DIGITAL PHASE DEMODULATION IN HETERODYNE SENSORS

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.