PLC development gets active

May 1, 2005

Planar lightwave circuits (PLCs) have established a niche for themselves within such passive devices as arrayed-waveguide-grating(AWG)-based multiplexers/demultiplexers and splitters. However, the technology has always been touted for its integration capabilities. Developers appear to have finally unlocked this door, based on recent product introductions and technology announcements that feature not only passive waveguides integrated with active light sources, but also PLC structures integrated within active components to create a new generation of laser devices.

Much of the activity focuses on the PON market. Companies such as NKT Integration and NeoPhotonics (San Jose, CA) have pursued PLC-based transceivers for PON optical-network terminals. NKT’s parent company decided to abandon this pursuit and sold NKT to Ignis Photonyx (Horten, Norway) in January. Ignis has decided to focus on pairing its in-house expertise in polymer technology, particularly as applied to variable optical attenuators, with its new asset’s strengths in AWG-based mux/demux technology, according to company chief executive Even Zimmer.
ColorChip’s use of glass instead of silicon creates circular waveguides that more easily integrate with light sources.

NeoPhotonics, however, has maintained its pursuit of PON transceivers. The result was the debut of PLC-based biplexers and triplexers at OFC/NFOEC in March. The devices cover BPON, GPON, and GE-PON applications. While reluctant to describe how the technology is applied, other than to say that an AWG is not involved, the company touts low manufacturing cost and high yield as the benefits PLCs bring to the design. NeoPhotonics is working with Photon Technology (Shenzhen, China) on the device. In fact, NeoPhotonics has established an equity relationship in the Chinese firm, which supplies the light sources for the transceivers. A press release announcing the deal described the transceiver collaboration as “the integration of active lasers with passive filtering and alignment on a PLC substrate.”

ColorChip (Or-Akiva, Israel) is more open about its approach to PON transceivers. The company showed prototypes of its upcoming EPON transceiver, based on its trademarked “SystemOnGlass” (SOG) technology. As its name implies, SOG uses glass instead of silicon, says chief executive Moshe Price and marketing and sales director Ari Mizrachi. Glass enables round waveguides, which make matching to active components or fiber more efficient, the spokesmen assert. For the transceiver, the waveguides perform light collimation and wavelength filtering as well as photon transport. ColorChip uses surface-mount technology to integrate such components as laser diodes, photodiodes, and other electronic components along with fiber into the substrate without the need for subsequent sealing. The resultant device should provide less than half the insertion loss of bulk-optics designs, Price and Mizrachi say. The company expects to provide the first samples of its small-form-factor-sized Meteor transceivers (the biplexer Meteor II and triplexer Meteor III) in July.

While these two companies are combining light sources and PLCs to create devices for access networks, Redfern Integrated Optics (RIO-Santa Clara, CA) has taken a longer view when it comes to the integration of PLCs into light sources, at least when it comes to reach. The company has used silica-on-silicon PLC technology to make a 10-Gbit/sec external-cavity laser capable of 80-km reach via a maximum output power of 2 dBm. The device integrates a high-speed gain chip and a PLC with a Bragg grating. The design can provide either low chirp or negative chirp; the product’s design obviates the need for a wavelength locker on a 50-GHz grid, according to a company spokesman.

The company has samples to what it calls “tier one customers” in a TOSA format, with volume production expected in the last quarter of this year. Both the 40-km RIO1501 and 80-km RIO1511 were designed to meet the XFP and XMD package requirements and provide a dispersion power penalty of <2 dB. RIO’s roadmap includes a 2.5-Gbit/sec device. The company also is thinking about adding electronic dispersion compensation to extend transmission distances to 120 or even 160 km. Tunable devices are expected to appear next year.

While PLCs will continue to find application within splitter and mux/demux devices, it appears the technology’s evolution into the wide range of applications initially forecasted for it has begun.

One significant integration challenge for planar-lightwave-circuit (PLC) developers is finding a way to efficiently couple a light source with their waveguides. While some designers will look to their own processes, the PLC community now has some help from the other side of the fence. Eblana Photonics (Dublin, Ireland) says its discrete-mode lasers are perfect for integration with PLC-based ­devices.

Eblana uses what it calls “photon bandgap technology” to create light sources using standard InP processes. The company recently announced an agreement with Vitesse Semiconductor (Camarillo, CA) whereby the IC maker will manufacture Eblana’s devices using its HBT process and production line. According to Eblana’s president, James O’Gorman, the company etches photonic bandgaps into the laser structure; the resulting laser, which can be based on a Fabry-Perot design, produces a stable device that emits at the target frequency without the need for isolators. The fact the company uses standard IC toolsets and processes ensures both high yield and low cost, O’Gorman asserts, particularly when compared to standard laser processes that require multiple growth steps.

“Because we’re etching these photon bandgaps into the top of the laser, the result is you basically have a standard wafer architecture for the different laser wavelengths. You’re essentially making a Fabry-Perot diode upon which you’re impressing these features, which give you the higher functionality of a singlemode device,” he says.

The technology works particularly well with PLCs for several reasons, according to O’Gorman. For example, the resultant laser emits a circular beam that is easier to couple into waveguides, thus maximizing optical power. The stable performance means that the integrator can count on a consistent beam pattern from device to device, thus improving manufacturing efficiency. The laser also exhibits significant resistance to backreflection, particularly in comparison to DFB devices, O’Gorman claims.

While Eblana currently is using its photon-bandgap technology exclusively on lasers and delivering them to at least one customer, O’Gorman envisions applying the technique to other devices. “It’s an enabler of PLC devices today-and ultimately this type of technology will migrate into the PLC itself in the future as you go forward,” he says. O’Gorman sees hybrid devices in which such functions as the laser, drive circuitry, a detector photodiode, and perhaps even multiplexing and demultiplexing appear within an InP assembly that is flip-chip bonded to a silicon bench.

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