Beat length measurement study of few-mode polarization-maintaining fiber based on low coherence off-axis digital holography

Abstract A method based on low coherence off-axis digital holography is proposed to measure the beat length of few-mode polarization-maintaining fiber (PMF). The real-time holograms are recorded based on cross-correlation in time domain. The energy of polarization modes is extracted by frequency-domain filtering. The mode energy fluctuation curve is constructed from a series of holograms. The mode field distribution and time distribution of polarization modes are reconstructed from the holograms recorded at the peaks of mode energy fluctuation curve. The beat length is calculated by analyzing the delay between orthogonal polarization modes. The low coherence off-axis digital holographic experimental system is established and the beat length of single-mode PMF is achieved. The beat length is obtained at multiple wavelengths. The result shows that beat length is proportional to wavelength. We measure the beat length of few-mode PMF. The beat length of high order mode and fundamental mode are obtained at the same time. Our method has some advantages including simple experimental setup and high precision.