Mode Beating Analysis by Sample Stretching and Wavelength Sweeping in a Few-Mode Fiber

We analyze the beating of coherently copropagating modes in a few-mode optical fiber using two complementary techniques, which involve respectively stretching the sample under examination and sweeping the wavelength for different polarizations of the incoming light. We show that analyzing the spectral composition of the intensity fluctuations allows for a direct measurement of the difference between the mode propagation constants and between their group delays, and we observe the lifting of the mode degeneracy, most possibly induced by residual anisotropies and birefringence. The outlined methodology can be used for the modal characterization of few-mode fibers employed in sensing and in spatially-multiplexed communications.