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Demonstration of birefringent optical fibre frequency shifter employing torsional acoustic waves

A fibre-optic frequency shifter utilising torsional acoustic waves to produce a travelling perturbation which couples the polarisation eigenmodes of a linearly birefringent optical fibre, is demonstrated. The shift in optical frequency was 3.195MHz and an optical coupling efficiency of 6% was obtained with 780mW of electrical power. 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.