Driven by the explosive growth of AI, HPC, and hyperscale data center infrastructure, switch ASIC bandwidth and power consumption continue to scale rapidly. Traditional pluggable optical module architectures are increasingly challenged by bandwidth, power, and thermal constraints. In response, Co-Packaged Optics (CPO) is gaining momentum as a key technology for next-generation AI switches.
A common feature of modern CPO systems is the use of Polarization-Maintaining Fiber (PMF) between the External Laser Source (ELS) and the Silicon Photonics Optical Engine (OE). Buy why is PMF used in this link?
Why Do CPO Architectures Use ELS?
An external laser source (ELS) is a separate pluggable module that houses continuous-wave (CW) lasers and delivers optical power to silicon photonics engines through optical fibers. A typical ELS is mostly used for mainstream CPO architectures. Rather than integrating lasers into the CPO module, using an ELS avoids temperature-related performance issues and simplifies maintenance.
1) Thermal Management:
Modern AI switch ASICs can consume several hundred watts of power, creating a challenging thermal environment for temperature-sensitive lasers. Separating the continuous-wave lasers from these high-power areas prevents performance degradation and ensures the lasers remain in a cooler thermal zone.
2) Improved Serviceability and Manufacturing Yield
Lasers are more sensitive to lifetime and reliability than silicon photonic devices. In a fully integrated design, laser failure may require replacing the entire CPO module or even the switch card, increasing maintenance cost and downtime.
The ELS architecture separates the laser from the optical engine, enabling easier maintenance and independent testing, which helps improve manufacturing yield and production efficiency.
Why Does the ELS Link Require PMF?
The ELS typically outputs high-power continuous-wave (CW) laser light. In silicon photonics systems, many components – such as waveguides, modulators – are highly sensitive to the polarization state of the input light.
If the polarization state varies randomly during transmission, the coupling efficiency into the silicon photonics chip will degrade, leading to increased insertion loss and reduced link stability. Therefore, maintaining a stable polarization state between the ELS and the silicon photonics engine is critical.
What is polarization? Light is an electromagnetic wave. Although its oscillation is not directly visible, it can be understood using a simple analogy: a vibrating rope. If the rope is shaken up and down, the wave oscillates vertically; if it is shaken left and right, the oscillation changes direction accordingly.
Similarly, light has an electric field component that oscillates in a specific direction. This direction is defined as the polarization state. When the electric field oscillates in a fixed orientation - such as purely vertical – the light is referred to as linearly polarized light.
In practice, standard single-mode fiber (SMF) cannot maintain a stable polarization state. As light propagates through the fiber, it is affected by external perturbations such as bending, mechanical stress, temperature variations, and vibration. These effects cause continuous and random changes in the polarization state.
As a result, light that initially maintains a given polarization direction may gradually rotate or drift into other orientations during transmission. This phenomenon is known as polarization drift.
In traditional optical communication systems, polarization variations are usually not critical. However, silicon photonics devices – such as waveguides, modulators, and couplers – are highly polarization-dependent and typically optimized for a specific polarization state.
Any change in the input polarization can reduce coupling efficiency, increase insertion loss, and degrade link stability.
Therefore, in CPO systems, silicon photonics engines, and ELS links, maintaining a stable polarization state is essential, which is why PMF is required.
How Does PMF Preserve Polarization?
PMF is designed to preserve the polarization state of input light during transmission, rather than generating polarized light itself.
In standard SMF, the highly symmetrical circular structure makes it easy for external disturbances to alter the polarization state, leading to random polarization changes during propagation. In contrast, PMF intentionally breaks this symmetry to suppress polarization mixing.
The most common structure is the Panda-type PMF. Its cross-section resembles a panda face, with a central fiber core and two stress-applying regions on either side, hence the name “Panda-type PMF”.
The two stress regions apply constant mechanical stress to the fiber, creating different propagation properties along two orthogonal axes. As a result, light traveling along these directions experiences different refractive indices, leading to a phenomenon known as birefringence.
In simple terms, the fiber introduces two distinct propagation paths: a fast axis and a slow axis. Due to the large difference in propagation constants, coupling between the two polarization states is significantly reduced.
As a result, light launched along one axis tends to remain aligned with that axis during propagation, rather than randomly coupling into the orthogonal direction. This is the fundamental mechanism by which PMF preserves the polarization state.
Why Do CPO Systems Use a PMF + SMF Hybrid Architecture?
In CPO architectures, a hybrid PMF+SMF approach is commonly adopted to balance performance and cost. PMF is significantly more expensive than standard SMF due to its manufacturing complexity. It requires highly precise stress-applying structures and strict polarization-axis alignment during splicing and connector assembly. Even a few degrees of rotational misalignment can lead to noticeable performance degradation.
As a result, PMF is used only in polarization-sensitive sections—typically between the External Laser Source (ELS) and the silicon photonics engine—where stable polarization is critical. Standard SMF is used elsewhere to reduce system cost and manufacturing complexity.
Beyond CPO systems, PMF is widely used in applications that require high polarization stability, including fiber optic gyroscopes, coherent communications, laser systems, and quantum communications.
As silicon photonics, CPO architectures, and AI infrastructure continue to evolve, demand for PMF and other high-precision polarization-maintaining components is expected to grow significantly.
