By Jesse Emspak: Subscribe to Jesse's RSS feed
October 25, 2010 11:49 AM EDT
Telephone calls long ago moved to fiber optic cables because they can carry thousands of times as much information as a copper wire. But computer components still use metal wires to talk to each other. That is likely to change.
At Duke University, a team led by electrical engineering professor Nan Marie Jokerst has built a set of tiny lasers on silicon chips. The lasers are laid on a silicon substrate, just like ordinary microchip components. A layer of a polymer is laid down on top, and that acts as a waveguide for the laser light. At the other end is a photodetector.
The result is an integrated circuit on a chip that can be used to transmit data. Just like in a fiber-optic cable, the photons emitted by the laser are forced to travel a certain path by the polymer. And since the laser can be tuned to multiple wavelengths, many more messages can be carried. Electrons in the gold or copper wires in a computer can only carry a single message, since they can't be tuned.
"Chip to chip is the biggest win," said Jokerst. "The big bottleneck is always between processor and memory." With optical communications, that bottleneck disappears, enabling computers to run much faster.
The lasers are only microns thick, and are made from diodes similar to those found in laser pointers. Jokerst said her team simply used an etching process to strip away everything in the diode that wasn't used to make the laser light. The diodes themselves start off about a half a millimeter thick.
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The process does require that the lasers be efficient, and well-made. Jokerst noted that she had the idea for integrating lasers on a chip years ago, but it took a lot of trial and error to get the lasers to work consistently. Part of the problem is the polymer. The polymer reduces the reflectivity of the tiny mirror at one end of the laser diode, increasing the amount of power it needs to generate coherent light. "When you slop polymer on a laser, sometimes it stops working," she said.
Lasers produce heat, so Jokerst also had to solve the problem of keeping the laser cool. Inserting thin layers of metal between the laser and the silicon substrate it sits on helped to keep the laser's temperature down, so it would keep working. She also had to make sure that the lasers didn't use a lot of power. "Our lasers are designed to be ultra low-power dissipation so you could run them on a battery," she said.
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