The optical frequency comb generator (OFCG) is an attractive optical reference source for
various applications such as optical frequency metrology, precision spectroscopy and
telecommunications [D.J. Jones, S.A. Diddams, J.K. Ranka, A. Stentz, R.S. Windeler, S.T.
Cundiff, Science 288, 635–639 (2000); T. Udem, R. Holzwarth, T.W. Hänsch,
Nature 416, 233–237 (2002); T.W. Hänsch, J. Alnis, P. Fendel, M. Fischer, C.
Gohle, M. Herrmann, R. Holzwarth, N. Kolachevsky, Th. Udem, M. Zimmermann, Philos. Trans.
R. Soc. A 363, 2155–2163 (2005)]. In particular, the OFCG can be used as
source for absolute frequency measurement, providing a precise ruler for length metrology.
In the present work we describe the results of absolute frequency measurements of primary
wavelength standards at 633 nm on the sixth components, d, e, f, g, h and i of the R(127)
11-5 hyperfine transition of the 127I2 molecule, at the Spanish
Centre of Metrology, CEM. The values obtained with a femtosecond frequency comb (FC1500,
Menlo Systems) at CEM are compared with the values recommended by the Consultative
Committee for Length (CCL) [T.J. Quinn, Metrologia 40, 103–133 (2003)]. This
determination was made by beat frequency method between a femtosecond laser comb and an
iodine-stabilized He-Ne laser. The difference between the mean frequency of the sixth
components of the standard laser and those of CCL recommended values for the same
components was found to be 6.557 kHz.