Over the last few years, a heterodyne or a fringe-scanning interferometry12) has been widely used in optical testing because of its fast and accurate phase-measurement. One can perform direct phase measurement by obtaining three or more intensity readings of fringe patterns when the phase difference between the two interfering beams is varied in some known manner either changing in discrete steps or charging linearly in respect to time. As a frequency shifter, a piezoelectric shifter, rotating gratings etc. have been employed. In contrast to using such mechanical moving Darts, frequency modulation of a laser diode by changing the injection current is very simple). We propose a heterodyne interferometer with a frequency-modulated laser diode source, which is based on a integrated-bucket method with continuous wavelength change by linear variation of injection current. The method is described and some experimental results are shown.
"Heterodyne Interferometry With A Frequency Modulation Of A Laser Diode", Proc. SPIE 0813, Optics and the Information Age, (1 January 1987); doi: 10.1117/12.967139; https://doi.org/10.1117/12.967139