Heterodyne interferometer scheme using a double pass in an acousto-optic modulator

We introduce a heterodyne interferometer scheme utilizing the double-pass arrangement of the deflected beams of an acousto-optic modulator. To prove our new idea, the interferometer has been applied for measuring small amplitude vibrations. The output intermediate frequency signal from the interferometer is processed by using an I/Q demodulator. In theory, it can provide the quantum noise limited sensitivity, whereas, in our present Letter, the sensitivity of the phase measurement is limited by the resolution of the 16?bit A/D converter used for processing the I/Q signals. Details of our interferometer scheme are discussed.