Want to Get an Extra 50% out of Your Dispersion-managed Subsea Fiber? Here’s How.

Submarine network operators are currently seeing significant growth in bandwidth demand. According to Telegeography’s 2022 The State of The Network Report, international bandwidth more than doubled between 2018 and 2020, while international internet bandwidth grew at a compound annual rate of 45% between 2017 and 2021. One approach for addressing this growth is the deployment of new submarine cable systems, with 75 cable systems in Telegeography’s submarine cable map having ready-for-service dates between 2022 and 2026.

A complementary approach is maximizing the capacity of the 400+ already deployed submarine cable systems. These cable systems required huge investment and took many years to plan and deploy, hundreds of millions of dollars and five to seven years in the case of a typical transoceanic cable system. Submarine network operators therefore naturally want to extend their lives and maximize their capacities. These deployed cable systems fall into several categories, including dispersion-managed, uncompensated, and space-division multiplexing (SDM). In this blog, I will focus on some of the specific challenges and solutions for maximizing the capacity of the 200+ dispersion-managed cables that were deployed between approximately 2000 and 2015.

Table 1: 200+ cables with 2000-2015 ready-for-service dates, indicating they are dispersion managed

In the pre-coherent era, high chromatic dispersion was bad

Chromatic dispersion occurs because different frequencies travel at different speeds through the fiber – even different frequencies of the same wavelength travel at slightly different speeds and eventually distort the signal, as illustrated in Figure 1. Before the advent of coherent optical technology, which has the ability to digitally compensate for dispersion in the DSP, chromatic dispersion was a key challenge for direct detect (i.e., 10G) wavelengths. To address this challenge, the subsea industry developed and deployed dispersion-managed cables.

Figure 1: Chromatic dispersion was a key challenge in the pre-coherent era

First-generation dispersion managed

Leveraging non-zero dispersion-shifted fiber (NZDSF), the first generation of these cables, deployed between approximately 2000 and 2010 (150+ cable systems in the submarine cable map) used approximately nine lengths of fiber with chromatic dispersion of -2 ps/nm/km for each one of +18 ps/nm/km. However, due to the frequency-dependent nature of chromatic dispersion, there are large variations in chromatic dispersion across the C-band. As shown in Figure 1, while at the center wavelengths (~1550 nm) chromatic dispersion stays close to zero, over longer distances chromatic dispersion becomes highly positive (~1565 nm) or highly negative (~1530 nm) toward the edges of the C-band.

Figure 2: First-generation dispersion managed (c. 2000-2010)

Second-generation dispersion managed

Deployed between approximately 2010 and 2015 (80+ cables in the submarine cable map), the second generation, commonly referred to as “slope managed,” typically alternated positive-dispersion fiber (+18 ps/nm/km) with negative-dispersion fiber (-18 ps/nm/km). As shown in Figure 2, while chromatic dispersion was a little higher at the center frequencies (~1550 nm) relative to the first generation, across the C-band, differences in chromatic dispersion were minimized.

Figure 3: Second-generation “slope managed” (c. 2010-2015)

In the coherent era, low chromatic dispersion is bad

However, in the coherent era, low chromatic dispersion creates a key challenge for dispersion-managed cables. Low chromatic dispersion increases the likelihood that symbols on different wavelengths propagate together, changing the refractive index of the fiber at the same time though the Kerr effect, causing nonlinear effects such as cross-phase modulation (XPM).

ICE6 toolkit for dispersion-managed cables

Infinera’s ICE6 and ICE6 Turbo optical engines include a comprehensive submarine toolkit. Key features for dispersion-managed cables are shown in Figure 4. To address the challenge of low chromatic dispersion resulting in high nonlinearities, ICE6 supports specialized 4D (ME-8QAM) and 8D (FD-eBPSK, FD-2.5QAM, FD-3QAM) multi-dimensional modulation formats that leverage a set of rules for how these dimensions can be combined in order to minimize nonlinearities in dispersion-managed fibers. For example, in the case of ME-8QAM, a high-power constellation point on one polarization is balanced by a low-power constellation point on the other polarization, thus minimizing variations in power, which are a key cause of nonlinearities.

Figure 4: Key ICE6 features for dispersion-managed submarine cables

In addition, ICE6’s SD-FEC gain sharing feature addresses the challenge of chromatic dispersion variability found in first-generation dispersion-managed cables. And while ICE6’s ultra-high baud rates have many benefits, as higher baud rates consume more spectrum, reduce margin a little (for the same spectral efficiency) and are less tolerant of chromatic dispersion, ICE6’s tuneable baud rate provides the option of lower baud rates that can be more optimal for dispersion-managed fibers. Other ICE6 features that can be valuable on dispersion-managed cables include Nyquist subcarriers, frequency-domain hybrid modulation (i.e. 4/3-QAM), conventional QPSK, a shared wavelocker for tight channel spacing, super-channels, polarization-dependent loss (PDL) mitigation, and a high-gain 33% FEC option.

Up to 50% capacity increase over previous-generation technology

As one example of the value ICE6 can bring to dispersion-managed submarine cables, on a 2,940-km dispersion-managed subsea cable between Hong Kong and Singapore, Telstra was able to increase capacity by 45% over previous-generation technology and by 20 times the original design capacity of the cable. As a second example, on TPG Telecom’s dispersion-managed PPC-1, an approximately 7,000-km cable system connecting Australia and Guam, ICE6 was able to increase capacity by 50%, from 8 Tb/s per fiber pair to 12 Tb/s. By maximizing the capacity of these cables with improved spectral efficiency and reducing the cost per bit, ICE6 is extending their economic life. Furthermore, ICE6 can reduce power consumption by up to 70% and footprint by up to 50% on dispersion-managed fibers compared to the previous generation of coherent technology.

For more details on this topic, download the new Infinera application note, ICE6 for Submarine Networks.