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Optoelectronics & Communications


Telecom puts the focus on integration and all-optical networking.

From oemagazine December 2001
30 December 2001, SPIE Newsroom. DOI: 10.1117/2.5200112.0008

The 2001 market downturn has prompted telecom service providers to optimize existing networks rather than pursue the upgrades and expansions that characterized the previous two years. In 2002 these economic conditions are likely to continue, causing the telecommunications industry to focus on ways to increase the density of existing fiber networks without moving toward early adoption of the technically challenged 40-Gb/s (OC-768) equipment.

"A smaller footprint is something we need desperately because [of equipment] collocation at facilities. Power considerations [for next-generation equipment] can be up to three times as much but only use one-third of the space," says Chuck Dunsey, vice president of networking engineering at Verizon (New York, NY). In 2001, manufacturers of erbium-doped waveguide amplifiers (EDWA) gained serious interest from those in the industry because these chips are significantly smaller than their counterpart erbium-doped fiber amplifiers (EDFAs). They also can be constructed in arrays to accommodate the many fibers that compose a WDM or DWDM network.

As the market recovers and the metropolitan area networks emerge as industry drivers, cost reduction via chip-level integration is going to become critically important. EDWA technology is compatible with techniques for integrating active and passive components on chip, which cuts costs, simplifies automation to enable further economizing, and minimizes optical coupling loss.

Erbium-doped phosphate glass and erbium-doped silica-on-silicon are both promising for the mass-production of EDWA arrays. Springboarding off technology developed in conjunction with the Technical University of Denmark (Lyngby, Denmark), Cisilias (Lyngby, Denmark) has adapted plasma-enhanced chemical-vapor-deposition (CVD) manufacturing processes to include erbium doping, which avoids inefficient erbium ion clusters while keeping doping levels high as required by EDWAs.

The amplifiers deliver up to 12 dB of gain across the C-band (1528 nm to 1565 nm) at 120-mW pump power; comparable phosphate-based devices typically deliver no more than 10 dB. Jacob Philipsen, CEO of Cisilias, says that EDWAs are designed to replace fiber amplifiers in complex networks, such as DWDM installations where multiple fibers converge and real estate is at a premium. While the device is platform and speed agnostic, it is likely that the tight power budgets imposed by OC-768 will add to the product's appeal because of reduced sizes and lower optical losses due to the integrated passive optical interconnects. Future improvements include full power taps on chip, including couplers and monitor photodiodes. Integrated side- or surface-emitting pump sources may be up to two years away. According to at least one source, Cisilias is expected to produce an eight-port IC amplifier for close to the same cost as the single-port ICs produced by other companies.

ejecting the electric

No crystal ball for fiber-optic communications would be complete without a look at the key barriers to the all-optical network. According to Verizon's Dunsey, the all-optical cross-connect is the bridge to future fiber-optic networks.

Robert Nolan of Communications Industry Researchers Inc. (Charlottesville, VA) agrees. "[We believe] that service providers will begin to make significant deployments of optical cross-connects during the next 12 months, and this will generate significant revenues for optical-switching components manufacturers," Nolan says. CIR projects that as a result of the new trends in optical switching, the $247-million U.S. optical-switching component and subsystems market (2000) will turn into a $2-billion market by 2004. Before this market can be realized, however, several challenges remain. At the switch level, microelectromechanical-systems (MEMS) technologies rule the headlines, but yield issues remain a problem.

Perhaps more important is the need to solve all optical wavelength conversion to ensure proper wavelength provisioning in dense networks, explains market analyst Sterling Perrin with IDC (Framingham, MA), pointing to technology like that of Luxcore (Atlanta, GA). The company has developed a transponder that provides localized, all-optical provisioning in DWDM systems to maximize the efficiency of channel use and ensure that two signals with the same wavelength do not take the same path. The system achieves this task through either difference frequency mixing or cross-phase modulation. Each method offers its own strengths: Difference frequency mixing is compatible with 40-Gb/s data rates, while cross-phase modulation is not. Cross-phase modulation can, however, provide retiming and reshaping (2R) regeneration with amplification and signal shaping to improve the bit error rate (BER), according to Calvin Martin, vice president of optical systems engineering at Luxcore. At data rates below 40 Gb/s, cross-phase modulation can act as a link extender, improving signal quality, but not jitter.

By using provisional wavelengths outside the C-band, the transponder ensures that two channels with the same wavelength do not head down the same port. At the heart of the system is a semiconductor optical amplifier (SOA) based on indium gallium arsenide/indium phosphide (InGaAs/InP) materials. The technology will see initial deployment in 10-Gb/s and 12.5-Gb/s systems. Viney notes, however, that the same device will work just as well in 40-Gb/s systems—assuming that the conversion method can meet the bandwidth limitations mentioned above.

In today's market, carriers are focused on getting maximum performance for minimum cost. Technologies like integration and all-optical switching will continue to grab attention as the months roll on. oe

Winn Hardin

Winn Hardin is a freelance technology writer based in northern Florida.