Torsional ultrasound modes can couple the optical polarization states in a birefringent fiber. Polarization coupling produced by interaction with a higher-order torsional mode slightly above its cutoff may provide a route to producing an in-fiber isolator suitable for use in narrow-band high-power fiber amplifiers. This paper describes a model of a transducer generating torsional modes in a cylindrical fiber. This model predicts that almost all of the power applied to the transducer is radiated into the desired mode. The paper also discusses effects produced by acoustic absorption and the dependence of the acoustic velocity on temperature. 1. Introduction By matching the optical polarization beat length in a birefringent silica optical fiber to the axial wavelength of a traveling torsional wave of angular frequency propagating in the fiber, it is possible to couple light from one polarization to the other [1, 2]. When light is coupled from the slow polarization to the fast polarization, it is down-shifted in frequency by if the light and sound are copropagating, but up-shifted if the light and sound are counter-propagating. This nonreciprocal frequency shift can be used to make an in-fiber acousto-optic (AO) isolator. This requires placing two torsional transducers on the fiber separated by , where is the group velocity of light in the fast polarization. These transducers propagate sound in the same direction. One watt of acoustic power from each transducer is sufficient to couple 50% of the light from one polarization to the other over a few centimeters [2]. The polymer jacket is removed from the fiber in the interaction regions to prevent acoustic damping. The relative acoustic phase of the two transducers is adjusted such that in the forward direction all of the light at the wavelength of interest returns to the slow polarization, whereas in the backward direction all of the light at this wavelength goes into the fast polarization. Polarizers at both ends, such as 45-degree tilted fiber Bragg gratings [3], remove light in the fast polarization to make an isolator. To date an isolator based on torsional acoustic waves has not been made, but 100% polarization switching has been demonstrated using the lowest-order ( ) torsional mode [1]. However, an analogous isolator was demonstrated using the lowest-order flexural mode at ？MHz [4]. In this case coupling was between the two transverse modes of a two-mode fiber. Although a 22？dB extinction ratio was reported, the large D？=？11？m spacing between the transducers makes this approach unattractive for
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