An optical low-coherence interferometry technique has been used to simultaneously resolve the mode profile and to measure the intermodal dispersion of guided modes of a few-mode fiber. Measurements are performed using short samples of fiber (about 50 cm). There is no need for a complex mode-conversion technique to reach a high interference visibility. Four LP mode groups of the few-mode fiber are resolved. Experimental results and numerical simulations show that the ellipticity of the fiber core leads to a distinct splitting of the degenerate high-order modes in group index. For the first time, to the best of our knowledge, it has been demonstrated that degenerate LP11 modes are much more sensitive to core shape variations than the fundamental modes and that intermodal dispersion of high-order degenerate modes can be used for characterizing the anisotropy of an optical waveguide.
Ma, Y. Z., Sych, Y., Onishchukov, G., Ramachandran, S., Peschel, U., Schmauss, B., & Leuchs, G. (2009). Fiber-modes and fiber-anisotropy characterization using low-coherence interferometry. Applied Physics B, 96(2-3), 345-353. https://doi.org/10.1007/s00340-009-3517-9