The BIPM optical comb facilities have been used for absolute frequency calibrations in the key comparison BIPM.L-K11, in the campaigns of November 2005 and May 2006. This extends the series of absolute frequency measurements to include some 37 lasers. In addition, the participating lasers have been compared to the BIPM reference laser BIW 167 for a determination of their coefficients of sensitivity for parameters such as power, pressure and modulation effects.
In preparation for the discontinuation of the activities of the Length Section, a consultation was undertaken among laboratories with an interest in pursuing BIPM.L-K11, and on the engagement of NMIs in piloting or hosting this comparison. Based on the responses to this consultation, the staff of the Length Section will re-organize this key comparison and provide initial support to the future pilot laboratories.
The Length Section provided calibration and measurement services for some BIPM users and for some specific external needs. In particular, for the frequency-stabilized lasers used in the gravimeters participating in the ICAG comparisons made at the BIPM in 2005, which were calibrated on site.
The BIPM offers a commercial service of assembling and filling iodine cells for use in stabilized-lasers and spectroscopy. During the period of this report, 18 cells have been prepared. Efforts have been made to further improve the quality of this service and to respond to the customer needs of the NMIs.
In dimensional metrology, the compact diode-pumped laser systems show excellent performance characteristics as sources for use in interferometry. For this purpose, a new type of iodine cell has been designed, fabricated by an external company and tested. They are to be used in the calculable capacitor and watt balance projects, and in a redesigned and more reliable absolute gravimeter. The seventh International Comparison of Absolute Gravimeters (ICAG-2005) took place at the BIPM in early 2005. Sixteen countries participated in measurements with nineteen absolute gravimeters. Relative measurements have also been carried out with fifteen relative gravimeters from ten countries. The BIPM absolute gravimeter has been upgraded, and is undergoing improvement.
Support was provided to the Joint CCL/CCTF Working Group on Secondary Representations of the Second, in which some members of the staff have responsibilities. The section has been involved in discussions with experts from NMIs concerning the need for improvement in the performance of remote comparison techniques to take advantage of the performance of optical clocks for the international time scale.
An external audit of the quality procedures of the Length Section took place in April 2006. As a result, the auditor found that the work was well organized, and suggested only some minor changes in the quality documents.
Summary
