Optical frequency combs: Coherently uniting the electromagnetic spectrum

An optical timekeeper Optical clocks, based on optical transitions of atoms, operate at much higher frequency than the microwave atomic clocks currently used as our timing standards. They have been shown to exhibit better stability and are poised to redefine the second. The development of stable, self-referenced optical frequency combs that span the microwave to optical wavelengths has been key to these efforts. Diddams et al. reviewed developments and refinements of these optical combs over the past 20 years and provide an overview of where they are finding application, from precision timing to high-resolution spectroscopy and imaging, ranging, and navigation. Science this issue p. eaay3676 BACKGROUND The generation and control of coherent electromagnetic waves, such as those provided by electronic oscillators in the radio frequency domain or lasers in the optical domain, have had an unparalleled impact on human society over the past century. For example, precise timing with radio waves referenced to atomic transitions undergirds navigation with the Global Positioning System. And modern communication systems are built around the properties of such waves to carry data through the air or within optical fibers that circumnavigate the globe. Such technical advances rely upon an exceptionally well developed and unified theoretical understanding of radio and optical waves, as well as the devices for their generation and control. Nonetheless and surprisingly, just 20 years ago, radio and optical technology realms remained largely distinct and isolated from one another. Although light waves could be modulated at radio rates and likewise electrical currents could be produced by demodulating optical signals, a simple coherent connection between radio and optical fields did not exist. As a result, many common technologies for the synthesis and control of radio frequencies, including those central to navigation, communications, and measurement, seemed futuristic for optics. Conversely, the full potential of optics for time standards, metrology, and science was not accessible, despite a decades-long recognition of the opportunities and attempts to harness their technological impact. This situation, which arose as a consequence of the enormously high frequencies of electromagnetic waves in the optical domain, thereby limited scientific progress and technical capabilities. ADVANCES The invention of the laser in 1960 represented an optical analog to radio oscillators, which were invented much earlier. This development motivated efforts to create a coherent bridge between the radio and optical realms with multiple oscillators of successively higher frequencies being chained together. However, the sheer complexity and size of such approaches made it clear that these systems would never be widely available and that their capability would be limited. Such efforts were upended after nearly four decades of work when an unanticipated breakthrough enabled by combined advances in femtosecond laser technology, nonlinear optics, and precision frequency metrology finally solved this problem. The key to overcoming these issues was a new approach to generate and control the spectrum of a mode-locked laser, which was called an optical frequency comb in accordance with the regularly spaced comb of frequencies it contained. Even though mode-locked lasers and optical frequency combs had existed previously, in 2000 it was demonstrated how their spectra could be expanded over an octave of optical bandwidth. This critical advance enabled the technique of self-referencing, by which the optical frequencies of the comb are locked in an absolute sense to a radio frequency reference. In a simple and elegant way, this produced an optical clockwork, analogous to a gearbox (see the figure), that provided the bidirectional coherent connection between the optical and radio frequency domains. Moreover, because this connection is simultaneously established to many thousands of optical frequencies within the comb, self-referencing makes the coherent translation of frequencies possible across hundreds of terahertz of the optical spectrum. OUTLOOK In the two decades since the introduction of the frequency comb, entirely new scientific and technology vistas have been opened. New applications are leveraging an unprecedented sharing of the respective strengths of electronics and photonics, as well as a new freedom to work seamlessly across the broad optical frequency expanse. Frequency combs are used to realize and compare ultraprecise optical clocks at the 19th decimal place, which provides powerful approaches to test relativity and quantum theory as well as the search for physics beyond our present understanding. In addition, combs transfer this exceptional precision across the spectrum to perform massively parallel spectroscopy, generate the lowest-noise microwaves and few-cycle attosecond waveforms, and even help astronomers search for Earth-like exoplanets. Just as applications of frequency combs have expanded, there are new developments in frequency comb technology. Among these are combs that do not use mode locking for comb formation, as well as new approaches built on integrated photonics and microresonators that enable frequency combs on a silicon chip. Indeed, it is now clear that the once-enormous gap between optics and electronics will be further blurred as the coherent electromagnetic spectrum is united by frequency combs. An optical clockwork. A frequency comb is a type of laser that functions in a manner analogous to a set of gears, as illustrated here, to link radio frequencies to a vast array of optical frequencies (“the comb”) with values precisely determined by the expression fn = nfr + f0, where fr and f0 are radio frequencies and n is an integer on the order of 105. Optical frequency combs were introduced around 20 years ago as a laser technology that could synthesize and count the ultrafast rate of the oscillating cycles of light. Functioning in a manner analogous to a clockwork of gears, the frequency comb phase-coherently upconverts a radio frequency signal by a factor of ≈105 to provide a vast array of evenly spaced optical frequencies, which is the comb for which the device is named. It also divides an optical frequency down to a radio frequency, or translates its phase to any other optical frequency across hundreds of terahertz of bandwidth. We review the historical backdrop against which this powerful tool for coherently uniting the electromagnetic spectrum developed. Advances in frequency comb functionality, physical implementation, and application are also described.