Getting Tuned Up

 
 
By eweek  |  Posted 2001-02-12 Email Print this article Print
 
 
 
 
 
 
 

New lasers will speed optical networks

This quarter, most large carriers are testing tunable lasers. These devices speed ones and zeroes down optical networks and jump from wavelength to wavelength faster than an eyeblink.

The market for tunable lasers is expected to grow from $100 million last year to about $1.5 billion in the next couple of years, according to the research firm ElectroniCast.

"Everyone is saying that tunable is going to be a winner," says Stephen Montgomery, market research consultant for optical communications at ElectroniCast. "Theres going to be a huge demand for this stuff over time as copper is replaced by fiber."

Nortel Networks and Lucent Technologies are market leaders, each offering lasers that can tune to 20 different wavelengths.

But Santa Barbara, Calif., start-up Agility Communications claims the lead in breadth of tunability, with a laser in trials that can switch between 100 wavelengths. And Altitun, a Swedish start-up acquired in May by ADC Telecommunications, claims to be the first to market with a tunable laser that can switch wavelengths in nanoseconds, so fast that it can shuffle packet by packet, simplifying the switches on optical networks.

Tunable lasers are key to a faster, fatter Internet. They let networks operate at maximum capacity, switch bottlenecked traffic to uncrowded wavelengths, and potentially can make many costly optical switches unnecessary.

A fiber-optic strand is just skinny glass until it is "lit" when a laser is turned on and, teamed with a modulator, starts blinking signals down the thread of fiber.

A fiber strand is thinner than a human hair. Anything that thin seemingly would be doing well enough to carry just one lane of voice, video and data traffic.

But as demand for bandwidth mushroomed, physicists came up with a lower-cost alternative to digging trenches and laying more conduits in the ground. They developed Dense Wavelength Division Multiplexers (DWDM), which use mirrors or bubbles to divide fiber into several differently colored wavelengths of light.

Each color can carry its own traffic because each has a very precise and never-varying wavelength — and thus never, or very rarely, bumps into another colors traffic. Carriers are beginning to deploy DWDM gear that can divide light into as many as 64 wavelengths. In laboratories, scientists have divided light into more than 1,000 wavelengths.

Each wavelength that carries traffic needs a laser to blink on and off billions of times per second and bring to life the ones and zeroes that make up the bits and bytes, the e-mails, songs, movies and charts traveling across a network.

Most lasers on fiber-optic networks today are fixed — that is, once theyre assigned to a color, they stay with it forever. They cant help ease the overflow of traffic if, say, the new Victorias Secret online fashion show is planned for the same day as the corporations big videoconference, and theyre sharing the same overcrowded path through several switching stations.

Tunable lasers, however, switch traffic to wavelengths that arent overloaded, easing the bumper-to-bumper conditions.

The goal is a laser that can be tuned across all channels of a DWDM system, but which also can switch channels in a flash and has the oomph to carry the signal a long distance over the networks core. So far, most tunable lasers can only perform one or two of these functions.

100 Points of Light

Agilitys 100-channel laser is the size of a grain of salt and runs on five volts of power. It can tune to 100 different channels of light, taking about 10 milliseconds to switch from one channel to another.

Agility has tested the 100-channel laser with eight companies, including Lucent, Nortel, PhotonEx and Zaffire, and plans to have it in the hands of a couple of dozen more by the end of the year.

"Were in the lead, but its a very big market and there are a whole lot of companies that are going to be chasing it," says Arlon Martin, vice president of marketing at Agility.

Altitun says its 80-channel lasers can switch channels in under 10 nanoseconds and a nanosecond is 1 million times faster than a millisecond, which is the speed at which most of todays gear operates.

Altitun makes lasers in four parts, separating the lasers active function of generating the light from its passive function of defining the wavelength. A grating is etched into the laser to allow for tuning to different wavelengths. A phase module fine-tunes the wavelength and a coupler transfers some wavelengths from the bottom to the top of the wave guide. When pumped with electricity, most of the light spews from the front of the laser and is funneled into the fiber, but some is reflected back into the laser to be recycled. By changing the current that goes through the etched grating, the laser can be tuned to any of dozens of wavelengths.

"It has several advantages," says Robert Plastow, chief technology officer at Altitun. "Its a single chip with no moving parts. Everything is controlled by the current. And its extremely fast."

Millisecond speed is fine for todays applications, says ElectroniCasts Montgomery, noting that a millisecond is a lot faster than the time it takes to manually lay another conduit or insert another card into a 7-foot rack. "The competition is a truck roll."

"Millisecond is not an issue yet. If and when it becomes one, Agility will probably have a solution," says Tom Hausken, a senior analyst at Strategies Unlimited.

But Plastow says nanosecond speed, by allowing a network to switch on a packet-by-packet basis, "will let you utilize the network capacity much more efficiently. Its a year or two out.? When chips can shift wavelengths that fast, the true advantage of a mesh network, combining the strengths of tunable lasers and optical switches, will emerge.

Carriers are looking for tunable lasers that can handle the shakes, rattles, rolls and temperature changes in an optical network. However, price is important, too. "They dont want to pay more than 1.5 or two times the premium of a fixed laser," Montgomery says. "The tunable lasers are more expensive than that now. Theyre hard-pressed to get their prices down, but they can do it with more customers and volume."

Altitun has completed 150,000 hours of tests and has supplied samples of its 80-channel chips to most of the major carriers in the world, Plastow said.

Changing Their Tune

NTT Electronics of Japan also makes tunable lasers that can change from one wavelength to another in about 10 nanoseconds. Wavelengths in the middle of the spectrum tend to have stronger signals. NTT uses Super-Structure Grating Distributed Bragg Reflector technology to correct for that by changing the current and thus changing the amount of light being generated. The Distributed Bragg Reflector technology uses a device in which light is bounced back and forth along a horizontal cavity running the length of the laser, ensuring that it resonates at the required frequency. Grooves etched into the surface of the laser keep the laser beam finely focused.

Lucents Microelectronics Group touts the fiber-optic industrys first 20-channel continuous wave tunable laser module for both short- and long-haul high-speed optical networking systems. Lucent has patented its "wavelength locker" that locks the laser emission to any of 20 adjacent 50-gigahertz channels. The 1.5-micron Distributed Bragg Reflector laser, called a tunable electroabsorptive modulated laser, has a booster amplifier and a photo detector.

Marconi makes a Sample Grating Distributed Bragg Reflector, a technology that offers wide tuning range, but limited power output.

Coretek, which was acquired by Nortel Networks, uses Vertical Cavity Surface Emitting Laser technology in its tunable laser, bouncing light between two mirrors at either end of a vertical hole to keep it resonating at the required wavelength. The top mirror has a hole in it to let light escape into a fiber strand. It operates at millisecond speed.

 
 
 
 
 
 
 
 
 
 
 

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