Optical Isolator with Si Guiding Layer Fabricated by Photosensitive Adhesive Bonding

Abstract

INTRODUCTION In optical communication systems, an optical isolator is indispensable in protecting active photonic devices from unwanted reflected light. An optical isolator with a Si guiding layer is a significant device for silicon photonic integrated circuits. In this paper, the authors report on an optical isolator with the Si guiding layer fabricated by photosensitive adhesive bonding. DEVICE STRUCTURE Figure 1 shows an optical isolator with a Si guiding layer employing a nonreciprocal guided-radiation mode conversion [1]. A magnetic garnet cladding layer is bonded with the Si guiding layer by photosensitive adhesives. A cerium-substituted yttrium iron garnet (Ce:YIG) is used as the magnetic-garnet cladding layer [2]. The optical isolator is comprised of a straight rib waveguide with a Ce:YIG/Si/SiO2 structure. An external magnetic field is applied to the magneto-optic waveguide in film plane. TM modes travelling in the magneto-optic waveguide have distinct propagation constants for the forward- and the backward-travelling waves owing to a nonreciprocal phase shift. By adjusting waveguide parameters, the following relationship is satisfied: β b y < β c x < β f y (1) where β f y and β b y denote the propagation constants of the forward- and the backward-traveling TM modes and β c x denotes the cutoff of TE modes. Only the backward-traveling TM modes can couple to the TE radiation modes so that the device acts as a TM-mode optical isolator. The optical isolator employing the nonreciprocal guided-radiation mode conversion was designed at 1.55 μm. The nonreciprocal phase shifts in the magneto-optic waveguides with various gap widths were calculated. It was confirmed that the nonreciprocal phase shift was reduced with increasing the gap width. The relationship of the waveguide parameters that satisfies equation (1) was investigated. Once the rib height was set, the range of the rib width for the isolator operation, which means that the propagation constants satisfy the relationship denoted by expression (1), was given. It was found that smaller gap width was desired for large tolerance of the waveguide parameters. ACKNOWLEDGMENT This work was partially supported by the SIT Research Center for Green Innovation. REFERENCES 1. H. Yokoi, K. Sasaki and T. Aiba, Jpn. J. Appl. Phys., 48, 062202 (2009). 2. T. Shintaku, T. Uno and M. Kobayashi, J. Appl .Phys., 74, 4877 (1993). Figure 1