Acoustic modes propagating in cylindrical solid rods are considered. A review of the properties of several mode groups is given, and computed results are shown for a wide range of material properties. The lower mode of each group is relevant for an all-fiber-optic frequency shifter recently demonstrated. For these modes dispersion relations, mode patterns, power relations, and acoustooptic coupling strength are calculated. Some experimental results are compared with the calculations, and alterations to improve the frequency shifter are proposed.
An all-fiber-optic frequency shifter is demonstrated that uses mode coupling between the LP 01 and LP 11 modes by a traveling acoustic flexural wave guided along the optical fiber. The input and output leads of this frequency shifter are single-mode fibers. Unity mode-conversion efficiency for cw operation is achieved at 8-MHz frequency shift with about 0.25 W of electrical input power. Carrier and image sideband suppression of 15 and 35 dB, respectively, are demonstrated.Single-sideband optical frequency shifters in waveguide form have been sought for many years for use in fiber-optic systems such as gyroscopes, heterodyned sensors, signal processing and coherent communications. By replacing its bulk-optic counterpart (Bragg cell) with a guided-wave frequency shifter, one can make such fiber-optic systems mechanically rugged and compact, and critical optical alignment procedures can be avoided. A number of different approaches have been demonstrated that use either birefringent optical fibers 1 ' 2 or integrated-optical waveguides. 3 -5 These devices make use of periodic optical coupling between two orthogonal polarization modes that have different propagation velocities in a singlemode waveguide using acousto-optic or electro-optic effects or take the form of a serrodyne frequency shifter in which a linear phase ramp simulates frequency shifting. Integrated-optical devices were demonstrated with broad bandwidth and better than 30-dB sideband suppression, but reduction of relatively high insertion loss of the devices and further suppression of unwanted sidebands are yet to be seen. Fiber-optic frequency shifters with similar sideband suppression have also been demonstrated that use a traveling acoustic wave and high-birefringence fiber, but they require a large acoustic energy to achieve good optical coupling efficiency.In this Letter, we report an all-fiber-optic singlesideband frequency shifter utilizing periodic optical coupling between two spatial modes in circular optical fibers, instead of two polarization modes, with the coupling provided by a traveling acoustic wave guided by the optical fiber. In contrast to frequency shifters using high-birefringence fiber, the present device does not require any critical angular alignments of fiber axes or other components. Also, since the optical fiber actually guides the acoustic wave, thy acoustic energy can be more efficiently employed without compromising the carrier and sideband suppression ratios.With a weakly guiding approximation, the two lowest-order modes of an optical fiber are the LPo 1 and LP 11 modes with propagation constants of Ioi and f1, (
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