Under the right environmental conditions, optical interferometers can measure macroscopic distances to nanometer accuracies. As a result, they are used for precision applications from semiconductor-wafer manufacturing to LIGO (the Laser Interferometer Gravitational-Wave Observatory; Hanford, WA, and Livingston, LA). But standard distance-measuring interferometers (DMIs) have a disadvantage: they can only measure relative--not absolute--distances. This limitation means that to measure any distance at all, an interferometer mirror must be moved along the entire distance while fringes are counted. One way around this procedure is to exploit the output of a femtosecond pulsed laser, which consists of many monochromatic modes phase-locked in the optical-frequency domain; for example, an approach based on synthetic-wavelength interferometry (SWI) was capable of measuring absolute distances in a special tunnel of up to 240 m at a 50 (mu)m resolution. But is there a way to achieve nanometer-scale resolution while simultaneously measuring long (meter- or kilometer-scale) absolute distances?
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