Active heterodyne interferometric displacement measurement using optical feedback effects of laser diodes

A novel method is presented for precision displacement mea- surement based on active heterodyne interferometry. A pair of laser di- odes, each of which is equipped with an external cavity, are used as light sources. A small displacement of a piezoelectric transducer, which car- ries the reflecting mirror of the external cavity, causes lasing frequency shift due to optical feedback on the laser diode. Detection of the optical frequency shift is accomplished by a heterodyne arrangement. Direct counting of the heterodyne beat frequency enables precise measure- ment of a dynamic displacement without any complex numerical calcu- lation. Excellent performance of the presented method is illustrated by experimental results on the measurement for small displacement with an accuracy of 65 nm and a time resolution of 33 ms.