Chapter 20 - Acousto-Optic Interaction in Few-Mode Optical Fibers

Abstract

This chapter presents a brief introduction to the optical and acoustic properties of few-mode fibers and describes acousto-optic interaction in such fibers relevant for practical devices. Optical fibers are characterized as single-mode or multimode. Multimode in general means a large number of modes, typically several hundred. The chapter discusses that two-mode fibers are primarily used for sensing purposes, using the phase change between the two spatial modes induced by a measurand such as strain, pressure, or temperature. The sensitivity to temperature and strain is comparable with polarimetric sensors, but the pressure sensitivity is poor. In addition to sensing purposes, few-mode fibers can be incorporated into all-fiber systems to manipulate the propagating beam without having to leave the fiber environment. Thus, examples of frequency shifting, optical filtering, and add/drop multiplexing functions have been reported. For such purposes, coupling between the LP01 mode and the LP11 mode or between the LP01 polarization modes has been used. In addition, the LP02 mode has been used for dispersion compensation and for sensing purposes. However, acousto-optic coupling into the LP02 mode is considered difficult and inefficient. A number of acousto-optic two-mode fiber components have been proposed and demonstrated, including frequency shifters, tunable filters, and fast scanning heterodyne interferometers.