Tiny tunable laser lights way to high-speed data networks perhaps 100 times faster than today's fastest data communication systems, UC Berkeley researchers say

by Robert Sanders

BERKELEY -- A microscopic tunable laser etched directly on a computer chip could be the key to making extremely high-speed networks, boosting network speeds beyond the gigabit rate that is set to hit the market later this year.

Standards for gigabit-per-second ethernet - the new state-of-the-art standard for local area networks (LANs) - are being determined now, and some companies already have them available to replace the previous top-speed network connections of 100 Megabits per second. The gigabit networks use lasers that transmit a single stream of data through optical fibers.

UC Berkeley electrical engineering professor Constance Chang-Hasnain hopes to push the speed 10-1,000 times higher with semiconductor diode lasers that transmit information on tens to hundreds of different wavelength channels simultaneously through the same optical fiber.

The tunable laser that can do this is a vertical cavity surface-emitting laser (VCSEL), which she and her colleagues at UC Berkeley etched directly onto a gallium arsenide chip using the techniques commonly employed to make integrated circuits and microprocessors.

"Tunable lasers are available today, but they are about 100 million times larger and a million times slower than this," Chang-Hasnain says. "An integrated device that includes the laser and associated electronics would all fit on the same chip, and would be significantly cheaper."

She will discuss the micromechanical VCSEL on Thursday, Mar. 12, at the annual Industrial Liaison Program conference hosted by UC Berkeley's College of Engineering. She also published her findings in the January 1998 issue of IEEE Photonics Technology Letters (Vol. 10, No. 1), a publication of the Institute of Electrical and Electronics Engineers.

Another future application for such tunable lasers involves networks that switch data based on wavelength. Called wavelength division multiplexed (WDM) systems or wavelength routing, signals can be switched and routed to different directions depending upon their wavelength, allowing faster and more efficient data flow. An array of tunable lasers, each emitting at a different and constantly changing wavelength, could transmit data to many different destinations along the same fiber network.

"We are the first to propose using micromechanical lasers to do such multiplexing," Chang-Hasnain says.

Such systems could be used to speed communication between processors in a computer made up of many parallel processors, for example, eliminating connections to a motherboard.

Through a scanning electron microscope the laser looks like a disk-shaped lollipop floating above the chip surface, both lollipop and surface acting as a mirror to reflect light. The space between the lollipop and the surface is the resonant cavity, with the distance determining the wavelength of light given off by the laser.

Tuning is as easy as levering the disk higher or lower, changing the distance between it and the surface. The near infrared wavelength range of the laser is continuously tunable between 910 nanometers and 950 nanometers, with a power output of about one milliwatt, the level of a common telecommunication laser.

The cantilevered laser is produced by depositing layers of aluminum-gallium-arsenide semiconductor and gallium-arsenide onto a gallium-arsenide substrate (using molecular beam epitaxy growth or MBE), and then etching away the middle layer of gallium-arsenide to leave two precise and optically polished surfaces that act as a resonant cavity.

"The big challenge has been to remove the layer just right to get the right sized cavity," she says. "The beauty of this device is that we can grow very precise gap sizes down to a precision of a few Ångstroms, the size of an atom."

Each laser is about 150 microns by 200 microns (.15 x .2 mm), with the cantilever about 100 microns long, or four-thousandths of an inch.

The next steps, Chang-Hasnain says, are to make the device smaller with faster tuning, tunable over an even wider range, and more reliable.

Coauthors on the project, funded by the Defense Advanced Research Projects Agency (DARPA), include graduate students Melissa Li, Wu-Pen Yuen and Gabriel Li.

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