How does polarization-maintaining fiber preserve linearly polarized light?

There is a significant refractive index difference (birefringence) between the orthogonal "slow" and "fast" axes of a polarization-maintaining (PM) fiber, and this birefringence is the reason PM fiber is effective in preserving the polarization state of input linearly polarized light. However, the input linear polarization state can only be preserved if it is aligned parallel to one of the fiber's axes.

Because PM fibers are birefringent, there are different velocity, or more accurately propagation constant, requirements for light polarized parallel to the fiber's slow vs. fast axes. In order for light to switch to being polarized parallel to the orthogonal axis, the light would have to change its velocity (propagation constant) to meet the requirements of the orthogonal axis. This creates such a barrier that a switch is unlikely to occur unless the fiber's birefringence is reduced.

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Figure 2: Bow-tie polarization-maintaining fibers use two wedge-shaped stress rods to place the core in tension and make it birefringent. The stress is directed along the slow axis, and it results from the stress rods contracting more than the cladding as the fiber cooled after fabrication.

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Figure 1: PANDA polarization-maintaining fiber uses two cylindrical stress rods to place the core in tension, making it birefringent. The stress, which is directed along the slow axis, results from the stress rods contracting more than the cladding as the fiber cooled after fabrication.

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Figure 4: The elliptical shape of the core is sufficient to induce birefringence in elliptical-core polarization-maintaining fiber.

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Figure 3: To minimize microbends, spool fiber by winding it loosely in parallel rows (top). Microbends result from winding fiber so that it crosses over the bumpy surface created by deeper layers of the wound fiber (bottom).

A Stressful Situation
One approach to creating a PM fiber is to apply a mechanical stress to the fiber's core, since stress causes glass to become birefringent (photoelasticity). The two most common stress-birefringent fibers, PANDA and bow-tie, apply tension to the fiber's core.

In these designs, glass structures called stress rods extend down the length of the fiber, parallel to fiber's core. In cross section, the stress rods and the core of the fiber are linearly arranged, as shown in Figures 1 and 2. As the fiber cools after fabrication, the glass in the stress rods contracts more than the glass in the surrounding cladding. The pull from the contraction of the stress rods creates a line of tension (slow axis) across the core, with comparatively little stress applied in the orthogonal direction (fast axis). This creates an index difference between the two axes.

Stress Relief is not Always a Good Thing
The tension across the core in stress-birefringent PM fibers is temperature dependent, since the stress results from the glass in the stress rods and the glass in the cladding having different rates of thermal expansion (CTEs). The tension provided by the stress rods decreases as the operating temperature increases. Since this reduces the birefringence, and therefore the fiber's ability to preserve polarization, it can result in a reduced extinction ratio (ER).

The tension in the core can also be reduced by stress from handling, such as coiling the fiber in a small-diameter ring, routing it around sharp corners, and fixing it to a bumpy surface. Microbends at localized stress points scatter light into the orthogonal polarization state, which reduces ER. Winding a fiber across itself (Figure 3), or pressing a bare fiber against a surface, can cause microbending.

Attaching fiber connectors typically reduces ER, since the cured potting compound that secures the fiber can induce asymmetric stress, hardened bubbles within the compound can press into the fiber, and there can be contact between the fiber and the ferrule's bore. To increase the maximum ER the fiber can provide, manufacturers typically take steps to suppress these sources of stress, but they cannot be eliminated.

Form is Function
If the temperature-dependence of stress-birefringent fibers is unacceptable, form-birefringent fibers offer a largely temperature-insensitive alternative. These PM fibers are birefringent due to their elliptically shaped cores, rather than tension induced by stress rods (Figure 4).

Form-birefringent fibers, which include PM photonic crystal fibers, are not well-suited to every application. Their elliptical cores, attenuation, and small mode sizes are not ideal for telecommunications applications, and they find most use in fiber optic sensors.

References
[1] Chris Emslie, in Specialty Optical Fibers Handbook, edited by Alexis Mendez and T. F. Morse (Elsevier, Inc., New York, 2007) pp. 243-277.
[2] Malcolm P. Varnham et al., "Analytic Solution for the Birefringence Produced by Thermal Stress in Polarization-Maintaining Optical Fibers," J. Lightwave Technol., LT-1(2), 332-339 (1983).
[3] Zhenyang Ding et al., "Accurate Method for Measuring the Thermal Coefficient of Group Birefringence of Polarization-Maintaining Fibers," Opt. Lett., 36(11), 2173-2175 (2011).
[4] M. Shah Alam and Sarkar Rahat M. Anwar, "Modal Propagation Properties of Elliptical Core Optical Fibers Considering Stress-Optic Effects," World Academy of Science, Engineering and Technology, Open Science Index 44, International Journal of Electronics and Communication Engineering, 4(8), 1170 - 1175 (2010).

Date of Last Edit: Sept. 11, 2020