Digital phase demodulation is being developed as a flexible heterodyne
demodulation technique for on-line dynamic displacement detection. Th
e digitized modulated carrier and reference carrier signals from a het
erodyne interferometer are demodulated for surface displacements using
computer algorithms. Since the signal demodulation is performed in th
e computer, the heterodyne beat frequency or center frequency of the c
arrier signals can be easily optimized for the desired detection cente
r frequency and the heterodyne beat frequency does not have to be stab
le. The resulting measured displacements are calibrated with respect t
o the wavelength of light used in the interferometer and remain accura
te for displacements larger than the wavelength of light. No assumptio
ns are made about the sideband frequency content or the amplitude of t
he surface displacements. The research presented here introduces digit
al phase demodulation concepts and shows initial experimental results
from a heterodyne interferometer while monitoring vibrational and ultr
asonic displacements. The initial results show that the heterodyne sen
sor system is capable of measuring displacements greater than three wa
velengths of light and has a single-shot resolution better than 7 nm.
Thus digital phase demodulation will allow a single instrument to moni
tor displacements produced by low-amplitude, high-frequency ultrasound
and by high-amplitude, low-frequency vibrations.