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A digital-to-analog converter setup used in the lab of the UCL Optical Networks Group. [Image: Courtesy of Ivona Kostadinova/UCL]

Scientists in the Optical Networks Group at University College London (UCL), U.K., have demonstrated a coherent-receiver design for passive optical networks that they say could significantly reduce the number of optical elements in the “last-mile” network endpoints known as optical network units, or ONUs (J. Lightw. Tech., doi: 10.1109/JLT.2015.2507869). The scheme—which relies on a kind of signal encoding borrowed from wireless communication to eliminate the need for some bulky, polarization-dependent optical components—could, in the view of the UCL team, make next-generation, ultrahigh-bandwidth fiber-to-the-home (FTTH) more cost-effective.

Coherence, complexity and cost

Coherent detection, which offers higher sensitivity than direct detection, has emerged as a practical necessity for the stringent performance demands of next-gen, high-speed access networks. But the intradyne digital receivers employed in core networks are packed with bulky optical elements, especially polarization beam splitters (PBSs) and 90-degree optical hybrids, and cannot be easily integrated into devices with a compact footprint.

That optical complexity makes intradyne receivers impractical—and much more expensive—relative to the slower direct detectors commonly used for consumer ONUs at the end of current-generation FTTH access networks, where cost and compactness are key. And, while current intradyne receivers might be reconfigured to remove the PBS, the resulting setup would need to be redesigned to optically track the state of polarization of the incoming signal, a laborious process that in turn would introduce new complexity into the system.

Eliminating polarization dependence

That dilemma has spurred a search for simpler receivers for ONUs. One possibility, explored by the UCL team, lies in using heterodyne detection and, in particular, in somehow erasing polarization dependence from the signal processing. Together, these steps could eliminate the need for PBSs and 90-degree optical hybrids and, in principle, dramatically simplify the receiver setup.

The UCL team achieved this by borrowing a signal coding scheme used in some wireless networks. The scheme, known as polarization-time block coding (PTBC), introduces redundancy into the signal to allow polarization information to be mathematically embedded into the digital signal itself. As a result, the x and y polarization modes—uncorrelated in conventional dual-polarization signals—become correlated. The system effectively collapses two polarizations into one, making transmission independent of polarization rotation, and eliminating the need for polarization tracking.

Dramatic simplification, next-gen speeds

The result, the researchers say, is a simpler receiver that consists mainly of a 3-dB coupler and a single balanced photodiode, and that contains 75 to 80 percent fewer components than conventional core-network receivers. The group successfully tested such a device over a distance of 80 km in a single-mode-fiber, wavelength-division-multiplexed passive optical network, at next-gen FTTH speeds of 10 Gb/s.

“We have designed a simplified optical receiver,” the paper’s lead author, Sezer Erkılınç, concluded in a press release, “that could be mass-produced cheaply while maintaining the quality of the optical signal.” To move toward that kind of commercialization, the team is now working to address stability issues in the receiving unit’s laser, a step to be followed by field trials.