The L Band: The Inevitable Destination of Long Haul Optical Networking

Long haul optical networks are approaching the Shannon limit in the work horse C band raising the inevitable question of how to keep increasing bandwidth. Right now the 64QAM modulation scheme achieves spectral efficiency of 6.5 bit/s/hz. But the upper bound on an optical channel according to the Shannon limit is 6.9 bit/s/hz. In other words, we are getting damn close. Obviously, it is a technological triumph. In very few fields of technology has mankind reached one of Nature's own speed limits.

But the practical question is where do we go from here? If you look at the chart, the answer is obvious. The L band (1563-1630) is adjacent to the C band and current long haul DWDM equipment can handle these frequencies without any substantive modification (just software upgrades). Furthermore, as the chart shows, the L band optical loss is only slightly higher than the C band. The line marked 'experimental' showed the actual measured optical loss as a function of wavelength. The C band is where it hits bottom, but the subsequent L band increase in optical loss is gradual.

An early pioneer in the use of the L band is Arelion. Its expansion into North America involves buying IRUs on long haul dark fibre. These are expensive. Plus there are significant operating expenses at each optical amplifier hut in terms of leasing space and power consumption. So typically Arelion purchases just one or two pairs on most long haul routes and squeezes as much bandwidth juice as possible out of them. DWDM on steroids and blood doping. Mid-2023 Arelion started carrying traffic on the L band on a fibre route connecting Ashburn Equinix to Atlanta. Arelion management has stated that the L band doubles the bandwidth potential of a fibre pair. In 2022 Colt deployed 800G wavelengths in the L band. Sparkle stated in 2023 that use of the C and L band frequencies would allow it to achieve up to 38.4 terabits a second per fibre pair. I believe that the L band will sharply increase the rate of return on carrier fibre optic networks as the operating expenses in terms of gear and software license is probably small to the potential revenue impact on potential revenue. However, if the L band just leads to more supply and demand is price inelastic, then it is possible that the inevitable price declines more than offset increased bandwidth consumption.