Using an iodine-stabilized He-Ne laser as a transfer oscillator, we compare absolute measurements of the optical frequency from a traditional frequency synthesis chain based on harmonic generation and from the frequency division technique of an ultrawide bandwidth femtosecond frequency comb. The agreement between these two measurements, both linked to the Cs standard, is 220+/-770 Hz, yielding a measurement accuracy of 1.6x10(-12). We report 473 612 353 604.8+/-1.2 kHz as a preliminary updated value of the absolute frequency of the " f" component for the He-Ne laser international standard at 633 nm.

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.

Coincidences between, the sodium D1 and Dz lines and two corresponding ' ' ' I 2lines have permitted the determination of the absolute wavelengths of the hyperfine components of the sodium D lines by using a cw tunable dye laser. The laser frequency, locked in turn on the "0" and "n" hyperfine components of the P 114 (14-1) and P 38 (15-2) iodine lines, was measured at B.I.P.M. on the Michelson interferometer. The values of the wavelengths of these iodine lines are respectively: 589756661.7fmand 589158022,Ofm.

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.

After the adoption of the new mise en pratique of the de® nition of the metre in 1992, a wide-ranging program me of laser comparisons at l 633 nm, the most used of the recommended wavelengths, was carried out involving national laboratories and the Bureau International des Poids et Mesures (BIPM ). During the period July 1993 to Septem ber 1995, six grouped and ® ve bilatera l comparisons were made involving thirty-® ve national laboratories. This report will comprise six different parts. The ® rst part, this paper, describes the general conditions for such comparisons and the results of the unpublished bilateral comparisons, and in the ® ve following parts, the detailed results of the grouped comparisons will be presented and discussed. Except for a few lasers which exhibited some malfunction, the frequency differenc es between the lasers being compared lay inside the standard uncertainty (2,5 parts in 10 11 ) given in the mise en pratique, once the lasers had been adjusted to have the recom mended parameter values.