Optical link and surface-emitter laser arrays achieve gigabit speeds

Optical link and surface-emitter laser arrays achieve gigabit speeds


Working through a fiber-optic bidirectional data link, an integrated array of 10 vertical cavity surface-emitting lasers developed by Motorola GmbH in Munich delivers data transfer rates of 4 gigabits per second over distances to approximately 300 meters.

This data link, known as Optobus, is expected to begin volume production this August in Motorola`s Phoenix, AZ, facility, where it was designed. European telecommunications, however, represents a market driving force for Optobus, according to German telecommunications engineers.

Several factors have brought these lasers into a competitive position in the optoelectronics marketplace. Low threshold currents, high-density packages, excellent beam characteristics and high bandwidths make surface emitters a preferred choice for optical storage and computing, backplane interconnect, fiber in the loop, high-capacity switching systems and smart pixel array applications. The use of an integrated array of these surface-emitting lasers in a bidirectional optical data link establishes gigabit-per-second optoelectronics.

Supporting multimedia markets

The Optobus link demonstrates how vertical cavity surface-emitting laser technology can support multimedia markets. High bandwidth leads to low-cost solutions, where huge amounts of data have to be transferred via backplanes, twisted-wire pairs or coaxial cables. The interconnected equipment includes massively parallel computers, file servers, data storage devices and high-definition television sets, as well as telecommunications systems using asynchronous transfer mode, or ATM, and synchronous digital hierarchy, or SDH, in data transport and signal processing.

Karl Lange, logic operations manager at Motorola, explains that "the bidirectional link consists of two identical transceivers connected by two ribbons with 10 multimode fibers, each transferring 400 megabits per second. The densely packaged laser-diode array, as well as 10 transmitter and 10 receiver channels, complementary metal-oxide semiconductor laser driver, ECL receiver and surrounding logic, are implemented in a 96-lead pin grid array package.

"The transceivers are protocol-independent and enable physical data transfer. Hence, the Optobus link may be readily adapted to bus architectures," Lange says.

According to Dieter May, product marketing manager at Motorola, "Optobus is not intended to be a competitive alternative to such standards as Fibre Channel, high-performance parallel interface, ATM and SDH because these protocols can be tied into the Optobus link by application-specific adaptations.

"Therefore, users do not need optoelectronics skills. The module drops into a socket on the system interface board. Currently, we are in the last stage of determining the specifications for market entry in the third quarter of 1996," says May.

He notes that there are companies manufacturing products similar to the Optobus optical link, but claims that Motorola is at least a year ahead of companies such as Hitachi, IBM and AT&T. "However, because many of these competitors do not have all of the technologies available, they have to work together, and this is time-consuming," May says. Among those groups are the Optoelectronic Technology Consortium, including IBM, AT&T, Honeywell, Martin-Marietta and the U.S. Department of Energy; and Jitney, which includes IBM, Lexmark and the U.S. Department of Energy.

The Parallel Optical Link Organization is aiming to develop multichip modules that will compete with the Optobus product. These modules will employ vertical cavity surface-emitting lasers that will act as transmitter diodes. The consortium includes institutes such as the University of Southern California and companies such as Hewlett-Packard Co. and DuPont.

Surface emitters

In operation, the lasers emit light vertically from the semiconductor`s surface rather than from its edge. These devices provide improved beam characteristics and lower threshold currents than the traditional edge-emitting laser diodes. Other advantages such as dense packaging and reduced costs achieved by on-wafer testing make surface-emitting lasers attractive for optical-link applications.

Because testing and packaging comprise more than 80% of semiconductor laser costs, Optobus succeeds in delivering low-cost, high-performance data transfer. Coupling efficiencies to 90% are possible--30% more than normally achieved with edge-emitting lasers because of less-divergent laser beam characteristics.

Of the various surface designs considered, vertical cavity surface-emitting lasers furnish low thresholds and high-density packages that suit optoelectronic applications. The low threshold current of 5 milliamps for one microlaser of the array at room temperature allows streams of non-return-to-zero data to directly drive the laser diodes of the Optobus link. The side mode compression ratio is comparable to, or better than, most edge-emitting distributed feedback lasers--even for high drive currents.

High-modulation bandwidths of several gigahertz per laser are possible because surface emitters can be driven far above threshold with injection currents. In this case, the complementary metal-oxide semiconductor, or CMOS, laser driver is the reason why a single fiber of the Optobus link carries only 400 Mbits/sec. The manufacturer relies on a mature CMOS standard technology that cannot handle higher bit rates. Future products will use enhanced laser drivers for higher performance.

The gallium arsenide-based array structure used in the link emits light at 850 nanometers, an established technology. Efforts to fabricate vertical cavity surface-emitting lasers based on indium phosphide with continuous-wave operation at room temperature, however, have not yet succeeded. Long-wavelength semiconductor lasers emitting at 1.3 or 1.55 microns suit long-haul optical telecommunications because these wavelengths meet the dispersion minimum and the attenuation minimum requirements of optical fibers. However, it is unlikely that the well-established distributed-feedback edge emitters will be replaced by vertical cavity surface-emitting lasers until such wavelengths are developed.

The one-dimensional array consists of 10 independently addressable microlasers, each serving as the light source for one fiber. To channel the optical signal from the diodes to the fibers, a special connection technique combines the optical waveguide, the diode array and the female side of the connector assembly. The connectors of the waveguide cable are fabricated with a center-to-center alignment of ۬ microns using multimode fibers with a core diameter of 62.5 microns.

Fibre Channel

The Optobus chips can be integrated with conventional components like those used for the serial Fibre Channel. The transceiver modules can accommodate serial data transfer methods such as ATM and enterprise systems connection architecture, an IBM connection method. Hence, Optobus is a pluggable unit. The costs for a Fibre Channel arbitrated loop, including transmitter and receiver, are approximately $1000 to $2000, or $1500 to $2000 if a connection to a switch is included.

The distance of two Fibre Channel units connected via fiber can be 10 or 100 km in a Fibre Channel arbitrated loop. Once the Optobus link is mass-produced, Motorola estimates that one connection will cost $100 per gigabit. q

Achim Strass writes from Munich, Germany.

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