Wideband phase demodulation based on Fourier-transform techniques in optical heterodyne interferometry

A method for computer-aided wideband phase-demodulation has been developed for taking out any time-varying phase in optical heterodyne interferometry. The generated beat-photocurrent is memorized in a digital transient memory so that its phase can be demodulated with the aid of a computer. The Fourier transform of the photocurrent obtained gives the photocurrent spectra from which only the beat-photocurrent spectra needed can be extracted. The Fourier inversion of the extracted spectra gives the significant parameters that can determine the time-varying phase of interest. This method makes it possible to measure any mode of vibration, not limited to a sinusoidal vibration, over 10 MHz in bandwidth. The frequency response of a vibrating mirror can be examined with a random Gaussian signal as one possible application