Growing relevance of Polarization
Maintaining Fibers

The optical performance of several systems, like sensors, fiber laser and electro-optic modulators, is impacted by the polarization of the light travelling through the fiber. This Polarization-Dependent Loss (PDL) can be addressed with specialized fibers called Polarization Maintaining (PM) Optical Fibers.

What is Polarization Maintaining Optical Fiber?

Polarization Maintaining Optical fiber is a type of single-mode fiber specially designed so it preserves the original polarization of the input light.

Polarized light vibrates only in one direction in a single plane, while unpolarized light vibrates in more than one direction. Polarization is important in fiber optic communication systems because it impacts the system's performance and reliability.

In a regular single-mode fiber, the vertical and the horizontal planes of the orthogonally polarized fibers have the same propagation constant, which means they travel at the same speed. Essentially, this means that the optical energy is easily able to transfer or cross-couple from one mode to the other in case of changes in external temperature, fiber core geometric irregularities or stress on the fiber. Because of this, a typical single-mode fiber is unable to maintain the polarization state of single-mode laser beam.

However, the Polarization Maintaining Optical fibers are designed in such a way that the two orthogonally polarized modes are forced to travel at different velocities with different propagation constants. This ensures minimal or no cross-coupling, thus preserving the polarization state of the transmitted light. PM fibers are created by making changes in the anisotropy of the fiber core.

Polarization Maintaining Optical Fibers are also known as birefringent fibers because they prevent cross-coupling by inducing the same birefringence across the fiber. Bigger birefringence is more effective in preventing cross-coupling.

Applications of Polarization Maintaining Fibers

Polarization is a crucial feature as it impacts the system's functioning as well as reliability. PM fibers are used in several applications where the polarization plane of the optical signal is crucial, like transmission lines for optical sensors and coupling for optical and electrical integrated circuits. They are also part of the devices that require a consistent polarization state even when there is a change in external conditions, for instance, an increase or decrease in temperature.

There are several applications in which the use of polarization maintaining optical fibers is critical:

• - Several fiber optic components, such as external modulators, require polarized light to function.
• - They are also apt for use in fiber optic sensors that depend on changes in the polarization state of light to measure physical quantities such as temperature, stress, and pressure.
• - PM fibers are also used in coherent optical transmission systems. Fiber optic sensing, interferometry and slab dielectric waveguides also use PM optical fibers.
• - PM Fibers are also required for quantum communication systems
• - They are also used in LIDAR systems and other remote sensing applications that demand polarization control for precise measurements.
• - Several medical imaging systems also use PM optical fibers to ensure the same polarization state of light for the best possible image quality.
• - Medical Field – Used in endoscopic imaging, Optical Coherence Tomography (OCT)

However, Polarization Maintaining Optical Fibers are rarely used for long-distance transmission because of the high cost.

Types of PM Optical Fibers

Essentially, there are two types of PM fibers: Panda PM Fiber and Bow-Tie PM Fibers. In the Panda design, two silica stress rods are on both sides of the core. These rods are made of different types of glass that expand differently under heat. When the fiber cools during manufacturing, the stress rods strain the core, induce birefringence, and help maintain the polarization of light propagation throughout the fiber.

On the other hand, Bow-Tie Polarization Maintaining optical Fiber has bow-tie-shaped stress elements on both sides of the core, which leads to birefringence. While Panda fiber is typically used for telecom use cases, the bow-tie fiber are generally deployed for sensor applications.

In Closing

Polarization Maintaining (PM) Optical Fibers play a vital role in applications like fiber optic communication, quantum communication, LIDAR, and medical imaging. With designs such as Panda PM Fibers and Bow-Tie PM Fibers, they ensure superior polarization stability and reliability. As industries demand more precision, PM optical fibers are becoming essential for high-performance optical systems and advanced photonic solutions.