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.
Ways of Realizing Polarization-maintaining Fibers
A commonly used method for introducing strong birefringence is to include two (not necessarily cylindrical) stress rods of a modified glass composition (typically boron-doped glass, with a different degree of thermal expansion) in the preform on opposite sides of the core (Figure 1). When a fiber (called a PANDA fiber) is drawn from such a preform, the stress elements cause some mechanical stress with a well-defined orientation. With other techniques (see the article on fiber preforms), one can make bow-tie fibers, where the stress elements have a different shape and reach closer to the fiber core, so that a stronger birefringence can be achieved. Another variant of that approach is to have an elliptical cladding of different glass around the core; this leads to an elliptical-stress-layer fiber. One may also combine both methods .
Another technique, not relying on mechanical stress, is to use an elliptical core causing so-called form birefringence . Here, the elliptical form itself, even without any mechanical stress, produces some level of form birefringence.
In a photonic crystal fiber (PCF), very strong birefringence can be obtained with an asymmetric arrangement of air holes, but stress elements (which may be index-matched) can also be used . In any case, the polarization beat length can be so small (a few millimeters or even less) that additional stress effects can cause only a low level of mixing of the polarization states. The index contrast can be several times 10−3, whereas in all-glass PM fibers it is typically only a few times 10−4.
Single-mode and Few-mode Fibers
Polarization-maintaining fibers are mostly single-mode fibers, only in rare cases few-mode fibers , and apparently never highly multimode fibers. This is because it is difficult to produce sufficiently strong and uniform birefringence in the fiber glass over a sufficiently large core area where many modes can be guided.
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.
The polarization extinction ratio of light coming out of a polarization-maintaining fiber may be lower than that at the fiber input. This can occur as a result of imperfect alignment of the polarization direction at the input, but also be due to some residual degree of mode mixing. The latter effect can be strongly increased by mechanical stress (e.g. in a fiber connector). For applications requiring a very high polarization extinction ratio (e.g. in interferometry), it can be necessary to use an additional high-quality polarizer after the fiber.
Polarization-maintaining fibers should not be confused with single-polarization fibers, which can guide only light with a certain linear polarization.
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See also: polarization beat length, fibers, spun fibers, single-polarization fibers, single-mode fibers, photonic crystal fibers, birefringence, polarization of light, specialty fibers, mode coupling, fiber polarization controllers, The Photonics Spotlight 2007-05-19
and other articles in the category fiber optics and waveguides
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!