The beat-length of polarization-maintaining few-mode-fiber measurement based on polarized interference

Polarization-maintaining few-mode fiber (PM-FMF) is a new kind of fiber that is envisioned to have applications in optical communications and sensors. But characterization methods for PM-FMF have not been established. In this paper, the method of beat-length measurement based on polarized interference is extended to the PM-FMF and the beat-length calculation formula of PM-FMF is deduced for the first time. Mode coupling, which exists in PM-FMF is not present in single-mode polarization-maintaining fiber (PMF), can be effectively differentiated by Fourier transform of the modulated spectrum. The measurement system is quite simple and easy to be implemented. The beat-length of each mode can be tested.