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Coaxial acousto-optical fiber frequency shifter

A fiber-optic frequency shifter utilizing torsional acoustic waves to produce a travelling perturbation which couples the polarization eigenmodes of a linearly birefringent optical fiber, is demonstrated. The frequency shifter configuration is such that the acoustic wave generator and optical fiber are co-axial. The device is thus more rugged than previous 'free fiber' frequency shifters which have incorporated a T-junction between acoustic generator and optical fiber. The shift in optical frequency 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. The theory of operation of this device is presented using a coupled mode approach, and the coupling coefficient for a fundamental torsional mode is derived in terms of its peak angular displacement.