We demonstrate a compactly integrated polarization beam splitter (PBS) with high polarization extinction ratios greater than 20 dB over the full C-band wavelength range based on a simple bridged silicon nanowaveguide directional coupler. The PBS device is designed via three dimensional finite-difference time-domain (3D-FDTD) simulation, and fabricated experimentally. The optimum dimension of the bridge waveguide is determined to be 7.5-μm-long and 500 nm-wide for 250-nm thick silicon core. At the 1,550-nm wavelength, the measured polarization extinction ratios (PERs) of the PBS device are 22.5 dB and 22.9 dB for TE and TM polarization modes, respectively, and its corresponding insertion losses (ILs) are about 2.1 dB and 1.8 dB, both PERs and ILs within the maximum error range of ± 2.0 dB.

We have investigated the ultimate limits of nonuniform grating couplers (NGCs) for optimized fiber coupling to silicon waveguides, compared to uniform grating couplers (UGCs). Simple grating coupler schemes, which can be fabricated in etching steps of the conventional complementary metal-oxide semiconductor (CMOS) process on silicon-on-insulator (SOI) wafers without forming any additional overlay structure, have been simulated numerically and demonstrated experimentally. Optimum values of the grating period, fill factor, and groove number for ultimate coupling efficiency of the NGCs are determined from finite-difference time-domain (FDTD) simulation, and confirmed with experimentally demonstrated devices by comparison to those for the UGCs. Our simulated results indicate that maximum coupling efficiency of NGCs is possible when the minimum pattern size is below 50 nm, but the experimental value for the maximum coupling efficiency is limited by the attainable fabrication tolerance in a practical device process.

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An integrated polarization rotator is demonstrated experimentally by forming a strip waveguide with an asymmetric trench on a silicon-on-insulator wafer. The trench is located asymmetrically in the strip waveguide. It induces the evolution of an orthogonal polarization mode upon a linearly polarized beam input, and thus causes polarization rotation. The device is fabricated using a conventional complementary metal oxide semiconductor process with a single dry etching step. The fabricated device shows a maximum transverse electric (TE)-to-transverse magnetic (TM) polarization conversion efficiency of 21.3 dB and an insertion loss of −0.95 dB at a 1550 nm wavelength with a device length of 67 μm. The device exhibits a polarization conversion efficiency and insertion loss of 21.1 dB and −2.12 dB, respectively, for the TM-to-TE polarization conversion. The optimum parameters for the waveguide size and trench size are investigated by performing numerical simulations, and by demonstrating experimental fabrication and measurement.

Bismuth-substituted yttrium iron garnet (Bi:YIG) films were prepared by using spin coating processes with metal-organic-decomposition-method-based solutions on crystalline silicon (Si) substrates, and their magneto-optic properties at the 1550-nm wavelength region were investigated by performing various thermal treatments. The maximum Verdet constant of the Bi1Y2Fe5O12 film on the Si substrate with a middle buffer layer of Bi2Y1 Fe5 O12 was measured to be 1 072 038°/T/m at 1550-nm wavelength in the unsaturated linear magnetization region by accounting for the negative Verdet constant of the silicon substrate. The optimum thermal treatment condition was observed at the maximum annealing temperature of 700 °C and the annealing speed of 3 °C/min. These spin coating enabled processes may be included to the conventional complementary metal-oxide semiconductor fabrication processes to demonstrate integrated optical waveguide-type isolators on silicon-on-insulator wafers.