Absolute laser frequency measurements
DOI: 10.1063/1.3070804
If we knew a precise value for the frequency of the length standard, or, alternatively, for the wavelength of the frequency standard, we would have a good way to define the velocity of light c. But until now the hitch has been that wavelength measurements, although fairly easily done in the visible through the near infrared (to three microns or so), are difficult at longer wavelengths, whereas frequency measurements have until recently been limited to the microwave region. Now a group at the National Bureau of Standards in Boulder, Colorado, mixing their oscillators by means of a metal–metal diode originally used at the Massachusetts Institute of Technology, reports the highest “absolute” frequency measurement to date.
