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In page Dye laser:
"Tunable lasers are used in swept-frequency metrology to enable measurement of absolute distances with very high accuracy. A two axis interferometer is set up and by sweeping the frequency, the frequency of the light returning from the fixed arm is slightly different from the frequency returning from the distance measuring arm. This produces a beat frequency which can be detected and used to determine the absolute difference between the lengths of the two arms.[9]
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