Quadrature interferometers have been used for more than 20 years for detection of small surface motions. The performance of these interferometers was initially limited by the quality of electronic components that were available (limited bandwidth & dynamic). With the recent improvement of electronic components, quadrature interferometers have now reached maturity, exhibiting high-bandwidth and high sensitivity. The principle is the following: the probe laser beam is divided into 2 beams: Probe beam and Local Oscillator beam. The probe beam is incident on the sample. The light scattered by the surface of the sample is the signal beam collected by and whose phase contains the surface displacement information d(t). The signal beam and the local oscillator beam are mixed using a beam splitter. The detector produces a time-varying analog voltage that is proportional to the instantaneous surface displacement at ultrasonic frequencies. This conventional homodyne detection set-up allows to collect one or few speckle on the photodetector surface.

Using a photodetector array coupled with an innovative "random quadrature" demodulation scheme allows QUARTET to increase the effective collection efficiency (increase of sensitivity & reduction of signal dropout caused by speckles) without reducing the depth-of-field. Our proprietary multi-detection system (Patent US 7,978,341 B2, US 7,864,338 B2, US 7,729,881 B2 and US 20090027688 A1) allows QUARTET to achieve high sensitivity. QUARTET is the equivalent of multiple quadrature interferometers used in parallell.

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