Ultra Wideband and Ultrahigh Speed Optical Modulation Devices with Five-Layer Asymmetric Coupled Quantum Wells

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The aim of this research project was to study and develop wideband and ultrahigh speed optical modulators/switches with the giant electrorefractive (ER) effect of the five-layer asymmetric coupled quantum well (FACQW) in the core layer. The FACQW can produce giant electrorefractive index change Δn in the transparency wavelength region far from the absorption edge. We obtained the results as follows :1.GaAs/AlGaAs FACQW and its application to optical modulation devices(1)Observation of giant electrorefractive index change in the FACQWWe fabricated an FACQW phase modulator using molecular beam epitaxy (MBE), and for the first time, giant ER index change was successfully observed, as theoretically expected. Polarization dependence of the FACQW was also investigated theoretically and experimentally.(2)Fabrication and characterization of FACQW Mach-Zehnder modulatorWe designed and fabricated a Mach-Zehnder modulator with multiple FACQW in its core layer. The driving voltage of the modulator was successfully reduced.2.InGaAs/InAlAs FACQW for 1.55 micron and its application to optical modulation devices(1)Theoretical analyses of ER effect in InGaAs/InAlAs FACQWER effect of InGaAs/InAlAs FACQW was theoretically studied using k-p perturbation theory. As a result, we found a new structure that can produce giant ER effect as GaAs/AlGaAs FACQW.(2)Fabrication of FACQW using MBE and its characterizationWe grew InGaAs/InAlAs FACQWs using MBE, and characterized their crystal structures and optical properties. Results of photocurrent measurements showed the tendency similar to the calculated absorption spectra.(3)Fabrication and characterization of FACQW Mach-Zehnder modulatorWe developed fabrication techniques for a compact Mach-Zehnder modulator with multi-mode interferers (MMI). MMI devices were successfully fabricated using wet-etching technique.