How scaling optical networks is soon to change

Carrier division multiplexing and spatial division multiplexing (CSDM) are both needed, argues Lumentum’s Brian Smith.

The era of coherent-based optical transmission as is implemented today is coming to an end, argues Lumentum in a White Paper. 

Brian Smith

The author of the paper, Brian Smith, product and technology strategy, CTO Office at Lumentum, says two factors account for the looming change.

One is Shannon’s limit that defines how much information can be sent across a communications channel, in this case an optical fibre.

The second, less discussed regarding coherent-based optical transport, is how Moore’s law is slowing down.

”Both are happening coincidentally,” says Smith. “We believe what that means is that we, as an industry, are going to have to change how we scale capacity.”

Accommodating traffic growth

A common view in telecoms, based on years of reporting, is that internet traffic is growing 30 per cent annually. The CEO of AT&T mentioned over 30 per cent traffic growth in its network for the last three years during the company’s last quarterly report of 2023.

Smith says that data on the rate of traffic growth is limited. He points to a 2023 study by market research firm TeleGeography that shows traffic growth is dependent on region, ranging from 25 to 45 per cent CAGR.

Since the deployment of the first optical networking systems using coherent transmission in 2010, almost all networking capacity growth has been achieved in the C-band of a fibre, which comprises approximately 5 terahertz (THz) of spectrum.

Cramming more data into the C-band has come about by increasing the symbol rate used to transmit data and the modulation scheme used by the coherent transceivers, says Smith.

The current coherent era - labelled the 5th on the chart - is coming to an end. Source: Lumentum.

Pushing up baud rate

Because of the Shannon limit being approached, marginal gains exist to squeeze more data within the C-band. It means that more spectrum is required. In turn, the channel bandwidth occupied by an optical wavelength now goes up with baud rate such that while each wavelength carries more data, the capacity limit within the C-band has largely been reached.

Current systems use a symbol rate of 130-150 gigabaud (GBd). Later this year Ciena will introduce its 200GBd WaveLogic 6e coherent modem, while the industry has started work on developing the next generation 240-280GBd systems.

Reconfigurable optical add-drop multiplexers (ROADMs) have had to become ‘flexible’ in the last decade to accommodate changing channel widths. For example, a 400-gigabit wavelength fits in a 75GHz channel while an 800-gigabit wavelength fits within a 150GHz channel.

Another consequence of Shannon’s limit is that the transmission distance limit for a certain modulation scheme has been reached. Using 16-ary quadrature amplitude modulation (16-QAM), the distance ranges from 800-1200km. Doubling the baud rate doubles the data rate per wavelength but the link span remains fixed.

“There is a fundamentally limit to the maximum reach that you can achieve with that modulation scheme because of the Shannon limit,” says Smith.

At the recent OFC show held in March in San Diego, a workshop discussed whether a capacity crunch was looming.

The session's consensus was that, despite the challenges associated with the latest OIF 1600ZR and ZR+ standards, which promise to send 1.6 terabits of data on a single wavelength, the industry is confident that it will meet the OIF’s 240-280+ GBd symbol rates.

However, in the discussion about the next generation of baud rate—400-500GBd—the view is that while such rates look feasible, it is unclear how they will be achieved. The aim is always to double baud rate because the increase must be meaningful.

“If the industry can continue to push the baud rate, and get the cost-per-bit, power-per-bit, and performance required, that would be ideal,” says Smith.

But this is where the challenges of Moore’s law slowing down comes in. Achieving 240GBd and more will require a coherent digital signal processor (DSP) made using a 3nm CMOS process at least. Beyond this, transistors start to approach atomic scale and the performance becomes less deterministic. Moreover, the development costs of advanced CMOS processes - 3nm, 2nm and beyond - are growing exponentially.

Beyond 240GBd, it's also going to become more challenging to achieve the higher analogue bandwidths for the electronics and optics components needed in a coherent modem, says Smith. How the components will be packaged is key. There is no point in optimising the analogue bandwidth of each component only for the modem performance to degrade due to packaging. "These are massive challenges," says Smith.

This explains why the industry is starting to think about alternatives to increasing baud rate, such as moving to parallel carriers. Here a coherent modem would achieve a higher data rate by implementing multiple wavelengths per channel.

Lumentum refers to this approach as carrier division multiplexing.

Capacity scaling

The coherent modem, while key to optical transport systems, is only part of the scaling capacity story.

Prior to coherent optics, capacity growth was achieved by adding more and more wavelengths in the C-band. But with the advent of coherent DSPs compensating for chromatic and polarisation mode dispersion, suddenly baud rate could be increased.

“We're starting to see the need, again, for growing spectrum,” says Smith. “But now, we're growing spectrum outside the C-band.”

First signs of this are how optical transport systems are adding the L-band alongside the C-band, doubling a fibre’s spectrum from five to 10THz.

"The question we ask ourselves is: what happens once the C and L bands are exhausted?” says Smith.

Lumentum’s belief is that spatial division multiplexing will be needed to scale capacity further, starting with multiple fibre pairs. The challenge will be how to build systems so that costs don’t scale linearly with each added fibre pair.

There are already twin wavelength selective switches used for ROADMs for the C-band and L-bands. Lumentum is taking a first step in functional integration by combining the C- and L-bands in a single wavelength selective switch module, says Smith. "And we need to keep doing functional integration when we move to this new generation where spatial division multiplexing is going to be the approach."

Another consideration is that, with higher baud-rate wavelengths, there will be far fewer channels per fibre. And with growing fibre pairs per route, that suggests a future need for fibre-switched networking not just wavelength switching networking as used today.

"Looking into the future, you may find that your individual routeable capacity is closer to a full C-band," says Smith.

Will carrier division multiplexing happen before spatial division multiplexing?

Smith says that spatial division multiplexing will likely be first because Shannon’s limit is fundamental, and the industry is motivated to keep pushing Moore's law and baud rate.

“With Shannon’s limit and with the expansion from C-band to C+L Band, if you're growing at that nominal 30 per cent a year, a single fibre’s capacity will exhaust in two to three years’ time," says Smith. "This is likely the first exhaust point.”

Meanwhile, even with carrier division multiplexing and the first parallel coherent modems after 240GBd, advancing baud rate will not stop. The jumps may diminish from the doublings the industry knows and that will continue for several years yet. But they will still be worth having.