Design of Optical Isolator with TiO 2 /(CeY) 3 Fe 5 O 12 Guiding Layer
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Design of Optical Isolator with TiO2 / (CeY)3Fe5O12 Guiding Layer Hideki Yokoi, Shintaro Ikeya and Tsuyoshi Imada Department of Electronic Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo, 135-8548 JAPAN, ABSTRACT An optical isolator with a TiO2/(CeY)3Fe5O12 guiding layer was studied. A nonreciprocal phase shift was calculated in the magneto-optic waveguide with the TiO2/(CeY)3Fe5O12 guiding layer at a wavelength of 1.55 μm. By employing a multimode interference coupler as coupling devices, the total device length of approximately 600 μm was achieved. An interferometric optical isolator with distinct layer structures, which could be operated in a unidirectional magnetic field, was also designed. INTRODUCTION In optical communication systems, an optical isolator is indispensable for protecting active devices from unwanted reflected light. In the near-infrared region, magnetic garnet crystals are necessary components of an optical nonreciprocal device because of their transparency and large magneto-optic effect. An optical isolator employing a nonreciprocal phase shift is attractive because there is no need for phase matching or complicated control of the direction of magnetization [1]. As described in ref. [2], the nonreciprocal phase shift originates from an asymmetric structure of a waveguide. Therefore, a large nonreciprocal phase shift is expected when a magneto-optic waveguide has a TiO2/magnetic garnet guiding layer [3]. In this paper, we report on an optical isolator with a TiO2/magnetic garnet guiding layer obtained using the nonreciprocal phase shift. The nonreciprocal phase shift in the magneto-optic waveguide is calculated at a wavelength of 1.55 μm. A 1×2 multimode interference (MMI) coupler in the optical isolator is designed to make a compact device. An interferometric optical isolator with distinct layer structures, which can be operated in a unidirectional magnetic field, is also described. DEVICE STRUCTURE Figure 1 shows the optical isolator with the TiO2/magnetic garnet guiding layer that is obtained by using the nonreciprocal phase shift. The optical isolator has a TiO2/(CeY)3Fe5O12 (Ce:YIG) guiding layer and a (GdCa)3(GaMgZr)5O12 (GCGMZG) lower cladding layer. The nonreciprocal phase shift occurs in transverse magnetic (TM) modes that travel in magneto-optic waveguides in which magnetization is aligned transverse to the light propagation direction in the film plane. The nonreciprocal phase shift indicates that the amount of the phase shift is not identical for both the forward- and backward-traveling waves propagating into the magneto-optic waveguide; hence, the forward- and backward-traveling waves have distinct propagation constants.
External magnetic field 90O Reciprocal p.s. O (90 Nonreciprocal p.s.) Output MMI coupler
Input (Gd,Ca)3(Ga,Mg,Zr)5O12 3Fe5O12 (CeY) TiO2
Figure 1. Optical isolator with TiO2/Ce:YIG guiding layer employing nonreciprocal phase shift.
Nonreciprocal phase shift (rad/mm)
The isolator includes an optical interferometer that consists of two
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