NeoPhotonics : Coherent Hot Pluggable Module Evolution and Future Outlook

By Winston Way, Ph.D. on August 19, 2021 | Leave a Comment

Mechanical Form Factor Evolution

When coherent 40 Gb/s transponder was first introduced in 2008, it was a line-card with all components spread out on multiple printed circuit boards (PCBs), as shown in Figure 1. To increase the number of interoperable vendors, the Optical Inter-Networking Forum (OIF) developed Multi-Source-Agreements (MSA) for 100 Gb/s long-haul transponder form factors, as shown in the left two pictures of Figure 2. These MSA modules are not hot pluggable, but are fastened through screws and electrical connectors to the host PCB during line card assembly. As time evolves, there have been many proprietary embedded modules, similar to the MSA modules, developed for advanced system performance, including those used in today's 90⁺-Gbaud-based 800Gb/s line-cards.

Coherent hot pluggable modules were developed to take advantage of their low power, low cost, field replaceable, and pay as you grow features. The first such coherent pluggable was the CFP module introduced around 2016, which evolved to a smaller CFP2-DCO, and finally to today's QSFP, QSFP-DD, and OSFP modules, as shown in Figure 2. The next-gen OSFP-XD and QSFP-DD can accommodate 30+W of power, and are preparing for 1.6T per module. The significant size and power consumption reduction of these hot pluggable modules in turn improves drastically the port density on switches, routers, and DWDM transport equipment.

Figure 1 Ciena's 40Gb/s line-card in 2008.

Figure 2 Module size and power consumption evolution from MSA to hot-pluggable modules (courtesy of Atul Srivastava, NEL America)

Optical System Performance Evolution

Figure 3 shows the rOSNR difference between hot-pluggable and embedded modules. Since embedded modules are large and can tolerate a very high power consumption of up to ~100W, the general trend is that embedded modules were first developed to lead the best system performance by using a high-power DSP and the best-in-class optoelectronic components. Then 3~4 years later they were followed by hot-pluggable modules which use the next-generation DSP chip and more power efficient optoelectronic components to achieve a similar system performance (aiming for metro or long-haul applications), but at a much lower power consumption. As shown in Figure 3, we have compared the rOSNR for the coherent pluggable and embedded modules for 100, 200, and 400Gb/s net data rates. For each data rate, although the rOSNR numbers represent different generations of DSP, baud rate, and FEC, we can see that the rOSNR performance for pluggable and embedded modules is essentially the same for 100 and 200Gb/s. In the case of 400 Gb/s, the best rOSNR for a state-of-the-art pluggable module is about 2.5dB worse than the state-of-the-art embedded module. This is because the pluggable module uses 69Gbaud (requiring an efficient 75GHz-spacing DWDM multiplexer and demultiplexer), while the embedded module uses 90~95Gbaud (requiring a twice as wide 150GHz-spacing DWDM multiplexer and demultiplexer, and consequently a lower spectral efficiency).

Figure 3 Required optical signal-to-noise ratio (rOSNR) of various pluggable and embedded modules at 100, 200, and 400Gb/s.

References

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[10] Internal test data.