Waveguide Optical Isolators Fabricated by Wafer Bonding
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Waveguide Optical Isolators Fabricated by Wafer Bonding Tetsuya Mizumoto1,3 and Hideki Yokoi2,3 1 Tokyo Institute of Technology, Dept. of Electrical and Electronic Engineering, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8552 JAPAN 2 Shibaura Institute of Technology, Dept. of Electronic Engineering, 3-9-14 Shibayura, Minato-ku, Tokyo, 108-8548 JAPAN 3 CREST-JST ABSTRACT Wafer bonding of a magnetooptic garnet crystal to III-V compound semiconductor and LiNbO3 is discussed for the application to waveguide optical isolators. Two types of waveguide isolators, an interferometric isolator and a semileaky isolator, are discussed. The interferometric isolator uses nonreciprocal phase shift and is composed of the GaInAsP guiding layer. The isolator has the advantage of integratability with optical active devices. The semileaky isolator composed of a magnetooptic garnet guiding and LiNbO3 upper cladding layer has the advantages of large fabrication tolerance and wide operating wavelength range. The performance of isolators is also demonstrated. INTRODUCTION In optical fiber communication systems, an optical nonreciprocal device such as an optical isolator and an optical circulator is indispensable to protect an optical active device from unwanted reflected light. The waveguide isolator is desired for an application to photonic integrated circuits, and is expected to contribute to the cost reduction of devices. In the near-infrared region, magnetooptic garnet crystals are commonly used to construct the optical nonreciprocal devices owing to their large magnetooptic effect and low absorption loss. When one considers integrating the optical isolator with the optical active device, a critical issue exists in integrating a magnetooptic garnet with a compound semiconductor. We succeeded in bonding such dissimilar crystals, and demonstrated an isolator operation in the interferometric isolator [1]. Since the isolator is composed of a GaInAsP guiding layer, it has the advantage of integratability with optical active devices. The isolator employs a nonreciprocal phase shift in a Mach-Zehnder interferometer. Also, we succeeded in bonding LiNbO3 on a magnetooptic garnet to construct a semileaky waveguide isolator, which has the great advantages of relaxed fabrication tolerance and wide operating wavelength range [2]. In this paper, the wafer bonding of a magnetooptic garnet crystal to III-V compound semiconductors and LiNbO3 is discussed together with the effectiveness of surface treatment for hydrophilic bonding. Also, the surface activation bonding is addressed. The designs of the interferometric and the semileaky isolator are discussed together with experimental demonstration of their performance. In a preliminary experiment, an isolation of 5dB was obtained in the interferometric isolator composed of a GaInAsP guiding layer, to which Ce:YIG was directly bonded as an upper cladding layer. In the semileaky isolator, LiNbO3 is bonded to a Ce:YIG rib waveguide. The lightwave of TE modes is confined in the guiding layer, while TM mode
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