Increasing the bandwidth of telecommunications systems and developing photonic crystal structures that could allow for the development of all-optical circuits that minimize electronic components, novel types of optical fiber with unique capabilities, semiconductor quantum “dots” with potential for highbroadband optical amplifiers, and nonlinear optical switches are among the long-term goals of photonics research.

Applied and Engineering Physics Professor Chris Xu, who helped create a world-record transmission capability for fiber optic communication, has established, with help from the fiber optic industry, a state-of-the-art fiber test bed that studies various modulation formats and nonlinearity management techniques for long-haul optical communications and new techniques for fiber optic access networks.

“The information-carrying capacity of optical fibers has been growing exponentially since the advent of fiber optical networks,” Xu says. “Much of the capacity improvement in the last decade came from the implementation of dense wavelength division multiplexing (DWDM) and the increase in the perchannel data rate. Although the demonstrated total capacity is impressive, it is still far from the physical limits of optical fibers.”

Professor Michal Lipson in Electrical and Computer Engineering has demonstrated the ability to guide and bend light in air and in a vacuum, as well as the ability to connect nanophotonic chips to optical fiber. The light is routed on-chip by silicon waveguides controlled by electro-optical switches, and off-chip by a “pin hole” lens connecting to normal optical fibers.

While other researchers have succeeded in building nanoscale photonic devices with square waveguides that confine light by total internal reflection, they have done this only in materials with a high index of refraction, such as silicon. Lipson’s Nanophotonics Group can guide and bend light in low-index materials, including air or a vacuum.

“In addition to reducing losses, this opens the door to using a wide variety of low-index materials, including polymers, which have interesting optical properties,” Lipson says.

Alexander Gaeta, professor in Applied and Engineering Physics, is investigating novel optical waveguides known as photonic crystal fibers. Unlike conventional fibers that guide light in glass, certain types of photonic crystal fibers can confine light within a hollow core, which allows for intense lightmatter interactions with atomic or molecular gases over unprecedented lengths.

“For relatively modest powers it is possible to achieve high intensities over long interaction lengths, allowing nonlinear optical processes such as selfphase modulation, parametric four-wave mixing, and harmonic generation to occur efficiently,” he says. “Our efforts have been devoted to understanding the propagation of ultra-short light pulses under conditions in which these nonlinear processes occur and in certain cases using these nonlinear interactions to create new all-optical devices.”

Professor Alyssa Apsel in Electrical and Computer Engineering expects to see optics in computers in the near future.

Apsel, who heads the Optoelectronic VLSI (Very Large System Integration) Laboratory, designs low-power arrays of optical interconnects for short distance and chip-to-chip communication. Optical processing and communication complement the computational power of standard electronic CMOS (complementary metal-oxide semiconductor) systems.

“The focus of this work is the development of low-power integrated CMOS systems that utilize the speed and computational benefits of optical processing and communication,” she says.

“The limit for shrinking the integrated circuit is real, and improving system performance beyond scaling limits calls for a different way of thinking, perhaps including elements from photonics,” Apsel says. “We’ll see basic systems with higher performance pushed not by scaling, but by the integration of other technologies—with advantages in communication speed, data density, and electrical isolation. These kinds of benefits can be realized through the combination of photonics with more conventional electronics and new approaches to circuit design.”