International comparisons of He-Ne lasers stabilized with 127I2 at λ ≈ 633 nm -: Part VII:: Comparison of NORAMET 127I2-stabilized He-Ne lasers at λ ≈ 633 nm

This paper reports the seventh 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 (CCDM, now the Consultative Committee for Length, CCL), during the periods July 1993 to September 1995 and March to July 1997. The results of this comparison, involving six lasers from three countries of North America and from the BIPM, 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 (1 sigma) 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 x 10(-11)) when the lasers compared meet the recommended values of the parameters. The lasers were first compared with the BIPMP3 laser, with all the lasers set to the parameter values normally used in each laboratory; the results then ranged from -10.1 kHz to 19.0 kHz. After checking and correcting the values of all the parameters, the range was reduced to -10.6 kHz to +2.6 kHz. Under the latter conditions, the average frequency difference of the group of lasers, with respect to the BIPM4 laser, was +0.3 kHz with a standard uncertainty (1 sigma) of 5.3 kHz. The best relative frequency stabilities, with Allan standard deviations of about 6.2 x 10(-12), 5.1 x 10(-13) and 1.4 x 10(-13), were observed with sampling times of 1 s, 100 s and 1000 s, respectively. The overall best value was 9.1 x 10(-14) for a sampling time of 10 000 s.