For polarization-maintaining (PM) optical fiber, assuming that the polarization direction of the input linearly polarized light is in the middle of the fast axis and the slow axis, it can be decomposed into two orthogonal polarization components. As shown in the figure below, the two light waves initially have the same phase, but because the refractive index of the slow axis is greater than that of the fast axis, their phase difference will increase linearly with the propagation distance.
In the figure above, the phases of the two waves and their difference are in the range of 0 to 2π (minus the part of multiples of 2π). The green square indicates the position where the phase difference is 2π times, and the beat length is the length of the phase difference increased by 2π, which is equal to the ratio of the vacuum wavelength (λ) and the birefringence (B):
In this orthogonal coupling situation, the polarization in the PM fiber will change periodically with the beat length: linear polarization, elliptical polarization, circular polarization, elliptical polarization, linear polarization... and the beat length can be measured through this polarization evolution process. This is because for the periodically changing polarization state, the scattered light overflowing from the side of the fiber is like being spatially modulated, and the beat length can be measured in the periodic structure of the light and dark areas.
The greater the birefringence of the fiber, the shorter the beat length and the better the polarization-maintaining performance. Currently, PM fibers with beat lengths less than 1 mm have elliptical cores and very small mode field diameters (significantly smaller than standard single-mode fibers). Many applications require circular cores and larger mode field diameters, and PM fibers that meet the requirements usually have beat lengths between one and several millimeters. Standard single-mode fiber also has a measurable beat length, but the value can reach several meters, which is not suitable for use as PM fiber. To illustrate the polarization-maintaining performance of PM fiber, manufacturers usually provide beat length specifications instead of output extinction ratio, and specify wavelength and temperature range. This is because many factors affect the extinction ratio, including fiber length, wiring method and input polarization direction. However, beat length is not related to the above factors, so it is more suitable for quantifying the polarization-maintaining ability of the fiber, but the actual performance and output extinction ratio ultimately depend on the details of use.