When measuring the duration of high-speed physical phenomena, a good stopwatch can only get you so far, and while oscilloscopes can pick up electrical signals with frequencies of a few GHz, measuring incredibly fast optical phenomena requires something more—a system called an optical frequency comb. Normal lasers are monochromatic sources only containing a single frequency of light; in contrast, frequency combs contain many frequencies, equally spaced in the frequency domain, which look very much like the teeth of a comb. Frequency combs are used extensively as a type of ‘optical ruler’ since they can measure rapidly varying signals by interfering the ‘teeth’ of the frequency combs with the signal they want to measure, which consequently converts those signals into more manageable radio frequency signals.