(A) study on multiple quantum well electroabsorption modulator with traveling wave electrode

Abstract

A MQW electroabsorption modulator is one of the key elements for high-speed optical communication, because it has merits such as low operating voltage, small size and easy integration with semiconductor devices. However, for high-speed operation, the size of the device should be small, which requires integration with a lossy passive waveguide for handling. In addition, the short device length results in low extinction ratio and poor power handling ability. Recently, there have been many efforts to overcome the resistance-bandwidth limit and a high bandwidth has been achieved using the TWEAM. In this dissertation, we deal with the characterization and analysis of the microwave properties of TWEAM. This work has been motivated by the attempts to understand the operation of TWEAM as a microwave device as well as an optical one. Chapter 2 presents the design and fabrication of an EAM. Multiple quantum well structure was designed by simulation. The absorption spectrum of the designed epi-structure was measured by fabricating vertical photo-detectors. Lumped EAM devices were fabricated and characterized. In Chapter 3, we deal with the fabrication and the measurement of TWEAM. Extinction ratio of 14, 18 and 25 dB at 1.5V reverse voltage was measured for 400, 600 and 800 μm-long devices, respectively. S-parameter measurement was performed for the devices with three types of feeding line and their microwave loss characteristics were compared. The 3 dB frequency of electrical to optical response was observed to be 10.5, 8.1 and 4.6 GHz for 400, 600 and 800 μm-long devices, respectively. In Chapter 4, the microwave characteristics are analyzed to understand high-frequency response of TWEAM. Slow-wave mode in which TWEAM operates was introduced and the microwave behavior in the mode was shown by approximated equations. For exact analysis of the microwave behavior on the active electrode, de-embedding was done using measured S-parameter data. The calculated characteristic i...