Self-mixing type of phase-locked laser diode interferometer

A new type of phase-locked laser diode interferometer which uses self-mixing interference in the optical system is described. The self-mixing interference signal is obtained easily by the optical feedback or feeding back laser beam from the object to the laser diode. The interference signal can be detected by a photodiode which is in the package of the LD. Therefore, a beam splitter, reference mirror and external photodetector are not required, and a kind of interference signal is obtained by a simple optical system. But the signal is not the same as that which is obtained in a conventional interferometer. We used the phase-locked technique to measure the phase change in the self-mixing interference signal. This technique enables us to detect the displacement of an object. We describe our principle, experimental setup, and experimental results.