Heterodyne Detection System

Abstract

Abstract In a heterodyne detection system (Martin and Wickramasinghe 1987), shown schematically in Fig. 7.1, the first beam splitter divides into two components. One passes through an acousto-optic modulator that shifts the beam frequency by Om , and the other is reflected onto a mirror as a reference beam. The beam with the shifted frequency, serving as the signal beam, passes through a polarizing beam splitter, a quarter-wave plate, and finally, a microscope objective that focuses it onto the lever supporting the force-sensing tip. The lever reflects the beam back through the microscope objective and quarter-wave plate, which rotates the polarization on the two passes by 90°. The polarizing beam splitter then deflects the beam through an analyzer that adjusts the relative power of the beam incident on the photodetector. The reference beam is deflected by a second mirror, passes through the polarizing beam splitter and analyzer, and is incident on the same photodetector. The reference and signal beams interfere on the photodetector, which generates a current consisting of a spectrum of frequencies. The photocurrent is fed into a single side-band receiver driving a phase-sensitive detector that provides the signal used to display the force acting on the tip.