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Infinera's InP demo heralds 10 Tbit/s systems

21 Sep 2007

ECOC 2007 sees Infinera promote the latest advances in its photonic integrated circuit (PIC) technology, as it adds semiconductor optical amplifiers to its multi-channel receivers.

Infinera is hoping to increase the bandwidth of optical communication networks to 4–10 Tbit/s on a single fiber, by adding semiconductor optical amplifiers to its photonic integrated circuits, or PICs.

“We now have PIC networks that can span the entire fiber bandwidth, without a fiber-based optical amplifier,” explained Radha Nagarajan, Infinera's director of advanced PIC development.

Nagarajan demonstrated the first results from semiconductor optical amplifier (SOA) PICs in an existing Infinera 10-channel receiver at ECOC 2007 in Berlin on 18 September.

Talking to compoundsemiconductor.net, Nagarajan commented, “The paper shows that the performance is consistent, that this technology is just not a one-off device.”

Although the company declined to discuss detailed plans for the commercial introduction of the SOAs, it was positive that the technology gives it a key advantage and will be deployed.

“I don't think there's anything to stop us,” commented Jeff Ferry, Infinera's senior director of communications. “You will see future generations of photonic integrated circuits from Infinera with greater functionality and greater bandwidth and you'll see that over the next several months and years ahead.”

Nagajaran pointed out that SOAs could deliver 4–10 Tbit through a single fiber because they allow lasers and amplifiers to be tuned to transmit data across the entire spectrum that can pass down a fiber.

“If you want wavelengths all the way from 1300–1550 nm the only medium that will allow you to do it is SOA,” he said.

SOAs' Achilles heel
Erbium-doped fiber amplifiers, or EDFAs, are currently the most popular means of performing the optical amplification that Infinera is looking to achieve with SOAs. They usually come in 100 mm square packages, making them rather too large to include in PICs.

By contrast, the SOA is only 0.9 mm long, and comes integrated with an arrayed waveguide grating and a photodetector, on a single InP receiver chip.

The reason Infinera hasn't used them before is that SOAs are difficult to implement, due to the role played by TE (transverse electric) and TM (transverse magnetic) polarization in the incoming light.

“That has always been the Achilles' heel, because to build a receiver SOA, to have polarization-independent behaviour is always the tricky issue,” Nagajaran commented. “The hard part is to be able to design the SOA to have the same gain for both the TE and TM polarization.”

Another reason that EDFAs have been more popular than SOAs is that they traditionally have lower noise and higher gain. In his talk, Nagarajan presented a gain distribution measured over 300 channels, in which the median gain was 23 dB, versus a noise figure of 4 db – not exceeding the performance of EDFAs, but certainly approaching it.

To do this, the Sunnyvale, California, optical networking firm has used the kind of proprietary epitaxial and chip design know-how it's gained developing InP PICs. Understandably Ferry and Nagarajan were keeping the chip-scale details under wraps – although Nagarajan was clearly so proud of the achievement, he couldn’t help but give a loose explanation.

“It's a combination of picking the right active region material and the correct waveguide structure,” he said. “You have two knobs – one is the active region and the other is the waveguide. With these two knobs, you dial them until you net the polarization dependence to zero – or close to zero.”

“By doing this, we now have a high-gain, polarization independent, and high saturation power output amplifier integrated together with the 10-channel or a 40-channel receiver chip.”

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