Ultraprecise 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 ${10}^{-18\mathrm{}}$ 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 ${\nu }_{\mathrm{SH}}$ at 532 nm to the fundamental ${\nu }_0$ at 1064 nm, ${\nu }_{\mathrm{SH}}/{\nu }_0=2.000\mathrm{}000000000000001\times (1\pm 7\times {10}^{-19\mathrm{}})$.