Sinusoidal phase modulating interferometer using the integrating-bucket method

We describe a sinusoidal phase modulating interferometer in which a CCD image sensor detects four values by integrating the time-varying intensity in an interference pattern for intervals of one-quarter period of the phase modulation. The optimum amplitude and phase of the sinusoidal phase modulation are determined. The measurement error caused by the additive noise and the deviation from the optimum phase modulation is analyzed. The experimental results for surface profiles of magnetic sliders show that the sinusoidal phase modulating interferometer proposed here yields a measurement accuracy of the order of 1 nm.