Fabrication and Characterization of Vertical Ge Thermooptic Modulators
Date Issued
2009
Date
2009
Author(s)
Chen, Tsang-Long
Abstract
Now the optical communication is world-widely used for high-speed internet and communication. The optical modulator is one of the most important devices of modern high-speed optical communication system. The two methods in optical modulation are the direct modulation of a semiconductor laser and the external modulation by using an external modulator. The advantages of direct modulation are simple and compact since no other component than the laser itself is used in modulation and the advantages of the external modulation are high speed, large contrast ratio, and low distortion due to its low chirp phenomenon. There are various types of external modulators available, including electro-optic modulators, electro-absorption modulators, and thermo-optic modulators. The research and develop on fabrication and characterization of a vertical germanium (Ge) thermooptic modulator are the key thrusts in this thesis. The thesis is divided into three parts: (1) the fabrication of Ge-on-insulator (GOI) structure, (2) the fabrication of the vertical Ge thermooptic modulator based on GOI wafer, and (3) the experimental setup and static and dynamic measurement of devices. Comparing with other semiconductors, Ge has a steep optical absorption curve at C-band (1530 nm-1565 nm) and therefore the curve can move fast with varying temperature; however, the use of Ge as the device material is beneficial for optical communication and optoelectronics-electronics integrated circuits (OEIC). In this thesis, we establish a process of low-temperature GOI bonding technology and fabricate a vertical Ge thermooptic modulator based on the GOI wafer. The device utilizes the lateral self-resistive-heating method to raise the device temperature for changing the absorption coefficient of Ge, and therefore the modulated light which passing through the device exhibits a red-shift response in the spectrum for an absorption-type modulation. In order to optimize the device, the volume of Ge is decreased for heat capacity reduction and a SiO2 layer is deposited on device as the anti-reflection coating (ARC) layer. The above processes can improve the operation speed and the contrast ratio of device. The experiment setup up is used to measure the static electro-optic curve and dynamic modulation. When this modulator is biased at 1 V (0.7 mW heating power) at the wavelength of 1553 nm , the modulation contrast ratio, rise time and fall time are 5.4 dB, 540 ns and 600 ns, respectively. When the heating power is raised to 4 mW and at 1554.4 nm, the modulation contrast ratio reaches 7.1 dB, the rise time and fall time are down to 240 ns and 210 ns, respectively. This study demonstrated that the vertical Ge thermooptic modulator is a useful device with low power consumption and it’s suitable for optical communication and OEIC.
Subjects
thermooptic
optical modulation
external modulator
Ge
absorption
GOI
Type
thesis
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