A device that facilitates and greatly simplifies the measurement of very high frequencies, particularly in the optical range from 10¹⁴ to 10¹⁵ Hz. Extremely stable and reproducible optical frequencies f_opt in this range are produced by trapped-atom and trapped-ion atomic clocks. Frequency-comb technology is based on the train of short-duration pulses of optical radiation emitted at a stable and measurable repetition rate f_R of 100 MHz to 1 GHz by a femtosecond laser. Such a pulse train can be equivalently represented as a continuously produced “comb” of frequencies. The “teeth” of the comb are discrete phase-related frequencies spaced by f_R from one another, and the frequency of the mth tooth is mf_R, where m is an integer (see illustration). The frequencies produced typically span a rather narrow wavelength range of 30 nm to around 800 nm, but this span can be increased to a factor of 2 by passing the pulsed light through an optical fiber made from a highly nonlinear material, enabling a comb frequency to be matched to many different optical frequency standards.