A novel method for the measurement of small displacements of a light beam and for optical ranging

A sensitive method for measuring small displacements of a light beam is described which does not require the use of light probes or multielement detectors. Instead, the light beam whose displacement is to be measured, as well as a comparison light beam, is passed alternately through a rotating light chopper and then both beams are made to overlap on the surface of a photodetector. If the intensity of the two beams is made equal and if the phase (relative to the light chopping sequence) is properly adjusted, the photodetector output will be constant. Any beam displacement perpendicular to the edge of the rotating light chopper will disrupt the established phase relationship and produce a signal component in quadrature (i.e., 90 degrees out of phase) with that which would be produced if only the intensity of one beam were changed. Theory shows the relationship between the amplitude of the quadrature component and the beam displacement. Measurements as small as 5 x 10(-6) cm have been made and another more sensitive configuration is suggested. By reinterpreting the transverse spatial dimensions as distances parallel to the light beam, theory shows that this technique should result in a very sensitive optical ranging device.