The Sixth International Comparison of Absolute Gravimeters was held from 5 June to 28 August 2001 at the Bureau International des Poids et Mesures (BIPM), Sèvres. Seventeen absolute gravimeters were used to make measurements at five sites of the BIPM gravity network. The vertical gravity gradients at the sites and the ties between them were also measured using seventeen relative gravimeters. For the first time the ties were also measured using absolute gravimeters. Various methods of processing the absolute and relative data were tested to calculate the results. The final results of ICAG-2001 are presented. The acceleration due to gravity at a height of 0.90 m is given as (980 925 701.2 ± 5.5) µGal* and (980 928 018.8 ± 5.5) µGal for sites A and B, respectively,

Results of an international comparison of iodine absorption cells organized by the BIPM are reported. Ten of the thirteen member laboratories of the Comité Consultatif pour la Définition du Mètre (CCDM) participated. Two experimental methods were used to estimate the purity of the iodine in the cells: first, the cells were incorporated in BIPM He-Ne lasers and the laser frequencies thus obtained at λ = 633 nm were compared by the beat frequency method with those of the BIPM reference lasers and, second, direct measurements of iodine purity by a laser-induced fluorescence method were made at λ = 502 nm. Of the thirty-nine cells investigated, more than half are pure enough to realize a frequency reproducibility to within 10 kHz (2 parts in 1011 in relative terms). A contamination of 129I2 has been observed in certain cells containing 127I2. The comparison confirms that observed frequency shifts of the hyperfine components in iodine are strongly linked to impurities in the iodine cells.

An international comparison of 12712 He-Ne stabilized lasers, used as length standards, has been made. Five laboratories from Sweden, Denmark (two laboratories), Finland and Germany were represented. In addition, two lasers from the Bureau International des Poids et Mesures took part in order to relate the results to earlier comparisons. The reproducibility of the frequency mean of the four components d, e, f and g in the transition R(127)ll-5 in iodine 127 for the different lasers was found to be 15 kHz, taken as the standard deviation of the distribution of frequency among the group of ten lasers. Using laser BIPM4 as reference the frequency average of the group of lasers was +3,6 kHz, making this a good reference laser for the current ensemble. Measurements of frequency dependence on modulation amplitude, iodine pressure and, for some lasers, intracavity power are reported. The relative frequency partition of the four components d, e, f and g of each laser was also determined and shown to exhibit characteristics which are new for this type of stabilized laser.

The fifth in the series of International Comparisons of Absolute Gravimeters (ICAG) was held at the Bureau International des Poids et Measures (BIPM) in November 1997. Fifteen absolute gravimeters participated in the comparison. The mean gravity value obtained at station A (0.9 m) at the BIPM was found to be 980 925 707.8 µGal with a standard uncertainty of 2.8 µGal. This is consistent with the results obtained in previous comparisons at this site. Conclusions based on the analysis of the present results and proposals for future activities are presented.

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The results of an international comparison of iodine-stabilized He-Ne lasers at 633 nm, made by beat-frequency techniques, between the NRLM (Japan) and the BIPM are presented. During two weeks the frequency of the two lasers NRLM1 and BIPM4 were compared twice a day. The mean frequency difference was determined to be Δf = [0.9 ± 2.6] kHz, corresponding to a relative frequency difference of [2 ± 5] · 10-12. Parameters which affected the frequency of the lasers such as modulation width, iodine pressure and laser power were also investigated. As the laser BIPM4 was compared with the laser BIPM2 at the BIPM before and after the travel to the NRLM, we can link our results to the previous international comparisons in which the BIPM has participated.

An international comparison of eight I -stabilized semiconductor laser systems (DLs) has been carried out. Five of the DLs were extended-cavity lasers (ECLs) using extra-cavity saturation spectroscopy; another was a microlens-mounted diode modified to have weak optical feedback, stabilized using the same technique; the seventh ECL was stabilized using frequency-modulated spectroscopy. The final DL was a simple laser diode at 635 nm locked with a digital system on a linear absorption of iodine. The P(33) 6-3 transition of iodine was first used to compare the first seven DLs with a He-Ne laser stabilized on the R(127) 11-5 transition of iodine. The relative frequency stability of these lasers was between 5 parts in and 7 parts in for a sampling time of 1 s, with the best results less than 2 parts in over 1000 s. The frequency repeatability measured during one week was of the order of a few tens of kilohertz. This large fluctuation was caused by poor adjustment of the electronic offset of two of the lasers. For the well-corrected lasers, the repeatability was within a few kilohertz. A study of stabilization on the strong absorption group of transitions R(60) 8-4, R(125) 9-4 and P(54) 8-4, located about -12 GHz from the R(127) 11-5 transition, was also carried out. For the first time, a short-term frequency stability better than that of the classical He-Ne laser around 633 nm has been achieved with a relative frequency stability of 4 parts in for 1 s.

This second Northern European comparison between seven lasers exhibits closer results than the ® rst comparison. A standard uncertainty of 5 kHz was obtained against 15 kHz in 1990, while the frequency average of the group of lasers was about the same with 3,9 kHz, using the BIPM4 laser as reference. A large frequency difference on a laser observed during the ® rst com parison was again observed; partial explanations of the origin of this differenc e were found. This differenc e has been excluded from the calculation of the average. Typical frequency stabiliti es using Allan standard deviations of about 1,6 10 ±11 and 1,7 10 ±12 were observed on 1 s and 100 s sampling tim es, respectively. * are the characteristics of the M 1 m irror located on th e iodin e cell sid e of the lasers. ** are th e characteristics of th e M 2 m irror located on the gain tube sid e of the lasers. * T he m odulatio n w idth is alw ays give n in M H z peak-to -peak. ** E xterna l powe r of the laser.