Frequently Asked Questions
Polarization Maintaining Fibers (PM Fibers or PMFs) exhibit birefringence throughout the propagation length of the fiber. As a result of this birefringence only two states of polarization (modes) are sustained within the fiber, each of which maintains their own phase velocities.
Birefringence is defined as a difference in the refractive index along different directions or axes within the same material. Light traveling through birefringent materials will witness different amounts of retardation, depending on its direction of propagation or its orientation. This causes the polarization of light to vary during propagation. Birefringent materials are also called anisotropic.
In order to evaluate the performance of polarization-maintaining fibers, manufacturers rely on a parameter known as the beat length. The beat length is defined as the distance over which the phase difference is equal to 2π. Here, the phase difference is defined as the phase shift between two linearly polarized waves traveling in different directions in a birefringent material. The beat length depends on the wavelength as well as the refractive index difference between the fast and slow axes of the material. Shorter beat lengths indicate a better polarization-maintaining performance.
Polarization-maintaining fibers are useful in many applications where the polarization of light must not be changed, for example in in fiber lasers and sensors. They also find applications in laser coupling, optical switching, and optical amplification.
Most polarization-maintaining fibers can only support one single mode making them single-mode fibers. However, some designs can operate in the few-modes regime, but never in the multimode regime.
A Panda fiber is a type of polarization-maintaining fibers. It is a single-mode fiber where the core is sandwiched between two stress rods. These stress-inducing tubes are made of different material, so that when it expands due to changes in temperature, it induces stresses in the core of the fiber and gives rise to a uniform birefringence. This ensures that the polarization is conserved as the light propagates down the fiber.
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