Ultra-precise measurement of optical frequency ratios
We developed a novel technique for frequency measurement and synthesis, based on the operation of a femtosecond comb generator as transfer oscillator. The technique can be used to measure frequency ratios of any optical signals throughout the visible and near-infrared part of the spectrum. Relative uncertainties of $10^{-18}$ for averaging times of 100 s are possible. Using a Nd:YAG laser in combination with a nonlinear crystal we measured the frequency ratio of the second harmonic $ν_{SH}$ at 532 nm to the fundamental $ν_0$ at 1064 nm, $ν_{SH}/ν_0 = 2.000 000 000 000 000 001 \times (1 \pm 7 \times 10^{-19})$.
physics.optics↗