An optical frequency comb is a spectrum composed of a series of evenly spaced frequency components on the spectrum, which can be generated by mode-locked lasers, resonators, or electro-optical modulators. Optical frequency combs generated by electro-optic modulators have the characteristics of high repetition frequency, internal interdrying and high power, etc., which are widely used in instrument calibration, spectroscopy, or fundamental physics, and have attracted more and more researchers’ interest in recent years.

Recently, Alexandre Parriaux and others from the University of Burgendi in France published a review paper in the journal Advances in Optics and Photonics, systematically introducing the latest research progress and application of optical frequency combs generated by electro-optical modulation: It includes the introduction of optical frequency comb, the method and characteristics of optical frequency comb generated by electro-optic modulator, and finally enumerates the application scenarios of electro-optic modulator optical frequency comb in detail, including the application of precision spectrum, double optical comb interference, instrument calibration and arbitrary waveform generation, and discusses the principle behind different applications. Finally, the author gives the prospect of electro-optic modulator optical frequency comb technology.

01 Background
It was 60 years ago this month that Dr. Maiman invented the first ruby laser. Four years later, Hargrove, Fock and Pollack of Bell Laboratories in the United States were the first to report the active mode-locking achieved in helium-neon lasers, the mode-locking laser spectrum in the time domain is represented as a pulse emission, in the frequency domain is a series of discrete and equidistant short lines, very similar to our daily use of combs, so we call this spectrum “optical frequency comb”. Referred to as “optic frequency comb”.
Because of the good application prospect of optical comb, the Nobel Prize in Physics in 2005 was awarded to Hansch and Hall, who made pioneering work on optical comb technology, since then, the development of optical comb has reached a new stage. Because different applications have different requirements for optical combs, such as power, line spacing and central wavelength, this has led to the need to use different experimental means to generate optical combs, such as mode-locked lasers, micro-resonators and electro-optical modulator.

FIG. 1 Time domain spectrum and frequency domain spectrum of optical frequency comb

Since the discovery of optical frequency combs, most optical frequency combs have been produced using mode-locked lasers. In mode-locked lasers, a cavity with a round-trip time of τ is used to fix the phase relationship between longitudinal modes, so as to determine the repetition rate of the laser, which can generally be from megahertz (MHz) to gigahertz (GHz).
The optical frequency comb generated by the micro-resonator is based on nonlinear effects, and the round-trip time is determined by the length of the micro-cavity, because the length of the micro-cavity is generally less than 1mm, the optical frequency comb generated by the micro-cavity is generally 10 gigahertz to 1 terahertz. There are three common types of microcavities, microtubules, microspheres and microrings. Using nonlinear effects in optical fibers, such as Brillouin scattering or four-wave mixing, combined with microcavities, optical frequency combs in the tens of nanometers range can be produced. In addition, optical frequency combs can also be generated by using some acousto-optic modulators.