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Axonlink: A New Take on TunabilityAxonlink: A New Take on Tunability

Could its tunable receiver technology be a breakthrough for access network equipment?

February 5, 2003

4 Min Read
Axonlink: A New Take on Tunability

What could turn out to be a breakthrough technology for optical receivers was unveiled yesterday by Axonlink Inc. (see Axonlink Intros AWDM).

The technology, called optical heterodyning, promises to eliminate the need for expensive filters and multiplexers in access networks, by allowing the receiver to pick out one channel in the presence of other channels. It's not a new technology -- in fact, it works in the exact same way as ordinary radio receivers -- but this appears to be the first time it's been commercialized for optical applications.

This is the first time Axonlink has talked about its plans since it was spun out of Israel's ECI Telecom Ltd. (Nasdaq/NM: ECIL) in late 2000. Axonlink was founded by Joshua Piazetsky and Hertzel Yehezkely, who are the CTO and deputy CTO, respectively, at ECI. Yehezkely served as Axonlink's CEO until just this week, when he takes on the positions of president and COO, handing over the CEO reins to newcomer Dror Nahumi.

According to Nahumi, there was plenty of research into optical heterodyning going on in the 1980s. "The motivation at the time was completely different to today," he says. "One of the properties is increased receiver sensitivity, because you are also doing amplification of the incoming signal." Back then, Erbium Doped-Fiber Amplifiers (EDFAs) hadn't been invented, and researchers were using the extra receiver sensitivity to make optical signals go as far as possible. When EDFAs were invented in 1987, most of the work in the field stopped.

Today, the main motivation for developing the technology is to achieve maximum performance at minimum cost. Increased receiver sensitivity allows cheaper types of detectors to be used. Tunability in the receiver, as noted, eliminates the need for some other components in the network. And it also makes the receiver a perfect companion for a tunable laser.

The best way to understand how optical heterodyning works is with the aid of this diagram, supplied by Axonlink:

27795.gifHeterodyning works by mixing the incoming signal -- in this case a bunch of DWDM channels carrying data -- with another signal of similar, but not identical, wavelength. This is implemented in the receiver by combining light from the input fiber with light from an on-board laser (the combiner is shown as a circle).

All of the light then falls on a standard PIN photodetector, which converts it into an electronic signal. This information can be filtered electronically to extract the data from any one of the incoming DWDM channels.

The filtering relies on a process called "beating", which makes the amplitude of the electronic signal wobble at a certain frequency -- called the intermediate frequency (IF) -- that is always equal to the difference in frequency (wavelength) between the two incoming optical signals. To pick out one channel and ignore the others, the receiver simply needs to know where to look -- in other words it just needs to know the intermediate frequency.

In practice, the intermediate frequency is fixed, and the receiver tunes by shifting the wavelength of the onboard laser. Axonlink does this in the current version of its device by heating and cooling standard Distributed Feedback (DFB) Lasers, creating a tuning range of a few nanometers.

One way of cutting costs is to split the output of the on-board laser and use some of it in the receiver and the rest of it to power a transmitter. Indeed, that is what's shown in the diagram.

Sharing the laser in this way does mean that the transceiver will send and receive signals at different wavelengths. But that's okay, says Nahumi. "For applications where you want to have bidirectional capabilities -- sending signals in both directions on the same fiber -- you like to have channels at different wavelengths." This type of module is likely to find applications in access and fiber-to-the-home scenarios. In other applications, he adds, where it's important to send and receive at the same wavelength, two lasers can be used inside the module.

Nahumi says the company recently demonstrated a tunable transceiver like the one in the diagram to potential customers. Now it is exploring the business case for different applications, while also looking for a partnership with a larger vendor.

Axonlink has gotten first-round funding of $12 million from Benchmark Capital, ECI Telecom, and Innovacom.

— Pauline Rigby, Senior Editor, Light Reading

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