A frequency comb is a special kind of laser light whose spectrum, or set of frequencies, resembles the teeth of a comb. Instead of containing just one colour (one frequency), it emits a large number of evenly spaced frequencies. The spacing is extremely regular.
One way to make a frequency comb is using a mode-locked laser that emits very short pulses of light, repeating again and again at a steady rate.
A frequency comb is useful because it lets scientists compare an unknown light frequency to a stable reference with extraordinary precision. Say you have some mystery light, e.g. light produced by a new laser that you have built, and you need to find out its exact frequency. You can shine the new laser light and light from a frequency comb on a common light sensor.
The sensor will produce an electrical signal. If one comb tooth is very close in colour to your unknown laser, the two light waves don’t line up perfectly. Sometimes their peaks line up, so the sensor signal gets stronger; sometimes they don’t and the signal gets weaker. You can thus figure out what colour your laser is producing by combining the frequency (or tooth) it’s closest to with how much it is off by. The sensor signal will reveal both these details.
Frequency combs are an important modern tool to calibrate atomic clocks and measure shifts in light caused by gravity, among various other applications.
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