Transverse Faraday effect in multimode fibers

The weak guidance approximation suppresses the longitudinal field component E_z, and with it an entire class of linear magneto-optic interactions. We derive and experimentally verify a transverse Faraday effect in step-index multimode fibers that couples E_y into E_z. Using speckle decorrelation, we isolate the linear Faraday signal from the quadratic Cotton-Mouton background, confirming four predictions: the effect vanishes without a strong index step, scales with launch angle, is completely polarization-selective relative to the field, and tracks Verdet dispersion. Under 0.24 T, we observe Δρ≈0.16 for perpendicular input and a complete null for parallel input.