EXFO Optics instruments with PMF configuration

Electromagnetic energy consists of two orthogonally polarized components representing both electric and magnetic fields, referred to as Transverse Electric (TE) and Transverse Magnetic (TM) modes, being perpendicular to the direction of travel.

Natural imperfections in optical fibres themselves or external influences such as mechanical stress will cause changes in the State of Polarization (SOP) and differences in the refractive index profile affecting these modes; one mode will therefore on average travel faster along what is referred to as the 'fast axis' whilst the other will propagate more slowly along the 'slow axis'. This difference in mode propagation speeds is measured by Differential Group Delay (DGD) and leads to Polarization Mode Dispersion (PMD).

Polarization-Maintaining Fiber (PMF) is carefully constructed optical fiber using special cladding profiles to achieve a minimal DGD and maintain the Principle State of Polarization (PSP) of the propagating electromagnetic energy, maintaining the fast and slow axes as in the image above and being referred to as 'weak mode coupling'.

The TE mode is typically aligned with the slow axis and the TM mode with the fast axis, since the TE mode being aligned with the slow axis maintains the best ratio of mode powers, referred to as the Polarization Extinction Ratio (PER) and minimizes polarization drift / the SOP rate of change.

EXFO Optics instruments with the PM/PMF option therefore align the slow axis / TE mode (vertically) with the alignment key of their FC optical connector.