Measurement of the speed of light II. Wavelength measurements and conclusion

Abstract

Wavelength measurements have been carried out on the radiation at 9.3 pm from a stabilized CO 2 laser. The wavelength in vacuum was determined as 9 317 246 348 femtometres (fm), with a standard error of the mean of 11 fm (1.14 parts in 10 9 ) and a total systematic uncertainty of ± 13 fm ( ± 1.4 parts in 10 9 ). An upconversion technique of optical mixing in a non-linear crystal was used, so that the interferometric wavelength measurement was carried out on visible difference-frequency radiation at 679 nm. Its wavelength was determined relative to that of an iodine-stabilized He-Ne laser at 633 nm, by using plane-parallel Fabry-Perot interferometers. When combined with the result of the frequency measurement described in part I, this wavelength measurement leads to a value for the speed of light in vacuum of 299792459.0 ± 0.6 m/s (Blaney et al. 1974). This result is in agreement (within the quoted uncertainties) with the value recommended internationally by the 14th General Conference of Weights and Measures in 1975.

Some Basics of Measurement

“…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).…”

Laser wavelength measurements

“…Usually several interference maxima from each source are recorded simultaneously (by chopping at different frequencies) as the path difference changes slowly. Air pressure scanning, either at atmospheric pressures with a piston [Rowley and Wallard, 1973;Blaney et al, 1977] or from vacuum with a controlled leak Hanes et al, 1973;Ito and Tanaka, 1978;$chweitzer et al, 1973], is the most usual way of achieving a smooth linear scan. Problems of nonlinearity arise from the linear rate of volume (rather than pressure) change with piston scanning and from temperature changes induced by the pressure changes.…”