the substrate, a larger size of the patch-antenna and stronger electric field across narrow-gap can be obtained. The measured sensitivity of the proposed devices becomes twice compared to the LiTaO_3-based optical modulators at the microwave operational frequency of 26 GHz. Techniques for improving effective sensitivity are also discussed for microwave/ millimeter-wave frequency operations.%X-cut LiNbO_3基板上に、狭いギャップ(～5μm)を設けた長方形パッチアンテナと光導波路を作製した電気光学変調器について報告する。この光変調器の上部からマイクロ波無線信号を照射すると、パッチアンテナにより無線信号が受信されて、パッチ導体表面に高周波電流が流れる。このとき、パッチアンテナの中央に狭いギヤップを設けると、電流の連続性のために、変位電流が流れることになり、ギャップ間に強い電界が誘起される。ギヤップ間に光導波路を作製すれば、誘起電界によって光が変調され、無線-光信号変換が得られる。本報告では基板にx-cut LiNbO_3を用いることで従来型のz-cut LiTaO_3基板を用いたものに比べて変換効率が6 dB向上(動作周波数26GHz)することを示す。