Measurement of the speed of light I. Introduction and frequency measurement of a carbon dioxide laser

Abstract

This, and part II following, describe a determination of the speed of light made by measuring the frequency and wavelength of radiation from a CO 2 laser. This laser was operated on the 9.3 μm R(12) transition, and stabilized by reference to fluorescence in an external CO 2 absorption cell. The laser frequency was measured via a sequence of harmonic-mixing stages involving the HCN and H 2 O lasers as transfer oscillators, and was found to be 32 176 079 482 ± 14 kHz ( ± 4.2 parts in 10 10 ). A correction to this value of — 10 kHz is necessary to obtain the centre frequency of the CO 2 reference transition. The wavelength measurements and value of c are described in part II.

Some Basics of Measurement

“…Such a current realisation of the metre can have an uncertainty in frequency of one part in 10 11 . Figure 2.4 shows an iodine-stabilised heliumÀneon laser held at NPL.…”

“…With a so-called femtosecond comb, the frequency standard from an atomic clock can be directly coupled to optical frequencies such as those emitted by stabilised lasers, removing the requirement for the laborious frequency chain comparisons from the 1970s, which Blaney [11,12] used to provide values for input into Eq. (2.1).…”