Optical data link delivers more for less

DN Staff

March 3, 1997

4 Min Read
Optical data link delivers more for less

Palo Alto, CA--When Hewlett- Packard enters a market, it rarely does so with a "me too" product. The Gigabit Link Module is no exception. Designed to facilitate high-speed digital data communication between large computers, this new optical link card is claimed to be ten times more reliable than competing products while costing just 1/6th as much and drawing half the power. Based on an optical transmitter port that uses a more efficient vertical cavity surface emitting laser (VCSEL) of HP's design, it also offers the potential for transmitting over greater distances.

Optical links work by converting electrical signals to a modulated laser-light beam aimed along an optical fiber. At the receiving end, the process is reversed. Long distances are their forte, and at the limit, they can sustain transfer rates of 1 GHz without resorting to exotic cables. "This port design could someday support a ten-kilometer transmission," beams David Sears, quality program manager and designer of the Gigabit Link Module, "regular optical ports are limited to about two kilometers." And at 1 GHz, a coaxial cable begins to choke after just 50 meters.

Sears' design stands out for the way in which it addresses the competing drivers of cost and reliability. "Our objective was $20, whereas the competition costs $120," he says. In addition, he says it pumps up the mean-time-between-failure (MTBF) rate from the usual 10,000 hours--to 100,000 hours, more than 11 years.

Job one was to simplify the manufacturing and assembly process. He also reduced the high cost of materials, components, and processes down to a critical few--but then pays for those few.

For example, conventional optical links use a high-performance gradient index (GRIN) lens to get lots of laser power into the fiber. The lens itself costs $5.50. Sears replaced it with an extremely precise 2.5-mm-diameter sapphire ball bearing made in Switzerland. "They vary no more than a few thousandths of a millimeter between balls," he says, "and cost just 50 cents." To address the inefficiencies brought on by spherical aberration, he specs a combination of coatings to bring the ball lens' performance to within 5% of that attained by the GRIN lens.

The lens is mounted into the single component where Sears concentrated all the high-precision machining, the port housing. "We press-fit the ball precisely along the optical axis of the fiber," Sears explains, "and take into account the deformation of the port housing." Tolerance from the image point to the back end of the ball is 0.050 mm.

At the other end of the port housing lies the fiber receptacle. Competing designs specify a precise ceramic insert--$2.00 a pop--to receive the ceramic ferrule on the tip of the optical fiber. Instead, Sears machined the receptacle from metal and worked with machinists to develop a repeatable process that leaves the critical last 1.5 mm of the bore dimensioned to within about plus or minus 10 microns. The hole naturally tapers out slightly, leaving an opening that provides easy entry but captures the fiber snugly at the tip. A layer of hard plating provides a tough surface to mate with the ceramic ferrule. "By learning how to make this receptacle, we saved two bucks," says Sears.

The VCSEL laser also breaks new ground. Fabry-Perot lasers, the kind found in most optical links, are made in extremely large quantities for consumer CD players, and last about 1,000 hours; hand-selecting the good ones can boost that figure to 10,000 hours. By contrast, VCSEL lasers last upwards of 100,000 hours. They are more efficient, too, requiring a bias current of 5 to 10 mA instead of the Fabry-Perot's 20 to 40 mA. At higher currents, a VCSEL could greatly extend the Gigabit Link's transmission distance--the 10-km figure Sears hinted at--but eye-safety issues need to be addressed for this to happen.

Using an interactive process, the laser assembly is precisely positioned. Technicians tack the assembly in place with a UV cure adhesive and then stake it with epoxy. The scheme is remarkably robust. "The only thing that kills them is thermal shock--like dropping them into liquid nitrogen," says Sears.

Possibly his biggest challenge was developing a way to monitor the light output from the VCSEL. Sears' patented solution was to mount the laser right in the middle of a large photo detector. A second custom coating on the sapphire lens reflects a portion of the laser's light back to the detector. The detector produces a signal, proportional to the light received, that's used in a feedback loop to control the VCSEL's output. By carefully selecting the coatings applied to the ball lens, Sears can maintain high laser output for a good signal-to-noise ratio, provide a high reflected portion of light for the photo detector, yet keep the final output to the fiber within eye-safety limits.

Currently in low production, Gigabit Link Modules are already a great success, says Sears. "The final coupled power is maybe two to three percent lower than the Fabry-Perot design," says Sears. "But, it's much, much cheaper and lasts longer."

Additional details...Contact David Sears, Hewlett-Packard Components Division, 1501 Page Mill Road, MS 4L7, Palo Alto, CA 94304, (415) 857-5665.

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