International comparison of two iodine-stabilized frequency-doubled Nd : YAG lasers at lambda=532 nm

An international comparison of two I-2-stabilized frequency-doubled Nd:YAG lasers at lambda = 532 nm has been performed between the Istituto di Metrol ogia "G. Colonnetti" (IMGC)/Politecnico di Milano (PoliMI) and the Physikal isch-Technische Bundesanstalt (PTB). Both lasers used saturated absorption of the P(54) 32-0 iodine transition as a frequency reference. The signals f or stabilizing the lasers were obtained using the frequency modulation spec troscopy technique with an external iodine cell. A minimum value for the re lative Allan standard deviation of 7 x 10(-14) was reached after an integra tion 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 r ange of frequency differences for the iodine cells IMGC(QU) and PTB1/98 bet ween 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 repeatabilit y of the optical standards was within 2.5 kHz.