Technique boosts data rate in light pipes Like runners racing to a finish line, a

burst of photons fired into some glass fibers arrive at the end as a drawn-out blur. In today’s computer networks, those optical fibers in which such smearing, or dispersion, is great are used to carry only a limited volume of data per fiber and on- ly for relatively short hauls. Nonetheless, these fibers align easily with lasers, so systems using them are comparatively in- expensive and popular.

Now, Howard R. Stuart of Lucent Tech- nologies’ Bell Labs in Holmdel, N.J., has found a way to transform the Achilles heel of these so-called multimode fibers into a source of new strength. Using sev- eral lasers to send data and as many de- tectors to receive it, Stuart expects to multiply bit flow by the number of lasers, he says. No longer a curse, dispersion be- comes the key to unraveling the data threads at their destination, he notes.

The new scheme is “intellectually beautiful. It’s really, really nice stuff,” comments George Papen of the Universi- ty of Illinois at Urbana-Champaign. Many network-equipment developers are fran- tically pursuing new ways to boost data flow across multimode connections. Such links are used in data networks that cover areas up to the size of a col- lege campus or business park.

Stuart says his method, which he de- scribes in the July 14 SCIENCE, has great potential but remains far from becoming a product. Because the latest multimode fibers eliminate dispersion and optical networking is in rapid flux, “it’s not clear [Stuart’s] technique will keep up with the rate of change,” Papen remarks.

Stuart’s laboratory prototype, with two lasers, two detectors, and a circuit that sorts out the received signals, has mixed and separated data streams. It has yet to double the transmission rate of a one- laser system, however.

An optical fiber consists of a glass strand coated with a thin layer of another glass with different optical properties. The mismatch causes light to remain con- fined primarily to the core. Physicists at- tribute the spread of light traveling through fibers to different patterns, or modes, of oscillation that electromagnet- ic waves can experience in the glass. The modes travel at different velocities through the fiber.

To achieve high data flows over long distances, network designers depend on so-called single-mode fibers. These have cores 10 micrometers (pm) or less in di- ameter, and the light they carry doesn’t disperse. But aligning laser beams to the narrow fibers adds cost.

In contrast, fibers with cores roughly 50 to 60 ym across often host hundreds of modes-hence the name multimode. In theory, engineers can design optical equipment that taps each mode as a data

channel, but “there is really no practical way of doing it,” Stuart explains. Instead, he took cues from an innovation in wire- less communications.

The radio signals used by wireless equipment bounce off buildings and oth- er obstacles en route to a receiver. Multi- ple copies of the signal arrive at slightly different times due to the multiple paths the signal takes, usually creating a nui- sance. In 1998, other researchers at Bell Labs demonstrated a way to exploit such scattering to cram extra radio channels into a single frequency band. In their set-

up, separate antennas broadcast differ- ent parts of each signal. At least as many antennas pick up the jumbled broad- casts, feeding them into an unscram- bling circuit.

Stuart’s brainstorm was to note that the spread of pulses in a multimode fiber was analogous to the effect of multiple paths on radio waves. In principle, his op- tical scheme can boost flow through mul- timode fibers without limit as new lasers and detectors are added. In practice, how- ever, losses of light intensity at the fiber entrance and a rapidly growing load on the signal-processing circuitry may limit the increase to about 4 to 8 times the sin- gldaser rate, he predicts. -P Weiss

Possible Alzheimer’s vaccine seems safe The first tests in people of a vaccine

that may slow or prevent the devastation of Alzheimer’s disease have so far shown that the approach is safe, scientists an- nounced this week at a Washington, D.C., meeting. It’s still too early to gauge whether the vaccine actually works, how- ever, say the researchers.

Newly reported animal tests added to the cautious optimism about the new vaccine strategy. In mice genetically engi- neered to develop brain lesions similar to those seen in Alzheimer’s disease, the vaccine averted expected declines in cer- tain memory and learning skills.

Last year, Dale Schenk of Elan Pharma- ceuticals in South San Francisco stunned neuroscientists with the news that sim- ply injecting such mice with a protein fragment called beta-amyloid keeps natu- rally-occurring amyloid from accumulat- ing in the brains of the rodents and even eliminates preexisting amyloid deposits known as plaques (SN: 7/10/99, p. 20).

Many scientists believe that the ab- normal buildup of amyloid brings about Alzheimer’s disease, and Elan’s vaccine offers the first possible treat- ment for this apparent root cause of the illness.

The vaccine elicits antibodies that bind to beta-amyloid. While the antibodies generally circulate in the bloodstream, some apparently leak across the blood- brain barrier. Once in the brain, the anti- bodies seem to mark amyloid plaques for clearance by microglia, which are im- mune cells that patrol the brain.

At this week’s World Alzheimer Con- gress 2000, Schenk offered new evidence to support this theory. He and his col- leagues added microglia to slices of ro- dent brain tissue riddled with amyloid plaques. The immune cells took up the amyloid, but only if the researchers also introduced antibodies that bind to beta- amyloid. The microglia then degraded the protein fragments. “The amyloid liter- ally disappears,” says Schenk.

Some scientists have expressed con- cern that stimulating anti-amyloid re-

sponses will itself lead to brain damage. To address that issue, the Elan team ex- amined the vaccine’s safety in mice, guinea pigs, rabbits, and rhesus mon- keys. “We saw virtually no signs of prob- lems in the animals,” says Schenk.

These encouraging results prompted Elan to begin safety tests on people with mild-or-moderate Alzheimer’s disease. The researchers recently completed tests on 24 patients in the United States, who each received a single injection of beta-amyloid. No obvious side effects have emerged. “There’s no question the vaccine was well tolerated,” says Schenk.

To determine the vaccination schedule that induces the strongest antibody re- sponse, Elan has started a new trial in England, where about 80 people will re- ceive multiple injections of beta-amyloid. If that trial confirms the safety of the vac- cine, the company plans in 2001 to start trials designed to reveal whether the vac- cine actually arrests, or even reverses, Alzheimer’s disease.

“Our first goal is to block further pro- gression” in Alzheimer’s patients, says Schenk. “We don’t know if they will re- cover.”

Other scientists at the meeting report- ed they had confirmed and extended Elan’s initial mouse study. Christopher G. Janus of the University of Toronto and David G. Morgan of the University of South Florida in Tampa each described immunizing genetically engineered mice that normally develop amyloid plaques and lose memory and learning skills. Janus’ vaccinated mice actually im- proved their performance on one maze test; Morgan’s immunized rodents didn’t acquire the same memory problems with age as his untreated animals did.

Bill Thies, the Alzheimer’s Association vice president of medical and scientific affairs, calls Janus’ and Morgan’s results “very significant” but also expresses caution about their relevance to people. “It’s very hard to look at mouse behavior and compare it to Alzheimer’s disease,”

-J Travis he says.

38 SCIENCE NEWS, VOL. 158 JULY 15,2000