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  • Signals amplified by light alone

Photonics Spectra
Dec 2012
MINNEAPOLIS/ST. PAUL – The discovery that the force of light in a nanoscale switch is strong enough to move an optical waveguide without having to rely on the device’s mechanical structure could dramatically increase Internet download speeds while also consuming less power.

The microscale optical device, developed at the University of Minnesota, uses the force generated by light to flop a mechanical light-based switch on and off at very high speeds. This development could lead to advances in computation and signal processing using light instead of electrical current. “This device is similar to electromechanical relays but operates completely with light,” said Mo Li, an assistant professor of electrical and computer engineering in the College of Science and Engineering.

University of Minnesota researchers led by Mo Li have invented a novel microscale mechanical switch of light on a silicon chip. The team says the technology could dramatically increase Internet download speeds while also consuming less power.

Li and collaborators discovered in 2008 that nanoscale light conduits can be used to generate optical forces strong enough to mechanically move an optical waveguide. With their new device, they found that its mechanical properties can be completely dominated by the optical force.

“This is the first time that this novel optomechanical effect is used to amplify optical signals without converting them into electrical ones,” Li said.

Glass optical fibers carry many communication channels, each assigned a different color of light so they don’t interfere with each other. This noninterference characteristic ensures the efficiency of a single optical fiber to transmit more information over very long distances. But this advantage also harbors a disadvantage: When considering computation and signal processing, optical devices could not allow the various channels of information to control each other easily – until now.

The new device has two optical waveguides, each carrying a signal. Placed between the waveguides is an optical resonator in the shape of a microscale doughnut. In the optical resonator, light can circulate hundreds of times, gaining intensity.

Using this resonance effect, the optical signal in the first waveguide is significantly enhanced in the resonator and generates a very strong optical force on the second waveguide. That waveguide is released from the supporting material so that it moves in oscillation, like a tuning fork, when the force is applied on it. This mechanical motion of the waveguide alters the transmission of the optical signal. Because the power of the second optical signal can be many times higher than the control signal, the device functions like a mechanical relay to amplify the input signal.

The new optical relay device currently operates 1 million times per second, but the researchers expect to improve it to several billion times per second. The mechanical motion of the current device is sufficiently fast to connect radio-frequency devices directly with fiber optics for broadband communication.

Li’s team includes graduate students Huan Li, Yu Chen and Semere Tadesse, and former postdoctoral fellow Jong Noh. The project received funding from the University of Minnesota College of Science and Engineering and the Air Force Office of Scientific Research.

The results were published online in Nature Communications (doi: 10.1038/ncomms2103).

For more on Mo Li and his 2008 work on nanoscale light conduits, see “Light Drives Nanomachines,”

optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
optical waveguide
Any structure having the ability to guide the flow of radiant energy along a path parallel to its axis and to contain the energy within or adjacent to its surface.
A volume, bounded at least in part by highly reflecting surfaces, in which light of particularly discrete frequencies can set up standing wave modes of low loss. Often, in laser work,the resonator contains two facing mirrors that may either be flat (Fabry-Perot resonator) or have some spherical curvature, which together bind the lasing material that is referred to as the gain medium, and hence the optical cavity of a laser is where lasing occurs.  
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