Self-Mixing Vibrometer has picometer sensitivity by exploiting the FM Channel

Abstract

Using a Mach-Zehnder interferometer (MZ) as an edge filter, we read the frequency modulation (FM) of a laser diode (LD) self-mixing interferometer (SMI), and obtain much better performance respect to the normally used amplitude modulation (AM) signal. The MZ filter has a conversion factor of 19 (GHz)−1 and is tuned at half-fringe on the laser wavelength with a feedback loop acting on the laser bias current. On small amplitude vibrations measured with the converted-FM signal, we attain a noise-equivalent-displacement (NED) of 1.3-pm/VHz, a factor 130 better than AM channel or a normal SMI.