Heterodyne interferometer

Claim 1

A heterodyne interferometer for measuring the amplitude of vibration of a vibrating object which comprises: (a) a laser for emitting a monochromatic light wave; (b) a beam splitter for splitting the light wave into an object light wave and a reference light wave, each having the same frequency; (c) a modulator for changing the frequency of the object light wave and the reference light wave so as to produce a predetermined offset between the frequency of the object light wave and the frequency of the reference light wave, said modulator being so positioned with respect to the beam splitter that the object light wave and the reference light wave pass through the modulator thereby changing their frequencies; (d) means for directing the object light wave onto the vibrating object, said means so positioned between the object and the modulator that at least a portion of the object light wave is directed onto and reflected off of the object; (e) a photodetector for producing, at a frequency equal to the predetermined offset, a beat signal varying in accordance with variations in the interference resulting from combining at least a portion of the reflected object light wave and the reference light wave wherein the power of the reference light wave is greater than the power of the portion of the reflected object light wave, the photodetector being so positioned with respect to the object and the beam splitter that it detects and measures the interference; and (f) means for processing the beat signal to measure variations in its phase or amplitude, said means being electrically connected to the photodetector in order to receive the beat signal.