Autumn 2018 Optical Connections Magazine



could maintain 16QAM modulation and increase the transmission baud rate to obtain the same capacity results without the reduction in reach associated with an increased modulation order. To help illustrate this scenario, Figure 2 shows a theoretical plot of three equivalent 400G capacity links using different modulation orders, assuming the same forward error correction (FEC) coding is used. Rather than moving from 16QAM to a higher order 32QAM modulation (moving from blue to red line in Figure 2), a link could remain at 16QAM and support 400G by increasing the transmission to 64Gbaud. 32QAM or 64QAM are also options, however an optical signal-to-noise ratio (OSNR) penalty of approximately 2dB (red arrow in Figure 2) or 4dB (orange arrow) are incurred, respectively. Returning to the topic of 64QAM, for shorter reaches such as DCI-edge links, the capability of having 64QAM is important for these links where raw capacity is desired. By having the capability of 64QAM, 64Gbaud (or higher) transmission one could “open the flood gates,” maximising both modulation order and baud rate, to

Eugene Park, senior manager, Technical Marketing, at Acacia Communications looks at the progress of coherent optical technology and how the 600G era provides differentiating capabilities to enhance transmission performance and flexibility.

C oherent optical interconnect improvements in higher orders of modulation, higher baud rates, and increased performance. The latest generation adds another element— that of advanced flexible transmission capabilities that enable network optimisation. These advanced capabilities are meant to help network operators maximise line-side capacity utilisation within their networks by filling capacity gaps. technology continues to advance with each generation, bringing GENERATIONAL ACHIEVEMENTS Figure 1 illustrates the main coherent optical technologies that undergo improvements from generation to generation. TRANSMISSION PERFORMANCE IN THE 600G ERA Advances in sampling rate and effective number of bits (ENOB) of high-speed

to today’s 64QAM. The ability to modulate at these high baud rates also brings about a new performance option where a high-baud rate, low modulation order combination can provide a high-capacity link over a long distance. A first line of reasoning to increase link capacity may be to increase the modulation order since the bit- per-symbol efficiency improves with moving to a higher modulation order. However, the capacity increase results in a reduced reach. Alternatively, one

mixed-signal analogue-to-digital converters (ADCs) and digital-to- analogue converters (DACs) have

contributed to the increase of baud rates beyond 64Gbaud, and the advancement of modulation capability from 16QAM

Figure 1. Generational advances in coherent optical technologies and where we are in today’s “600G Era”. 64QAM constellation shown.


| ISSUE 14 | Q3 2018

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