A fiber-optic frequency shifter utilizing a traveling torsional acoustic wave to couple the polarization eigen-modes of a highly linearly birefringent optical fiber is demonstrated. The frequency-shifter configuration is such that the acoustic-wave generator and optical fiber are coaxial. The device is thus more rugged than previous free-fiber frequency shifters, which have incorporated a T junction between the acoustic generator and optical fiber. The optical frequency shift was 3.204 MHz, and an optical coupling efficiency of 12% was obtained with 4 W of electrical drive power applied to the acoustic transducer.
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Berwick et al. (1992) studied this question.
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