Specifications of polarization-maintaining fiber

Polarization-maintaining (PM) fibers are single-mode fibers designed to preserve the linear polarization of light, featuring high birefringence, defined slow and fast axes, and precise structural designs such as PANDA or bow-tie stress rods.

Fiber Types and Structure

PM fibers are typically single-mode fibers with built-in birefringence to maintain polarization. The most common designs are PANDA and bow-tie fibers, distinguished by the shape of stress rods embedded parallel to the core. PANDA fibers use cylindrical rods, while bow-tie fibers use trapezoidal prism rods. These stress elements induce linear birefringence, creating two orthogonal polarization modes with distinct phase velocities, which minimizes cross-coupling and preserves the input polarization state over long distances .

Key Optical Specifications

• Birefringence and Beat Length: The beat length (Lb) is the distance over which one polarization mode accumulates a phase difference of one wavelength relative to the orthogonal mode. Typical beat lengths are a few millimeters and scale with operating wavelength .
• Mode Field Diameter (MFD): Defined as the beam diameter at the 1/e² power level in the near field, MFD is specified nominally and varies with wavelength .
• Numerical Aperture (NA): Indicates the light acceptance angle of the fiber; values are nominal and depend on the fiber design and wavelength .
• Attenuation: PM fibers are designed for low optical loss, with specialized coatings available for high-power or flame-retardant applications .
• Wavelength Range: Standard PM fibers cover visible, near-infrared (NIR), and telecom wavelengths, typically from 350 nm to 1550 nm, with specialized fibers for short wavelengths or dispersion compensation .

Practical Considerations

• Alignment: When splicing or connecting PM fibers, the slow and fast axes must be precisely aligned to avoid polarization crosstalk. Specialized fusion splicers and keyed connectors are used to maintain axis orientation, often with alignment accuracy better than 0.5° .
• Environmental Stability: PM fibers are sensitive to stress, bending, and temperature changes. Some fibers are designed to be bend- and temperature-insensitive, suitable for applications like fiber-optic gyroscopes .
• Specialty Fibers: Variants include photosensitive fibers for Fiber Bragg Gratings, dispersion-compensating fibers, and fibers optimized for ultrafast lasers or space applications .

Applications

PM fibers are used in systems where polarization stability is critical, including:

• Fiber interferometers and modulators
• Fiber-optic gyroscopes and accelerometers
• Polarization-sensitive laser systems
• Sensing and metrology applications requiring low polarization drift . In summary, PM fibers combine structural birefringence, precise axis alignment, and controlled optical properties to maintain polarization, with specifications tailored to wavelength, environmental conditions, and application requirements.