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The metre is one of the seven base units of the International System of Units, known as the SI. At NPL we use frequency-stabilised lasers to realise the metre.
Based on the definition of the metre, which refers to the speed of light in a vacuum, the length scale for the UK is based around the wavelength emitted by any one of a group of frequency-stabilised lasers at NPL. These lasers are operated according to international guidelines which guarantee that the frequency of the emitted light is known to a few parts in 1011, based on quantum transitions in iodine vapour. These primary reference lasers are used to calibrate lasers of lower accuracies by simple beat frequency comparison. Since the speed of light in vacuum is a fundamental constant, calibration of the laser output frequency determines the vacuum wavelength of the laser. The laser can then be used in an interferometer to provide a traceable length scale.
The metre is the SI unit of length, find out more about the definition
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