ECOC 2022: imec, Ghent, and Nokia Bell Labs drive toward 100G networks

Group presents first 100 Gbit/s amplifier for flexible passive optical networks to meet ever-growing demands.

At last week’s European Conference on Optical Communication (ECOC), in Basel, Switzerland, researchers from IDLab (an imec research group at Ghent and Antwerp universities, in Belgium) and Nokia Bell Labs jointly presented what they called “the first upstream linear burst-mode transimpedance amplifier (TIA) chip that accommodates 50 Gbit/s NRZ and 100 Gbit/s PAM-4 modulation.”

The chip enables optical line terminals (OLTs) to cope with upstream packets’ varying signal strength and quality degradation; effects that are compounded by the high speeds at which next-generation passive optical networks (PONs) now operate.

The partners say that the new TIA chip “will be critical to making next-gen flexible PON deployments – and 100G PON networks in particular – technically and economically viable”. PON technology brings high-speed broadband to residential and business subscribers, while supporting services such as 5G mobile fronthaul and backhaul.

PON networks are characterized by a tree-like network topology that allows to serve multiple customers with a common strand of fiber connected to a single optical line terminal (OLT) in the operator’s central office. This makes for a cost-effective deployment scenario, but it also impacts how data packets travel the network: downstream traffic is sent over the network continuously, while upstream traffic is transmitted in bursts.

With 10G PON sales skyrocketing and the first 25G PON solutions being commercially introduced, PON technology is evolving rapidly. But the partners note that several challenges remain to be addressed, especially in the upstream direction, before next-gen (50G and 100G) PONs can be rolled out.

Upstream packets arriving at an OLT’s receiver may exhibit a large dynamic range of optical powers. This is due to the differential path loss of the optical distribution network, as well as the variation in optical network terminal (ONT) transmitters’ launch powers. Secondly, the distance between an OLT and its ONTs is a determining factor too, ranging anywhere between a few hundreds of meters, up to a few tens of kilometers.

As those effects are compounded even further by the high speeds at which next-gen PONs operate, it will be crucial to make sure that all packets arriving at the OLT end up having roughly the same signal strength. This must be achieved with minimal overhead – meaning no more than a few tens of nanoseconds,” said Gertjan Coudyzer, senior researcher in analog/mixed signal IC design at IDLab.

He added, “Our new chip does exactly that, allowing us to use each packet – and the network as a whole – to the fullest, maximizing its speed, reach and throughput.

Chip supports 50 G NRZ and 100 G PAM-4

Coudyzer said, “During our experiments, we have been able to validate the chip’s linear burst-mode operation; a linearity not only enabling signal equalization, but also paving the way for PAM-4 as a future PON modulation format – doubling the bitrate compared to the use of NRZ. This world’s first is a breakthrough to facilitate the future roll-out of large-scale 100G PONs.”

Peter Ossieur, program manager, high-speed transceivers at IDLab, commented, “Rolling out a fiber network is a huge investment. This announcement confirms that operators’ networks are capable of supporting higher bandwidths should the need arise. We are looking at the ITU-T [international telecoms standards setting body] to pick up our development and include it in their standardization efforts.

Tod Sizer, Optical System and Device Lab Leader, at Nokia Bell Labs, said, “The demonstration of our burst-mode TIA chip, results from our fruitful and long-standing collaboration with imec. The results are timely, as targets for 50G upstream PON are being defined in the ITU G.9804 standard. Thanks to its linearity, the TIA chip also supports 100G flexrate PON using higher order modulation.

The TIA chip is fabricated in a 0.13 μm SiGe process. It comes with a power consumption of 275 mW (on average) from a 2.5 V supply. Its total settling time is well under 150 ns, meeting the typical PON target preamble time.

Ossieur concluded, “Future challenges will continue to be closely linked to the growing distance between end users and the central office – as telecom operators try to expand their customer base to more rural areas. Hence, strategies to flexibly provide different customers with different PON flavors from the same OLT will continue to gain momentum, requiring new digital signal processing approaches.