Caltech-led engineers solve longstanding problem in photonic chip technology

Aug 04, 2011
Caltech engineers have developed a new way to isolate light on a photonic chip, allowing light to travel in only one direction. This finding can lead to the next generation of computer-chip technology: photonic chips that allow for faster computers and less data loss. Credit: Caltech/Liang Feng

Stretching for thousands of miles beneath oceans, optical fibers now connect every continent except for Antarctica. With less data loss and higher bandwidth, optical-fiber technology allows information to zip around the world, bringing pictures, video, and other data from every corner of the globe to your computer in a split second. But although optical fibers are increasingly replacing copper wires, carrying information via photons instead of electrons, today's computer technology still relies on electronic chips.

Now, researchers led by engineers at the California Institute of Technology (Caltech) are paving the way for the next generation of computer-chip technology: photonic chips. With that use light instead of electricity, photonic chips will allow for faster computers and less data loss when connected to the global fiber-optic network.

"We want to take everything on an and reproduce it on a photonic chip," says Liang Feng, a postdoctoral scholar in electrical engineering and the lead author on a paper to be published in the August 5 issue of the journal Science. Feng is part of Caltech's group, led by Axel Scherer, Bernard A. Neches Professor of Electrical Engineering, , and Physics, and co-director of the Kavli Nanoscience Institute at Caltech.

In that paper, the researchers describe a new technique to isolate light signals on a , solving a longstanding problem in engineering photonic chips.

An isolated light signal can only travel in one direction. If light weren't isolated, signals sent and received between different components on a photonic circuit could interfere with one another, causing the chip to become unstable. In an electrical circuit, a device called a diode isolates electrical signals by allowing current to travel in one direction but not the other. The goal, then, is to create the photonic analog of a diode, a device called an optical isolator. "This is something scientists have been pursuing for 20 years," Feng says.

Normally, a light beam has exactly the same properties when it moves forward as when it's reflected backward. "If you can see me, then I can see you," he says. In order to isolate light, its properties need to somehow change when going in the opposite direction. An optical isolator can then block light that has these changed properties, which allows light signals to travel only in one direction between devices on a chip.

"We want to build something where you can see me, but I can't see you," Feng explains. "That means there's no signal from your side to me. The device on my side is isolated; it won't be affected by my surroundings, so the functionality of my device will be stable."

To isolate light, Feng and his colleagues designed a new type of optical waveguide, a 0.8-micron-wide silicon device that channels light. The waveguide allows light to go in one direction but changes the mode of the light when it travels in the opposite direction.

A light wave's mode corresponds to the pattern of the electromagnetic field lines that make up the wave. In the researchers' new waveguide, the light travels in a symmetric mode in one direction, but changes to an asymmetric mode in the other. Because different light modes can't interact with one another, the two beams of light thus pass through each other.

Previously, there were two main ways to achieve this kind of optical isolation. The first way—developed almost a century ago—is to use a magnetic field. The magnetic field changes the polarization of light—the orientation of the light's electric-field lines—when it travels in the opposite direction, so that the light going one way can't interfere with the light going the other way. "The problem is, you can't put a large magnetic field next to a computer," Feng says. "It's not healthy."

The second conventional method requires so-called nonlinear optical materials, which change light's frequency rather than its polarization. This technique was developed about 50 years ago, but is problematic because silicon, the material that's the basis for the integrated circuit, is a linear material. If computers were to use optical isolators made out of nonlinear materials, silicon would have to be replaced, which would require revamping all of computer technology. But with their new silicon waveguides, the researchers have become the first to isolate light with a linear material.

Although this work is just a proof-of-principle experiment, the researchers are already building an optical isolator that can be integrated onto a silicon chip. An optical isolator is essential for building the integrated, nanoscale photonic devices and components that will enable future integrated information systems on a chip. Current, state-of-the-art photonic chips operate at 10 gigabits per second (Gbps)—hundreds of times the data-transfer rates of today's personal computers—with the next generation expected to soon hit 40 Gbps. But without built-in optical isolators, those chips are much simpler than their electronic counterparts and are not yet ready for the market. Optical isolators like those based on the researchers' designs will therefore be crucial for commercially viable photonic chips.

Explore further: Laser device may end pin pricks, improve quality of life for diabetics

Related Stories

Photonics: Pump up the bandwidth

Jun 21, 2006

U.S. scientists say they've developed an optical amplifier based on silicon that works across a wide range of frequencies.

Time Lens Speeds Up Optical Data Transmission

Sep 28, 2009

(PhysOrg.com) -- Researchers at Cornell University have developed a device called a "time lens" which is a silicon device for speeding up optical data. The basic components of this device are an optical-fiber ...

Team develops tiny optical switch

Dec 20, 2007

A team of researchers at the University of St. Andrews has developed one of the smallest optical switches ever made.

Recommended for you

Laser makes microscopes way cooler

Aug 15, 2014

(Phys.org) —Laser physicists have found a way to make atomic-force microscope probes 20 times more sensitive and capable of detecting forces as small as the weight of an individual virus.

User comments : 8

Adjust slider to filter visible comments by rank

Display comments: newest first

gmurphy
2 / 5 (1) Aug 04, 2011
I'm a little dubious about the assertion that "10 gigabits per second" is "hundreds of times the data-transfer rates of today's personal computers". My GTX480 has a memory bandwidth of 177.4 GB/sec : http://www.nvidia..._us.html
jamesrm
5 / 5 (3) Aug 04, 2011
Just a guess but GTX480
Memory Interface Width 384-bit
Memory Bandwidth (GB/sec) 177.4
177.4/384=0.461979167 GBs

So imagine you bus widths times 40GBs about 15TBs
?

rgds
James
Skultch
not rated yet Aug 04, 2011
Maybe they're comparing it to the PCI bus speed. ???
Vendicar_Decarian
1 / 5 (1) Aug 04, 2011
It was a meaningless claim... However..

"My GTX480 has a memory bandwidth of 177.4 GB/sec"...

Over a 128 bit bus to RAM. So a little over 10 Ghz per data line.
Guy_Underbridge
not rated yet Aug 05, 2011
Current, state-of-the-art photonic chips operate at 10 gigabits per second (Gbps)


Actually, 100Gb (OTU4) waves are already becoming telecom backbone standard in some areas.
Bob_Kob
not rated yet Aug 05, 2011
How are they going to manage to guide light through 90 degree bends? Im not an expert in whatever waveguides they use for this but it seems that either it will be reflected back or attenuated at a small angle bend.
Skultch
not rated yet Aug 05, 2011
How are they going to manage to guide light through 90 degree bends? Im not an expert in whatever waveguides they use for this but it seems that either it will be reflected back or attenuated at a small angle bend.


To isolate light, Feng and his colleagues designed a new type of optical waveguide, a 0.8-micron-wide silicon device that channels light.


I guess there's no attenuation or at least a predictable attenuation.
Jimee
not rated yet Aug 05, 2011
Which is itself dubious.