Columbia, MD. - A novel photorefractive material discovered by two Israeli scientists is being incorporated into an all-optical dense wavelength-division multiplexed (DWDM) switch. Trellis Photonics Ltd.-a startup founded by Anan Agranat and Elon Ittwitz to commercialize the technology-is engaged in a major expansion, fueled by $25 million in funding.

The company plans to build fab lines here and also in Jerusalem to manufacture its Intelligent Lambda Router switch, which uses light, rather than micromirrors or liquid crystals, to redirect optical beams.

Trellis Photonics' optical router is an all-solid-state device that uses optically induced diffraction gratings inside a potassium lithium tantalate niobate (KLTN) ferroelectric crystal. The compound, a variant on the lithium niobate crystal that is a staple of nonlinear optical systems, supports a spatially varying refractive index that depends on the distribution of electrons within the crystal lattice.

But the electron distribution itself can be controlled by incident light, so that a virtual diffraction grating defined by the varying refractive index can be instantly written into the crystal. The basic effect, called electroholography, was discovered 12 years ago by Agranat.

The method is ideal for using holographic methods to route light. Multiple holographic gratings can be written into a single device, making it possible to scale the switch size without any physical changes. The induced diffraction gratings can also deflect light at wavelengths different from those of the beam used to write them. That makes it possible to switch light at 1.55 microns, which is the wavelength used in current optical networks.

The crystal-based switch has some fundamental advantages over current micromirror or liquid-crystal-based optical routers. It is inherently simpler, needing no micromechanical parts, and it operates in the nanosecond speed range, since the gratings are formed by light.

In addition to those basic advantages, the switch also has another significant function, which arose as a side effect of the diffraction gratings used to redirect light.

"No diffraction grating is 100 percent efficient, so a small part of the signal-about 5 percent-travels straight through the switch," explained Tom Cahall, Trellis' chief executive officer. "We use that attenuated signal in an optical-feedback loop to do two things. First, it can be used to control the voltage on the crystal, which allows us to perform dynamic attenuation on a wavelength basis." Second, the attenuated signal allows a full diagnostic capability.

Dynamic attenuation addresses the problem of amplifying a group of wavelength channels that may have different powers. Currently, erbium-doped fiber amplifiers are used as simple amps in DWDM networks. However, they are nonselective, leading to either the danger of overamplifying the stronger signals in the fiber or underamplifying the weak signals. In addition to rerouting channels, the dynamic-attenuation feature can be used to balance the power in all wavelength channels as they go through the switch.

The diagnostic capability is another plus. "With the optomechanicals-the micromirrors, liquid crystals, bubbles and so on-you have no way of knowing what is actually going on inside the various-wavelength channels," Cahall said. By contrast, "We are able to use the attenuated beam to look inside a channel for monitoring purposes."

The switch is set up as a matrix with the rows representing wavelengths and the columns representing fibers. One row at the top is reserved for diagnostic purposes. "That allows us to drill down through the switch to actually see the Internet Protocol headers or Sonet frames," he said.

DWDM optical networks use wavelength to multiplex signals. The actual content of the channel is unknown to the system. For example, all-optical add-drop multiplexers simply syphon off or add a wavelength channel with no knowledge of its contents. Similarly, micromirror-based switches reflect an incoming optical beam to an output port without any analysis of the information content of the beam.

These components introduce some routing flexibility into optical networking, but fall short of network-based protocol switching, which still has to be done electronically.

The solid-state optical switch will introduce a new level of control and diagnostics into optical networks by not only making signal content visible, but also by allowing nanosecond response to repair problems.

"Basically, we can identify a problem and create a duplicate of the original signal which is free of distortion,"Cahall said.

Since they are similar to the early electromechanical switches of the telephone system, MEMS-based switches actually have reintroduced the same fault-isolation techniques that were used in the late 1800s. "You can't put a number of these optomechanicals concatenated together in a real network because if anything goes wrong, you are in deep trouble," Cahall said. "You have to fall back on the same fault-isolation techniques that went out in the early 1970s, when people started putting in network management centers and software-based diagnostics. That is why you see a lot of trials with these switches, but few working systems."

Test ports

The all-optical switching capability of the Trellis system will be used with an online monitoring and diagnostic system that continually collects data, looking for anomalies. When a problem is detected, the switch can automatically route the problem channel to a test port.

By looking inside the switch and constantly monitoring network activity, it is possible to be more proactive in isolating problems. "One common problem is the slow degradation of power-a line is kinked or a connector is out of alignment-and you can detect this kind of trend and locate the problem before it affects the network," Cahall explained.

The marketing plan is to first talk to Internet service providers to get a complete set of relevant functions based on the capabilities of the technology. "We will be selling directly to the equipment OEMs," said Cahall. "Basically, service provides don't want to get into managing networks. They have gotten into some network management because they have to, but basically they want to concentrate on delivering services. So we don't expect them to be enthusiastic about incorporating another unique network component."

The goal, he said, is for "the OEMs to be able to deliver working network systems that offer the higher-level monitoring and diagnostic functions that ISPs need." The software and support systems are mature and represent an investment that ISPs have already made.

Trellis expects to have the first switching products in beta version in the second quarter of next year. "Its difficult to compare this technology with electromechanicals, since they talk in ports. Basically, we just talk in terms of fibers," Cahall said. "We do all the DWDM muxing inside the switch fabric."

The first chips will have 40 wavelengths per fiber, which will represent a 240 x 240-port switch. "But that will be easily scalable to 1,920 x 1,920," he said. "By January of 2002 we should have the U.S. facility up to full volume and by then we expect to be at 3,840 x 3,840."

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