Abstract
An international comparison of two I2-stabilized frequency-doubled Nd:YAG lasers at λ = 532 nm has been performed between the Istituto di Metrologia "G. Colonnetti" (IMGC)/Politecnico di Milano (PoliMI) and the Physikalisch-Technische Bundesanstalt (PTB). Both lasers used saturated absorption of the P(54) 32-0 iodine transition as a frequency reference. The signals for stabilizing the lasers were obtained using the frequency modulation spectroscopy technique with an external iodine cell. A minimum value for the relative Allan standard deviation of 7 × 10-14 was reached after an integration time of 40 s. The hyperfine line separations were measured by the beat-frequency method and compared with the frequency values recently reported by Ye and co-workers. Three different iodine cells were investigated. The range of frequency differences for the iodine cells IMGCQU and PTB1/98 between the two lasers was from -9 kHz to 7.3 kHz with a mean of 1.2 kHz and a standard deviation of 3.5 kHz. Taking into account the measured frequency offsets of the different iodine cells, the evaluated frequency repeatability of the optical standards was within 2.5 kHz.