The extended surface forces apparatus. Part III. High-speed interferometric distance measurement

Abstract

In this communication, we describe a photoelectric setup used to monitor the wavelength of a spectral line at sampling rates exceeding 1 kHz. A split photodiode was chosen as the position-sensitive element. Its operation, calibration, and performance are discussed in terms of the interferometric distance measurement in the surface forces apparatus. We demonstrate how the normalized difference signal from the photodiode can be transformed into surface separation and that the extension of the experimental window allows for the acquisition of up to ten force-versus-distance curves per second. Provided that the detector is sufficiently small, we find a linear relationship between the surface separation and the normalized photodetector signal. We illustrate the experimental setup in detail, including the necessary signal-evaluation circuitry. The main difficulties associated with this high-speed approach are low light levels and noise. Optimizing the spectral intensity, keeping the detector area small, designing the evaluation circuits carefully, as well as normalizing the photodetector signal are some of the measures that were found to improve overall performance. Calibration of the device against an absolute distance measurement using a charge coupled device camera as well as its statistical and systematic errors are discussed. The curved nature of the interference fringes and associated implications on the distance determination are considered.