The wave polarized parallel to the slow axis has a shorter period, and therefore propagates over a shorter distance for a given number of oscillations, than the light polarized parallel to the fast axis.

But, the waves do not stay in phase as they propagate, since the refractive index of the slow axis is larger than that of the fast axis ( n slow > n fast ). If this is done, the orthogonally polarized waves will have the same amplitude and be in phase when they enter the fiber. One way these waves can be excited in the fiber is by coupling in monochromatic, linearly polarized light, with the input light's polarization angle oriented midway between the fiber's fast and slow axes. The concept of a PM fiber's beat length can be visualized by considering waves propagating along the fiber's two orthogonal axes, fast and slow. The beat length is the distance over which the accumulated phase difference is 2, and this is also the distance between adjacent markers. Note that this phase is generally not acquired over a whole number of periods. Between adjacent markers, the slow-axis wave accumulates an extra 2 in phase compared with the fast-axis wave. The green diamonds on all three plots mark the points at which both waves share the same phase on a 0 to 2 scale. The difference between the waves' phase values (bottom, also shown on a 0 to 2 scale) increases linearly with distance. Each wave's phase is plotted (middle) on a 0 to 2 scale, but in reality the two wave's absolute phases differ by a factor of 2. Note that distance illustrated in this figure is longer than the distance shown in Figure 1. Figure 2: The amplitudes of the waves shown in Figure 1 are both plotted with respect to propagation distance (top).

Cellprofiler measure fiber length