Optical Frequency Measurement Using Frequency Multiplication and Frequency Combs

We review the development of phase-coherent optical frequency measurement from its start with the harmonic frequency chain to the present use of optical comb generators. We demonstrate that the frequency fluctuations of a femtosecond comb laser—used in optical frequency measurement—can be eliminated completely from the measurement process, allowing ultimate frequency measurement performance. This principle was applied to the frequency measurements of the Ca cloud standard and the Yb + single-ion standard. The latter measurements confirm the suitability of these optical standards as future atomic clocks.