Accurate wavelength measurement on up-converted CO₂ laser radiation

Abstract

WAVELENGTH measurements, which are part of an accurate determination of the speed of light¹, have been made on the radiation from a carbon dioxide laser, stabilised to the R(12) transition at 9.3 µm by saturated fluorescence in an external CO₂ cell, which has a measured frequency² reproducible to better than one part in 10⁹. The problems of a direct intercomparison of infrared and visible wavelengths were avoided by mixing the CO₂ radiation with light from a 10-mW He–Ne laser at 0.63 µm to give a difference frequency sideband at a wavelength of 0.68 µm. This mixing process, known as up-conversion, was performed in a cooled, single crystal of proustite^3,4. The wavelength of the CO₂ radiation was then determined using the relationship λ_9.3 = λ_0.63/(1—R), in which it is assumed that c is the same for the two visible radiations. The wavelength measurements were thus of the ratio R = λ_0.63/λ_0.68 and the value of λ_0.63 with respect to the primary length standard.