Is Silicon Photonics a Competitive Technology to Enable Better and Highly Performing Networks?

This chapter focuses on the fundamental and high-speed characteristics of small-footprint integrated optical modulators designed and fabricated on the basis of the silicon-photonic platform to assess their key performance factors in applications related to high-capacity energy-efficient optical networks transmitting data in various modulation formats. The design and characteristics of high-speed silicon rib-waveguide phase shifters, which are most essential in the high-speed optical modulators, are described. A low-loss quasi-single-mode silicon rib-waveguide phase shifter with reduced RC delay is highlighted along with its design features and fundamental performances in terms of optical loss and on/off dynamic response. Free-carrier plasma dispersion is reviewed as a physical process for performing optical modulation, which allows a reduction in thermal drift and frequency chirping. The plasma dispersion has a unique property in that signal distortion due to residual intensity modulation cancels with the nonlinear voltage dependence of the optical phase, thereby being useful for zero-chirp optical modulators to eliminate transmission impairments. The on-off keying performance of a silicon optical modulator using a single Mach-Zehnder interferometer waveguide is described in the first example of optical network applications with emphasis on a 10-Gb/s dispersion tolerance comparable to that of a commercial lithium niobate modulator. The advantage of silicon-photonic integration is remarkable, in particular, for the ultrasmall-footprint silicon optical modulator consisting of a pair of IQ nested Mach-Zehnder interferometers for two orthogonal polarization components and a polarization multiplexer monolithically integrated on a silicon chip. Such a chip is presented with respect to applications in digital coherent communication in optical-fiber links up to 1000 km long at a bit rate as high as 128 Gb/s.