This paper reports the ninth 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 Consultative Committee for Length (CCL), formerly the Consultative Committee for the Definition of the Metre (CCDM), for the periods July 1993 to September 1995 and March 1997 to March 2001. As with the previous eight comparisons, this one 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 six lasers from five European countries and the BIPM, meet the goals set by the CCDM in 1992 and in 1997 and adopted by the International Committee for Weights and Measures (CIPM) the same year. The standard uncertainty (1 σ) of the frequency of the He-Ne laser stabilized on the saturated absorption of ¹²⁷I₂ at λ = 633 nm is reduced to a level of 12 kHz (2.5 parts in 10¹¹) 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 -97.6 kHz to +30.4 kHz. After checking and correcting when possible the values of all the parameters, the range was reduced to -16.4 kHz to +10.4 kHz. Under the latter conditions, the average frequency difference of the group of lasers, with respect to the BIPM4 laser, was 5.6 kHz with a standard uncertainty (1 σ) of 9.9 kHz. The best relative frequency stabilities, with Allan standard deviations of about 9.7 × 10^-12, 6.0 × 10^-13 and 2.0 × 10^-13, were observed with sampling times of 1 s, 100 s and 1000 s, respectively. The overall best values were 1.0 × 10^-13 and 9.0 × 10^-14 for sampling times of 3000 s and 12800 s, respectively.