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Microwave photonic (MWP) signal processors, which process microwave signals based on photonic technologies, bring advantages intrinsic to photonics such as low loss, large processing bandwidth, and strong immunity to electromagnetic interference. Optical microcombs can offer a large number of wavelength channels and compact device footprints, which make them powerful multi-wavelength sources for MWP signal processors to realize a variety of processing functions. In this paper, we experimentally demonstrate the capability of microcomb-based MWP signal processors to handle diverse input signal waveforms. In addition, we quantify the processing accuracy for different input signal waveforms, including Gaussian, triangle, parabolic, super Gaussian, and nearly square waveforms. Finally, we analyse the factors contributing to the difference in the processing accuracy among the different input waveforms, and our theoretical analysis well elucidates the experimental results. These results provide guidance for microcomb-based MWP signal processors when processing microwave signals of various waveforms.

RF photonic transversal signal processors, which combine reconfigurable electrical digital signal processing and high-bandwidth photonic processing, provide a powerful solution for achieving adaptive high-speed information processing. Recent progress in optical microcomb technology provides compelling multi-wavelength sources with a compact footprint, yielding a variety of microcomb-based RF photonic transversal signal processors with either discrete or integrated components. Although they operate based on the same principle, the processors in these two forms exhibit distinct performances. This paper presents a comparative investigation of their performances. First, we compare the performances of state-of-the-art processors, focusing on the processing accuracy. Next, we analyze various factors that contribute to the performance differences, including the tap number and imperfect response of experimental components. Finally, we discuss the potential for future improvement. These results provide a comprehensive comparison of microcomb-based RF photonic transversal signal processors implemented using discrete and integrated components and provide insights for their future development.

Microwave transversal filters, which are implemented based on the transversal filter structure in digital signal processing, offer a high reconfigurability for achieving a variety of signal processing functions without changing hardware. When implemented using microwave photonic (MWP) technologies, also known as MWP transversal filters, they provide competitive advantages over their electrical counterparts, such as large operation bandwidth, strong immunity to electromagnetic interference, and low loss when processing signals at high frequencies. Recent advances in high‐performance optical microcombs provide compact and powerful multi‐wavelength sources for MWP transversal filters that require a larger number of wavelength channels to achieve high performance, allowing for the demonstration of a diverse range of filter functions with improved performance and new features. Here, a comprehensive performance analysis for microcomb‐based MWP spectral filters based on the transversal filter approach is presented. First, the theoretical limitations are investigated in the filter spectral response induced by finite tap numbers. Next, the distortions are analyzed in the filter spectral response resulting from experimental error sources. Finally, the influence of input signal's bandwidth on the filtering errors is assessed. These results provide a valuable guide for the design and optimization of microcomb‐based MWP transversal filters for a variety of applications. A comprehensive analysis of microcomb‐based microwave spectral filters based on the transversal filter approach is presented. Theoretical limitations in the filter spectral response from finite tap numbers, experimental errors, and the influence of the input signal's bandwidth are all analyzed. This work provides a useful guide to the design of microwave photonic filters based on microcombs.

Soliton crystal microcombs, as a new type of Kerr frequency comb, offer advantages such as higher energy conversion efficiency and a simpler generation mechanism compared to those of traditional soliton microcombs. They have a wide range of applications in fields like microwave photonics, ultra-high-speed optical communication, and photonic neural networks. In this review, we discuss the recent developments regarding soliton crystal microcombs and analyze the advantages and disadvantages of generating soliton crystal microcombs utilizing different mechanisms. First, we briefly introduce the numerical model of optical frequency combs. Then, we introduce the generation schemes for soliton crystal microcombs based on various mechanisms, such as utilizing an avoided mode crossing, harmonic modulation, bi-chromatic pumping, and the use of saturable absorbers. Finally, we discuss the progress of research on soliton crystal microcombs in the fields of microwave photonics, optical communication, and photonic neural networks. We also discuss the challenges and perspectives regarding soliton crystal microcombs.

Optical frequency combs, a revolutionary light source characterized by discrete and equally spaced frequencies, are usually regarded as a cornerstone for advanced frequency metrology, precision spectroscopy, high-speed communication, distance ranging, molecule detection, and many others. Due to the rapid development of micro/nanofabrication technology, breakthroughs in the quality factor of microresonators enable ultrahigh energy buildup inside cavities, which gives birth to microcavity-based frequency combs. In particular, the full coherent spectrum of the soliton microcomb (SMC) provides a route to low-noise ultrashort pulses with a repetition rate over two orders of magnitude higher than that of traditional mode-locking approaches. This enables lower power consumption and cost for a wide range of applications. This review summarizes recent achievements in SMCs, including the basic theory and physical model, as well as experimental techniques for single-soliton generation and various extraordinary soliton states (soliton crystals, Stokes solitons, breathers, molecules, cavity solitons, and dark solitons), with a perspective on their potential applications and remaining challenges.

Gas detection, encompassing both species identification and concentration quantification, is a critical capability. However, within a compact photonic chemical sensing unit, the simultaneous realization of these two objectives still remains elusive. Here, leveraging silicon photonics as the foundational platform, we introduce a high-precision multi-species gas mapper that integrates advances in nanoscience and fiber sensing. In this scheme, on-chip Kerr soliton dual-microcombs simultaneously drive and demodulate a network of nanomaterial-functionalized micro fiber Bragg grating (μFBG) detectors. This on-chip & on-fiber hybrid system achieves individual identification for 12 gas components with remarkable sensitivity. Within this single-laser-source, single-fiber architecture, responses across diverse nanomaterial-functionalized μFBGs are inherently independent, enabling each microcomb-line-driven sensor to exhibit high specificity and sensitivity to its target gas, attaining a record-low detection limit of 24.3 parts per billion (ppb) in single-shot measurements and 2.1 ppb post-averaging. Furthermore, the system enables high-fidelity fingerprint analysis of complex gas mixtures, with a maximum measurement error below 2.27%. This synergistic fusion of chip-scale dual-microcomb photonics with nanomaterial-enhanced optical sensor arrays represents a significant cross-disciplinary advancement, paving a way towards intelligent, miniaturized opto-chemical analysis platforms.

Designing and engineering microresonator dispersion are essential for generating microresonator frequency comb. Microresonator frequency combs (microcombs, Kerr frequency combs) offer the potential for various attractive applications as a new type of coherent light source that is power efficient and compact and has a high repetition rate and a broad bandwidth. They are easily driven with a continuous-wave pump laser with adequate frequency tuning; however, the resonators must have a high quality ( ) factor and suitable dispersion. The emergence of cavity enhanced four-wave mixing, which is based on third-order susceptibility in the host material, results in the generation of broadband and coherent optical frequency combs in the frequency domain equivalent to an optical pulse in the time domain. The platforms on which Kerr frequency combs can be observed have been developed, thanks to intensive efforts by many researchers over a few decades. Ultrahigh- whispering gallery mode (WGM) microresonators are one of the major platforms since they can be made of a wide range of material including silica glass, fluoride crystals and semiconductors. In this review, we focus on the dispersion engineering of WGM microresonators by designing the geometry of the resonators based on numerical simulation. In addition, we discuss experimental methods for measuring resonator dispersion. Finally, we describe experimental results for Kerr frequency combs where second- and higher-order dispersions influence their optical spectra.

The combination of integrated optics technologies with nonlinear photonics, which has led to growth of nonlinear integrated photonics, has also opened the way to groundbreaking new devices and applications. In a companion paper also submitted for publication in this journal, we introduce the main physical processes involved in nonlinear photonics applications and discuss the fundaments of this research area. The applications, on the other hand, have been made possible by availability of suitable materials with high nonlinear coefficients and/or by design of guided-wave structures that can enhance a material’s nonlinear properties. A summary of the traditional and innovative nonlinear materials is presented there. Here, we discuss the fabrication processes and integration platforms, referring to semiconductors, glasses, lithium niobate, and two-dimensional materials. Various waveguide structures are presented. In addition, we report several examples of nonlinear photonic integrated devices to be employed in optical communications, all-optical signal processing and computing, or in quantum optics. We aimed at offering a broad overview, even if, certainly, not exhaustive. However, we hope that the overall work will provide guidance for newcomers to this field and some hints to interested researchers for more detailed investigation of the present and future development of this hot and rapidly growing field.

Coherent frequency microcombs, generated in nonlinear high-Q microresonators and driven by a single continuous-wave laser, have enabled several scientific breakthroughs in the past decade, thanks to their high intrinsic phase coherence and individual comb line powers. Here, we report terabit-per-second-scale coherent data communications over a free-space atmospheric link, using a platicon frequency microcomb, employing wavelength- and polarization-division multiplexing for next-generation optical wireless networks. Spanning more than 55 optical carriers with 115 GHz channel spacing, we report the first free-space coherent communication link using a frequency microcomb, achieving up to 8.21 Tbit/s aggregate data transmission at a 20 Gbaud symbol rate per carrier over 160 m, even under log-normal turbulent conditions. Utilizing 16-state quadrature amplitude modulation, we demonstrate retrieved constellation maps across the broad microcomb spectrum, achieving bit-error rates below both hard- and soft-decision thresholds for forward-error correction. Next, we examine a wavelength-division multiplexing free-space passive optical network as a baseline for free-space fronthaul, achieving an aggregate data rate of up to 5.21 Tbit/s and a field-tested spectral efficiency of 1.29 bit/s/Hz in the microcomb-based atmospheric link. We also quantify experimental power penalties of ≈ 3.8 dB at the error-correction threshold, relative to the theoretical additive white Gaussian noise limit. Furthermore, we introduce the first-ever demonstration of master–slave free-space carrier phase retrieval with frequency microcombs, and the compensation for turbulence-induced intensity scintillation and pointing error fluctuations, to improve end-to-end symbol error rates. This work provides a foundational platform for broadband vertical heterogeneous connectivity, terrestrial backbone links, and ground-satellite communication.

Thin-film lithium niobate (TFLN) has enabled efficient on-chip electro-optic modulation and frequency conversion for information processing and precision measurement. Extending these capabilities with optical frequency combs unlocks massively parallel operations and coherent optical-to-microwave transduction, which are achievable in TFLN microresonators via Kerr microcombs. However, fully integrated Kerr microcombs directly driven by semiconductor lasers remain elusive, which has delayed integration of these technologies. Here, we demonstrate electrically pumped TFLN Kerr microcombs without optical amplification. With optimized laser-to-chip coupling and optical quality factors, we generate soliton microcombs at a 200 GHz repetition frequency with an optical span of 180 nm using only 25 mW of pump power. Moreover, self-injection locking enables turnkey initiation and substantially narrows the laser linewidth. Our work provides integrated comb sources for TFLN-based communicational, computational, and metrological applications.