Acousto-optic coupling in optical fibers

The status of acousto-optic coupling in optical fibers as used for manipulation of the optical power is reviewed. Some resulting effects of this process are frequency shifting, filtering, switching and dynamic equalization. The paper will start with basic considerations on the propagation of various acoustic modes in cylindrical structures. This includes torsional, longitudinal as well as flexural modes. Also the propagation characteristics and other properties of optical modes in optical fibers are reviewed. Next the coupling between optical modes due to acoustic modes propagating along the same fiber is considered. Coupling equations and mode considerations are basic elements in this presentation. Special fiber properties related to the coupling will be considered. To achieve efficient coupling requires high acoustic intensities. For this purpose acoustic horns are widely used. Typically the acoustic energy is generated by a transducer at the thick end of the horn which is a multi-mode region while the other end being connected to the fiber is a single-mode region. General properties of such horns are analyzed and a few designs are discussed. Finally, several practical device and system applications will be reported.