Abstract

Measurements of distance using methods based on phase information have the characteristic property of an ambiguity range—a consequence of the phase periodicity of the used wave. Therefore, the resolution and operating range of such measurements are linked via the resolution with which the phase measurements are performed. Typically, phase resolutions of approximately λ/100 to λ/ 1000 can be achieved. Consequently, measuring absolute distances in the meter range with nanometer resolution requires a multi-step technique. An unknown distance is determined by subsequent measurements with decreasing ambiguity range. Each step uses the information obtained by its predecessor. Our system uses a three step method. The technique used in the coarse region is to measure the phase of the beat frequency of an amplitude-modulated laser signal after it has been reflected by the object to be located. The oscillator that drives the amplitude modulation serves as the reference. The measurement method in the middle region is a two-wavelength technique using two semi-conductor lasers with a stabilized wave-length difference of 3-6 nm.

© 1992 Optical Society of America