A frequency comb is a precise and regular series of equally spaced spectral lines, or frequencies, that are generated with great accuracy. The term "frequency comb" is often associated with the Nobel Prize-winning technique known as frequency comb spectroscopy, developed by John L. Hall and Theodor W. Hänsch in the 1990s. The technology has since become a powerful tool in various scientific and technological applications.
Key points about frequency combs:
Origin and development: The development of frequency combs originated from the field of precision spectroscopy and optical frequency metrology. The technique was initially developed for improving the accuracy of optical frequency measurements and played a crucial role in the development of optical atomic clocks.
Mode-locked lasers: Frequency combs are typically generated using mode-locked lasers. In a mode-locked laser, the modes or longitudinal resonances of the laser cavity are phase-locked together, producing ultrashort pulses of light. The spectrum of these pulses forms a comb-like structure in the frequency domain.
Equally spaced lines: The key characteristic of a frequency comb is the equally spaced and coherent spectral lines. Each line corresponds to a specific frequency, and the spacing between the lines is determined by the repetition rate of the mode-locked laser pulses.
Applications in spectroscopy: Frequency combs revolutionized spectroscopy by providing a direct and accurate link between optical and radio frequencies. The comb's equally spaced lines serve as a precise ruler for measuring optical frequencies, enabling high-resolution spectroscopy in various spectral regions, including the visible and infrared.
Optical atomic clocks: Frequency combs are essential components in optical atomic clocks, which use optical transitions in atoms to keep time. The accuracy of optical clocks surpasses that of traditional microwave atomic clocks.
Laser frequency stabilization: Frequency combs are employed to stabilize the frequency of lasers, enabling precise and stable laser sources for applications such as interferometry, sensing, and communications.
Broadband light sources: Frequency combs can be used as broadband light sources for applications in nonlinear optics, where the equally spaced lines provide a wealth of well-defined frequencies for various nonlinear processes.
Attosecond science: The ultrashort pulse durations associated with frequency combs have opened up the field of attosecond science, where researchers study extremely short timescales in the range of attoseconds (10-18 seconds).
Frequency combs have had a profound impact on various scientific and technological disciplines, ranging from precision measurement and spectroscopy to timekeeping and the exploration of ultrafast processes in physics and chemistry.