Mode‐beating spectroscopy in a few‐mode optical guide

A mode‐coupling modulator is scanned along an optical waveguide. At the output of the guide spatial beats are observed. From their spatial frequencies ‖βm−βn‖ a self‐consistent set of propagation constants βm of the guided modes is derived in a manner similar to that used in atomic spectroscopy. The method is illustrated by magneto‐optic modulation of the six lowest modes of an optical fiber. Assignment of the βm to modal fields yields the core ellipticity and internal stress of the fiber.