Scanning differential-heterodyne-interferometer with acousto-optic deflectors

A heterodyne interferometer principle is applied for a noncontact optical profilometer that scans the probe by means of variation of the driver frequency of an acousto-optic deflector. The reference and measuring beams are focused on the same surface under test to reduce environmental influences. The phase of the beat frequency of the interfering return beams is a measure of the height difference between reference and measuring beam. A height sensitivity of 5 nm was achieved with a breadboard set-up and a 550 mu m line-scan length. Measurements on different samples and their features are discussed,