Self-Mixing Displacement Measured by a Two-Color Laser in 66-nm Steps

We demonstrate a new self-mixing interferometer (SMI) simultaneously working on two wavelengths, the fundamental at 1064 nm and the second harmonic at 532 nm delivered in the same beam. The beam is sent to the remote target in the usual way, and in the return path, two photodiodes detect the two self-mixing signals without crosstalk. As the source, we were able to use a diode-pumped neodymium-doped yttrium vanadate (Nd:YVO4 laser crystal) with potassium titanyl phosphate (KTP nonlinear crystal) two-color laser of a commercial pointer. With simple processing of the two self-mixing signals, we are able to obtain the interferometric signals $\cos (2ks)$ and $\sin (2ks)$ at 532 nm, from which the unambiguous measurement of arbitrary target displacement follows with counts in units of $\lambda /8$ of the second-harmonic wavelength or 66 nm.