International comparisons of He-Ne lasers stabilized with 127I2 at λ≈633 nm (July 1993 to September 1995) Part IV:: Comparison of Western European lasers at λ≈633 nm

This paper reports the fourth set of results of a series of grouped laser comparisons from national laboratories undertaken by the Bureau International des Poids et Mesures (BIPM) at the request of the Comite Consultatif pour la Definition du Metre (CCM; now the Consultative Committee for Length, CCL) during the period July 1993 to September 1995. The results of this comparison, involving eleven lasers from eight countries and the BIPM, again meet the goals set by the CCDM in 1992 and adopted by the Comite International des Poids et Mesures (CIPM) the same year. The standard uncertainty of the frequency of the He-Ne laser stabilized on the saturated absorption of I-127(2) at lambda approximate to 633 nm is reduced to a level of 12 kHz (2.5 parts in 10(11)) when the lasers compared meet the recommended values of the parameters. The lasers were first compared with the BIPM4 laser with the parameters set to the values normally used in each laboratory; using the BIPM4 laser as a reference the results ranged from -15.4 kHz to 36.8 kHz. After checking and readjusting the values of all the parameters, the range was reduced to -8.6 kHz to 14.0 kHz, Under the latter conditions, the average frequency difference of the group of lasers, with respect to the BIPM4 laser, was 2.7 kHz with a standard uncertainty (1 sigma) of 8.1 kHz. The best relative frequency stabilities, with Allan standard deviations of about 5.5 parts in 10(12) and 4.6 parts in 10(13), were observed with sampling times of 1 s and 100 s, respectively.