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High extinction ratio and broad band DQPSK demodulation on silicon-on-insulator platform

The extinction ratio of a Mach-Zehnder interferometer, a cascade of two directional couplers, is a critical parameter for the bit error rate performance of differential quadrature phase shift keying (DQPSK) demodulation. The coupling length of a directional coupler is also strongly dependent on polarization and wavelength. A Mach-Zehnder directional coupler including the bending effects was then proposed on a 5-µm thick silicon-on-insulator waveguide. A 30-dB extinction ratio and 0.5-dB polarization dependent loss over C+L bands of the lightwave communication wavelengths were demonstrated for DQPSK demodulation applications.