Experimental revealing of acousto-optic polarization-governed LP-mode switching in optical fibers

We experimentally study a recently predicted vectorial acousto-optic effect in circular few-mode optical fibers in which a linearly polarized fundamental flexural acoustic wave is externally excited. We reveal that when a condition of acousto-optic resonance is met at some specific fiber length, incident LP0y and LP1y modes, in which the polarization direction is perpendicular to the acoustic one, experience a total transformation to the modes LP1y and LP0y, respectively, with azimuthal numbers upshifted or downshifted by unity. Simultaneously, orthogonally polarized modes LP0x and LP1xemerge from the fiber in their initial states. This finding strongly validates a revised theoretical approach to an acousto-optic coupling in optical fibers proposed in Opt. Lett.44, 598 (2019)10.1364/OL.44.000598. We also show that the demonstrated mode swap mimics the logic of a controlled-NOT gate for the LP_ℓ modes.