Why PAM4 and WDM Are the Future of Rugged Optical Transceivers

Aerospace and defense systems must ingest, transmit, and process ever-increasing volumes of data to support sensor fusion, but size and weight constraints limit bandwidth scalability. For example, today’s 3U VPX processor modules integrate 500 to 700 Gb of optical bandwidth. That’s an exceptional amount of throughput designed to keep up with the data generated by modern sensors.

As bandwidth demands grow, system architects face new pressure to move more data within tight space and weight constraints. That’s where PAM4 signaling and WDM transceivers come in. Today we’ll explore how these technologies—along with advances in wavelength division multiplexing systems—are reshaping what’s possible with rugged optical links.

Where We’re Coming From: NRZ Limitations

NRZ (Non-Return to Zero) is traditional signaling that uses two logical levels—on and off—to transmit binary data. It’s simple, robust, and widely used across generations of optical links in both commercial and defense systems.

However, as aerospace and defense applications demand greater density in less space, NRZ begins to show its limitations. While NRZ has scaled from 10 Gbps to 25 Gbps and beyond on a single channel, it can only go so far before additional optical cables must be added to increase bandwidth.

Where We’re Heading: Wave Division Multiplexing (WDM) Systems

Wave division multiplexing (WDM) systems allows multiple wavelengths of light to be transmitted through a single fiber, with each wavelength carrying its own independent data stream. It’s like having multiple optical lanes running parallel in the same strand. Optical transceivers combine wavelengths (multiplexing) and later separate them (demultiplexing) at the endpoint.

WDM transceivers can carry multiple signals on a single strand, vastly increasing density without adding more fiber—making it a compelling option for aerospace and defense applications where size and weight are constrained.

Currently, however, WDM is typically used on single-mode fiber, which is common in data centers but not rugged platforms, where multi-mode fiber is more prevalent. As the technology evolves, we may see a shift toward WDM-compatible fibers in rugged systems—or the development of compact WDM transceivers and miniaturized mux/demux solutions that can operate reliably in harsh environments.

It’s also important to note that receiver sensitivity plays a major role in wavelength division multiplexing system performance. As signals pass through multiple connectors and fibers, the ability of the receiver to detect and accurately interpret light levels becomes increasingly critical—especially across wide temperature ranges.

Where We’re Heading: PAM4 Signaling

Where WDM sends data over multiple wavelengths on a fiber, Pulse Amplitude Modulation 4-level (PAM4) signaling increases bandwidth by encoding data with four logical light levels. It is another approach to increasing optical bandwidth density in the system.

With PAM4, you can either cut the number of cables in half or double the bandwidth of existing fiber, whichever the application calls for. That’s a major advantage for SWaP-constrained aerospace and defense platforms.

In NRZ signaling, the laser operates with just two states: fully on or fully off (1 or 0). But with PAM4, you can have four different levels—for example, 100%, 75%, 25%, and 0%. However, this approach requires the receiver to reliably discern fine variations in light intensity, which gets much more difficult at extreme temperatures and demands a tight optical link budget.

The technology is already proven in data centers, but its environmental performance still lags behind existing rugged options. That said, vendors are actively working to ruggedize PAM4-capable transceivers. As these designs mature, PAM4 will likely become a viable solution for increasing bandwidth without expanding physical footprints.

The Future: Combining PAM4 and WDM

PAM4 doubles the bandwidth per lane, and WDM increases the number of lanes on a single fiber. Together, these technologies can achieve massive throughput with minimal footprint, making this combination promising for SWaP-constrained embedded systems used in aerospace and military applications.

In addition, work is being done in silicon photonics and integrating optical transceivers in the package with signal processors to further increase optical bandwidth density and reduce SWaP.

In the near future, we might see dozens or even hundreds of gigabit lanes entering a 3U VPX module to power sensor fusion—but temperature, shock, and vibration concerns must be addressed before these technologies can be reliably integrated in rugged optical transceivers.

Rugged optical transceivers enable high performance in modern aerospace and defense systems, but as data demands grow, we’re beginning to reach the limits of what traditional technologies can handle. PAM4 and WDM offer a path to dramatically increase bandwidth without increasing physical footprint. As these technologies become ruggedized, they will define the next generation of high-speed data movement for mission-critical sensor fusion.

If you’re exploring high-bandwidth solutions for rugged environments, our team is here to help. Let’s talk.