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Optical integration moves back onto the agenda

16 Jun 2006

Photonic integration was one of the hot new ideas that emerged during the telecoms boom, but it was subsequently shelved by cash-strapped component vendors. Now, as Roy Rubenstein reports, the technology is back on the table and could be set for a big revival.

Considering that the market for 40 Gbit/s transmission is yet to take off, Infinera's recent announcement is nothing short of extraordinary. The optical networking equipment maker has detailed a lab demonstration of an optical chip comprising 40 channels, each one operating at 40 Gbit/s. "The reasoning [for such a device] is purely economic," explains Jagdeep Singh, CEO of the US systems vendor. "It is the lowest cost way to deliver bandwidth."

Even though this 1.6 Tbit/s device will not be deployed for several years, it highlights the performance hike that can be achieved with a photonic integrated circuit (PIC) made entirely from indium phosphide (InP). More significantly, Infinera's PIC strategy could signal a forked path in the optical networking industry.

InP is the material of choice for active optical devices, accounting for more than 60% of the global telecom market for discrete optical components, according to market analyst Ovum-RHK. Its dominance stems from its energy bandgap - which spans the full range of optical-networking wavelengths (920-1650 nm) - and its high refractive index, which yields more compact devices.

Infinera's PICs exploit monolithic integration, in which a single material system is used to implement all the component's optical functions. InP is ideal for monolithic integration because it can support all the key optical functions, including lasers, gain blocks, detectors and passive waveguides. There are issues with yield however - see "InP manufacturing yields").

The alternative approach is hybrid integration, in which several different materials are used, each one best suited to a particular optical function. The challenge with hybrid integration lies in developing the assembly techniques and a platform to host the different materials.

High speed fuels PICs

Today, development of monolithically integrated InP circuits is being driven by high-speed transmission, and in particular the move towards 40 Gbit/s and new 100 Gbit/s standards.

Indeed, the most widely deployed InP PIC to date is an electro-absorption modulated laser (EML), which combines a laser and modulator in InP. Tunable lasers also integrate several functions in a single InP device. "Tunable lasers are selling in reasonable volumes - several tens of thousands a year - while EMLs are in the hundreds of thousands," says Vladimir Kozlov, the founder of transceiver market analyst Lightcounting.

Apogee Photonics, US, is one company concentrating on the 10 Gbit/s laser market. Apogee has its origins in two InP start-ups: ASIP and ThreeFive Photonics, the latter having brought to market such monolithic devices as a multiwavelength receiver and an optical performance monitor.

Apogee's focus is to exploit its selective-area growth and asymmetric twin-waveguide technologies to optimize the optical functions in its range of laser products. "We connect functions using tapers and, with precise control of the composition and structure, we can determine the performance of the laser and modulator," says Milind Gokhale, Apogee's CTO.

One example is Apogee's uncooled EML, which operates over a wide temperature range. "We are using one of the best integration platforms to make more down-to-earth products that the market needs in large quantities," says Erik Pennings, Apogee's director for product marketing.

Cyoptics, US, is another InP specialist making EMLs, as well as tunable lasers under contract for other firms. Like Apogee, its staff has made some exotic PICs in the past. "We made a distributed Bragg grating with phase control, a power detector and a modulator for a tunable laser over the C-band," says Robert Hartman, Cyoptics' vice-president for device design and development. "We even had a version with an SOA that was finished in development." But the market wasn't ready for such devices, he says, and didn't want to pay any more than a 10% premium for single-wavelength (untuned) devices.

Now, however, Cyoptics says that it is seeing renewed interest in PICs, particularly in the emerging 100 Gbit/s Ethernet standard. Two schemes are being considered for 100 Gbit/s Ethernet: four channels at 25 Gbit/s or one 50 Gbit/s channel with modulation. If a phase-modulation scheme is chosen, integration will be needed to realize the complex detector and waveguide structures that will be required.

For now, though, carriers - and by implication, system vendors - want to maintain flexibility by using only a fraction of a system's capacity. More channels are lit, or a channel is upgraded from 2.5 Gbit/s to 10 Gbit/s, only when the need arises.

"For any WDM system, our philosophy is pay-as-you-grow," comments Emmanuel Desurvire, senior director for photonic technologies in Alcatel's photonic-network product group. Such an approach does not favour multichannel PIC designs - such as "32 lambdas at once" - for optical networking. "Also, in case of a single-channel failure, the corresponding line card can be replaced without hitting the full (e.g. 320 Gbit/s) traffic," adds Desurvire.

System PICs

When designing a new system, vendors such as Nortel Networks look at the platform specification in terms of the interfaces - and the density of interfaces wanted by the carrier - and the reach. Depending on the maturity of the integration process, Nortel will consider the technology if it delivers power, size and, in some cases, improvements in system performance.

Alcatel stresses that adequate trade-offs between different performance criteria must be made when considering integrated products. Adopting an integrated device will probably require the line card to be redesigned, and that adds to the cost. "The technology also has to be mature and proven - we don't take chances in the field," says Desurvire. "A highly integrated device is not necessarily the best solution. Throwing away what you have can have an impact across the [network] architecture."

With Nortel and Alcatel selling their optical component divisions, it has also become more complicated to explore how integration can benefit system design. "It is difficult to predict what optical component players are doing and for them to second guess systems performance," says Michel Belanger, senior technical advisor in Nortel's next-generation optical network group.

Indeed, Infinera's decision to make systems allows the company to put its PIC technology at the heart of its design. "If you don't have control of the optical technology, you can't have a differentiated product," says Dave Welch, Infinera's chief strategy officer.

Infinera's DTN platform addresses the cost issue of optical-electrical-optical conversions by integrating discrete transponder functionality into its transmit and receive PICs. Infinera will not detail the resulting cost-savings, but one carrier suggests that its DTN platform is 30% cheaper than equivalent DWDM systems.

Welch claims that Infinera's InP manufacturing process is robust enough to achieve high yield. "We make DFB lasers in a similar fashion to everyone else, as we do our modulators," he says. What is different is that Infinera gets its engineers to design around the process. "We design what the process can manufacture."

Infinera's design also trades off optics and electronics. Forward-error correction and electronic dispersion-compensation techniques are used to relax the optical specifications, placing more of the link-budget burden on the electronics. Nortel makes a similar point about the importance of integrated components - not just optical, but also analogue and digital silicon chips.

"Infinera basically can do what others thought impossible because they expanded the domain over which they did their design trade-offs to span systems to processing," says Karen Liu, research director for components at Ovum-RHK.

And while the industry is focused on a pay-as-you-grow approach, Infinera's strategy is to deliver wavelength blocks - in chunks of 10 × 10 Gbit/s - cheaply enough, whether or not they are all needed. Welch argues that a line card costs roughly the same, regardless of what is on it, and the company's PIC shifts the industry from 10 to 100 Gbit/s on a line card. The company's prototype 40 × 40 Gbit/s PIC will deliver a further ten-fold hike in three to four years' time.

As to whether the pace of monolithic integration will hasten in the next five years, most believe not. But that doesn't mean there won't be exciting developments. Perhaps the most exciting is the placing of a tunable laser within an XFP package that supports line-side transmission distances of 80 km and greater. Agility (recently bought by JDSU) has an InP monolithically integrated tunable laser that is sufficiently small to fit within an XFP package.

This development indicates that a variety of transceiver types will converge to one form factor, with one laser and one receiver. Unit volumes will go up while the price of such a tunable, pluggable DWDM interface will dip below $1000 (€780).

Meanwhile, Infinera believes that it is only a matter of time before someone breaks away from the pack to make a PIC triplexer. At the same time, advances in hybrid technology continue to reduce packaging costs. Only time will tell which integration method will win.

• A version of this article originally appeared in the May 2006 issue of FibreSystems Europe in association with Lightwave Europe.

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