Optical Waveguides a Focus at Cornell
Peter Singer, Editor-in-Chief -- Semiconductor International, 3/1/2004
Michal Lipson, an assistant professor at Cornell University (Ithaca, N.Y.), described recent research by the Nanophotonics Group in Cornell's School of Electrical and Computer Engineering at the annual meeting of the American Association for the Advancement of Science (AAAS) in Seattle last month. Her talk was part of a symposium on "21st Century Photonics."
Lipson's group has made waveguides with two parallel strips of a material with a high refractive index placed ~50-200 nm apart, with a slot containing a material of much lower refractive index. In some devices, the walls are made of silicon with an air gap, and others have SiO2 walls with a silicon gap. In both cases, the refractive index of the medium in the gap is much lower than that of the wall, up to a ratio of ~4:1.
When a wavefront crosses two materials of very different refractive indices, and the low-index space is very narrow in proportion to the wavelength, nearly all the light is confined in the "slot waveguide." Theory predicts that straight slots will have virtually no loss of light, and smooth curves will have only a small loss. This has been verified by experiments.
Slot waveguides can be used to make ring resonators, already familiar to nanophotonics researchers. When a circular waveguide is placed very close to a straight one, some of the light can jump from the straight to the circular waveguide, depending on its wavelength. "In this way, we can choose the wavelength we want to transmit," Lipson said. In fiber-optic communications, signals often are multiplexed, with several wavelengths traveling in the same fiber, each carrying a different signal. Ring resonators can be used as filters to separate these signals.
Like the transistor switches in conventional electronic chips, light-beam switches would be the basic components of photonic computers. Lipson's group has made switches in which light is passed in a straight line through a cavity with reflectors at each end, causing the light to bounce back and forth many times before passing through. The refractive index of the cavity is varied by applying an electric field; because of the repeated reflections, the light remains in the waveguide long enough to be affected by this small change. Lipson is working on devices in which the same effect is induced directly by another beam of light.
Connecting photonic chips to optical fibers can be a challenge because the typical fiber is vastly larger than the waveguide. Most researchers have used waveguides that taper from large to small, but the tapers typically have to be very long and introduce losses. Instead, Lipson's group has made waveguides that narrow almost to a point. When light passes through the point, the waveform is deformed as if it were passing through a lens, spreading out to match the larger fiber. Conversely, the "lens" collects light from the fiber and focuses it into the waveguide. Based on experiments at Cornell, the device could couple 200 nm waveguides to 5 µm fibers with 95% efficiency, she reported. It can also be used to couple waveguides of different dimensions.
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