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The Secret Sauce for Breaking the 400G ZR+ Record on Arelion’s Live Network

portrait of Fady Masoud

November 28, 2023
By Fady Masoud
Senior Director, Solutions Marketing

As the optical networking industry closed the curtains on ECOC 2023 in Glasgow, one of the accepted papers from Infinera, titled “1800 km 16QAM Transmission with a 400G QSFP-DD Coherent Pluggable” explained how a new record was set in transmitting a 400 Gb/s wavelength using a coherent pluggable in a QSFP-DD form factor over an 1,800 km distance – approximately 35% greater than the previous record. In this blog, I will explain the story behind this market-leading optical performance.

In the world of sports, setting a world record is a rare and incredible achievement. It requires unmatched skills, relentless determination, and a lot of hard work. Last March, Infinera’s team went through this journey, setting two new industry records a few days apart {Link 1, Link 2}. This industry breakthrough raised the bar of innovation to an entirely new level, achieving capacity and reach usually associated with long-haul deployments using pluggables and successfully overcoming numerous technical challenges such as fiber impairments. This milestone has positive implications for service provider economics, as intelligent coherent pluggables like Infinera’s ICE-X can significantly reduce optical transport costs and enhance networking flexibility.

The longest field transmission distance of 400 Gb/s using a QSFP-DD over a live production network

This milestone was achieved across Arelion’s production transmission network on routes between Dallas, Memphis, and Chattanooga, with Infinera’s ICE-X 400G ZR+ intelligent coherent pluggable successfully transmitting 400 Gb/s over 1,800 km with concurrently deployed wavelengths from third-party solution providers. This is a major breakthrough in the evolution of optical networking. Let me explain. Up until now, transmitting 400 Gb/s over distances around 1,000 km has been limited to embedded optical engines built in sleds/modules and equipped in full-fledged optical transport platforms that are a few rack units high. Sleds provide the required space and adequate air flow to host and cool the larger, more power-hungry digital signal processors (DSPs) and the sophisticated opto-electronic modules that constitute the embedded optical engine. Such sleds typically are the size of a paperback book. Achieving such a distance using a 400G QSFP-DD form factor, which has the width of a USB thumb drive and the length of a ballpoint pen, is a major achievement from both engineering and operational perspectives.

About the field trial

The field trial took place in the United Stages between the cities of Dallas (DAL), Memphis (MPHS), and Chattanooga (CHTG), as depicted on this U.S. map. The Arelion and Infinera teams tested five different links with various data rates and modulation formats, as listed in Table 1.

Table 1: All tested links during the field trial

The average distance between amplifiers is ∼92 km with a mean fiber loss of ∼0.204 dB/km. Finally, apart from the ROADM-to-ROADM connections, all spans utilize hybrid/Raman amplification. For example, the 900 km link consists of nine spans equipped with hybrid/Raman amplifiers. As this is a production network, ICE-X traffic coexisted with Infinera’s 600 Gb/s-per-wavelength, 16-nm CMOS DSP-based optical engine using CHM2T sleds and Infinera’s 800 Gb/s-per-wavelength, 7-nm CMOS DSP-based ICE6 optical engine on the G42 platform, as well as traffic over an 800 Gb/s-per-wavelength optical engine from a third-party optical equipment vendor. The channel allocation of the live network is depicted in Figure 1.

Channel allocation in Arelion’s live network and optical spectrum between Dallas and Memphis for the 1,800 km linkFigure 1: Channel allocation in Arelion’s live network and optical spectrum between Dallas and Memphis for the 1,800 km link

The industry record was set on the 1,800-km Memphis-Dallas-Memphis link with an optical loopback in Dallas using Infinera’s ICE-X 400G ZR+ QSFP-DD operating at 400 Gb/s with 16QAM modulation.

What’s the secret sauce?

As a matter of fact, there are multiple driving factors behind Infinera’s record-breaking performance in this 400G ZR+ application. Let me briefly describe each one.

  1. High signal-to-noise ratio (SNR) building blocks: SNR is a key measure of signal quality in optical networking as it reflects signal degradations caused by amplified spontaneous emission (ASE) noise added by optical components such as amplifiers along the transmission link. It is the ratio of the signal power to the noise power of an optical channel after passing through an optical network. It provides an estimate of the impact of noise power on signal power. The higher the SNR, the better it is for the system’s optical capacity-reach performance. Infinera’s sophisticated digital and opto-electronic components built into our ICE-X 400G ZR+ coherent pluggables are designed to deliver a high SNR to provide a higher tolerance to link impairments such as ASE, filter narrowing, nonlinear effects, and many others. As a result, the 400G wavelength can be transmitted through all intermediate optical building blocks such as amplifiers, filters, ROADMs, etc. There are two main building blocks in Infinera’s ICE-X 400G ZR+ coherent pluggable that significantly contribute to high SNR:
    • TROSAHigh-performance transmit/receive optical sub-assembly (TROSA): The TROSA consists of a single monolithic photonic integrated circuit (PIC) mounted on a carrier, high-speed analog circuitry to be the interface between the digital and analog/photonic domains, free-space optics for coupling in and out of the TROSA package, and flexible cabling to connect to the transceiver module board. It sets a new industry record in the high-density integration of discrete functions (including PIC, analog ASIC, and thermoelectric cooling) into a single submodule, and it provides leading optical performance with 0 dBm launch power enabled by an integrated semiconductor optical amplifier (SOA) in the PIC, low out-of-band noise enabling deployment over colorless ROADM architectures, high Tx OSNR, and high Rx sensitivity. Infinera’s TROSA stands out through its high-density monolithic integration using an absolute minimum (one of each) subcomponent count to deliver industry-leading performance, higher reliability, and high-volume manufacturability. Current third-party TROSAs leveraging InP or SiPh use two to three PICs, while a single Infinera PIC integrates multiple functions (lasers, splitters, amplifiers, Mach-Zehnder I/Q modulator). Also, current TROSAs use many analog ASIC couplings and TEC/fiber couplings, compared to one or two in Infinera’s. This high-density monolithic integration enables outstanding optical performance with low noise.
    • DSPHigh-performance digital signal processor (DSP): The DSP acts as the brain of the coherent pluggable and it plays a key role in enabling high optical performance. Typically, the DSP in coherent pluggables generates the different modulation schemes, such as QPSK, 8QAM, and 16QAM, to maximize wavelength capacity over any given fiber link, and performs advanced digital signal processing algorithms to compensate for distortions in the optical signal caused by dispersion and other factors. It also performs forward error correction (FEC) to ensure error-free transmission of data over the optical fiber. The 400G ICE-X DSP implemented in ICE-X pluggables stands out from the other DSPs in the industry for its large compensation for chromatic dispersion and its support for digital subcarriers, high-performance DAC/ADC, and high-bandwidth RF interconnect, which all contribute to ensure leading optical performance.
  1. Digital subcarriers: In order to increase the data rate on a single optical carrier, most dense wavelength-division multiplexing (DWDM) equipment vendors are forced to use very high single-carrier baud rates – 32 gigabaud (Gbaud) and upward. However, nonlinear impairments, such as self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), have different distortion penalties as baud rate increase. While FWM will decrease as baud rate increases, SPM actually rises, as depicted in Figure 2. This leads to the creation of a nonlinear “sweet spot” between about 4 and 16 Gbaud where distortion penalty, corresponding to the sum of all nonlinear fiber impairments, is at a minimum. Infinera introduced to the optical networking industry the concept of digital subcarriers. Using advanced digital signal processing, digital subcarrier technology divides a single-carrier wavelength into multiple lower-bandwidth subcarriers generated by a single coherent laser/transceiver. These subcarriers have a symbol rate that operates in the distortion penalty sweet spot mentioned earlier. For example, with Infinera’s ICE-X intelligent coherent pluggable, a 60.5 Gbaud 400G wavelength is divided into 16 x 25 Gb/s subcarriers, each having a symbol rate of 3.87 Gbaud with 16QAM modulation while occupying approximately 4 GHz of spectrum. With higher tolerance to nonlinear fiber impairments, digital subcarriers contribute to raising optical performance. By combining the benefits of digital subcarriers increasing tolerance to nonlinear effects and the chromatic dispersion compensation enabled by the DSP, a potential gain of 1.5-2.0 dB SNR can be achieved.

Total distortion penaltyFigure 2: Total distortion penalty

In addition, the use of digital subcarriers significantly reduces Chromatic Dispersion (CD). For example, by using 16 subcarriers, CD is reduced by a factor of 256. This has a number of benefits. For example, with the same amount of CD compensation circuitry in the DSP, a much greater tolerance for CD is achieved, thus enabling much greater reach. Alternatively, for the same amount of CD tolerance, less power is consumed. Furthermore, an additional benefit relates to the equalization-enhanced phase noise (EEPN) that is a byproduct of compensating for CD. With significantly reduced CD to compensate, this EEPN is greatly reduced, improving the overall performance of the optical engine,

3. Fully programmable intelligent coherent pluggables: There are many operational challenges when it comes to deploying coherent pluggables in live production networks. They include the optical loss associated with ROADM sites and line amplifiers, the need to go through fixed wavelength filters, fixed-grid legacy networks, and many others. To overcome these challenges, coherent pluggables must be designed and built around the principle of complete remote software programmability of the different functions required to successfully deploy and operate them. Infinera’s ICE-X is the industry’s first fully programmable intelligent coherent pluggable, with many key functions that can be set through software to enable successful deployment and optimized performance over any given link in any given application. Programmable functions include modes (ZR, ZR+, XR), baud rate, spectrum, per-digital subcarrier launch power, modulation format (8QAM, QPSK, 16QAM), fiber plant (single fiber or fiber pair), and traffic flows (asymmetrical, symmetrical). Support for full programmability in coherent pluggables is critical to overcome current operational challenges while enabling high optical performance.

Why does high optical performance matter?

Optical performance is of paramount importance in optical networking due to its direct impact on CapEx, OpEx, and network efficiency. High optical performance is crucial for maximizing network capacity and minimizing costs. Ensuring higher data throughput in a compact form factor reduces footprint and power consumption at terminal sites. Moreover, higher optical reach leads to the reduction, and often the elimination, of regeneration sites along the span, which further reduces capital and operating costs in addition to enhancing network reliability by reducing the number of resources for failure.

Now that this record is in the book, on to the next one! Visit Infinera’s ICE-X Intelligent Coherent Pluggables web site for more details.