Fibers for polarized light
If polarized light is launched into a conventional single mode optical fiber the state of polarization will be rapidly lost after a few meters. To maintain the state of polarization a polarization preserving optical fiber, or polarization maintaining optical fiber should be used. The most common type of polarization maintaining fiber is the high birefringence type. In this type of optical fiber an asymmetric stress is applied around the core of the fiber that gives slightly different refractive indices to two orthogonal axes. This fast and slow axis will maintain the polarization state launched into the fiber over long distances. This type of optical fiber is used for many sensor designs for example in optical fiber gyroscopes.When using this type of fiber it is necessary to align the axes of the fiber with the polarized light. If this is not possible or is inconvenient then it is possible to use low birefringence fiber to transmit polarized light. In this case the fiber is made with a much higher degree of symmetry than standard single mode fiber. The fiber must have perfect geometry and be completely symmetrical along the optic axis. In some sensor designs the fiber itself is used as the sensing element and in the case of a Faraday sensor a conductor passing through a coil of fiber will cause a rotation in the polarized light in the coil. An ideal fiber will give a rotation proportional to the current and be insensitive to temperature variations.
