optical amplifiers

• Finisar revealed its in-house silicon photonics design capability at ECOC
• The company also showed its latest ROADM technologies: a dual wavelength-selective switch and a high-resolution optical channel monitor.
• Also shown was an optical amplifier that spans 400km fibre links 

These two complementary technologies [VCSELs and silicon photonics] work well together as we think about the next-generation Ethernet applications.

Rafik Ward

Finisar demonstrated at ECOC its first optical design implemented using silicon photonics. The photonic integrated circuit (PIC) uses a silicon photonics modulator and receiver and was shown operating at 50 Gigabit-per-second.

The light source used with the PIC was a continuous wave distributed feedback (DFB) laser. One Finisar ECOC demonstration showed the eye diagram of the 50 Gig transmitter using non-return-to-zero (NRZ) signalling. Separately, a 40 Gig link using this technology was shown operating error-free over 12km of single mode fibre.

"Finisar, and its fab partner STMicroelectronics, surprised the market with the 50 Gig silicon photonics demonstration,” says Daryl Inniss, practice leader of components at Ovum.

"This, to our knowledge, was the first public demonstration of silicon photonics running at such a high speed," says Rafik Ward, vice president of marketing at Finisar. However, the demonstrations were solely to show the technology's potential. "We are not announcing any new products," he says.

Potential applications for the PIC include the future 50 Gig IEEE Ethernet standard, as well as a possible 40 Gig serial Ethernet standard. "Also next-generation 400 Gig Ethernet and 100 Gig Ethernet using 50 Gig lanes," says Ward. "All these things are being discussed within the IEEE."

Jerry Rawls, co-founder and chairman of Finisar, said in an interview a year ago that the company had not developed any silicon photonics-based products as the technology had not shown any compelling advantage compared to its existing optical technologies.

Now Finisar has decided to reveal its in-house design capability as the technology is at a suitable stage of development to show to the industry. It is also timely, says Ward, given the many topics and applications being discussed in the standards work.

The company sees silicon photonics as part of its technology toolkit available to its engineers as they tackle next-generation module designs.

Finisar unveiled a vertical-cavity surface-emitting laser (VCSEL) operating at 40 Gig at the OFC show held in March. The 40 Gig VCSEL demonstration also used NRZ signalling. IBM has also published a technical paper that used Finisar's VCSEL technology operating at 50 Gbps. 

"What we are trying to do is come up with solutions where we can enable a common architecture between the short wave and the long wave optical modules," says Ward. "These two complementary technologies [VCSELs and silicon photonics] work well together as we think about the next-generation Ethernet applications."

Cisco Systems, also a silicon photonics proponent, was quoted in the accompanying Finisar ECOC press release as being 'excited' to see Finisar advancing the development of silicon photonics technology. "Cisco is our biggest customer," says Ward. "We see this as a significant endorsement from a very large user of optical modules." Cisco acquired silicon photonics start-up Lightwire for $271 million in March 2012.

ROADM technologies

Finisar also demonstrated two products for reconfigurable optical add/ drop multiplexers (ROADM): a dual configuration wavelength-selective switch (WSS) and an optical channel monitor (OCM).

The dual-configuration WSS is suited to route-and-select ROADM architectures.

Two architectures are used for ROADMs: broadcast-and-select and route-and-select. With broadcast-and-select, incoming channels are routed in the various directions using a passive splitter that in effect makes copies of the incoming signal. To route signals in the outgoing direction, a 1xN WSS is used. However, due to the optical losses of the splitters, such an architecture is used for low node-degree applications. For higher-degree nodes, the optical loss becomes a barrier, such that a WSS is also used for the incoming signals, resulting in the route-and-select architecture. A dual-configuration WSS thus benefits a route-and-select ROADM design.

Finisar's WSS module is sufficiently slim that it occupies a single-chassis slot, unlike existing designs that require two. "It enables system designers to free up slots for other applications such as transponder line cards inside their chassis," says Ward. 

The dual WSS modules support flexible grid and come in 2x1x20, 2x1x9 and 2x8x12 configurations. "There are some architectures being discussed for add/ drop that would utilise the WSS in that [2x8x12] configuration," says Ward.

The ECOC demonstrations included different traffic patterns passing through the WSS, as well as attenuation control and the management of super-channels. 

Finisar also showed an accompanying high-resolution OCM that also occupies a single-chassis slot. The OCM can resolve the spectral power of channels as narrow as 6.25GHz. The OCM, a single-channel device, can scan a fibre's C-band in 200ms.

A rule of thumb is that an OCM is used for each WSS. A customer often monitors channels on a single fibre, says Ward, and must pick which fibres to monitor. The OCM is typically connected to each fibre or to an optical switch to scan multiple fibres.

"People are looking to use the spectrum in a fibre in a much more optimised way," says Ward. The advent of flexible grid and super-channels requires a much tighter packing of channels. "So, being able to see and identify all of the key elements of these channels and manage them is going to become more and more difficult," he says, with the issue growing in importance as operators move to line speeds greater than 100 Gig.   

Finisar also used the ECOC show to demonstrate repeater-less transmission using an amplifier that can span 400km of fibre. Such an amplifier is used in harsh environments where it is difficult to build amplifier huts. The amplifier can also be used for certain submarine applications known as 'festooning' where the cable follows a coastline and returns to land each time amplification is required. Using such a long-span amplifier reduces the overall hops back to the coast.