Interferometric technique with a reference scale.

A double interferometer technique is presented. One interferometer generates a reference scale, while the other one is used as a measuring tool. The presented apparatus makes it possible to overcome 2 ambiguity by monitoring the phase difference at short time or space intervals. The phase unwrapping procedure is very simple due to a homodyne reference scale time interval method. The accuracy of the method is of 1 nm. The drift of the laser power and unequal gain of the photodetectors do not affect the system performance. The precision of the technique can be improved significantly by using a high-resolution time interval analyzer. The application of the method for determining the profile of a curved surface is demonstrated.