An extremely stable laser developed at the Joint Institute for Laboratory Astrophysics, Boulder, Colorado, is providing a sensitive way to measure earth strain. Judah Levine and John L. Hall have built an interferometric strain gauge whose key element is a methane‐stabilized 3.39‐micron helium–neon laser at the Poorman's Relief Mine near Boulder. The stabilized He–Ne laser is beat against a second He–Ne laser locked to a 30‐meter Fabry–Perot interferometer and fluctuations in the beat frequency are a direct measure of changes in the 30‐meter path length.

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