Glossary entry

Polarization-maintaining fiber (PMF)

An optical fiber that preserves the linear polarization state of light launched along one of two principal axes. Used where polarization stability is essential.

Standard single-mode fiber is nominally polarization-insensitive — the two polarization modes are degenerate. In practice, small asymmetries (residual core ellipticity, stress, bending) cause unpredictable polarization rotation as light propagates, and the output polarization is unstable against temperature and mechanical perturbations.

Polarization-maintaining fiber deliberately introduces high birefringence between two orthogonal axes — typically called the slow axis (higher $n_\text{eff}$ ) and fast axis (lower $n_\text{eff}$ ). Light launched into one principal axis stays in that axis along the fiber length, because random perturbations are too weak to couple energy across the large $\Delta n_\text{eff}$ .

Standard PM fiber designs introduce stress-induced birefringence via stress-applying parts adjacent to the core:

Design	Geometry	Typical $\Delta n_\text{eff}$ at 1550 nm	Beat length
PANDA	Two cylindrical stress rods on opposite sides of the core	$\sim 5 \times 10^{-4}$	3 mm
Bow-Tie	Two fan-shaped stress regions	$\sim 5 \times 10^{-4}$	3 mm
Elliptical core	Geometric birefringence from elliptical core	$\sim 2 \times 10^{-4}$	8 mm
Microstructured / PCF	Asymmetric photonic crystal cladding	up to $\sim 10^{-3}$	1.5 mm

The standard launch reference is the slow axis, marked by an indicator on the fiber connector (typically the connector's keying notch is aligned with the slow axis).

Polarization extinction ratio (PER) is the standard quality metric for PM fiber:

\text{PER} \;=\; 10 \log_{10}\!\left( \frac{P_\text{principal}}{P_\text{cross-coupled}} \right).

Specifications:

PM fiber grade	Typical PER (per 10 m)
Standard PANDA	25 – 30 dB
High-PER PANDA	30 – 40 dB
Bow-Tie premium	30 – 35 dB
Microstructured PM	30 – 40 dB

PM fiber preserves PER best at controlled temperature and minimal bending. Bending the fiber, applying lateral stress, or excursions in temperature degrade the PER. Connectorization is the dominant PER degrader for short patch cords — each connector mate introduces angular alignment uncertainty that costs 5–15 dB of PER.

Applications:

Coherent receivers — local oscillator must be polarization-stable
Interferometric sensors — PMF prevents polarization drift from masquerading as signal
Frequency-doubling and SHG — typically requires specific input polarization
Polarization-multiplexed transmission — keeps the two channels from mixing
Atom traps and ion physics — laser polarization must be controlled and stable