The University Record, November 6, 1995

New photoreceiver would zip information across superhighway

By Sally Pobojewski
News and Information Services

 Researchers in the College of Engineering have developed a new type of photoreceiving chip that could increase the information carrying capacity of tomorrow's "information superhighway."

The photoreceiver integrates a light detector and amplifier in the same semiconducting layers on the microchip, eliminating the connecting wires that increase the fabrication cost of hybrid photoreceivers. Integrated technology also increases compactness and reliability.

According to Pallab K. Bhattacharya, professor of electrical engineering and computer science, the photoreceiver currently holds the world's speed record for high-speed optoelectronic signal detection. It can accept data transmitted as pulses of laser light flashing at speeds up to 24 gigabits or 24 billion pulses per second. Most current photoreceivers can only handle data transmission speeds up to about 11 gigabits per second, Bhattacharya says. At 19. 5 gigahertz, the device's bandwidth, or its ability to receive high-speed signals, is considerably higher than existing integrated photoreceivers.

Optoelectronics technology combines the speed and compactness of optical fibers with the switching and amplification capability of electronics. To switch signals back and forth between pulses of light and electrons without scrambling or losing bits of data, all components of the system must be compatible and capable of operating at very high speeds.

"One of the technological roadblocks to the effectiveness of fiber-optic communications systems has been the lack of receivers capable of getting data out of the system as fast as the laser can transmit data into the system," Bhattachary a said. "This new photoreceiver has the potential to ease that roadblock by increasing total system throughout, allowing us to receive more information in a given time interval."

According to Bhattacharya, the photoreceiver can detect a wide range of laser light signals from the strongest to the weakest. The microchips are produced in the Solid-State Electronics Laboratory with a one-step molecular beam epitaxy process commonly used in semiconductor manufacturing. Once the fabrication techniques are standardized, the device will be simple and less expensive to produce, Bhattacharya says.

The photoreceiver circuit includes a p-i-n photodiode to detect incoming light pulses and a hetero-junction bipolar transistor to amplify the high-speed electrical signals from the detector. Both components are fabricated from the same layers on a semiconductor wafer.

"In order to use compatible layers for both the photodiode and the transistor, we had to compromise the performance of individual components to a small extent," Bhattacharya says. "But the optimum performance of the total photoreceiver more than makes up for it."

Development of the photoreceiver was funded by the Advanced Research Projects Agency and the Army Research Office. Co-investigators include research fellows Augusto Gutierrez-Aitken and Kyounghoon Yang, and George I. Haddad, the Robert J. Hiller Professor of Electrical Engineering and Computer Science.