Heterodyne interferometer with a phase modulated source

Most practical interferometers used for precision sensing are based on a concept of heterodyne interferometer due to its high-precision readout afforded by the heterodyne signal processing. However, a typical optical heterodyne interferometer is limited in its resolution to a few nanometers by a parasitic leakage of optical signals called "polarization mixing," or more generally "self-interference", resulting in measurement non-linearity. The non-linearity is proportional to the ratio of the self-interference-induced heterodyne beat to the desired range-related interferometer signal, ordinarily on the order of several percent. A number of laboratory techniques have been proposed in the past to suppress the self-interference. All of these techniques are very difficult to implement with a precision required and their implementation often introduces its own noise into the interferometer. For example, cyclic averaging by reference position dithering or ramping of the frequency both degrade the inherent stability of the interferometer by destabilizing the reference arm or the carrier frequency, respectively. The present invention relates to a heterodyne interferometer system with a pre-processing of the target signal to isolate and remove self-interference signals using a known phase modulation of the carrier signal's frequency. Where self-interference signals do not include a time delay inherent in the target signal that travels to the target reflector, by selecting a modulation frequency tuned to the time delay and then filtering the resultant signal the target beam can be isolated and the self-interference signal can be effectively removed The system includes a modulation unit to apply a phase modulation to the carrier signal, and a mixing unit that demodulates the target signal at the modulation frequency to isolate the target beam.