I came here to post about LDVs but was beaten to it. https://en.m.wikipedia.org/wiki/Laser_Doppler_vibrometer https://en.m.wikipedia.org/wiki/Laser_Doppler_vib

I came here to post about LDVs but was beaten to it. https://en.m.wikipedia.org/wiki/Laser_Doppler_vibrometer https://en.m.wikipedia.org/wiki/Laser_Doppler_vibrometer They're sensitive to very small vibrations. A friend of mine used them while working at a hard drive manufacturer to better understand head and platter vibrations.