Transmission technologies have evolved at a blistering pace in recent years, but there are signs that Sonet might max out at 40 Gbit/s, the next big milestone, simply because the electronics in Sonet gear won’t work any faster.

An early-stage startup based in New Jersey, Ultra Fast Optical Systems Inc., says it's got a solution. It’s developing equipment based on technology called OTDM (optical time-division multiplexing) that does a similar job to Sonet -- packing lower bandwidth channels into wavelengths. However, it’s got optical rather than electronic innards, which means that it can operate at much faster speeds.

OTDM could support transmission speeds of 640 Gbit/s -- 64 times faster than current state-of-the-art Sonet equipment -- according to a paper presented at ECOC 2000, a recent European conference on optical communications.

And that’s 640 Gbit/s per wavelength. OTDM can still be used in conjunction with DWDM (dense wavelength-division multiplexing) systems to blast tens of terabits a second down a single fiber. It's likely to complement Sonet, which will still have a big role to play, feeding channels into the monster pipes supported by OTDM technology.

Researchers all over the world have been trying to develop OTDM devices for a decade or more, but one particular team -- Paul Prucnal and his co-workers at Princeton University -- reckons it's got a lead. It's already developed an all-optical demultiplexer -- a gadget that can extract one low-capacity channel from a high-speed system. The device could also be adapted to perform all-optical signal regeneration or wavelength conversion, according to Prucnal.

Enter the TOADUltra Fast Optical Systems Inc. intends to commercialize Prucnal's device, which he calls a TOAD (for Terahertz Optical Asymmetric Demultiplexer). The startup was founded by Maxon Sexton, director of NY investment bank Harmonic Research. Prucnal continues as a professor at Princeton, but maintains his link with Ultra Fast as head of its technical advisory committee.

The startup's president and CEO, Martin Singer, was previously with Safco Technologies Inc., a manufacturer of wireless test equipment that's now owned by Agilent Technologies Inc. (NYSE: A). Tom Curtis, Ultra Fast's VP of development, previously directed development of crossconnects at Lucent Technologies Inc. (NYSE: LU). An undisclosed amount of initial funding has come from Harmonic Research, an investment bank.

The best way of understanding how TOAD works is to consider a simple application -- one in which a bundle of eight channels is being carried by a wavelength, and one of the channels is to be split off and diverted onto a new path.

The data signals come into the TOAD as a cycle of light pulses, one for each channel. A separate timing mechanism, comprising regular pulses of light on a different polarization plane, enters the TOAD at the same time.

The timing system is adjusted so a clock pulse arrives at the TOAD at exactly the same moment as the data pulse to be diverted. This effectively barges the data pulse onto a different output path than the other signals. (Readers with a background in electronics will realize this is the optical equivalent of a logical AND gate.)

The TOAD isn't the only solution for making an OTDM demultiplexer, but it's the most practical, claims Prucnal. It has a timing window of just a few picoseconds, which means that optical pulses representing different data channels can be squeezed very closely together. A second advantage is its low power consumption. Finally, it can be manufactured out of fiber or as an integrated optical circuit. The final configuration hasn't been decided yet, Prucnal adds.

The switching element inside the TOAD is a semiconductor optical amplifier (SOA). Without going into details, this element can switch "on" in a matter of 100 femtoseconds (1 fs is one millionth of a nanosecond), which is much faster than any other optical technique (see Optical Switching Fabric). However, the SOA takes much longer to switch off again. "Other people who have attempted to make this kind of device have to wait for [the SOA] to recover. We don't have to do that," points out Prucnal. One of the key innovations in the TOAD is that it uses a second SOA. When this is switched on (which is fast), it cancels out the effect of the first SOA, effectively resetting the switch.

Right now, very few companies manufacture SOAs. Two known vendors are OptoSpeed SA in Switzerland and Alcatel SA (NYSE: ALA: Paris: CGEP:PA) through Opto+, its joint R&D venture with France Telecom SA. So Ultra Fast has had to make its own arrangements for sourcing SOAs -- it intends to buy them from a local company called Application Specific Integrated Photonics Inc. (ASIP), a startup founded by a Princeton buddy, Steve Forrest. "More companies manufacturing SOAs would be good news for us," says Prucnal.

-- Pauline Rigby, senior editor, Light Reading http://www.lightreading.com