Optical nonreciprocal devices for silicon photonics using wafer-bonded magneto-optical garnet materials
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Introduction An optical isolator allows light waves to propagate in a specified direction and not in the opposite direction. Because of this behavior, the isolator plays an essential role in preventing undesired back reflections from interacting with optical active devices.1 In an optical circulator, an optical signal input is transmitted from a first port to a second port, but an input at the second port is transmitted to a third port (rather than returning to the first port). Such functionality enables photonic circuits that process counter-propagating light waves. The demand for integrating isolators and circulators for building silicon photonic integrated circuits continues to increase. Current optical isolators and circulators employ the Faraday effect, in which a polarization plane rotates in different directions depending on the light propagation direction. The same principle of magneto-optical polarization rotation is applicable for realizing waveguide optical nonreciprocal devices.2–7 Note, however, that phase matching between the transverse electric (TE) and transverse magnetic (TM) modes must be realized in the waveguide to rotate the polarization. In order to achieve this, stringent control of waveguide parameters is
needed to balance the various contributions to the waveguide birefringence. Although it is possible, in principle, to control the birefringence, the tolerances in the manufacturing process make it difficult to achieve the required control. In contrast, an isolator based on the magneto-optical phase shift has a distinct advantage over one based on the polarization rotation, in that it can operate in a single polarization and it is therefore not necessary to achieve phase matching between TE- and TM-mode light waves.8,9 TM-mode light waves experience a magneto-optical phase shift caused by first-order magneto-optical effects when they propagate in a waveguide in which a magneto-optical material (placed as an upper cladding layer) is magnetized in the film plane transverse to the light propagation direction. A different phase shift is obtained depending on the propagation direction as well as the direction of magnetization. Another consideration in realizing optical nonreciprocal devices on a silicon waveguide platform is the integration of a magneto-optical material on silicon. In optical fiber communication wavelength bands, magneto-optical garnet (of the formula R3Fe5O12, where R is a rare-earth element) is the best
Tetsuya Mizumoto, Tokyo Institute of Technology, Japan; [email protected] Roel Baets, Ghent University, Belgium; [email protected] John E. Bowers, Departments of Electrical and Computer Engineering and Materials, University of California, Santa Barbara, USA; [email protected] doi:10.1557/mrs.2018.125
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