The rapid development of AI computing is changing the architecture of optical interconnects. As data rates move from 800G toward 1.6T and beyond, the industry is increasingly exploring technologies such as Co-Packaged Optics (CPO) and Near-Packaged Optics (NPO) to overcome the power consumption, signal integrity, and bandwidth-density limitations of conventional electrical interconnects. At the same time, a fiber technology that has existed for decades is gaining renewed attention: polarization-maintaining fiber (PMF).
This trend was particularly visible at CIOE 2026, where major Chinese optical-fiber manufacturers introduced dedicated PMF product brands and solutions for next-generation optical interconnects. The specialty fiber brand intended for applications including CPO/NPO, coherent optics, and high-speed optical interconnects. These announcements are significant because they show that PMF is moving beyond its traditional role in specialized optical systems and becoming increasingly connected with the emerging AI optical-interconnect ecosystem.
PMF itself is not a new technology. It has long been used in fiber optic gyroscopes, optical sensing, laser systems, interferometers, and other applications where maintaining a stable polarization state is important. What has changed is the optical architecture of modern computing systems. As photonic components move closer to high-performance computing chips, precise polarization control is becoming relevant to a much larger class of optical systems. Understanding this change requires first looking at what PMF actually does and how it differs from conventional single-mode fiber.
What Makes Polarization-Maintaining Fiber Different?
Light propagating through an optical fiber is an electromagnetic wave, and its electric field can have a particular polarization state. In a conventional single-mode fiber (SMF), however, the polarization state is not inherently fixed. Small asymmetries in the fiber structure, mechanical stress, bending, twisting, temperature changes, and other environmental effects can cause coupling between two orthogonal polarization states. Consequently, linearly polarized light launched into an ordinary SMF can gradually change its polarization state as it travels through the fiber.
This does not normally prevent SMF from performing its primary function as a communication medium. Modern telecommunications systems are designed to tolerate and compensate for polarization changes, and ordinary SMF remains the dominant transmission fiber for data centers, telecommunications networks, and access networks. Its excellent transmission performance, low attenuation, mature manufacturing technology, and relatively low cost make it extremely effective for moving optical signals over long distances.
PMF takes a different approach. Instead of trying to eliminate birefringence, it deliberately introduces strong and controlled birefringence into the fiber. The resulting fiber has two well-defined principal polarization axes, commonly referred to as the fast axis and slow axis. When polarized light is launched along one of these axes, the strong birefringence suppresses coupling between the two orthogonal polarization modes. The polarization state can therefore remain stable over the length of the fiber.
Many PMF designs achieve this effect by incorporating specially engineered stress regions into the fiber structure. The widely used PANDA design, for example, uses stress-applying regions positioned around the fiber core to create a controlled difference in refractive index along two orthogonal directions. This intentional asymmetry gives PMF its characteristic polarization-maintaining behavior.
The distinction between conventional SMF and PMF can therefore be summarized as follows:
| Characteristic | Standard SMF | Polarization-Maintaining Fiber |
|---|---|---|
| Primary purpose | Efficient optical signal transmission | Optical transmission with controlled polarization |
| Polarization state | Can change during propagation | Designed to remain stable |
| Birefringence | Relatively weak and uncontrolled | Strong and intentionally engineered |
| Polarization axes | Not strongly defined | Clearly defined fast and slow axes |
| Typical applications | Telecom, data centers, FTTx, general optical links | Lasers, sensing, gyroscopes, coherent optics, CPO/NPO |
| Manufacturing requirements | Mature and highly scalable | Tighter control of optical and mechanical characteristics |
One important parameter used to characterize PMF is beat length. In simplified terms, beat length describes the propagation distance over which the phase relationship between the two orthogonal polarization modes completes one full cycle. A shorter beat length generally corresponds to stronger birefringence. Other important parameters include polarization extinction ratio (PER), polarization crosstalk, attenuation, mode field diameter, and the accuracy of polarization-axis alignment.
This last point becomes particularly important when PMF is incorporated into an optical assembly. A PM fiber may have excellent intrinsic polarization-maintaining performance, but bending, splicing, connectorization, and inaccurate axis alignment can affect the performance of the finished assembly. Therefore, the challenge in advanced applications is not simply manufacturing a PM fiber with strong birefringence; it is maintaining its polarization characteristics throughout the entire optical path.
Why Is PMF Becoming Important for CPO and NPO?
The renewed interest in PMF is closely connected to the changing architecture of AI data centers. Conventional optical networking generally places the optical transceiver outside the switching ASIC. High-speed electrical signals travel from the ASIC to the pluggable optical module, where they are converted into optical signals before entering the fiber network. This architecture has significant advantages in terms of modularity, serviceability, and compatibility with existing networking standards, but the electrical connection between the ASIC and optical module becomes increasingly difficult to scale as data rates rise.
At 800G and beyond, electrical losses, power consumption, thermal management, and signal-integrity requirements become increasingly challenging. CPO and NPO attempt to address these limitations by moving optical conversion much closer to the switching ASIC. CPO integrates optical engines closely with the switch chip, while NPO places the optical engines near the package or ASIC without necessarily integrating them into the same package.
The underlying idea is straightforward: instead of transporting extremely high-speed electrical signals over relatively long distances before converting them to light, convert the signals to optical form as close to the computing device as practical. This can reduce the electrical reach and potentially improve bandwidth density and energy efficiency.
This architectural change also changes the role of optical fiber. In a conventional data-center link, the fiber is primarily a transmission medium between two optical endpoints. In a CPO or NPO system, however, some fiber segments may form part of the precision optical interface between a laser source, a fiber array, and a silicon photonic optical engine. At this point, optical characteristics that were relatively unimportant in conventional fiber links can become critical.
Polarization is one of them.
PMF and the Optical Engine
One particularly important use case is the connection between an external laser source and a silicon photonic optical engine. In some CPO and NPO architectures, the laser source can be separated from the optical engine for reasons related to thermal management, reliability, serviceability, and system architecture. Continuous-wave optical power generated by the laser must then be delivered to the optical engine through an optical path.
In a simplified architecture, the path can be viewed as:
External Laser → PMF → Fiber Array/FAU → Silicon Photonic Optical Engine
The reason PMF can be useful in this path is that the optical engine may have polarization-sensitive components. Silicon photonic devices, including certain modulators, filters, wavelength-management structures, and coupling interfaces, can exhibit different behavior depending on the polarization of the incoming light. If the polarization state changes unpredictably during transmission, the coupling and optical performance of the downstream photonic device can also vary.
PMF provides a controlled polarization state between the laser source and the optical engine. Rather than allowing the polarization to evolve unpredictably through an ordinary fiber, the PM fiber is designed to preserve a defined polarization orientation. This can make the optical coupling process more predictable and can help maintain consistent performance in polarization-sensitive photonic systems.
This is particularly relevant to the Fiber Array Unit (FAU), which provides a high-precision interface between optical fibers and a photonic integrated circuit. As the number of optical channels increases, fiber arrays must be positioned with extremely high mechanical accuracy. When PMF is used, the challenge includes another dimension: the polarization axis of each fiber must also be correctly controlled and aligned.
Consequently, the interface is no longer simply a matter of connecting fiber to fiber. It becomes a precision optical coupling problem involving fiber position, fiber orientation, mode-field matching, insertion loss, polarization extinction ratio, and long-term mechanical stability.
This is one of the reasons that the development of PMF for CPO and NPO is increasingly associated with FAUs, fiber arrays, optical engines, and silicon photonics rather than being treated as an isolated fiber-material technology.
PMF Does Not Replace SMF
The growing importance of PMF should not be interpreted as a replacement of conventional single-mode fiber. In most optical networks, SMF will remain the fundamental transmission medium because most communication links do not require strict preservation of polarization.
Instead, PMF and SMF are likely to coexist within advanced optical architectures, with each being used where its characteristics provide the greatest value.
A CPO or NPO optical system, for example, could use PMF in the polarization-sensitive section between the laser and optical engine, while conventional SMF is used for the subsequent data transmission path. This approach provides polarization control where it is necessary without adding the cost and manufacturing complexity of PMF to the entire network.
This distinction is important because PMF is a relatively specialized and precision-oriented fiber technology. Its manufacturing and assembly requirements are more demanding than those of conventional SMF, particularly when polarization-axis orientation, low crosstalk, high PER, and precise coupling are required. Therefore, the future growth of PMF does not depend on replacing billions of kilometers of existing SMF. Instead, it depends on the expansion of new applications where polarization control becomes an integral part of the optical architecture.
Why Is an Old Technology Suddenly Becoming Important?
The most interesting aspect of the current PMF trend is that the underlying technology is not new. PMF has been used for decades in applications where polarization stability is essential. Fiber optic gyroscopes are a good example. In sensing and interferometric systems, changes in polarization can affect measurement accuracy, making PMF a natural choice. Similar requirements exist in laser systems, optical instrumentation, and some coherent optical applications.
What has changed is the scale and location of the optical system.
Historically, PMF was primarily a specialized component used in relatively small volumes for demanding optical applications. The rise of AI computing introduces a very different market environment. High-performance GPUs and AI accelerators are driving massive increases in communication bandwidth within data centers. Switch ASICs are moving toward increasingly higher aggregate capacities, while the number of optical channels required by large-scale AI clusters continues to increase.
At the same time, silicon photonics is becoming an increasingly important platform for high-speed optical engines. These developments bring precision optical components much closer to the computing chip. As a result, technologies that were previously confined to specialized optical systems can become relevant to high-volume data-center infrastructure.
This is why PMF can suddenly become strategically important even though its basic operating principle has remained largely unchanged for decades.
The key change is not that PMF has become better at transmitting data. Rather, modern optical systems increasingly need to control the physical properties of light itself.
From Fiber-to-Fiber Connectivity to Fiber-to-Chip Connectivity
This shift represents a broader change in the optical communications industry.
For many years, optical connectivity was primarily discussed in terms of fiber-to-fiber connections. Engineers focused on connector types, insertion loss, return loss, fiber count, polarity, cable construction, bend radius, and transmission distance. These remain essential requirements for conventional networks.
CPO and NPO introduce another layer of complexity. The optical path increasingly extends from the laser source through specialty fiber and fiber arrays into a photonic integrated circuit. The critical interface is no longer only between two optical fibers; it can be between a fiber and a semiconductor photonic device.
This can be described as a transition from fiber-to-fiber connectivity toward fiber-to-chip connectivity.
Within this new architecture, PMF, FAUs, fiber arrays, silicon photonics, external laser sources, and high-density optical interconnects become closely connected technologies. The performance of the final system depends not only on the transmission properties of the fiber but also on how accurately the optical field is delivered into the photonic chip.
This is also why the development of dedicated PMF products for CPO and NPO is significant. The industry is not simply adapting an existing fiber to a new application. It is increasingly developing fiber characteristics, fiber assemblies, and alignment processes around the requirements of photonic packaging.
The Road Ahead for PMF and High-Speed Optical Interconnects
The transition from pluggable optics toward NPO and CPO will not happen overnight, and the industry is still working through important questions involving packaging technology, thermal management, manufacturing yield, optical coupling, standards, serviceability, and overall system cost. Different optical-engine architectures will also have different requirements for polarization management. PMF will therefore not necessarily be present in every CPO or NPO implementation.
Nevertheless, the overall direction is becoming increasingly clear. As AI systems demand more bandwidth and lower energy consumption, optical conversion is moving closer to computing devices. As silicon photonics becomes more deeply integrated into switching and computing platforms, the optical interface becomes more sensitive to precise alignment and optical-state control. In applications where the laser and photonic engine require stable polarization, PMF can become an important enabling technology.
The emergence of dedicated PMF brands and CPO/NPO-oriented products from major fiber manufacturers at CIOE 2026 is therefore worth watching. It reflects a broader transition in the optical industry: specialty fiber is moving from highly specialized instrumentation applications toward large-scale photonic computing infrastructure.
PMF should consequently be understood not as a replacement for conventional SMF, but as a complementary technology for the parts of the optical system where polarization control matters. SMF will continue to provide the foundation for long-distance and data-center optical transmission, while PMF can provide the precise polarization management required by specific optical engines and photonic interfaces.
The larger story is the evolution of optical connectivity itself. As 800G, 1.6T, and future higher-speed systems push optical technologies closer to the processor, the industry is moving beyond the traditional concept of fiber as simply a medium for transporting data. Increasingly, fiber is becoming part of a precision optical system that must control not only where light travels, but also how that light is coupled, aligned, and maintained as it enters a photonic device.
In that context, the renewed attention to polarization-maintaining fiber is not simply a revival of an old technology. It is a sign that the next generation of AI infrastructure will demand increasingly precise control of light—from the laser source, through the fiber and fiber array, into the optical engine, and ultimately onto the photonic chip.
Frequently Asked Questions
1. What is polarization-maintaining fiber (PMF), and how does it differ from standard single-mode fiber (SMF)?
Polarization-maintaining fiber is a specialty optical fiber designed to preserve the polarization state of light during transmission. Unlike standard SMF, which allows polarization to change due to bending, stress, temperature, and other environmental effects, PMF uses engineered birefringence to create stable fast and slow axes. SMF remains the primary choice for general optical transmission, while PMF is used where stable polarization is an important system requirement.
2. Why is polarization-maintaining fiber becoming more important for CPO and NPO?
CPO and NPO move optical conversion closer to high-speed switch ASICs, increasing the importance of precise optical coupling between lasers, fiber arrays, and photonic engines. In architectures that use polarization-sensitive components, uncontrolled polarization changes can affect coupling efficiency and optical performance. PMF can maintain a more stable polarization state, making it particularly valuable in specific laser-to-optical-engine paths.
3. Is PMF intended to replace conventional single-mode fiber in future data centers?
No. PMF and SMF serve different purposes and are more likely to be complementary technologies. SMF will continue to dominate general-purpose data-center and telecommunications transmission because of its low loss, mature manufacturing, and cost efficiency. PMF is more likely to be used selectively in polarization-sensitive sections, such as the optical interface between an external laser source and a CPO/NPO optical engine.
4. How does PMF maintain the polarization of an optical signal?
PMF deliberately introduces strong birefringence into the fiber structure, typically through specially designed stress-applying regions. This creates two well-defined orthogonal polarization axes and suppresses coupling between them. When the input light is properly aligned with one of these axes, its polarization state can remain stable as it propagates through the fiber.
5. Why can an optical engine or silicon photonics chip be sensitive to polarization?
Many silicon photonic components have polarization-dependent optical characteristics. Changes in the polarization state of the incoming light can influence fiber-to-chip coupling, modulation efficiency, filtering behavior, and other optical parameters. Maintaining a controlled polarization state can therefore improve the consistency and stability of the optical interface in applications where polarization sensitivity is significant.
6. What role does PMF play in the connection between an external laser source and a CPO optical engine?
In some CPO and NPO architectures, the laser source can be located outside the optical engine to improve thermal management, reliability, or system flexibility. PMF can provide the optical path between the laser and the optical engine while maintaining a controlled polarization state. The resulting chain can include the external laser, PMF, fiber array or FAU, and silicon photonic optical engine.
7. Why is PMF becoming important now if the technology has existed for decades?
PMF is not a new technology and has long been used in fiber optic gyroscopes, sensing, laser systems, and other specialized applications. What has changed is the scale and architecture of optical computing. The rapid growth of AI infrastructure, higher-speed optical interconnects, silicon photonics, and CPO/NPO is bringing polarization-sensitive optical components closer to high-volume data-center systems. As a result, an established specialty-fiber technology is finding new large-scale applications.
8. What PMF parameters are important for CPO and NPO applications?
Important parameters can include beat length, polarization extinction ratio (PER), polarization crosstalk, attenuation, mode field diameter, and polarization-axis alignment accuracy. For practical optical assemblies, fiber bending, splicing, connectorization, and mechanical alignment also need to be considered because the performance of the finished assembly depends on more than the intrinsic characteristics of the fiber itself.
