Glossary entry

Polarization mode dispersion (PMD)

Pulse broadening arising from random birefringence variations along a fiber, which produce a polarization-dependent group delay. A secondary but cumulatively significant limit on long-haul transmission.

In a perfectly cylindrically symmetric fiber, the two orthogonal polarization modes are degenerate. Real fibers have small residual birefringence (from elliptical core, internal stress, bending, twisting) that produces a slight group-velocity difference between the two principal axes — the differential group delay (DGD) $\Delta\tau$ .

The two principal axes vary along the fiber length due to varying birefringence orientation. Light initially in one principal axis couples randomly into the other along the propagation path. The end-to-end DGD therefore follows a Maxwellian statistical distribution rather than a deterministic value.

For a fiber of length $L$ , the mean DGD scales as

\langle\Delta\tau\rangle \;\propto\; \sqrt{L}.

This $\sqrt{L}$ scaling distinguishes PMD from chromatic dispersion (which scales as $L$ ). The proportionality constant defines the PMD coefficient with units of ps/ $\sqrt{\text{km}}$ .

Typical PMD coefficients:

Fiber generation	PMD coefficient
Pre-1990 deployed fiber	1 – 3 ps/ $\sqrt{\text{km}}$
1990s standard SMF	0.1 – 0.5 ps/ $\sqrt{\text{km}}$
Modern G.652 (post-2000)	$<$ 0.1 ps/ $\sqrt{\text{km}}$
Best modern SMF	$<$ 0.04 ps/ $\sqrt{\text{km}}$

For an 80 km link of modern fiber at 0.1 ps/ $\sqrt{\text{km}}$ : $\langle\Delta\tau\rangle \approx 0.9$ ps. At 10 Gb/s, this is 0.9% of the bit period — manageable. For 100 Gb/s coherent transmission, this is 9% of the symbol period and must be compensated digitally.

PMD compensation. Older 10 Gb/s links used adaptive optical PMD compensators that tracked the slowly-varying state of polarization. Modern coherent transceivers track and compensate PMD digitally in the receiver DSP, which simultaneously handles chromatic dispersion. This is one reason coherent systems superseded direct-detection systems for high-rate transmission.

PMD is unique among fiber impairments in being non-stationary — it varies on timescales of minutes to hours due to temperature, mechanical perturbations, and aging. Static compensation cannot fully address it; only adaptive (analog or digital) compensation works.

PMD is minimized by:

Improved manufacturing process control (modern fibers achieve $<$ 0.04 ps/ $\sqrt{\text{km}}$ )
Spinning the fiber preform during drawing — randomly orienting the birefringence axes along the length
Operating below dispersion thresholds where PMD-induced impairment is negligible