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.
A novel interferometer based on sampling the maxima and minima of intensity of an optical standing wave has been developed. The photoelectric detection of the standing wave is performed by using a partially transparent thin-film photodiode. The automatic bidirectional fringe counting is provided by a partially transparent and phase-sensitive detector which is realized by the integration of two stacked transparent photodiodes along the optical axis of the standing wave. To obtain the ideal sine and cosine signals, the transparent phase-sensitive detector has to be optimized by adjusting the thickness of the single layers. Some features of optimization will be presented and explained. Length measurements have been demonstrated by displacing the plane mirror and bidirectional fringe counting within the standing wave.
A novel interferometer concept will be presented which is based on an optical standing wave. This standing wave is scanned by a novel, partially transparent photodetector, which is designed as nip-photodiode and contacted with transparent conductive oxide (TCO). Two transparent photodiodes are integrated to a transparent phase-sensitive sensor. The photodiodes are longitudinally arranged on the optical axis of the standing wave and generate a sine and a cosine signal for the up- and down-counting of the intensity maxima and minima of the standing wave. The layer thickness of the transparent photodiodes has been designed so as to take appropriate coating into account. These measures are demonstrated by a number of experimental results. An incorrect phase relation between the photodiodes will be corrected using the Heydemann algorithm. The non-linearity of the interferometer at a length of <λ/2 will be discussed.