Method for Analogue Measurement of Interference Fringe Displacement

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This paper describes a method for analog measurement of interference fringe displacement and evaluation of its systematic error.The interference fringe of a two-beam interferometer is vibrated in a small amplitude by frequency modulation of a laser, which is applied either as a light source of the interferometer or by an oscillating glass plate inserted in one of the arms of the interferometer. The fringe is detected by a photoelectric system containing a reversible counter, a count-voltage converter and a low-pass filter. The reversible counter and the count-voltage converter convert the momentarily changing position of the vibrating fringe into voltage. The voltage is filtered to remove the modulation component and the conversion distortion. Then the output voltage is analogous to the fringe displacement.The systematic error of this measuring method is calculated numerically on some wave form models of the modulation signal. The amount of the error depends on the wave form of the modulation and its amplitude. For a triangle wave, a condition is found under which measurement is carried out without any amount of error. But in general, this measurement does include an error, the magnitude of which is less than λ/30.
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