Detection of low frequency, out-of-plane vibrations by the Talbot effect and adaptive photodetectors

Detection and measurement of low frequency, out-of-plane vibrations play a very important role in several metrological applications. Classical interferometry is well suited for measuring small amplitudes of vibrations, ranging from picometers up to micrometers, but its use is limited to a laboratory environment. Here we consider the Talbot effect and the so-called adaptive photodetectors based on the non-steady-state photo-electromotive force effect for the measuring of low frequency, out-of-plane of vibrations of flat objects with rough surfaces and with amplitudes of vibrations in the order of microns. The adaptive photodetectors produce an electrical current proportional to the square of the visibility of the vibrating intensity pattern impinging on them. In the method here proposed, the vibrating object with rough surface is illuminated with a beam diffracted by a grating (a Ronchi grating), the light reflected by the object is collected by a lens and imaged on to a home-made GaAs adaptive photodetector. The electrical current from the adaptive photodetector is proportional to the instantaneous position of the vibrating object; this signal is monitored and measured with an oscilloscope connected to a lock-in output. The method is very robust and adequate for environments subjected to perturbations and presents the possibility of adjusting its dynamical range by modifying the period of the grating employed. Experimental results that verify our proposal are presented.