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Polarization-maintaining Fibers

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Polarization-maintaining Fibers

  • 2025-06-06

Optical fibers always exhibit some degree of birefringence, even if they have a circularly symmetric design because in practice there is always some amount of mechanical stress or other effect which breaks the symmetry. As a consequence, the polarization of light propagating in the fiber gradually changes in an uncontrolled (and wavelength-dependent) way, which also depends on any bending of the fiber and on its temperature. 

Principle of Polarization-maintaining Fibers: 

The mentioned problem can be fixed by using a polarization-maintaining fiber, which is not a fiber without birefringence, but on the contrary a specialty fiber with a strong built-in birefringence (high-birefringence fiber or HIBI fiber, PM fiber). Provided that the polarization of light launched into the fiber is aligned with one of the birefringent axes, this polarization state will be preserved even if the fiber is bent. The physical principle behind this can be understood in terms of coherent mode coupling. The propagation constants of the two polarization modes are significantly different due to the strong birefringence, so that the relative phase of such copropagating modes rapidly drifts away. Therefore, any disturbance along the fiber can effectively couple both modes only if it has a significant spatial Fourier component with a wavenumber which matches the difference of the propagation constants of the two polarization modes. If this difference is large enough, the usual disturbances in the fiber are too slowly varying to do effective mode coupling. In quantitative terms, the polarization beat length should be significantly shorter than the typical length scale on which the parasitic birefringence varies.


Polarization-maintaining fibers are applied in devices where the polarization state cannot be allowed to drift, e.g. as a result of temperature changes. Examples are fiber interferometers, fiber-optic gyroscopes and certain fiber lasers.

Disadvantage of using polarization-maintaining fibers are the following:

  • They are more expensive.
  • Not not all kinds of fibers are available in polarization-preserving form. For example, availability can be limited for certain active fibers, for fibers with unusual values of the effective mode area, and for non-silica fibers.
  • Propagation losses are higher than for standard fiber.
  • Usually, an exact rotational alignment of the fiber with respect to the input polarization direction is required; the same holds for splicing of fibers. That makes the production of devices more cumbersome and expensive.

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