For the SOI-waveguide directional coupler (WDC), optical access loss (OAL) and polarization dependence (PD) are two critical performance specifications which seriously affect the adoptability and deployment of a device, including optical on-chip loss (OCL), polarization dependent loss (PDL) and extinction ratio of a 3dB-coupler based device. In this work, using a commercial software tool - FIMMPROP, the performance of an SOI-WDC is simulated. Simulations find that the curved waveguides for the turning sections of a 3dB WDC not only enlarge the footprint size, but also seriously deteriorate the device performance. For instance, the two curved waveguide sections of a WDC induce an unpredictably large change in the 3dB-coupling length, increase an OAL of 0.4-0.9dB, and seriously deteriorate the PD, and these performance changes radically depend on rib size. After a corner-turning mirror (CTM) structure is introduced to a 3dB SOI-WDC, the experiments show both the footprint length and 3dB-coupling length are unchanged, the OAL of the 3dB coupler is only 0.5dB which is close to the simulation value. Therefore, for a 3dB-coupler based Mach-Zehnder interference (MZI) structure, the OCL will be controlled to be <1.0dB in device design and will not depend on rib size.

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Keywords corner-turning mirror, Mach-Zehnder interference, optical access loss, polarization dependence, Silicon-on-insulator waveguide, waveguide directional coupler
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Conference Optoelectronic Devices and Integration III
Sun, D. (DeGui), Hasan, I. (Imad), Abdul-Majid, S. (Sawsan), Vandusen, R, Zheng, Q. (Qi), Hussien, A. (Ali), … Hall, T.J. (Trevor J.). (2010). Performance improvement to silicon-on-insulator waveguide directional-coupler based devices. Presented at the Optoelectronic Devices and Integration III. doi:10.1117/12.871723