Very low frequency Self-Mixing laser Diode vibrometer

We present an optical vibrometer especially designed to measure small amplitude (hundreds pm to several μm) vibrations at very low frequency, down to 0.01 Hz. The instrument is based on a Self-Mixing Interferometer of the fringe-lock type, to achieve high sensitivity and a wide dynamic range. As compared to previous design based on the same principle, our vibrometer is free from fringe-lock loss due to thermal drift of the laser wavelength; this result is obtained by exploiting a separate DC feedback loop acting on the Laser Diode bias. Experiments show a very good long-term stability, glitch-free operation, with rms error of 100-pm/sqrt(Hz) over a bandwidth from less than 20 Hz to 20 kHz.