Modulation characteristics of a novel InGaAs/InAlAs multiple quantum well (MQW) microring-enhanced Mach-Zehnder modulator (MRE-MZM) is investigated in detail and its low-voltage operation with high extinction ratio is demonstrated. The MZM has a single microring resonator in one arm and is driven by the change in electrorefractive index induced by the quantum-confined Stark effect in the MQW core layer. As the MQW, a multiple five-layer asymmetric coupled quantum well (FACQW) is used to obtain a large electrorefractive index change. The driving voltage of the proposed MZM is significantly reduced owing to the enhanced phase shift in the microring resonator. High-mesa waveguide structures are grown by solid-source molecular beam epitaxy and fabricated by inductively coupled plasma etching. A directional coupler with an asymmetric branching ratio is used as an input coupler to prevent the degradation of the extinction ratio of the MZM. The extinction ratio of the fabricated MRE-MZM is approximately 27 dB. The product of the half-wave voltage and phase shifter length, V(π) · L, is 1.7 Vmm in static modulation. This value is one-quarter that of a conventional MZM with the same waveguide structure.

Low-power-consumption thermo-optically controlled silicon-microring-resonator loaded Mach–Zehnder modulators (MRR-loaded MZMs) are demonstrated. We experimentally characterized a single microring and cascaded-multiple-microring resonators coupled to one arm of a Mach–Zehnder interferometer (MZI). The driving power consumption of the proposed MZM is significantly reduced owing to the enhanced phase shift in the MRR. The device was fabricated on a silicon-on-insulator (SOI) waveguide structure, and each microring is equipped with TiN microheater for thermo-optic tuning. The coupling efficiency between the microring and a busline waveguide was regulated by varying the gap between two waveguides at a directional coupler. The power consumption of single microring and cascaded MRR-loaded MZMs was approximately 0.4 and 1 mW, respectively. The phase-shift enhancement factor of up to 19 with a maximum extinction ratio of 18 dB was obtained experimentally. Multiple-wavelength operation was also demonstrated in the cascaded MRR-loaded MZM.

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