Measuring pulsed picometer-displacement vibrations by optical interferometry

Abstract

Optical interferometers are very sensitive if they operate at the point halfway between the peak and minimum of detected intensity. By introducing a 90° phase shift between orthogonal polarizations in the reference beam, the interferometer signals due to each polarization are in quadrature; squaring and summing the signals effectively stabilizes the operating point of the interferometer at the high-sensitivity point, independent of the large low-frequency variations in optical-path difference caused by ambient disturbances. This method has been used to make stable (±0.5 dB) measurements of 2.5-MHz ultrasound pulses of 1-pm displacement.