Glossary entry

Polarization-maintaining (PM) fiber

A single-mode fiber with strong built-in birefringence, so that linearly polarized light launched along one of its two axes stays on that axis despite bends, twists and temperature changes. Stress rods beside the core (PANDA, bow-tie) or an elliptical core create the birefringence.

Fiber & telecom Lab practiceUpdated October 2026

Polarization-maintaining (PM) fiber is a single-mode fiber made strongly birefringent on purpose, so that its two polarization axes, the slow axis and the fast axis, have clearly different effective indices, and linearly polarized light launched along either axis stays on it. The birefringence $B = n_{slow} - n_{fast}$ is about $5 \times 10^{- 4}$ in the common PANDA and bow-tie designs, which put the core under stress with two glass rods or two fan-shaped regions beside it. Ordinary single-mode fiber has only weak, random birefringence from residual core ellipticity, stress and bending, so its output polarization wanders with temperature and handling. PM fiber is used wherever a polarization-sensitive component sits at the end of a fiber: a lithium niobate modulator, a silicon photonic chip, a fiber-optic gyroscope coil or a frequency-conversion crystal.

How it holds the polarization

Light crosses from one polarization axis to the other only if a perturbation along the fiber varies with a period close to the beat length

L_{B} = \frac{λ}{B},

the length over which the two polarizations slip by one wavelength relative to each other: 3.1 mm at 1550 nm for $B = 5 \times 10^{- 4}$ . Bends, twists and temperature gradients change slowly over such lengths, so they move very little light between the axes, and light launched on one axis stays there.

The same birefringence makes the two axes travel at different speeds. Light launched between them splits into two components that keep a fixed power ratio while their relative phase drifts with temperature and strain, so the output is an ellipse that keeps changing: the fiber maintains polarization only for light launched on an axis.

The residual coupling a fiber does have is quoted as its h-parameter, the fraction of power that crosses to the other axis per unit length. After a length $L$ the extinction is about $- 10 \log_{10} (h L)$ while $h L$ is small, so a fiber with $h = 10^{- 5}$ per meter would reach 30 dB after 100 m.

Designs

Design	Source of the birefringence	Typical $B$ 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	The shape of the core	$\sim 2 \times 10^{- 4}$	8 mm
Microstructured	Asymmetric holes in a photonic crystal fiber cladding	up to $\sim 10^{- 3}$	1.5 mm

The stress regions are doped glass that contracts more than the surrounding silica as the fiber cools after drawing, which leaves the core under unequal stress along the two axes; the photoelastic effect turns that stress into birefringence. In PANDA and bow-tie fibers the core is stretched along the line through the two stress regions, and that line is the slow axis. Polarizing fiber is a related design with so much extra loss on one axis over its working band that only the other polarization survives, which makes the fiber itself a polarizer.

Axes, connector keys and extinction ratio

Light is usually launched on the slow axis, and most PM patch cords align the slow axis with the connector key. Fast-axis versions exist, so the alignment belongs in the cable's specification.

The quality of polarization maintenance is quoted as the polarization extinction ratio (PER):

PER = 10 \log_{10} ⁣ (\frac{P_{principal}}{P_{cross-coupled}})

In a working system the angular alignment at each launch, connector and splice usually sets the PER, more than the fiber does. Light arriving at an angle $θ$ to the axis puts $\sin^{2} θ$ of its power on the other axis, so a single joint limits the PER to $10 \log_{10} (\cot^{2} θ)$ :

Angular error	PER limit
1°	35.2 dB
2°	29.1 dB
3°	25.6 dB
5°	21.2 dB

The errors of successive joints combine, and tight bends, lateral stress on the cable and temperature changes lower the PER further. Measuring PER and PDL gives the procedure for measuring PER without being misled by a misaligned launch, and the polarization extinction ratio entry lists typical values for sources and components.

Splicing and connectorizing

A PM fusion splicer rotates the two fibers until their stress regions line up, judging the angle from a side view of the rods or from the way the rods focus light passed sideways through the fiber, and then fuses them; the residual angle sets the splice's PER through the formula above. A PM connector is assembled the same way, with the fiber rotated to put the slow axis on the key before the adhesive sets. Splicing PM fiber to standard single-mode fiber is an ordinary fusion splice, since the standard fiber has no axes to align; polarization maintenance ends at that splice.

Where PM fiber is used

Fiber-optic gyroscopes. Many use a PM fiber sensing coil, so that the two counter-propagating beams stay in one polarization and the phase reading stays stable.
Polarization-dependent components. Lithium niobate modulators, for example, are pigtailed with PM fiber on the input.
Silicon photonics. Grating couplers and most waveguide devices work for one polarization, so external laser sources feed co-packaged optics through PM fiber.
Coherent receivers. The local oscillator input is PM fiber.
Frequency conversion and polarized fiber lasers. Periodically poled crystals and waveguides convert one polarization, and fiber lasers and amplifiers with linearly polarized output are built from PM fiber.
Sensing and laboratory optics. In interferometric sensors, atom trapping and quantum optics experiments, polarization drift would read as signal or spoil the measurement.

Polarization-multiplexed coherent links, by contrast, run over standard single-mode fiber: the receiver's digital signal processing tracks the polarization as it wanders, so the link needs no PM fiber.

Common questions

What is the difference between PM fiber and standard single-mode fiber?

Both guide one spatial mode. Standard fiber has weak, random birefringence, so light launched into it comes out in an unpredictable polarization that drifts with temperature and handling. PM fiber has strong, deliberate birefringence with defined axes, and light launched on an axis comes out on that axis. Types of optical fiber covers the standard single-mode grades and the other specialty fibers.

What is the slow axis of a PM fiber?

The axis with the higher effective index, along which light travels more slowly. In PANDA and bow-tie fibers it runs through the stress regions, and most PM connectors align it with the key.

What is the beat length of PM fiber?

The length over which the two polarizations slip by one wavelength, $L_{B} = λ / B$ : about 3 mm at 1550 nm for PANDA and bow-tie fibers. A shorter beat length means stronger birefringence and more resistance to coupling between the axes.

Can PM fiber be spliced to standard single-mode fiber?

Yes, with an ordinary fusion splice, since the standard fiber has no axes to align. The light leaves the PM section on its axis and then evolves freely in the standard fiber.

References: J. Noda, K. Okamoto and Y. Sasaki, "Polarization-maintaining fibers and their applications," Journal of Lightwave Technology 4, 1071 (1986). K. Okamoto, Fundamentals of Optical Waveguides, 2nd ed. (Academic Press, 2006).