PLC technology arrayed for wider application

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Planar-lightwave-circuit (PLC) technology, which enables photons to pass through a wafer structure in much the same way they do through fiber, has become an accepted technology for WDM systems, largely in the form of multiplexer/demultiplexer modules based on arrayed-waveguide gratings (AWGs). However, PLCs—and their purveyors—have always promised more, particularly in the areas of multifunction integration at the wafer level and low-cost devices produced in high volumes. The confluence of several factors—especially that elusive high-volume application and the triumph of practicality over idealism when it comes to integration strategy—could make PLCs the technology of choice in a wider range of applications.

As is the case with other corners of the optical communications industry, device manufacturers see the burgeoning fiber to the premises (FTTP) market as the high-volume application that can kick-start their companies' return to growth. This notion is especially dear to many proponents of PLC technology who see the potential high volumes playing to one of their strengths: semiconductor-like manufacturability.Th 153255

Planar lightwave circuits (PLCs) have become very popular for splitters used in fiber to the premises (FTTP) applications. Because Asia is home to the most aggressive FTTP deployments, component vendors in that region have shown particular interest in the promise of PLCs.

"Once you've got the design and the process, you can make something very complicated, in large volumes, cheaply," says Ferris Lipscomb, vice president of marketing at NeoPhotonics (San Jose, CA), discussing PLCs in general. "Right now, FTTH [fiber to the home] is going strong in Asia. In fact, in Japan, something like 40,000 splitters a month are being consumed for FTTH. They're all being done with PLCs right now. So it's a natural thing for PLCs to do."

NeoPhotonics introduced a line of FTTP products in March, including splitters, couplers, triplexers, and WDM devices. In particular, Lipscomb says, FTTP splitters lend themselves to PLCs because of the matrix sizes required. While fused biconic taper (FBT) technology offers low cost and adequate performance in 1×2 and 2×2 configurations, the more complex FTTH configurations would require designers to cascade components. That degrades performance, increases cost, and results in a large device footprint, Lipscomb says, particularly in comparison to PLC-based devices.

"I think it's generally considered that for 1×8, 1×16, 1×32, PLCs are superior both in cost and performance. And obviously, the higher you go, the more superior they are," he offers. "A lot of the splitters in Japan are 1×4s, and they're made with PLC technology."

Jy Bhardwaj, general manager of the Waveguide Business Unit of JDS Uniphase (San Jose), agrees that PLCs have a role to play in FTTP systems. Price will prove the deciding factor, in his opinion. "We see PLC as potentially becoming even more significant down the road, as cost becomes an even greater objective for the FTTX area," he says. However, he believes the current spate of PLC-based devices for such applications are as much a reflection of current market realities as of technological development. "The interest in the market is intense because of the short-term opportunity to revenue," he says.

Richard Tompane, president and CEO of Gemfire (Fremont, CA), agrees and cautions that opportunities become limited when large numbers of competitors enter the market. Tompane says that PLC-splitter prices have dropped dramatically, and the number of competitors reduces volumes for everyone.

"The biggest problem in particular is since most of this deployment is being done in Asia, the Asian suppliers also have an inherent advantage in that marketplace. So it is a much more difficult thing for a company to break into if they don't have a lot of domestic operation there," he adds. "So Hitachi Cable and NEL are shipping a lot of parts into that category as well."

As an illustration of Tompane's point, two Asian companies are ready to enter the FTTP market with PLC-based devices. Central Glass (Tokyo) has delivered samples of the CWS-08E 1×8 splitter based on fluoride polymer PLC technology. The company also has bidirectional triplexers and diplexers (as well as filters and other devices). According to Tatsuya Mori, general manager of the Fine Chemical Business Development Department at Central Glass, the company's polymer material supports smaller component sizes and higher performance than conventional approaches. The company hopes to have its FTTP devices in production by the end of this year.

Meanwhile, in China, Broadex (Shanghai) also has PLC-based splitters, couplers, and fiber arrays in the sampling stage for FTTP applications. Its line of splitters ranges from 1×4 to 1×32. The company also has wavelength-insensitive couplers and AWG-based multiplexer/demultiplexer modules in its arsenal. Wei Zhu, Broadex's chief executive, says his company has developed an "optical rework" technique that enables Broadex to deliver yields on the order of 80%–90% while improving device performance, all using standard materials. While the company currently focuses on passive components, Wei envisions the inclusion of active devices next year.

Meanwhile, companies also have reduced their R&D emphasis on monolithic approaches to integration. Two factors have driven this decision. The first is the lack of standardized component requirements. "If every customer wanted the same thing, monolithic integration would be great. And customers normally do not want the same thing," JDSU's Bhardwaj concedes. "We think that today the market is not mature enough for one single [material] solution—or even two single solutions. And therefore, going down the monolithic route is not the best option, unless someone really has a volume requirement that's going to drive this."

The fact that no material provides optimal performance for every application also favors a hybrid approach. "One material handling everything, there's always going to be a compromise," Bhardwaj explains. "And the question is where do you actually take the hit?"

Therefore, companies now say that hybrid integration provides the best approach, since it enables device developers to customize their offerings where necessary and match the best material to each function. Gemfire in particular has embraced this philosophy, according to Tompane. The company has expanded its material line to include polysiloxane polymers, silica, silicon, indium gallium arsenide, and lithium niobate.

Hybrid integration enables a more complex set of functions to be incorporated into "intelligent," PLC-based modules. For example, JDSU has just announced a PLC-based reconfigurable add/drop multiplexer (see Lightwave, June 2004, page 22). Tompane says that PLC-based modules will meet growing tier one system-vendor requirements for intelligent devices that combine multiplexing/demultiplexing, add/drop, power monitoring, and overall module performance monitoring. Both Tompane and Bhardwaj foresee PLC technology as the basis for modules that accommodate channel counts of 32 and above, for which they report a growing interest.

Tompane envisions a day when PLC optical subsystems are designed much the way line cards are now. Passive waveguides would operate like copper trace (when they weren't operating as AWGs); they would connect photonic "chips" made of different materials that would provide various functions.

If such a vision does not become reality, it won't be for lack of trying.

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