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A laser interferometer for the study of earth strain is described. Changes in the length of an 800 m arm are measured by counting fringes in the interference pattern with a least count of 4×10−10. The output is linear and has a flat response from dc to 1 MHz and a dynamic range of 106. The laser wavelength is controlled by reference to a passive optic resonator contained in a stable environment. The wavelength stability is a few parts in 1010 for periods up to a laser lifetime (∼3000 h).
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Berger et al. (1969) studied this question.
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