Erbium-Doped Optical-Waveguide Amplifiers on Silicon
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Optical-Waveguide Amplifiers on Silicon RG. Kik and A. Polman
Introduction Thin-film integrated optics is becoming more and more important in opticalcommunications technology. The fabrication of passive devices such as planar optical waveguides, splitters, and multiplexers is now quite well-developed. Devices based on this technology are now commercially available. One step to further improve this technology is to develop optical amplifiers that can be integrated with these devices.1'2 Such amplifiers can compensate for the losses in splitters or other optical components, and can also serve as pre-amplifiers for active devices such as detectors. In optical-fiber technology, erbiumdoped fiber amplifiers,3'4 are used in long-distance fiber-communications links. They use an optical transition in Er3+ at a wavelength of 1.54 ^m for signal amplification, and their success has set a standard of optical communication at this wavelength. Using the same concept of Er doping, planar-waveguide amplifiers are now being developed. For these devices, silicon is often used as a substrate, so that optoelectronic integration with other devices in or on Si (electrical devices, or Si-based light sources, detectors, and modulators) may become possible. Figure 1 shows an example of a silicon-based optical integrated circuit5 in which a 1 X 4 splitter is combined with an amplifying section. It may seem straightforward to translate the concept of a fiber amplifier to that of an integrated planar waveguide. However in scaling down the device dimensions from the long fiber length (typically 40 m) to the small device dimensions of an optoelectronic integrated circuit, the Er concentration has to be in48
creased to achieve the same amount of optical gain. Physical processes that were unimportant in fiber technology become important in planar amplifiers. In the past few years, several Er-doped planar optical-waveguide materials have been explored, and in some cases optical gain has been achieved. The challenge today is to understand the processes that limit the gain, and to find materials and structures in which these processes have a minimum effect. This article discusses the most important of these processes
and gives an overview of the planar optical amplifiers obtained on silicon to date. Principle of Operation In most materials, erbium assumes the trivalent charge state (Er3+) with an electronic configuration of [Kr]4rf104/n5s25p6. Spin-spin and spin-orbit coupling in the incompletely filled 4/ shell give rise to a number of energy levels as depicted in Figure 2. Each degenerate level is Starksplit into a manifold of levels because of the presence of the host material. The transition from the first excited state (4I13,2) to the ground state (4Ii5,2) occurs at a wavelength of approximately 1.54 /im. The emission wavelength is relatively insensitive to the host material because the 4/shell is shielded from its surroundings by the filled 5s and 5p shells. In an optical amplifier, erbium is incorporated in the core of an optical waveguide
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