Wafer Bonding between Magnetic Garnet and Lithium Niobate for Semi-Leaky Isolator
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Wafer Bonding between Magnetic Garnet and Lithium Niobate for Semi-Leaky Isolator Hideki Yokoi, Tetsuya Mizumoto and Masafumi Shimizu Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8552 JAPAN, [email protected] ABSTRACT A semi-leaky isolator composed of a magneto-optic / anisotropic waveguide was fabricated by wafer bonding technique. Characteristics of the semi-leaky isolator were investigated. The magneto-optic / anisotropic waveguide exhibited the semi-leaky characteristics. The isolation ratio of 7.2 dB/cm was obtained at a wavelength of 1.55 µm. INTRODUCTION In optical communication systems, an optical isolator is indispensable in protecting optical active devices from unwanted reflected light. In the near infrared region, magnetic garnet crystals are necessary to construct the optical isolator. A waveguide-type optical isolator is desired for photonic integrated circuits. Various kinds of the waveguide-type optical isolator have been investigated by many researchers. A semi-leaky isolator was proposed by Yamamoto et al. in 1976 [1]. The semi-leaky isolator utilized unidirectional mode coupling in a magneto-optic / anisotropic waveguide. The semi-leaky isolator was attractive because of its compact mono-section structure and easy control of magnetization. However, the operation of the device was not confirmed due to poor optical contact between the magneto-optic guiding layer and the anisotropic cladding layer [1]. Kirsch et al. demonstrated the operation of the semi-leaky isolator by using a selenium intermediary layer between the magneto-optic layer and the anisotropic layer [2]. The authors have investigated wafer bonding between magneto-optic crystals and III-V compound semiconductors with the aim of integrating a semiconductor laser diode and an optical isolator [3,4]. It was considered that the optical contact between the magneto-optic crystal and the anisotropic crystal could be accomplished by this technique. In this paper, we report on a semi-leaky isolator fabricated by wafer bonding technique. Wafer bonding between the magneto-optic crystal and the anisotropic crystal was studied. Characteristics of the semi-leaky isolator fabricated by wafer bonding were investigated. DEVICE STRUCTURE Figure 1 shows the basic geometry of the semi-leaky isolator. Cerium-substituted yttrium iron garnet (Ce:YIG) and lithium niobate (LiNbO3) are assumed as a magneto-optic layer and an anisotropic layer, respectively. Faraday rotation coefficient of the Ce:YIG is approximately –4,500 deg/cm at a wavelength of 1.55 µm [5,6]. The optic axis of the lithium niobate is rotated in the film plane, which is done just by rotating the crystal. An external magnetic field is applied to the light propagation direction.
I5.17.1
LiNbO3 External magnetic field Output q Optic axis
Wafer direct bonding Input NOG substrate Ce:YIG
Figure 1. Basic geometry of semi-leaky isolator. The waveguide has a magneto-optic guiding layer and an anisotropic claddin
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