International comparisons of He-Ne lasers stabilized with I-127(2) at lambda approximate to 633 nm (July 1997) Part VIII: Comparison of NIM (China), NRLM (Japan), KRISS (Republic of Korea) and BIPM lasers at lambda approximate to 633 nm

This paper reports the eighth set of results of a series of grouped laser c omparisons from national laboratories undertaken by the Bureau Internationa l des Poids et Mesures (BIPM) at the request of the Comite Consultatif pour la Definition du Metre (CCDM, now the Consultative Committee for Length, C CL) during the periods July 1993 to September 1995 and March 1997 to July 1 997. This comparison, like the previous seven, is expected to be listed as a key comparison in the context of the ongoing BIPM.L-K10 series. The results of this comparison involving five lasers, four from three count ries of the Asia-Pacific group and one from the BIPM, meet the goals set by the CCDM in 1992 and adopted by the International Committee of Weights and Measures (CIPM) the same year. The standard uncertainty (1 sigma) of the f requency of the He-Ne laser stabilized on the saturated absorption of I-127 (2) at lambda approximate to 633 nm is reduced to the level of 12 kHz (2.5 x 10(-11)) when the lasers compared meet the recommended values of the para meters. The lasers were first compared with the BIPMP3 laser, with all the lasers s et to the parameter values normally used in each laboratory; the results th en ranged from -112.7 kHz to +24.9 kHz. After checking and correcting the v alues of all the parameters and replacing a contaminated iodine cell, the r ange was reduced to -31.5 kHz to +14.3 kHz. Under the latter conditions, th e average frequency difference of the group of lasers, with respect to the BIPM4 laser, was -1.9 kHz with a standard uncertainty (1 sigma) of 20.2 kHz . The best frequency stabilities, with relative Allan standard deviations o f about 6.6 x 10(-12), 6.0 x 10(-13) and 1.3 x 10(-13), were observed with sampling times of 1 s, 100 s and 1000 s, respectively. The overall best val ue was 9.5 x 10(-14) for a sampling time of 6000 s.