The interaction between acoustic waves and optical waves has been widely studied and has given rise to numerous applications in either bulk or waveguide media. In an acousto-optic interaction, an acoustic wave creates an index grating via the elasto-optic effect and causes coupling between optical modes. This interaction can also be viewed as a photon-phonon scattering process in which the total energy and linear momentum of the particles are conserved. Angular momentum (AM) is another physical quantity that has to be conserved in this interaction. To our knowledge, this issue has not been discussed in photon-phonon interactions because it is generally understood that phonons carry no intrinsic spin angular momentum (SAM). However, phonons can carry orbital angular momentum (OAM) by fomring vortices in the medium they propagate. Besides being the most widely studied optical waveguide structure, optical fiber is one of such media that are amenable to the propagation of acoustic vortices because of to its geometrical construction. In this talk, we show that an acousto-optic interaction in optical fiber can lead to a transfer of OAM from an acoustic vortex to an optical vortex and that spin and orbital angular momentum are conserved independently in this interaction. This renders the acousto-optic interaction a useful means to directly generate pure and stable optical vortices in the fiber medium starting from its core (fundamental) mode. Investigation of this interaction is interesting not only from a scientific viewpoint, but also has potential technological implications in optical trapping and quantum communication.
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