PM Fiber
Polarization Maintaining (PM) fiber is a special type of optical fiber designed to maintain the polarization state of light. While typical optical fibers cause the polarization state of light to change randomly during propagation, PM fiber can transmit light over long distances while preserving a specific polarization state. Many optical systems are sensitive to the polarization state of light, which is why PM fiber is necessary. For example, interferometers, polarization filters, and laser systems require light with a specific polarization state. In communication systems, changes in the polarization state can degrade signal quality; however, PM fiber ensures stable signal transmission. It is also used in optical sensors and optical measurement systems where precise control of the polarization state is essential.
Polarization Maintaining (PM) fiber utilizes birefringence to preserve the polarization state of light. Birefringence is the property of a material where the refractive index varies with the polarization of light. In standard optical fibers, the circularly symmetric core allows two orthogonal polarization modes to propagate at the same speed. As a result, the polarization state fluctuates randomly as light reflects and refracts within the fiber. However, PM fibers introduce artificial birefringence, causing the two orthogonal polarization modes to propagate at different speeds. This prevents the modes from interfering with each other, allowing them to maintain their respective polarization states.
Birefringence in PM fibers can be induced by altering the fiber geometry, such as introducing elliptical or asymmetric core shapes. Additionally, stress-induced birefringence can be achieved by applying mechanical stress in specific directions. Furthermore, incorporating anisotropic dopants into the fiber core can also induce birefringence.
Birefringence in PM fibers is often induced by incorporating stress rods. These micro-structured glass rods, embedded near the fiber core, apply mechanical stress to the core region, resulting in a controlled birefringence. Stress rods offer a robust and stable birefringence, enabling clear mode splitting and high polarization purity. The characteristics of the PM fiber, such as the beat length and polarization mode dispersion, can be tailored by adjusting the stress rods' size, position, and material.
A common type of PM fiber employing stress rods is the panda-type PM fiber. Its distinctive cross-sectional profile, resembling a panda's face, features an asymmetric core and two stress rods. This configuration is crucial for inducing sufficient birefringence and ensuring stable polarization mode propagation.
(The cross-section of an actual panda PM fiber, https://www.fujikura.co.jp/eng/products/optical/opticalfibers/04/2050104_12895.html)
As shown in the figure, a panda-type polarization-maintaining (PM) fiber exhibits two distinct polarization modes due to its asymmetrical core and cladding design. Stress rods apply an unbalanced force to the core, increasing the phase difference between the Transverse Electric (TE) and Transverse Magnetic (TM) modes. This effectively separates the two modes, enhancing polarization maintenance. In a panda-type PM fiber, the fast axis and slow axis are defined based on the propagation speed of each mode. The fast axis is the direction in which the TE mode propagates and experiences tensile stress from the stress rods, resulting in a relatively higher propagation speed. Light propagating along the fast axis travels a shorter optical path, generally resulting in faster performance. The slow axis is the direction in which the TM mode propagates and experiences compressive stress from the stress rods, resulting in a relatively lower propagation speed. Light propagating along the slow axis travels a longer optical path, resulting in relatively slower performance.
The various types of optical fibers discussed above are protected by two or three layers of thin, flexible polymer coatings. Including the thickness of the polymer protective coating (jacket), the cross-sectional diameter of the optical fiber is approximately 250 um. Multiple optical fibers are bundled together to form ribbon-type structures for multi-channel applications.
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