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The accuracy of a heterodyne interferometer is often limited by the periodic nonlinearity that arises mainly from imperfect separation of the two optical frequencies. This paper describes a new method for compensation of the nonlinearity in the heterodyne laser interferometer using the quadrature mixing technique for the phase measurement. The compensation technique is based on the elliptical fitting of two phase-quadrature signals obtained by a lock-in amplifier. The brief analysis and compensation scheme of the nonlinearity in the heterodyne interferometer, and the experimental result using a Zeeman stabilized He-Ne laser, have been presented. The results show that the suggested method can compensate for the nonlinearity of the heterodyne interferometer with sub-nanometre accuracy.

Dietary intake of vitamin C was associated with better hearing in the older population. Because less than one-half of elderly participants in this study consumed a sufficient amount of vitamins, and vitamin intake decreased with age, we should consider proper diet counseling to prevent hearing decline.