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Laser interferometers are important instruments for the measurement of length in today's mechanical engineering and manufacturing technology. The principle on which interferometers have operated to date is that of interference between beams with the same direction of propagation. However, optical beams can interfere with each other not only in the same direction of propagation but also in opposing directions. The name given to this type of interference is the standing wave. A beam of light strikes a plane mirror at 90° to it, is reflected and interferes with the beam currently being reflected at the mirror. The outcome of the interference is a standing wave in front of the plane mirror. The only way of detecting the maxima and minima of the intensity of a standing wave photoelectrically is to use a photoelectric detector which is partially transparent. The photoelectric detector is placed in path of the standing wave, which propagates through it. Phase-shifted signals can be received if two photoelectric detectors with a phase shift between them are positioned in the standing wave. These enable sin and cos signals to be registered so that bi-directional fringe counting can take place. The authors have named this assembly an optical Standing-Wave Interferometer. The form taken by the partially transparent photoelectric detectors is that of photodiodes based on amorphous silicon, in a TCO-pin-TCO structure. Phase-shifted signals are received by two components with a TCO1-(pin)1-TCO2-(pin)2-TCO3 composition, integrated at the engineering stage, which are called by the authors transparent phase selective photodiodes (TPS). The TPS have been used to carry out measurement of length in a technological setting in such a way that the standing-wave interferometer could be compared with a plane mirror interferometer.
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