Size of the stress ring in polarization-maintaining fiber

The stress ring (or stress rods) in polarization-maintaining fibers typically ranges from a few micrometers to tens of micrometers in diameter, depending on the fiber design and core size.

Overview of Stress Rings in PM Fibers

Polarization-maintaining (PM) fibers, such as PANDA and Bow-Tie fibers, maintain linear polarization by introducing stress elements in the fiber cladding that break the circular symmetry of the core . These stress elements, often called stress rods or stress rings, run parallel to the fiber core and induce birefringence, creating two orthogonal polarization axes (fast and slow axes) with distinct propagation constants . The stress rods are typically made of a material with a different thermal expansion coefficient than the surrounding silica, generating internal stress during fiber drawing.

Typical Dimensions

• In PANDA fibers, the stress rods are cylindrical and positioned symmetrically on either side of the core . Their diameters are usually 5–20 µm, depending on the fiber's core size and the desired birefringence.
• In Bow-Tie fibers, the stress elements are trapezoidal prisms or elongated rods, with dimensions comparable to PANDA rods, often 10–25 µm in width and extending along the fiber length .
• The distance from the fiber core center to the stress rod center is typically on the order of 10–30 µm, which affects the magnitude of induced birefringence and the polarization extinction ratio.

Functional Considerations

The size and placement of the stress rods directly influence the fiber's beat length, polarization extinction ratio, and sensitivity to bending or temperature changes . Larger or closer stress rods generally increase birefringence, improving polarization maintenance but potentially increasing fabrication complexity. The mode field diameter (MFD) of the fiber is also considered when designing stress rod dimensions to ensure effective polarization control without excessive optical loss .

Summary

While exact dimensions vary by manufacturer and fiber type, stress rods in PM fibers are typically a few micrometers in diameter and positioned tens of micrometers from the core, with PANDA and Bow-Tie designs being the most common. These stress elements are critical for maintaining linear polarization over long fiber lengths and minimizing crosstalk between polarization modes .