This is timely, as it were. Scientific American has an article on frequency combs that appears to be publicly accessible. Rulers of Light: Using Lasers to Measure Distance and Time
Optical frequency comb applications require exquisite control of light across a broad spectrum of frequencies. This level of control has been available for radio waves for a long time but is only now becoming possible for light. An analogy to music helps in understanding the required level of control. Before the development of combs, lasers could produce a single color, like a single optical tone. They were akin to a violin with only one string and no fingerboard, capable of playing only one note (ignore for the moment that musical notes are much richer than pure tones). To play even a simple piece would require many different instruments, each painstakingly tuned. Each violin would require its own musician, just as every single-frequency laser requires its own operator.
In contrast, one operator can use an optical comb to cover the entire optical spectrum, not merely like a pianist at a piano but like a keyboardist playing an electronic synthesizer that can be programmed to mimic any musical instrument or even an entire orchestra. Comb technology, in effect, enables symphonies of hundreds of thousands of pure optical tones.
As explained in the article, one key for usefulness in timing and frequency is that the comb spans an octave, i.e. a factor of two in frequency, so that a “tine” (spectrum line) of the comb at the low end can be frequency-doubled in a nonlinear crystal, and referenced to a line at the high end, making the comb stable — the frequency of any line is well-known. You can now reference a convenient optical transition to the comb and do clock measurements. Since the frequency is much higher, if a suitable (i.e. narrow) transition can be found the fractional error will be much smaller, and the measurements that much more precise.
One thing that was apparent from the Frequency Standards and Metrology conference is that combs are everywhere. A number of different atoms are being investigated, both neutrals typically trapped in an optical lattice, or ions trapped in, well, ion traps. Once you have a really nice clock, though, you need to have another very nice clock (or clocks) with which you can compare. Multiple clocks can be referenced to a comb, and this is being done in the larger labs. And there are also people investigating better techniques for comparing remote signals using fiber transmission of signals, to overcome the limitations of satellite comparisons.
