Magneto-optical isolator for silicon photonics

An optical isolator is one of developing devices in silicon photonic integrated circuits. The magneto-optical polarization rotation, which is used in conventional isolators and circulators, cannot be applied in... [ view full abstract ]

An optical isolator is one of developing devices in silicon photonic integrated circuits. The magneto-optical polarization rotation, which is used in conventional isolators and circulators, cannot be applied in Silicon-On-Insulator (SOI) waveguide platforms because of the phase matching issue between TE and TM modes. We have realized isolators in SOI Mach-Zehnder interferometer (MZI) waveguides based on the magneto-optical phase shift as shown in the figure. The magneto-optical phase shifters are installed in the MZI waveguide arms where a magneto-optical garnet (CeY)3Fe5O12 (Ce:YIG) is directly bonded on a silicon waveguide by using a surface activated bonding technique. A magnetostatic field is applied transverse to the light propagation direction in the film plane of Ce:YIG to saturate its magnetization. The first-order magneto-optical effect gives a different propagation constant for TM modes propagating in the waveguide depending on the propagation direction as well as the direction of the applied magnetostatic field. The phase difference is provided between two arms due to anti-parallel magnetostatic fields, which is set to be -90 and +90 deg in the forward and backward propagation, respectively. The magneto-optical phase difference is cancelled by the 90 deg phase bias installed in one of the arms in the forward propagation, whereas 180 deg phase difference is introduced in the backward propagation. Hence, constructive and destructive interferences occur in the forward and backward propagations, respectively, which result in the transmission and isolation in respective directions. A magneto-optical phase shift of 3.65 mm-1 is generated in a 450-nm-wide and 220-nm-thick SOI waveguide at a wavelength of 1550 nm using a saturation Faraday rotation of -4500 deg/cm for Ce:YIG. By virtue of the direct bonding technique, we can make full use of the large Faraday rotation of single-crystalline Ce:YIG. A maximum optical isolation of 30 dB is demonstrated together with a bandwidth of 8 nm for >20 dB isolation in a 1550 nm wavelength range. Also, a temperature insensitive isolation has been demonstrated in a temperature range of 20-60 degC by properly adjusting a phase bias. Reducing an insertion loss of 13 dB is under investigation.