Performance of an analog phase follower

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Abstract
We describe the performance of an analog phase follower used together with an active pendulum with interferometric sensing. The pendulum length is increased electronically by moving the suspension point horizontally by means of a piezoelectric transducer. This is driven by a feedback signal proportional to the relative displacement of the pendulum mass with respect to the suspension point. This displacement creates a phase shift Ψ in the optical path of the interferometer which in turn gives an output proportional to sin Ψ and not to Ψ, as this is required as the feedback signal. The device described here performs in an analog way the transformation sin Ψ ■ Ψ. At 0.5 Hz the dynamic range is ±500 rad corresponding to ±25-μm elongation of the pendulum mass.

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