Sensor signal processing is widely used on aircraft and spacecraft. The scheme employs multiple input/output nodes for data acquisition and CPU (central processing unit) nodes for data processing. To connect 110 nodes and CPU nodes, scalable interconnections such as backplanes are desired because the number of nodes depends on requirements of each mission. An optical backplane consisting of vertical-cavity surface- emitting lasers (VCSELs), VCSEL drivers, photodetectors, and transimpedance amplifiers is the preferred approach since it can handle several hundred megabits per second data throughput. Conventional electrical interconnects severely limit the performance of high-speed networks because of parasitic resistance and capacitance, which limit bandwidth and cause clock signals to skew. Moreover, they cause pin congestion and require large and bulky multi-pin connectors.

The next generation of satellite-borne systems will require transceivers and processors that can handle several Gb/s of data. These systems will significantly benefit from optical interconnect technology. This technique integrates laser diodes (LDs) and photodetectors (PDs) with RF/microwave electronic circuitry on a chip to form optical interconnects.

Optical interconnects have been praised for both their speed and functionality with hopes that light can relieve the electrical bottleneck predicted for the near future. High-speed, small-area metalsemiconductor- metal (MSM) photodetectors will allow dense photodetector arrays and provide added practicality to interconnects, without compromising overall performance. Optoelectronic interconnects provide a factor of ten improvement over electrical interconnects.

Optical interconnect requires operation at high frequency with low RF/microwave losses at both the input and output end of the link while maintaining a large dynamic range. It is expected that VCSELs have higher reliability than edge-emitting laser diodes, as well as low power consumption, singlemode operation, and relative ease of fabrication while having very large bandwidths. In an interconnect circuit, laser diodes are directly intensity-modulated by the RF/microwave signals, and the photodetectors detect the received intensity-modulated signals.

For free-space optical interconnects, VCSEL diodes provide significant advantages over traditional edge-emitting laser diodes. Edge-emitting diodes must be cut and mounted after being fabricated, while VCSEL diodes can lase directly on the wafer. It is also possible to create arrays of VCSEL diodes directly on the wafer, a virtual impossibility for the edgeemitting types. VCSEL diodes can be fabricated on the same wafer as the MSM photodetectors, increasing the functionality of an array of interconnects. Optical interconnect technology has applications mainly in interchassis chip-tochip interconnections. However, it can also find applications in intrachassis board-to-board for high-speed and EMIfree interconnections. Optical interconnects are also suited for large-capacity and high-density interconnection between information processing equipment.

This work was done by Rainee N. Simons, Gregory R. Savich, and Heidi Torres of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at under the Electronics/Computers category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18444-1.

NASA Tech Briefs Magazine

This article first appeared in the November, 2009 issue of NASA Tech Briefs Magazine.

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