TRIGGERING OF A HETERODYNE INTERFEROMETER

A method for triggering a heterodyne interferometer having two acousto-optical modulators in separate light paths, a receiver generating an analog signal and a downstream A/D converter converting the analog signal into a digital signal is described. The one acousto-optical modulator is triggered at a modulation frequency f1 and the other acousto-optical modulator is triggered at another modulation frequency f2, the difference between modulation frequencies f1 and f2 forming a heterodyne frequency fHet and the analog signal being converted into the digital signal in the A/D converter at sampling frequency fa. In such a heterodyne interferometer, a fixed ratio of modulation frequencies is maintained, and they are prevented from shifting due to aging and drift by forming at least two of the frequencies of modulation frequencies f1, f2 and sampling frequency fa from a fundamental frequency fqartz of a common oscillator. As a result, it is also possible for sampling frequency fa to be in a fixed phase ratio to the differential frequency of modulation frequencies f1, f2, of heterodyne frequency fHet. Measurement accuracy is increased because sampling is performed at a constant phase, independently of drift and aging.