A frequency comb is a tool used in physics and engineering to measure or generate frequencies. It consists of a series of equally spaced frequencies, usually created by combining the output of two lasers. The spacing between the frequencies is equal to the laser’s repetition rate.
Frequency combs have many applications, including precision measurement of optical frequencies, determination of the exact length of an optical fiber, and generation of microwave signals. They are also used in atomic clocks and as tools for studying ultrafast phenomena such as electron dynamics in molecules.
The first frequency comb was developed in 1999 by Theodor Hänsch and Nobel laureate John L. Hall at the Max Planck Institute for Quantum Optics in Garching, Germany. Their work was based on earlier ideas dating back to 1955 by Russian physicist Aleksandr Prokhorov.
Since then, frequency combs have been built using a variety of different technologies, including fiber lasers, microresonators, and nonlinear crystals. In each case, the basic principle is the same: two or more lasers are combined to create a series of evenly spaced frequencies (the “comb”).
Frequency combs have revolutionized precision measurement in optics and led to advances in many other fields including astronomy and telecommunications.