This paper reports the fifth 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 Comité Consultatif pour la Définition du Mètre (CCDM, now the Consultative Committee for Length, CCL), during the period July 1993 to September 1995.

The results of this comparison, involving six lasers from five countries and the BIPM, meet the goals set by the CCDM in 1992 and adopted by the Comité International des Poids et Mesures (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 or the CSIRO1 lasers, with all the lasers set to the parameter values normally used in each laboratory; the results ranged from -48.3 kHz to 13.8 kHz. After checking and readjusting the values of all the parameters, the range was reduced to -14.7 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 -0.5 kHz with a standard uncertainty (1 σ) of 8.5 kHz. The best relative frequency stabilities, with Allan standard deviations of about 3.5 parts in 10¹² and 3.5 parts in 10¹³, were observed with sampling times of 8 s and 2048 s, respectively.