Broadband spectroscopy combines wide spectral coverage with high resolution and accuracy, requirements that are often difficult to satisfy simultaneously using conventional approaches. Dual-comb spectroscopy addresses this challenge by using two optical frequency combs with slightly different repetition frequencies to map optical spectra directly into the radio-frequency domain. The method relies on time-domain interferometry and avoids mechanical scanning, enabling precise, rapid and broadband measurements. In principle, the achievable spectral resolution is set by the comb line spacing, defined by the laser repetition frequency. Because the measurement does not rely on geometrical constraints, the dual-comb interferometer offers a conceptual route towards broadband spectroscopy with resolution determined purely by temporal coherence. Over the past two decades, dual-comb spectroscopy has been implemented across the electromagnetic spectrum, from the terahertz to the visible, with ongoing efforts towards the ultraviolet. In this Primer, we present the fundamental principles of the technique, analyse the parameters governing its performance - including resolution, accuracy, signal-to-noise ratio and acquisition speed - and describe practical approaches to data acquisition and processing. We conclude by examining representative applications, current limitations and emerging directions for further development.

A platform for studying the spectral response of nanosystems combines fiber-based Fabry-Perot microcavities of high quality factors and dual-comb spectrally-resolved readout. Raster-scanning the sample through the cavity mode leads to multispectral spatial maps of nanoparticles.

An optical frequency comb of large repetition frequency (1 THz) generated in a microresonator provides a new tool for hyperspectral three-dimensional imaging, allowing thorough characterization of materials such as plastic polymers of environmental relevance.

We report the first experimental demonstration of a spectrometer freed from geometric limitations over a broad spectral bandwidth. A dual-comb interferometer records spectra spanning over 130 GHz at a resolution of 2.5 MHz.

Cavity-enhanced dual-comb spectroscopy combines high sensitivity, resolution and broad bandwidth, making it a powerful tool for gas-phase spectroscopy [1]. Existing implementations require complex dispersion management of the cavity mirrors and tight feedback locking for comb-cavity coupling. In this work, we present a novel approach to cavity-enhanced condensed-phase spectroscopy that enables spectrally and spatially resolved measurements without the need for dispersion management and feedback stabilisation.

Optical frequency combs have revolutionised time and frequency metrology [1, 2]. The advent of microresonator-based frequency combs ('microcombs' [3-5]) is set to lead to the miniaturisation of devices that are ideally suited to a wide range of applications, including microwave generation [6, 7], ranging [8-10], the precise calibration of astronomical spectrographs [11], neuromorphic computing [12, 13], high-bandwidth data communications[14], and quantum-optics [15, 16] platforms. Here, we introduce a new microcomb application for three-dimensional imaging. Our method can simultaneously determine the chemical identity and full three-dimensional geometry, including size, shape, depth, and spatial coordinates, of particulate matter ranging from micrometres to millimetres in size across nearly 10^5 distinct image pixels. We demonstrate our technique using millimetre-sized plastic specimens (i.e. microplastics measuring less than 5 mm). We combine amplitude and phase analysis and achieve a throughput exceeding 1.2 10^6 pixels per second with micrometre-scale precision. Our method leverages the defining feature of microcombs - their large line spacing - to enable precise spectral diagnostics using microcombs with a repetition frequency of 1 THz. Our results suggest scalable operation over several million pixels and nanometre-scale axial resolution. Coupled with its high-speed, label-free and multiplexed capabilities, our approach provides a promising basis for environmental sensing, particularly for the real-time detection and characterisation of microplastic pollutants in aquatic ecosystems [17].

Mid-infrared Kerr comb generation in the low-loss 4H-SiC platform is reported. Q factors are higher than $4\times 10^{5}$ at $2.5\ \mu \mathrm{m}$. Optical frequency combs at high repetition-frequency spanning over 10 THz are generated at 120 THz.

A dispersion-engineered SiC waveguide on a photonic chip simultaneously provides an f-2f interferometer and mid-infrared dispersive-wave frequency-comb generation at 120-pJ pulse energies. Accurate comb-assisted tunable-laser molecular spectroscopy is demonstrated at 3.6 μm.

Ultraviolet spectroscopy provides unique insights into the structure of matter with applications ranging from fundamental tests to photochemistry in the earth's atmosphere and astronomical observations from space telescopes. At longer wavelengths, dual-comb spectroscopy with two interfering laser frequency combs has evolved into a powerful technique that can offer simultaneously a broad spectral range and very high resolution. Here we demonstrate a photon-counting approach that can extend the unique advantages of this method into ultraviolet regions where nonlinear frequency-conversion tends to be very inefficient. Our spectrometer, based on two frequency combs of slightly different repetition frequencies, provides broad span, high resolution, frequency calibration within the accuracy of an atomic clock, and overall consistency of the spectra. We demonstrate a signal-to-noise ratio at the quantum limit and optimal use of the measurement time, provided by the multiplex recording of all spectral data on a single photo-counter. Our initial experiments are performed in the near-ultraviolet and in the visible spectral ranges with alkali-atom vapor, with a power per comb line as low as a femtowatt. This crucial step towards precision broadband spectroscopy at short wavelengths clears the path to extreme-ultraviolet dual-comb spectroscopy and, more generally, generates a new realm of applications for diagnostics at photon level, as encountered e.g., when driving single atoms or molecules.

Lensless three-dimensional hyperspectral imaging is performed with a Kerr comb of 100 GHz line spacing. Broad microcombs will enable an unprecedented combination of long axial range, high precision and fast acquisition rate.

Dual-comb spectroscopy is implemented in light-starved conditions at optical powers of a few tens of picowatts. Quantum-limited sensitivity is demonstrated for comb-line-resolved spectra of Doppler-broadened transitions in the ultraviolet 770-THz region.

We demonstrate a heterogeneously integrated III-V-on-silicon-nitride mode-locked laser with 710 MHz repetition rate. A versatile two-step micro-transfer printing approach is employed to enable low-loss integration on a commercial foundry low-index photonic platform.

Dual-comb spectroscopy [1] has emerged as a powerful tool for broad-spectral-bandwidth spectroscopy in the near-infrared and mid-infrared spectral regions. Exploring the ultraviolet spectral region will enable to reach electronic transitions in atoms and rovibronic transitions in molecules. Here, we demonstrate frequency-agile dual-comb spectroscopy [2], [3] in the near-ultraviolet region. Our dual-comb spectra exhibit a signal-to-noise ratio at the quantum limit, a comb line spacing and a center frequency which can be chosen by dialing a knob. We further show that photon-counting dual-comb spectroscopy [4] is a powerful approach for further extension at shorter wavelengths.

Optical microresonators are attractive comb sources due to their small form factor and stable broad optical spectra. We report on the first demonstration of microcomb-based digital holography. The large line spacing of microcombs promises an unprecedented combination of precision, fast update rate and ambiguity ranges on the scale of a few mm. Using a pulse-driven lithium niobate microcomb of 100 GHz line spacing and a scanning Michelson interferometer, we generate spectral hypercubes of holograms. Our first experimental results show that the amplitude and phase information of the object can be recovered for more than 100 comb lines.

A high-resolution broad-spectral-bandwidth spectrometer on a chip would create new opportunities for gas-phase molecular fingerprinting, especially in environmental sensing. A resolution high enough to observe transitions at atmospheric pressure and the simultaneous sensitive detection of multiple atoms or molecules are the key challenges. Here, an electro-optic microring-based dual-comb interferometer, fabricated on a low-loss lithium-niobate-on-insulator nanophotonic platform, demonstrates significant progress towards such an achievement. Spectra spanning 1.6 THz (53 cm-1) at a resolution of 10 GHz (0.33 cm-1) are obtained in a single measurement without requiring frequency scanning or moving parts. The frequency agility of the system enables spectrally-tailored multiplexed sensing, which allows for interrogation of non-adjacent spectral regions, here separated by 6.6 THz (220 cm-1), without compromising the signal-to-noise ratio.

Our ability to generate new distributions of light has been remarkably enhanced in recent years. At the most fundamental level, these light patterns are obtained by ingeniously combining different electromagnetic modes. Interestingly, the modal superposition occurs in the spatial, temporal as well as spatio-temporal domain. This generalized concept of structured light is being applied across the entire spectrum of optics: generating classical and quantum states of light, harnessing linear and nonlinear light-matter interactions, and advancing applications in microscopy, spectroscopy, holography, communication, and synchronization. This Roadmap highlights the common roots of these different techniques and thus establishes links between research areas that complement each other seamlessly. We provide an overview of all these areas, their backgrounds, current research, and future developments. We highlight the power of multimodal light manipulation and want to inspire new eclectic approaches in this vibrant research community.

Fourier transform measurements of line positions of the 3-0 band of CO are reported and compared to the corresponding best predicted Dunham estimates.

Near-infrared and visible frequency combs, combined with fast CMOS cameras and multi-heterodyne read-out, open up new opportunities for lens-less imaging, three-dimensional metrology and scan-free wavefront reconstruction.

A frequency comb, a spectrum of equidistant phase-coherent laser lines, can be harnessed for new approaches to interferometry. The dual-comb interferometer exploits the time-domain interference between two combs of slightly different line spacing. The instrument, which performs direct frequency measurements over a broad spectral bandwidth, opens up new perspectives in applications such as spectroscopy, distance metrology or holography.