Dissipative Kerr solitons in optical microresonators

Shrinking optical metrology The ability to generate laser frequency combs—light sources comprising equidistant laser lines spanning a large range of wavelengths—has revolutionized metrology and precision spectroscopy. The past decade has seen frequency combs being generated in optical microresonator circuits, offering the prospect of shifting precision metrology applications from the realm of national laboratories to that of everyday devices. Kippenberg et al. review the development of microresonator-generated frequency combs and map out how understanding and control of their generation is providing a new basis for precision technology. Science, this issue p. eaan8083 BACKGROUND Laser frequency combs, which consist of equidistant laser lines, have revolutionized time-keeping, metrology, and spectroscopy. Conventional optical frequency combs based on mode-locked lasers are still mostly confined to scientific laboratories. In recent years, there has been progress in the development of optical frequency combs based on compact, chip-scale microresonators (“microcombs”), with such microcombs operating in the dissipative soliton regime. Dissipative solitons rely on a double balance of nonlinearity and dispersion as well as dissipation and gain and are an example of self-organization in driven dissipative nonlinear systems. Dissipative temporal solitons are providing the long-sought-for regime of coherent, broadband microcomb spectra and in addition provide an experimental setting in which to study dissipative soliton physics. Microcombs are now capable of producing coherent, octave-spanning frequency combs, with microwave to terahertz repetition rates, at low pump power, and in chip-scale devices and have been used in a wide variety of applications, owing to bandwidth and coherence provided by the dissipative temporal soliton states. ADVANCES Underlying these recent advances has been the observation of temporal dissipative Kerr solitons (DKSs) in microresonators, which represent self-enforcing stationary localized solutions of a damped, driven, and detuned nonlinear Schrödinger equation, which was originally used to describe spatial self-organization phenomena. They correspond to solitons (localized patterns) in “open” systems—that is, systems that exhibit dissipation. DKSs, opposite to fiber solitons, therefore rely on a double balance of nonlinearity and dispersion as well as parametric gain and loss and depend on the laser-cavity detuning as a control parameter. Methods have been established that enable the reliable generation of such DKSs in a wide range of microresonator platforms, including platforms based on silicon nitride (Si3N4) that are compatible with photonic integration. In addition, a variety of previously unknown and nonanticipated soliton effects have been observed, such as soliton crystallization, Raman-Stokes solitons, and previously unseen soliton breather dynamics. Moreover, dissipative solitons have been shown to be suprisingly robust against imperfections in the resonator mode structure. Dissipative soliton states in microresonators have triggered a large number of applications, including in terabit-coherent optical communications, atomic clocks, ultrafast distance measurements, dual-comb spectroscopy, photonic integrated frequency synthesizers, and the calibration of astrophysical spectrometers for exoplanet searches. OUTLOOK The reliable generation of DKSs in microresonators has established a nascent research field at the interface of soliton physics, frequency metrology, and integrated photonics and provided impetus to microcomb sources. Emerging frontiers include using advances in nanofabrication and materials synthesis to realize ultralow-propagation-loss photonic nanostructures with unusual dispersion properties, which could explore dissipative solitons in new and unexplored parameter regimes and allow the synthesis of even broader spectra that in time domain could correspond to single-cycle pulses and whose spectral bandwidth could be extended to the mid-infrared or visible range. Beyond the narrow class of materials used for DKSs so far (Si, MgF2, SiO2, and Si3N4), many other materials exist with distinct advantages, such as III-V semiconductors, which are already widely used in light-emitting or laser diodes. Beyond existing applications, DKSs could be applied to optical coherence tomography, Raman spectral imaging, chip-scale atomic clocks based on optical transitions, or ultrafast photonic analog-to-digital conversion and have a potential to make frequency metrology and spectroscopy ubiquitous. DKSs in optical microresonators. (A) Principle of DKSs, representing a double balance of dispersion and nonlinearity as well as (parametric) gain and cavity loss. (B) Optical field envelope of a single DKS, containing the localized soliton on top of a continuous-wave (CW) background field. (C) Graphic image of dissipative soliton formation in a CW laser–driven photonic chip–based microresonator, generating a continuously circulating DKS, which in frequency space corresponds to a coherent optical frequency comb. The optical frequency comb is shown with two dispersive waves that arise from higher-order dispersion. CREDIT: SIMON HOENL, EPFL The development of compact, chip-scale optical frequency comb sources (microcombs) based on parametric frequency conversion in microresonators has seen applications in terabit optical coherent communications, atomic clocks, ultrafast distance measurements, dual-comb spectroscopy, and the calibration of astophysical spectrometers and have enabled the creation of photonic-chip integrated frequency synthesizers. Underlying these recent advances has been the observation of temporal dissipative Kerr solitons in microresonators, which represent self-enforcing, stationary, and localized solutions of a damped, driven, and detuned nonlinear Schrödinger equation, which was first introduced to describe spatial self-organization phenomena. The generation of dissipative Kerr solitons provide a mechanism by which coherent optical combs with bandwidth exceeding one octave can be synthesized and have given rise to a host of phenomena, such as the Stokes soliton, soliton crystals, soliton switching, or dispersive waves. Soliton microcombs are compact, are compatible with wafer-scale processing, operate at low power, can operate with gigahertz to terahertz line spacing, and can enable the implementation of frequency combs in remote and mobile environments outside the laboratory environment, on Earth, airborne, or in outer space.