At the Intersection of Pluggables and Subsea Networks

I still remember my very first visit to a cable landing station (CLS). I was a young engineering graduate who had just started working for a submarine cable operator. Back then, all I knew was that I was visiting a seaside facility packed with transmission equipment, where I would hopefully enjoy a nice seafood dinner at the end of the day.

The mission seemed simple enough. I was overseeing the commissioning of our very first 100G transponder cards, replacing older 40G units. To my novice eyes, it looked like a straightforward swap, removing the old card, installing the new card and making a few tweaks to the line. The two generations of transponder cards, which I saw for the first time, also looked remarkably similar side by side.

At the end of the day, the station manager walked me through the site’s legacy equipment. Stopping in front of an older, noisy 2.5G system, I was shocked (and a bit lost) when I learned that the single 100G card I had unboxed replaced several fully equipped shelves filled with discrete framer, driver, modulator, receiver and CDR cards. That moment, for me, was my very first “real-world” engineering crash course on technology evolution, miniaturization and commercial scalability.

Looking back, I often wonder what that young engineer would have thought if I handed them a modern coherent pluggable, a palm-sized device delivering the capacity that once needed full racks of equipment.

The evolution of coherent pluggables

Those coherent pluggables were initially developed for use as metro data-center interconnects, but are now transitioning for use in ocean-spanning subsea routes. To appreciate how this evolution occurred, several key developments must be examined.

The first factor is technology maturity. Current generation 800G coherent pluggables can support reaches of several thousand kilometers, typically between 2,500 km and 4,000 km at various rates, depending on system characteristics. In a recent trial on a repeatered subsea cable connecting the U.S. to Puerto Rico, coherent pluggables transmitted at 600G per wavelength over 2,841 km and, at 400G, achieved a record distance of 5,862 km. These distances make coherent pluggables extremely attractive for a broad range of submarine cables, including short- to mid-length digital line segments (DLSs) and unrepeated festoon links. Enhancements to 800G coherent pluggables are expected to further expand reach across all trans-Atlantic distances.

This progress was made possible by the adoption of smaller CMOS process nodes, which increased the processing power of the digital signal processor (DSP) while also reducing power consumption. When coupled with highly integrated, high-speed electro-optic components, it boosted baud rates and enabled advanced features, once considered exclusive to embedded transponders.

A progression began with 7 nm-based 400G pluggables, which introduced higher baud rates and advanced forward error correction (FEC) schemes. Building on this foundation, 800G coherent pluggables based on 3 nm DSPs have successfully ported other sophisticated optical technologies directly from embedded engines, including probabilistic constellation shaping (PCS) and high chromatic dispersion (CD) tolerance, which have increased both line rates and reach. Looking ahead, future pluggable generations are planned to support up to 1.6 -2.4 Tb/s per wavelength and integrate digital subcarrier multiplexing, helping boost capacity per wavelength and transmission performance.

Beyond technology readiness, the rapid evolution of submarine cable architectures, topologies and commercial models is creating the ideal environment for pluggables to address emerging needs of network operators.

More cables, more fibers, same subsea constraints

A glance at submarine cable maps from the past 25 years provides a visual representation of the expansion the industry has witnessed, driven by bandwidth growth. To meet this demand, operators have built numerous new cables, leveraging innovations by submarine line termination equipment and wet-plant providers. Spatial division multiplexing cables have boosted fiber counts from 4-6 pairs up to 24, expanding system throughput. While extra fibers enable more capacity per cable through more favorable trade-offs between spectral efficiency and cost per bit, quadrupling the fiber density at the CLS creates critical footprint and power bottlenecks. These constraints have driven the need for optimized solutions, creating a compelling case for adopting coherent pluggables.

Simultaneously, while new cables continue to be announced along traditional high-density trunk routes between major economic hubs, operators are also opening brand-new routes.

These routes are designed for resilience and low latency, often targeting historically underserved regions experiencing exponential growth. Most of these new cables are multi-regional systems featuring dozens of branching units (BUs) and digital line segments (DLSs). On high-demand main trunks, operators will continue to deploy embedded transponders from day one to maximize spectral efficiency. Conversely, unsaturated DLSs provisioned for growth or redundancy need a pragmatic option that optimizes Capex and Opex. Here, coherent pluggables emerge as the ideal complementary solution.

New architectures

Today, new generations of pluggables are reshaping CLS-to-CLS architectures. Deploying pluggables directly at the CLS restores signal regeneration and eliminates terrestrial optical signal-to-noise ratio (OSNR) degradation. Depending on the backhaul-to-subsea length ratio, this recovers 5% to 20% in subsea capacity while maintaining low space and power requirements.

With new-generation coherent pluggables, a novel approach to CLS-to-CLS architectures is emerging that takes advantage of the economic benefits of PoP-to-PoP connections without significantly compromising on the performance improvements typical of CLS-to-CLS connections using embedded transponders.

Furthermore, pluggables can perform wavelength conversion for the terrestrial backhaul to fully exploit C+L-band spectrum on those routes. This capability is particularly vital for managed optical fiber networks (MOFNs), where terrestrial fiber is often scarce and lease costs are high, as well as in jurisdictions where terminating traffic near a CLS is a regulatory requirement.

Coherent pluggables versus coherent embedded engines

Traditional embedded transponders are the de-facto standard for subsea connectivity, delivering top-tier performance, maximum spectral efficiency, clear physical demarcation, multi-client aggregation and robust service-level agreement monitoring and protection.

Coherent pluggables, by contrast, are most frequently discussed within the context of IPoDWDM architectures, which eliminate transponder shelves for maximum space and power savings. The trade-off is operational complexity. Physical demarcation points disappear, advanced performance monitoring is constrained, and network management system integration becomes more complex.

The rise of "thin transponders"

To bridge the operational gap between pluggables and chassis-based infrastructure, the "thin transponder" is rapidly becoming a popular choice. This approach combines the cost, size and power benefits of coherent pluggables with the robust management, operational simplicity and flexibility of traditional embedded transponders.

In a thin transponder model, a coherent pluggable is mounted onto a carrier sled that fits directly into a standard transponder shelf.

Compared to embedded solutions, this architecture can deliver up to a 50% reduction in footprint, a 35% drop in power consumption for identical capacity use cases, and an up to 40% in Capex savings, depending on the configuration. The trade-off is accepting a slight reduction in total fiber capacity compared to top-tier embedded transponders. However, the model also offers future-ready flexibility. Operators can easily swap a thin transponder sled for a full-featured embedded transponder when higher performance is required.

A new toolkit for subsea network evolution

Subsea networks have always been a fierce, highly competitive environment that demanded top-notch performance and state-of-the-art optical innovation. Today, emerging use cases and rapid network evolution are redefining what “competitive” really means, and there is no longer a single, one-size-fits-all answer.

Coherent pluggables are reshaping subsea network economics, delivering a compact, power-efficient and cost-optimized solution that is now expanding into transoceanic distances. By resolving critical space and thermal bottlenecks at CLSs, they offer unmatched flexibility for all scenarios where footprint and power are the strongest limiting factors. They are a natural complement to embedded transponders, which remain indispensable for ultra-long-haul links requiring maximum optical margin, peak spectral efficiency, complex client grooming, or capacity to be squeezed from aging wet plants

As deployment options expand, I like to think that somewhere in the world, in a not-so-distant future, a young engineer taking their first steps in a CLS and holding a modern subsea pluggable will feel that exact mix of enthusiasm, curiosity and “disorientation,” realizing how far technology has come while asking themselves, “How did we ever build subsea networks before we had these devices?”