SOI Waveguide Optical Nonreciprocal Devices with Directly Bonded Garnet
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SOI Waveguide Optical Nonreciprocal Devices with Directly Bonded Garnet Tetsuya Mizumoto and Ryohei Takei Dept. of Electrical and Electronic Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8552, JAPAN ABSTRACT Surface activated direct bonding of a magnetooptic garnet crystal to a Silicon-On-Insulator (SOI) wafer is discussed for the application to waveguide optical nonreciprocal devices. An interferometric waveguide isolator is discussed that uses nonreciprocal phase shift brought about by a first-order magneto-optic effect. In an SOI waveguide, the low refractive index of buried oxide layer enhances the magneto-optic phase shift. This contributes to reduce the device size together with the strong field confinement of high index contrast waveguide. The interferometric isolator can be extended to an optical circulator by adopting appropriate 3-dB directional couplers to construct a Mach-Zehnder interferometer. INTRODUCTION In optical fiber communication systems, optical nonreciprocal devices such as an optical isolator and an optical circulator are indispensable to prevent lightwave from propagating in unwanted directions. They protect active devices from unwanted reflected light. The waveguide nonreciprocal devices are desired for the application to photonic integrated circuits. In the near-infrared region, magnetooptic garnet crystals are commonly used to obtain the optical nonreciprocal function owing to their large magnetooptic effect and low optical absorption. When one considers integrating the optical isolator with other optical devices like a laser diode, a critical issue exists in integrating a magnetooptic garnet with commonly used optical waveguide platforms based on III-V compound semiconductors, silicon, and silica. It is needed to develop the technology for integrating a magneto-optic garnet with them. We developed a surface activated direct bonding technique to integrate a magneto-optic garnet on III-V, silicon and silica [1,2]. Using this technique, we fabricated an interferometric isolator that was composed of a GaInAsP guiding layer [3]. The isolator employs a nonreciprocal phase shift in a Mach-Zehnder interferometer. We also demonstrated an isolation of 21 dB at 1559 nm in the silicon interferometric waveguide isolator, where a Ce-substituted yttrium iron garnet (Ce:YIG) cladding layer was directly bonded on a silicon rib waveguide [4]. The surface activation process with plasma generated in oxygen is used for bonding the garnet onto silicon at 250 oC. In these cases, relatively low temperature process is crucial for bonding dissimilar crystals by circumventing the problems associated with a mismatch in thermal expansion between two crystals. In this article, we discuss the dependences of wafer pre-treatment and plasma conditions on the bonding of garnet and silicon. The direction-dependent phase difference in an interferometric waveguide is extended to realize a waveguide optical circulator. A silicon waveguide circulator is designed with a crosstalk < -20 dB an
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