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 \B(m)-B(n)\ a self-consistent set of propagation constants beta(m) 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 beta(m) to modal fields yields the core ellipticity and internal stress of the fiber.