Probleme d'asservissement pose en interferometrie a modulation de phase par l'instabilite du modulateur

By means of phase modulation interferometry, it is possible to measure phase retardations with a very small uncertainty. But a much greater error can be introduced because of the drift of the modulating system and this must be corrected. Direct detection of the cause of the drift requires a better sensitivity than that of the measurement and thus, to achieve correction, it is necessary to detect the effect of the drift on the phase retardation by means of a similar process to that of the measurement. A simple solution of this problem, applicable to birefringent or polarizing interference modulators, is described.