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Iodine-stabilized extended-cavity diode lasers at λ ≈ 633 nm: result of an international comparison

A comparison of three 127I2-stabilized extended-cavity diode lasers has been carried out between the Danish Institute of Fundamental Metrology (DFM) and the Bureau International des Poids et Mesures (BIPM). Two of the lasers are stabilized using intra-cavity saturation spectroscopy, the third one using extra-cavity saturation spectroscopy. All lasers are locked using the third harmonic detection technique on different hyperfine components of the P(33)6-3 transition. The range of frequency differences measured during the four days of the comparison was from -340 kHz to 150 kHz. The large fluctuations are due to the poor repeatability of the extended-cavity diode lasers with intra-cavity absorption cells (ECL4 and BIREL1-1). The frequency stability and repeatability of the diode laser (ECL1) using an external absorption cell is roughly one order of magnitude lower in performance than the He-Ne lasers. Using an iodine-stabilized He-Ne laser (DK3) as reference and the recommended values given in the practical realization of the definition of the metre, we found a mean offset from the expected frequency difference between ECL1 and DK3 of -12 kHz and a standard uncertainty of 10 kHz.