News Feature | November 3, 2016

Asymmetric Waveguide Makes For An Ultra-Efficient All-Optical Switch

By Jof Enriquez
Follow me on Twitter @jofenriq

The image above shows a weak control beam (narrow red line, far right) and a more intense laser signal (larger red line, far left) within an asymmetrical metawaveguide (purple box). Image courtesy of University at Buffalo.

Scientists at the State University of New York at Buffalo have designed a new optical switch that uses an asymmetric waveguide to let a weaker "switching" control beam control a stronger signal or "output" beam.

Typically, in an all-optical switch, the control beam needs to exceed a high threshold of power to be able to modulate an incoming signal beam. Even though others have developed switches that worked with equal-powered inputs, their high power consumption limits their practical use.

Now, researchers at Buffalo State say they have successfully demonstrated light-light switching that uses just one-third or less of the energy required to control a light source.

“Typically, symmetry connotes harmony and beauty. But not in this case. We’ve developed technology — an asymmetric metawaveguide — that enables a weak control laser beam to manipulate a much more intense laser signal,” says Liang Feng, Ph.D, assistant professor in the Department of Electrical Engineering at the University at Buffalo’s School of Engineering and Applied Sciences, and the study’s lead author, in a press release.

The 800-nanometer-wide silicon optical fiber waveguide works as an optical switch for a signal beam arriving from the left, and a control beam from the right. The waveguide has asymmetric notches carved along its length and a diamond-shaped pad—  consisting of chromium and germanium films — that lays on top. These two nanosized elements of the waveguide were designed to achieve asymmetric reflectivity, which makes low-power switching possible.

"The asymmetric reflectance was chosen to allow the control beam to have one-third the power of the signal beam, and yet the amplitude of the reflected light would match the amplitude of the transmitted light on both the left and right sides of the waveguide. In demonstrations, the team showed that the destructive interference between these outgoing waves resulted in a millionfold decrease (60 decibels) in the transmitted signal," according to APS Physics.

"This asymmetric control results from operating near an exceptional point of the scattering matrix, which gives rise to intrinsic asymmetric reflections of the metawaveguide through delicate interplay between index and absorption," adds the study abstract.

The designed metawaveguide reduces power requirements for all-optical switches by a factor of three, with further tweaking of materials promising even lower power interferometric light-light switching for the next generation of optical devices and networks.

The SUNY study was supported by grants from the U.S. Army Research Office, the National Science Foundation, and Boeing.