# Silicon Waveguide Optical Isolator with Directly Bonded Magneto-Optical Garnet
## 5 Integration of TE-TM Mode Converter  
Only the TM mode experienced the NPS in the configuration with vertically asymmetric waveguide and horizontal magnetization. An NPS was obtained for TE modes when the MO material was placed in a horizontally asymmetric manner and subjected to vertical magnetization [22; 23; 24; 25]. However, it required complicated fabrication processes or direct deposition of the MO material [10; 11]. An alternative approach to achieve isolator operation for the TE mode input was to integrate TE-TM mode converters on the same waveguide platform [26]. Here, we presented a configuration of TE-TM mode converter compatible with the fabrication process using a direct bonding technique [18]. A polarization rotator was integrated at the input waveguide of an MZI isolator. The configuration allowed not only input light of TE mode to go through but also to block backward reflections of both the TE and TM modes.  
Figure 8a shows the TE-TM mode converter consisting of two parallel waveguides. Because of air cladding waveguide without any prominent structure on the top Si, the photonic circuit was consistent with an optical isolator fabricated by direct bonding. The air cladding broke the vertical asymmetry, which was necessary for conversion between TE and TM modes. The two parallel waveguides with different width supported TE\({}_{0}\), TE\({}_{1}\), and TM\({}_{0}\) modes. The waveguides were then designed so that TE\({}_{1}\) and TM\({}_{0}\) modes had almost the same propagation constant. The TE input light injected into the narrow waveguide was efficiently converted into the TE\({}_{1}\) mode. After propagating a half-beat length \(L_{\pi}\) between the TE\({}_{1}\) and TM\({}_{0}\) modes, the light was converted to the TM\({}_{0}\) mode and output to the wide waveguide. By using a mode solver with finite element method, \(L_{\pi}\) was calculated to be 10.2 \(\upmu\)m when the two waveguide widths and gap were 630, 285, and 95 nm, respectively. A fabricated mode converter exhibited a mode conversion efficiency of 90%.  
Figure 8b shows the microscopic image of a fabricated MZI isolator integrated with the TE-TM mode converter at the input side. A 100-\(\upmu\)m-long tapered waveguide connec  
Figure 8: (**a**) Transverse electric (TE)–transverse magnetic (TM) mode converter consisting of two parallel waveguides. (**b**) Microscopic image of fabricated MZI optical isolator integrated with mode converter.  
of mode converter to the 450-nm-wide input waveguide of MZI isolator. The polarization of input light was adjusted to the TE mode, whereas that of backward reflections was unpolarized. The backward TM mode can be eliminated in the MZI isolator by the MO effect. The backward TE mode was also eliminated by the following two structural effects. The directional coupler in the MZI isolator had strong polarization dependence. It operated as 3-dB splitter and combiner for the TM mode, whereas it gave a higher loss for the TE mode. The TE-TM mode converter showed asymmetry in the input and output waveguides. The backward TE light injected to the wide waveguide was almost coupled to the TE\({}_{0}\) mode; therefore, it was neither converted to the TM mode nor coupled to the narrow waveguide. The integrated isolator was characterized to demonstrate a maximum isolation of 26.7 dB for the TE mode input and TM mode backward lights.