>"Using four-wave mixing
Four-wave mixing is a process where intense light at three frequencies f1, f2, f3 interact to produce light at a fourth frequency f4 = f1 + f2 - f3.

If the three frequencies are part of a perfectly spaced frequency comb, then the fourth frequency is mathematically required to be part of the same comb as well."

>"Low-frequency combs using electronics

A purely electronic device which generates a series of pulses, also generates a frequency comb.

These are produced for electronic sampling oscilloscopes, but also used for frequency comparison of microwaves, because they reach up to 1 THz. Since they include 0 Hz, they do not need the tricks which make up the rest of this article.

>"For many applications, the comb must be widened to at least an octave: [citation needed] that is, the highest frequency in the spectrum must be at least twice the lowest frequency. One of three techniques may be used:

o supercontinuum generation by strong self-phase modulation in nonlinear photonic crystal fiber or integrated waveguide

o a Ti:sapphire laser using intracavity self-phase modulation

o the second harmonic can be generated in a long crystal so that by consecutive sum frequency generation and difference frequency generation the spectrum of first and second harmonic widens until they overlap.""

>"Carrier–envelope offset measurement

Difference between group and phase velocity leading to carrier–envelope offset (https://en.wikipedia.org/wiki/File:Wave_opposite-group-phase...)

An increasing offset between the optical phase and the maximum of the wave envelope of an optical pulse can be seen on the right. Each line is displaced from a harmonic of the repetition rate by the carrier–envelope offset frequency.

The carrier–envelope offset frequency is the rate at which the peak of the carrier frequency slips from the peak of the pulse envelope on a pulse-to-pulse basis.""

(PDS: Reminds me of the following scene from Star Trek IV:

Computer: "Adjust the sine wave of this magnetic envelope so that anti-neutrons can pass through it but anti-gravitons can not!"

[Spock inputs the solution...]

Computer: "Correct!"

https://www.youtube.com/watch?v=EbEtN-e79Lo&feature=youtu.be...

Disclaimer: I have no idea as to what an "anti-neutron" or "anti-graviton" is... but they sound cool, don't they? <g>

Perhaps they exist as different wave envelope patterns -- but that's for future scientists to determine! <g>)

>"Applications

A frequency comb allows a direct link from radio frequency standards to optical frequencies.

(PDS: Or in theory, any EM frequency A to any EM frequency B -- but the frequency comb creator A -- must apparently be at a higher frequency than intersected with frequency B by apparently at least an octave, depending on the method used to generate the frequency comb...)

>"Current frequency standards such as atomic clocks operate in the microwave region of the spectrum, and the frequency comb brings the accuracy of such clocks into the optical part of the electromagnetic spectrum. A simple electronic feedback loop can lock the repetition rate to a frequency standard.""

(PDS: In theory you can generate any kind of stable clock signal, or perhaps more importantly, signal used for synchronization, using a frequency comb...)