Heterodyne laser Doppler vibrometer with squeezed light enhancement

An important feature of a heterodyne laser Doppler vibrometer (LDV) is the possibility of measuring an optical path length oscillation at a frequency f at a choosable frequency f_het ± f, at which the photo-electric measurement shows an optical quantum noise that is significantly greater than the detector dark noise. The full-squeezed light enhancement of a heterodyne LDV's signal-to-noise ratio has not been achieved so far. Here we use a sideband spectrum that is squeezed around f_het = 40 MHz and demonstrate the squeezing-enhanced measurement of an optical path length vibration at f = 1 MHz of about 3.5 dB while fully maintaining the signal power. The proof of principle we provide will enable the realization of ultra-precise LDVs over an extended signal bandwidth for probes or environments that require low intensities.