Phase coherent frequency measurement of visible radiation

Recently the Comite International des Poids et Mesures (CIPM) has recommended the frequency of the Ca-intercombination line ³P ₁-¹S₀ for the realization of the Metre to be ν=455 986 240.5 MHz with a standard uncertainty of 200 kHz (Δν/ν=4,5·10^-10). Up to now this frequency has been determined by an interferometric wavelength intercomparison. To take full advantage of its potential as a precise optical frequency/wavelength standard, we have measured this frequency with respect to the frequency of the primary standard of time and frequency, the Cs-clock. We have used the existing frequency chain at PTB to the infrared and extended it to a Ca stabilized dye laser. We have performed the first phase coherent frequency measurement of visible radiation referenced to the Cs clock and determined the absolute frequency of this transition with a relative standard uncertainty of 9·10^-13 . The mean of the high and low-frequency recoil components of the Ca atoms stored in a magneto-optical trap is 455 986 240 493.95 kHz with an estimated standard uncertainty of 420 Hz. The Ca stabilized laser now represents the optical frequency standard with the lowest uncertainty being more than an order of magnitude smaller than that of the iodine stabilized HeNe laser at λ=633 nm. The current status and prospects are reported