The absolute frequency of the 473-THz He–Ne laser (633 nm), stabilized on the g or i hyperfine component of the 127I2 11–5 R(127) transition, was measured by comparing its frequency with a known frequency synthesized by summing the radiation from three lasers in a He–Ne plasma. The three lasers were (1) the 88-THz CH4-stabilized He–Ne laser (3.39 μm), (2) a 125-THz color-center laser (2.39 μm) with its frequency referenced to the RII(26) 13C18O2 laser, and (3) the 260-THz He–Ne laser (1.15 μm) referenced to an I2-stabilized dye laser at 520 THz (576nm). The measured frequencies are 473 612 340.492 and 473 612 214.789 MHz for the g and i hyperfine components, respectively, with a total uncertainty of 1.6 parts in 1010. The frequency of the i component adjusted to the operating conditions recommended by the Bureau International des Poids et Mesures is 473 612 214.830 ± 0.074 MHz.
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