Polarization-maintaining fibers – key technology of the future?

Polarization-maintaining fibers – key technology of the future?

In modern communication systems, there are a number of applications that rely on the use of reliably linearly polarized light. This light cannot always be generated directly at the point of use and must therefore be transported there. However, commercially available single-mode glass fibers are not suitable for ensuring the preservation of linear polarization during transmission.

Polarization-maintaining fibers ensure stable light propagation in communications technology

When linearly polarized light is coupled into a glass fiber typically used in communications technology, the polarization changes uncontrollably and wavelength-dependently during propagation. This occurs, for example, due to mechanical stress-induced birefringence. In order to obtain light with a defined and reproducible polarization direction at the end of the transmission, a special type of optical fiber, the polarization-maintaining fiber, must be used. These fibers are characterized by the fact that, due to their special design, they maintain the polarization of linearly polarized light over technically relevant distances. To this end, the light to be transmitted is coupled along one of two directions defined by the fiber, its so-called “fast axis” or “slow axis.”

What is a PANDA fiber?

A subtype of polarization-maintaining fibers are the so-called PANDA fibers. These are single-mode fibers in which two round stress elements made of boron oxide-doped glass are incorporated into the cladding material on opposite sides of the core. Due to a different thermal expansion coefficient, these exert mechanical stress on the fibers, thus creating pronounced birefringence. The resulting birefringence causes the formation of two main axes, the “fast axis” and the “slow axis,” along which linearly polarized light can be transmitted while maintaining its polarization state. 

Advantages of PANDA fiber

• High polarization maintenance: The special structure maintains the polarization of light over long distances.
• Low attenuation: PANDA fiber has low attenuation, which results in efficient signal transmission.
• High temperature stability: Thanks to the materials used, the fiber is stable even at high temperatures.
• Compact design: PANDA fiber enables the compact design of optical components.

Assembly of PANDA fibers:

In order to utilize the positive properties of polarization-maintaining fibers, it must be ensured that linearly polarized light is coupled along one of the two main axes (usually the “slow axis”). For this purpose, the connectors at the fiber ends must be assembled in such a way that the alignment of the fiber is ensured within a small tolerance. As a result, during transmission within the connector, most of the linearly polarized light is coupled into one of the two axes and only a small portion into the second axis.

Rosenberger OSI has developed processes for assembling one or more polarization-maintaining fibers in PANDA design into various connectors, FC-APC, MTP^®, and EBO, with the required rotational alignment.

An important quality feature of an optical fiber cable with polarization-maintaining fibers is the polarization extinction ratio (PER), which indicates the ratio of the intensities of the desired to the undesired polarization direction. For most applications, it should be greater than 20 dB.