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This paper focuses on latest progress in experimental and theoretical studies on silicon-based carrier-depletion PNjunction phase shifters in terms of high modulation efficiency for energy-efficient photonic networks of high transmission capacity. Modulation efficiency of rib-waveguide phase shifters having various PN-junction configuration are characterized with respect to DC figure of merit defined for phase shifters using carrier-plasma dispersion as the physical principle of refractive-index modulation. In addition, RF drive voltage required for 10-Gb/s on-off keying is characterized for rib-waveguide phase shifters including lateral and vertical PN-junction configurations.

Latest computational and experimental studies on high-speed monolithic silicon-based Mach-Zehnder optical modulators are studied in the light of photonic integrated circuits for digital coherent communication at a bit rate as fast as 128 Gb/s per wavelength channel. Lateral PN-junction rib-waveguide phase shifters are elaborated with experimental characteristics of DC phase shifter response in comparison with computational characteristics. High-speed response in refractive-index dynamics including electron and hole transport in the PN junction is simulated to study speed limit of the phase shifters. The performance in quadrature phase-shift keying signal generation is characterized in experimental and computational constellation diagrams. Silicon waveguides for polarization-division multiplexing are designed in common design rules with the rib-waveguide phase shifters. Long-haul transmission in polarization-multiplexed quadrature phase-shift keying in 1000-km single-mode fiber link is confirmed with a monolithic silicon Mach-Zehnder modulator assembled with modulator drivers in a ceramic-based metal package.

Slot waveguide based ring resonators filled with atomic layer deposited (ALD) aluminum oxide (Al2O3) were fabricated and characterized. Our results demonstrate that ALD can be used to create slot waveguide ring resonators with relatively high Q-factors, which opens new possibilities for various photonic applications, such as optical sensing and all-optical signal processing.

Low-loss high-speed traveling-wave silicon Mach-Zehnder modulator with reduced series resistance is studied in microwave and optical measurements. Microwave impedance and propagation loss under reverse bias are characterized by S-parameter measurements. Resonant loss due to series inductance-resistance-capacitance coupling limits microwave performances of the traveling-wave modulator. High-speed optical performances are characterized, based on eyediagram measurements in on-off keying at 10-32 Gb/s and constellation and eye-diagram measurements in differential phase-shift keying at 20 Gb/s. Dispersion tolerance in error-free transmission in 10-Gb/s on-off keying and 20-Gb/s differential phase-shift keying is obtained as +/-950 ps/nm and +/-220 ps/nm, respectively by path-penalty measurements. Transmission performance in 10-Gbps on-off keying is comparable with lithium niobate Mach-Zehnder modulator.

Submicrometer channel and rib waveguides based on SOI (Silicon-On-Insulator) have been designed and fabricated with electron-beam lithography and inductively coupled plasma dry etching. Propagation loss of 8.39dB/mm was measured using the cut-back method. Based on these so-called nanowire waveguides, we have also demonstrated some functional components with small dimensions, including sharp 90° bends with radius of a few micrometers, T-branches, directional couplers and multimode interferometer couplers.