low birefringence fibers are optical fibers that can propagate optical radiation, preserving the state of polarization for long distances. Ideally this means that the fiber must have perfect geometry and be completely symmetrical along the optical axis. It must also be homogeneous along the axis. Any linear polarization can be represented by two linear polarizing modes orthogonal to each other. An ideal low birefringence fiber will propagate these two modes with identical velocity. In a real fiber there are a number of imperfections such as ellipticity, eccentricity, microbending, bending etc. Such imperfections lead to a difference in velocity between the two polarization modes of the fiber and hence a phase difference (retardance) between them.
standard telecomm type fibers typically have a large retardance that accumulates with length. The lb series fibers have a very low retardance that is not a linear function of length.