Frequency comparison of He-Ne/iodine lasers at ? = 633 nm between the NILPRP and the ISI, and traceability through the BIPM

We report the results of a bilateral laser frequency comparison between lasers from the National Institute for Laser, Plasma and Radiation Physics (NILPRP, Romania: laser RO2) and the Institute of Scientific Instruments (ISI, Czech Republic: laser INV3). The main objective was to correlate the results obtained from these two laboratories and the previous results obtained by each of these laboratories against the Bureau International des Poids et Mesures (BIPM: lasers BIPM4 and BIPMP3). The result of a set of ten direct measurements showed a medium frequency difference of ?fcorr = fRO2 - fINV3 = -15,1 kHz, with an estimated standard uncertainty (1?) of s = 2,6 kHz, compared with the expected mean value, resulting from earlier comparisons, of ?fcorr = fRO2 - fINV3 = -10,7 kHz with s = 7,4 kHz. Once corrections were applied, the standard uncertainty (repeatability) of the frequency measurements improved from 6,0 kHz to 2,6 kHz.