This paper celebrates the progress in optical frequency standards and measurement, won by the 40 years of dedicated work of world-wide teams working in frequency standards and frequency measurement. Amazingly, after this time interval, the field is now simply exploding with new measurements and major advances of convenience and precision, with the best fractional frequency stability and potential frequency accuracy now being offered by optical system. The new magic technology underlying the rf/optical connection is the capability of using femtosecond laser pulses to produce optical pulses so short their Fourier spectrum covers an octave bandwidth in the Visible. These white light pulses are repeated at stable rates (~100-1000 MHz, set be design), leading to an optical comb of frequencies with excellent phase coherence and stability and containing some millions of stable coherent optical frequencies. Optical-heterodyned differences between comb lines provides a frequency-related rf or microwave output with remarkably low added phase noise, such that in an optically based atomic clock, the phase noise of the standards-grade microwave frequency reference dominates over that of optical reference and the fsgear-box.
IEEE Transactions on Instrumentation and Measurement
femtosecond lasers, laser frequency control, optical frequency comb, optical frequency measurement
and Ye, J.
Optical Frequency Standards and Measurement, IEEE Transactions on Instrumentation and Measurement
(Accessed December 4, 2023)