Distance meter utilizing the intermode beat of a He-Ne laser.
A distance meter was built utilizing the intermode beat of a He-Ne laser instead of an optical modulater. Its performance was tested at distances between 10 and 1300 m on a 300-m baseline in a tunnel. The resolution was about 20 microm and the total uncertainty was within 0.1 mm, if the air was stable. The length of the tunnel was measured by a distance meter, and the annual periodic strain of the tunnel was observed with submillimeter precision.
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