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Got G.655 or Aerial Fiber? ICE6 Can Help with That.

headshot of Paul Momtahan

June 1, 2023
By Paul Momtahan
Director, Solution Marketing

In two previous blogs, I discussed how Infinera’s ICE6 coherent optical engines can help with terrestrial networks that face the challenges of high-loss spans and too many ROADMs. In this, the third and final blog in this series, I will focus on how ICE6 can address two further challenges that we often see in terrestrial networks: G.655 fiber with low chromatic dispersion and aerial fiber that can be impacted by lightning strikes.

G.655 Fiber and Low Chromatic Dispersion

G.655 fiber is the second most common type of terrestrial fiber after G.652. Common brand names include TrueWave RS, TrueWave Classic, and TrueWave REACH from OFS; LEAF and SMF-LS from Corning; and TeraLight from Prysmian. Offering low chromatic dispersion, it was introduced in 1993, with the first version of an ITU-T standard published in 1996. 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. For example, while G.652 fiber typically has chromatic dispersion of around 18 ps/nm/km, G.655 fiber typically has chromatic dispersion of around 6 ps/nm/km, approximately one-third that of G.652. At the time, chromatic dispersion was a key challenge for direct-detect (i.e., 10G) wavelengths, so this low chromatic dispersion was a very attractive fiber property for many network operators.

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

However, for coherent transmission, somewhat counterintuitively, low chromatic dispersion is actually a disadvantage. By causing the wavelengths to travel at slightly different speeds, chromatic dispersion reduces the buildup of nonlinear penalties. Thus, low chromatic dispersion makes optical networks based on G.655 fiber very challenging for high-performance coherent transmission.

ICE6 and G.655 Fiber

ICE6 has multiple features, including long-codeword probabilistic constellation shaping (PCS) and  advanced nonlinear compensation circuitry, that enable high performance even on G.655 fibers with low chromatic dispersion. And, while primarily used for dispersion-managed subsea applications, multi-dimensional 4D and 8D modulation formats that minimize nonlinearities at lower bits per symbol provide another option for the most challenging low-chromatic-dispersion scenarios. ICE6 therefore enables network operators to extend the life of their G.655 fiber assets, maximizing fiber capacity while simultaneously reducing cost per bit and power consumption significantly. For example, in a 2020 ICE6 field trial over G.655 Corning large-effective-area fiber (LEAF), Verizon was able to achieve 800 Gb/s over 667 km, 600 Gb/s over 2,283 km, and 400 Gb/s over 4,091 km.

Aerial Fiber and Lightning Strikes

Aerial fiber can be very cost-effective to deploy compared to buried fiber, especially when it is possible to leverage an existing aerial asset such as a power utility line. For example, Dgtl Infra recently put the cost per mile of underground fiber optic cable at between $60,000 and $80,000 USD, and the cost per mile of aerial fiber optic cable between $40,000 and $60,000 USD, or roughly a 25-33% savings.

OPGW cable with integrated optical fiberFigure 2: OPGW cable with integrated optical fiber

Aerial fiber deployments leverage optical ground wire (OPGW) cable, shown in Figure 2, which is run between the tops of high-voltage electricity pylons. OPGW is designed to attract lightning strikes. For example, in Central Europe, OPGW fiber can experience as many as 30 direct lightning strikes per 100 km per year, and an even greater number in other parts of the world, such as the Americas.

Global frequency of lightning strikes per square kilometerFigure 3: Global frequency of lightning strikes per square kilometer

However, because the conductor elements of OPGW are twisted like a wire cable, the current flows with a spiral component, which induces a magnetic field in the OPGW in the same direction as the optical fibers, causing the polarizations of the coherent wavelengths to rotate at great speed. The median rotation speed is 400,000 rad/s, with the worst 1% experiencing 2 million rad/s or more    . These high-speed rotations can result in temporary outages that violate service-level agreements (SLAs).

ICE6 and Aerial Fiber/Lighting Strikes

ICE6 provides extremely high tolerance to lightning strikes in aerial OPGW cables, with state-of-polarization (SOP) rotation tracking of up to 6 million rad/s. This is enabled by fast SOP tracking circuits that can be turned on for aerial fiber or turned off to reduce power consumption for buried fiber. Network operators can therefore maximize the capacity of their aerial fibers while simultaneously minimizing cost per bit and power consumption, but without running the risk of lightning-induced service interruptions.

A Comprehensive Toolkit for Challenging Conditions

ICE6 toolkit for challenging terrestrial conditionsFigure 4: ICE6 toolkit for challenging terrestrial conditions

Diverse terrestrial networks can create coherent performance challenges as a result of multiple factors, including high-span losses, high ROADM cascades, G.655 fiber with low chromatic dispersion, and aerial fiber with its susceptibility to lighting strikes. ICE6, including the 100 Gbaud ICE6 Turbo variant, provides a comprehensive toolkit to address these challenges. As shown in Figure 4, these tools include 400+ line modes, bandwidth virtualization, SD-FEC gain sharing, Nyquist subcarriers with dynamic bandwidth allocation (DBA), and a 33% FEC option. Together these features can maximize wavelength capacity-reach and spectral efficiency, minimizing cost, space, and power consumption under even the most challenging of terrestrial conditions, with a similar toolkit also available for challenging subsea conditions.

For more information on this important topic, download the Infinera application note, ICE6 for Challenging Terrestrial Conditions.