As data-bandwidth requirements continue to escalate in data-communications switches and routers as well as high-performance multiprocessor systems, moving data between systems and even within racks depends on ever-faster interfaces. Even hardwired backplane costs are escalating. As bus speeds rise, such backplanes may no longer be economical or practical. High-speed serial host adapters can move data from system to system at 10 Gbits/s. But they're still very expensive and not very cost-effective for short-range data transfers of 300 m or less. Furthermore, wide buses or flat cables that deliver differential data 128 bits at a time at speeds of 200 MHz or faster are often restricted to lengths of just a few meters. Creating such cables also isn't for faint-hearted designers.
To address these challenges, designers at Agilent Technologies have come up with parallel fiber-optic transmitter and receiver modules and a standard ribbon fiber-connector interface. Designers at the company took advantage of the dropping cost of moderate-performance optical interfaces. The interface cost has come down thanks to the availability of low-cost optical fibers, standardized transmitter and receiver modules, and the development of vertical-cavity surface-emitting laser (VCSEL) diodes. Additional help came from the availability of low-cost silicon to perform parallel-to-serial and serial-to-parallel conversions.
Agilent expects such modules to find a ready home in OC-192 10-Gbit/s very-short-reach (VSR) interconnects, Infiniband systems, and large multiprocessor system interconnects. The company also envisions them in use as even a common I/O that could replace legacy PCI and PCI-X interfaces.
Typically, the design of 10-Gbit/s serial adapters requires using expensive cleaved gallium-arsenide (GaAs) laser diodes and super-fast logic. Alternatively, high-performance backplanes often require creating complex controlled-impedance pc boards with over a dozen wiring layers and high-speed bus drivers. And, the high-speed cables must be well-designed to control impedances and provide shielding between adjacent-signal wires. Agilent's development is about to change all of that.
Several companies have already started offering portions of the solution that Agilent is providing. These companies are selling the silicon chips that perform the parallel-to-serial or serial-to-parallel conversion. Some have also begun offering the silicon along with the VCSEL emitters and photodetector arrays. Designers at Agilent, however, have gone several steps beyond this by introducing a more complete solution, including the parallel fiber-optic transmitter and receiver modules and a standard ribbon fiber-connector interface.
Not only can Agilent's modules deliver up to 12 independent high-speed transmitters or receivers in a single, very compact package, but they can do so at a very affordable price—about $25 to $30/gigabit of data bandwidth. Each channel in the HFBR-712BP transmitter module or the HFBR-722BP receiver module can transfer data at up to 2.5 Gbits/s. Therefore, a 12-channel module provides an aggregate throughput of 30 Gbits/s. The company also is working on a four-channel transceiver implementation.
Designed for very cramped quarters, the 12-channel transmitter or receiver modules are very compact, requiring a board area of only 38 by 14 mm (about 1.5 by 0.75 in.) per module (Fig. 1a). A full-bidirectional configuration (12 transmit and 12 receive channels) would occupy a board space of around 1.5 in. deep and about 3 in. wide, including a small space between adjacent modules.
The modules em-ploy a 6- by 12-pad ball-grid-array (BGA) connection/contact ar-rangement to the pc board on which they're mounted. The surface-mount-compatible BGA contact area uses a 1.27-mm ball pitch, which allows for a large number of connections in a small region. These connections include the interfaces to all 12 channels, the other signals required by the modules, and a large number of power and ground connections.
A single connector with a dozen optical fibers set to precisely align with the VCSEL array plugs directly into the socket that's part of the transmitter module (Fig. 1b). Similarly, the receiver module has a connector/socket arrangement that keeps the dozen fibers precisely aligned to the 12 optical detectors.
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