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We report the observation of four-wave mixing phenomenon in a simple silicon wire waveguide at the optical powers normally employed in communications systems. The maximum conversion efficiency is about -35 dB in the case of a 1.58-cm-long silicon wire waveguide. The nonlinear refractive index coefficient is found to be 9x10-18 m2/W. This value is not negligible for dense wavelength division multiplexing components, because it predicts the possibility of large crosstalk. On the other hand, with longer waveguide lengths with smaller propagation loss, it would be possible to utilize just a simple silicon wire for practical wavelength conversion. We demonstrate the wavelength conversion for data rate of 10-Gbps using a 5.8-cm-long silicon wire. These characteristics are attributed to the extremely small core of silicon wire waveguides.

We devised a silicon photonic circuit with polarization diversity that consists of polarization splitters and polarization rotators. The splitter is based on a simple directional coupler and the rotator has an off-axis double-core structure. Both devices can be made by using planar fabrication technology and require no complex proceses for the fabrication of three-dimensional structures. We fabricated a polarization-independent wavelength filter based on Si wire waveguides as an application of the polarization diversity. The filter consists of the polarization splitters, the rotators, and a ring resonator. The polarization-dependent loss of the filter is about 1 dB. A 10-Gbps data transmission with scrambled polarization is demonstrated.

We describe an ultrasmall polarization splitter based on a simple directional coupler consisting of silicon wire waveguides. The size is only 7 x 16 microm(2), and the polarization extinction ratio is about 15 dB for a single coupler. A double-coupler structure improves the extinction ratio to over 20 dB. The excess loss is smaller than 0.5 dB for both types of device. In the device, the shape of the high-speed waveform is retained at any angle of polarization. Our polarization splitter represents a first step towards accomplishing an ultrasmall optical circuit with polarization diversity based on silicon wire waveguides.

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