The Consultative Committee for Length has recommended that red (633 nm) unstabilized Helium-Neon lasers, operating on the 3s2¿_2p4 transition, should be included in the list of standard frequencies for realization of the meter. This article 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 of the transition, and potential problems associated with 640 nm contamination of the 633 nm transition. The uncertainty assigned to the wavelength of the 633 nm transition is sufficiently large that the wavelength value can be guaranteed independently of operating or construction details of the laser, and independent of the variations in isotopic mix that might be encountered in commercial laser tubes.
Pub Type: Journals
optical frequency standard, wavelength reference, wavelength standard