  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 $\nu_{SH}$ at 532 nm to the fundamental $\nu_0$ at
1064 nm, $\nu_{SH}/\nu_0 = 2.000 000 000 000 000 001 \times (1 \pm 7 \times
10^{-19})$.
