Polarisation-maintaining fibres – key technology of the future?

In modern communication systems, there are a number of applications that rely on the use of reliably linearly polarised 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 fibres are not suitable for ensuring the preservation of linear polarisation during transmission.

Polarisation-maintaining fibres ensure stable light propagation in communications technology

When linearly polarised light is coupled into a glass fibre typically used in communications technology, the polarisation 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 polarisation direction at the end of the transmission, a special type of optical fibre, the polarisation-maintaining fibre, must be used. These fibres are characterised by the fact that, due to their special design, they maintain the polarisation of linearly polarised light over technically relevant distances. To this end, the light to be transmitted is coupled along one of two directions defined by the fibre, its so-called ‘fast axis’ or ‘slow axis’.

What is a Panda fibre?

A subtype of polarisation-maintaining fibres are the so-called PANDA fibres. These are single-mode fibres 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 fibres, thus generating pronounced birefringence. The resulting birefringence causes the formation of two main axes, the ‘fast axis’ and the ‘slow axis’, along which linearly polarised light can be transmitted while maintaining its polarisation state. 

Advantages of PANDA fibre

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

Assembly of Panda fibres:

In order to utilise the positive properties of polarisation-maintaining fibres, it must be ensured that linearly polarised light is coupled along one of the two main axes (usually the ‘slow axis’). For this purpose, the connectors at the fibre ends must be assembled in such a way that the alignment of the fibre is ensured within a small tolerance. As a result, during transmission within the connector, most of the linearly polarised light is coupled into one of the two axes and only a small proportion into the second axis.

Rosenberger OSI has developed processes for assembling one or more polarisation-maintaining fibres in PANDA design into various connectors, FC-APC, MTP^® and EBO, with the required rotational alignment. An important quality feature of an optical fibre cable with polarisation-maintaining fibres is the polarisation extinction ratio (PER), which indicates the ratio of the intensities of the desired to the undesired polarisation direction. For most applications, it should be greater than 20 dB.