An auto-focusing approach for a dynamic quadrature interferometer

摘要

The measurement of surface vibrations with small amplitudes is often accomplished using interferometric techniques. For a good local resolution of the vibration amplitudes, the laser beam has to be focused on the surface through a high magnification lens. Owing to the small depth of focus of this lens, its distance to the surface has to be controlled accurately and precisely. In order to avoid the nonlinear components present in heterodyne interferometers (such as acousto-optic modulators) and to simultaneously eliminate the effects of noise sources such as fluctuations in the surface reflectivity of the device under test, we use a homodyne setup with additional path length modulation. The sensitivity of the setup is increased by quadrature interferometry, i. e., the synchronous measurement of the intensities detected by two photodiodes. We also present a signal processing technique for the control of the distance between lens and surface; this makes sure that the laser beam is optimally focused onto the surface and leads to a high overall signal-to-noise ratio.