Advice from the CCL on the use of unstabilized lasers as standards of wavelength: the helium–neon laser at 633 nm

The Consultative Committee for Length (CCL) has recommended that red (633 nm) unstabilized helium-neon lasers, operating on the 3s(2) -> 2p(4) transition, should be included in the list of standard frequencies for realization of the metre. This paper discusses several topics relating to the recommendation: motivations for including an unstabilized laser in the list of standard frequencies, considerations in assigning a value and an uncertainty for the frequency and vacuum wavelength of the 633 nm transition, and problems associated with the possible presence of 640 nm radiation in the laser output. The fractional uncertainty assigned to the vacuum wavelength of the 633 nm transition is 1.5 x 10(-6) (relative standard uncertainty). This uncertainty is sufficiently large to capture all sources of variability associated with construction or operating details of the laser and variations in isotopic mix that might be encountered in commercial laser tubes.