A demodulator for a two-wavelength interferometric transducer

An interferometer has been constructed for the purpose of measuring minute biological displacements, such as the ocular pulse and vibrations of the eardrum. This instrument consists of essentially two interferometers within one optical path, each with a laser diode illumination source of different wavelength. A description is given of the means of separating the interferometric fringe pattern into the effects of each wavelength, a process that involves both optical and electronic techniques. The implementation is novel and provides a way to make multiple fringe measurements with resolutions down to a small fraction of a fringe. This is important in measuring the ocular pulse, approximately 30 μm in the peak-to-peak displacement, because the fine features of this measurement contain valuable diagnostic information. Another advantage is the fact that optical interferometry enables the instrument to be noncontacting