PAUL MOMTAHAN COMPACT PLUGGABLES

HIGH-PERFORMANCE EMBEDDED COHERENT: WHAT’S NEXT?

Since its emergence in the late 2000s, coherent optical technology has revolutionised optical transport over long-haul, submarine, data centre interconnect (DCI), and metro networks, enabling huge increases in wavelength speed, spectral efficiency, and fibre capacity,

writes Paul Momtahan , Director, Solution Marketing, Infinera M ore recently, CMOS process node evolution and diverse market requirements have led the coherent optical engine performance segment evolve beyond the current 800 Gbps per wavelength

within 1 or 2 dB of the Shannon limit, the theoretical maximum spectral efficiency. But how much closer to the Shannon limit can we get? Next-generation embedded engines such as Infinera’s ICE7 improve spectral efficiency with features such as a continuous baud rate tuneability and a tighter roll-off. Continuous baud rate tuneability enables the wavelength’s spectrum to align with the available ROADM passband more closely, and to more optimally target a given link’s OSNR margin, which can be especially valuable in submarine applications. A tighter roll- off reduces the amount of additional spectrum required to accommodate the slopes and modes at the sides of the wavelength, enabling wavelengths to be packed closer together. Beyond ICE7, Infinera is developing enhanced algorithms for forward error correction (FEC), nonlinear compensation, and PCS to address the potential for spectral efficiency improvements of around 20%. For example, a recent Infinera trial with Australia-Japan Cable leveraging next- generation algorithms showed a 17% increase in capacity.

generation based on 7nm CMOS digital ASIC/DSP technology and 90 to 100 Gbaud photonics? In order to answer this question, it is important to start with the typical priorities for network operators in the long-haul and submarine applications for which high-performance embedded engines are typically the form factor of choice. These priorities include maximising fibre capacity, reducing cost, power consumption and footprint, and minimising operational costs. IMPROVED SPECTRAL EFFICIENCY Maximising fibre capacity is often the number one metric for long haul and submarine networks. One way to do this is with improved spectral efficiency, maximising the bps/Hz for a given reach/ path requirement to get the maximum capacity out of the available spectrum. Today’s 7nm embedded engines, such as Infinera’s ICE6, leverage multiple advanced features such as 64QAM- based probabilistic constellation shaping (PCS) and Nyquist subcarriers to get

market to bifurcate into two distinct segments, high performance embedded optical engines and compact coherent pluggables, as shown in Figure 1. Both types of optical engines have the same basic architecture as shown in Figure 2. However, coherent DSP designers have built ASICs optimised for low-power consumption and small footprint, with the current 7nm CMOS generation enabling 400 Gbps in QSFP- DD, OSFP, and CFP2 pluggable form factors. Meanwhile, high-performance engines leverage larger, more powerful, and more power-hungry digital ASICs able to deliver the highest possible baud rates and advanced features that maximise wavelength capacity-reach and spectral efficiency. These high- performance engines are embedded in transponders and are the form factor of choice for long-haul and submarine applications. But how will the high-

Figure 1 - Coherent Bifurcation: High-performance Embedded and Compact Pluggables.

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| ISSUE 34 | Q3 2023

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