Pushing the boundaries of optical network reach, capacity and speed

Sometime around 490 BC, the Greek runner Pheidippides is said to have sprinted from Marathon to Athens, about 26 miles, to announce the Athenians’ victory over the Persians. He then promptly passed away, becoming one of the earliest and most famous casualties of the speed vs distance conundrum.

While admittedly a loose analogy, Pheidippides’ failed attempt illustrates the challenge of simultaneously optimizing distance, speed and capacity —a problem all too familiar (and confounding) for today’s telcos and hyperscalers.

Source: IDC

At the heart of the issue is a perfect storm that starts with the exponential growth of data generation and consumption. According to IDC, by 2025, the amount of global data generated each year will reach 181 Zb; this is an increase of nearly 300%^[i] compared to 2020. This, in turn, is driving similar growth in required fiber capacity.

In 1989, the typical maximum fiber capacity for any distance was 2.5 Gbps. By 2019, it had increased by a factor of over 10,000 to 32 Tbps^[ii]. Notwithstanding this capacity growth, telcos and hyperscalers are still under increasing pressure to address the forecast growth for more data capacity throughout the network. The solution will include adopting new technologies for both the line systems and optical transmitters to enable ultra-high capacity, massive scalability, power efficiency, and lower cost per bit.

Before we look at what’s possible, let’s take stock of where we are from a technology perspective.  

Have current technologies reached their limits?

In the past, technology powering line-systems and transponders provided enough headroom to support incrementally higher demand for optical reach, speed and capacity. Backward Raman amplification, higher baud-rate transponders and the use of “bolt-on” L-band systems enabled networks to inch closer to the industry’s next significant milestone—efficient terabit networking. But, in their current form, they are reaching a plateau and are not enough to support the continued growth. 

Raman Amplification

Raman amplifiers allow the ROADM to reduce gain required from traditional erbium-doped fiber amplifiers (EDFA), resulting in a “cleaner” overall signal and longer reach. Many operators deploy a best-of-both-worlds strategy: reduce the noise by lowering the EDFA gain and offset this reduction using Raman amplifiers.

So far, Raman amplification was limited to backward-facing configurations where the Raman amplifier is on the receiving end of the fiber span. In Backward Raman amplification, the pump laser transmits in the direction opposite to signal propagation.

The ultimate goal for Raman amplification was to find a way to successfully deploy the amplification on both ends of the link—Forward and Backward Raman amplification. This combination allows transferring more EDFA amplifier gain to Raman amplifiers, further reducing the EDFA-induced noise and improving the generalized signal-to-noise ratio (GSNR). The challenge is that Forward Raman also introduces a significant degree of relative intensity noise (RIN) that, in effect, negates the benefits of the technology. Overcoming the added noise has been the key obstacle to unlocking Raman’s full potential—until now. (Spoiler alert).

Transponder technology

The most recent generation boasts a symbol rate of 90 to 100 Gbaud and operates at up to 800Gbps using 32 QAM. According to a recent post from Neophotonics, “The next step on this path will be to Class 70 components, supporting 130-140 Gbaud operation and supporting a 1 Tbps or more per wavelength.”^[iii]

Continuous C+L Operation

Another capacity-enabling technology is combining both the C and L bands. Terrestrial fiber systems have traditionally operated on the C-band (≈1530nm – 1565nm). But on very high-capacity and constrained routes, this approach is quickly reaching its limits, so the industry started using an adjacent optical spectrum, known as the L band, which runs from approximately 1565nm to 1625nm.

Aside from providing a much-needed capacity boost for high-capacity fiber routes, the L-band spectrum exhibits an attenuation comparable to the C-Band, making it an ideal complement. Due to the relatively low equipment and operating costs, the L band also provides key cost benefits.

However, much of those gains can be reduced due to inefficiencies in how this additional spectrum is deployed. Traditionally, telcos and hyperscalers tend to rely on their C-band spectrum to serve both single spans (≈80 – 120 km) and regional links (<600 km). As traffic demands build, particularly on the high-capacity links, C-band capacity quickly reaches its limit. To handle excess capacity demands, operators have deployed separate “bolt-on” L-band systems and used their control plane to off-load excess traffic to the L band as needed. This bolt-on deployment process is labor-intensive, inefficient and potentially disruptive. Duplication of the add/drop equipment, wavelength selectable switches (WSS) and amplifiers increase system complexity and management.

The next-gen optical platform

Solving the Forward Raman challenge

As previously mentioned, overcoming RIN signal degradation was the major obstacle to successfully implementing Forward Raman. Working with Furukawa Electric Co., Ltd., a key technology partner, Fujitsu has developed a Forward Raman amplifier which minimizes RIN distortion and further reduces the cost per bit per km. This significant achievement now makes it practical and highly beneficial to deploy Raman amplifiers in both directions to increase optical reachability or fiber capacity.

Solving the Forward Raman enigma also addresses the need for greater capacity. Given the same distance, a Forward Raman amplified signal benefit from a higher GSNR. This means a higher-order modulation (more bits per Hz) can be used within the spectral width, and hence enable an overall fiber capacity increase. Based on network and fiber conditions, combined Forward and Backward Raman amplification can increase spectrum efficiency by up to 40%.

135-Gbaud transponder

Another important advancement with the 1FINITY Ultra Optical System is the introduction of a 135-GBaud transponder. Featuring a baud-rate flexible DSP, it enables networks to connect any two locations up to 6000 km apart at 400Gb/s without any regeneration. This flexibility also allows multiple combinations of capacity, reach and spectral efficiency by supporting multiple baud rates lower than 135-GBauds as well as multiple density of m-QAM modulations.

An added advantage of the transponder is the higher density 5nm DSP chip which, when combined with an advanced hybrid liquid-air cooling system, consumes significantly less power than 96-GBaud transponders using 7 nm chips. As calls for greater sustainability within networks intensify, demonstrable power savings becomes increasingly important to telcos and hyperscalers who are working to meet aggressive environmental targets.   

The 150 GHz channel width of the new 135-GBaud technology also simplifies channel planning, enabling networks to allocate spectrum using mutually compatible spectral widths. This channel size is an integer multiple of 50Ghz and 75Ghz, which helps minimize spectral fragmentation through the network.

Continuous C+L

Finally, the Fujitsu 1FINITY Ultra Optical System enables networks to overcome the operational and management challenges of the traditional bolt-on method for L-band deployment. This advanced system combines C and L bands within all key ROADM components: WSS, add/drop complex, and all the amplifiers into a single system.

The benefit for telcos and hyperscalers is the ability to consolidate C- and L-band spectrum equipment into one simplified solution which is easier to deploy and manage and is fully optimized for gain and spectral flatness from Day One. It also results in a higher-efficiency, non-blocking architecture. The technology is expected to be an important step towards simplifying network capacity upgrades requiring the simultaneous use of C band and L band.

Finding the Best Balance of Speed, Reach, and Capacity (and Cost)

It is not possible to simultaneously optimize baud rate, channel count, distance and line rate. Simultaneously optimizing for everything was Phiedippides’ fatal mistake. Fiber transmission is filled with trade-offs, yet it is not an either/or proposition. With the right technology and engineering, operators can achieve a balance in which all variables are improved. This is precisely what the next-gen fiber platforms like the 1FINITY Ultra Optical System can provide.

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[i] Global DataSphere Forecast, 2021-2025; IDC; March 2021

[ii] “Evolution of Fiber-Optic Transmission and Networking toward the 5G Era.” iScience; December 20, 2019

[iii] Class 60 Coherent Components Enable 800G per Wavelength Optical Transport; Neophotonics, blog; July 9, 2021