International comparison of stabilized He-Ne lasers by the saturated absorption of I-127(2) at lambda approximate to 633 nm involving the CEM (Spain), the IPQ (Portugal) and the BIPM

The results of an international comparison of iodine-stabilized He-Ne lasers at lambda approximate to 633 nm, made by beat-frequency techniques, between the CEM (Spain), the IPQ (Portugal) and the BIPM are presented. Four lasers took part in the ten-day period of measurements, and were compared once or twice a day. The frequency differences between the lasers were all less than 6 kHz and the highest standard uncertainty of the mean of the frequency differences between two lasers was 7 kHz. These results lie inside the standard uncertainty given in the mise en pratique of the definition of the metre. Parameters which affect the frequency of the lasers, such as modulation width, iodine pressure and laser power, were also investigated.