Polarization Maintaining Fiber with Beat Length by Thorlabs

The beat length of a polarization-maintaining (PM) fiber can be understood by considering waves traveling along its two perpendicular axes, fast and slow. By inputting monochromatic, linearly polarized light with a polarization angle midway between these axes, the waves start with the same amplitude and phase. However, they don't maintain this phase due to the refractive index difference (n slow > n fast). The wave aligned with the slow axis has a shorter period, traveling less distance for a given number of oscillations. The phase difference between the waves increases linearly with distance (Figure 1.2). Points where the phase difference is a factor of 2 are marked green. The beat length is the distance over which this phase difference reaches 2. Notably, this phase difference isn't typically achieved over a whole number of oscillations by either wave.

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