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The field of integrated photonics has significantly impacted numerous fields including communication, sensing, and quantum physics owing to the efficiency, speed, and compactness of its devices. However, the reliance on off‐chip bulk lasers compromises the compact nature of these systems. While silicon photonics and III‐V platforms have established integrated laser technologies, emerging demands for ultra‐low optical loss, wider bandgaps, and optical nonlinearities necessitate other platforms. Developing integrated lasers on less mature platforms is arduous and costly due to limited throughput or unconventional process requirements. In response, a novel platform‐agnostic laser integration technique is proposed, utilizing a singular design and process flow, applicable without modification to a diverse range of platforms. Leveraging a two‐step micro‐transfer printing method, nearly identical laser performance is achieved across platforms with refractive indices between 1.7 and 2.5. Experimental validation demonstrates strikingly similar laser characteristics between devices processed on lithium niobate and silicon nitride platforms. Furthermore, the integration of a laser with a resonant electro‐optic comb generator on the thin‐film lithium niobate platform is showcased, producing over 80 comb lines spanning 12 nm. This versatile technique transcends platform‐specific limitations, facilitating applications like microwave photonics, handheld spectrometers, and cost‐effective Lidar systems, across multiple platforms. [ABSTRACT FROM AUTHOR]

Abstract Surface plasmon resonance (SPR) sensors are based on photon-excited surface charge density oscillations confined at metal-dielectric interfaces, which makes them highly sensitive to biological or chemical molecular bindings to functional metallic surfaces. Metal nanostructures further concentrate surface plasmons into a smaller area than the diffraction limit, thus strengthening photon-sample interactions. However, plasmonic sensors based on intensity detection provide limited resolution with long acquisition time owing to their high vulnerability to environmental and instrumental noises. Here, we demonstrate fast and precise detection of noble gas dynamics at single molecular resolution via frequency-comb-referenced plasmonic phase spectroscopy. The photon-sample interaction was enhanced by a factor of 3,852 than the physical sample thickness owing to plasmon resonance and thermophoresis-assisted optical confinement effects. By utilizing a sharp plasmonic phase slope and a high heterodyne information carrier, a small atomic-density modulation was clearly resolved at 5 Hz with a resolution of 0.06 Ar atoms per nano-hole (in 10–11 RIU) in Allan deviation at 0.2 s; a faster motion up to 200 Hz was clearly resolved. This fast and precise sensing technique can enable the in-depth analysis of fast fluid dynamics with the utmost resolution for a better understanding of biomedical, chemical, and physical events and interactions.

Surface plasmon resonance (SPR) sensors are based on photon-excited surface charge density oscillations confined at metal-dielectric interfaces, which makes them highly sensitive to biological or chemical molecular bindings to functional metallic surfaces. Metal nanostructures further concentrate surface plasmons into a smaller area than the diffraction limit, thus strengthening photon-sample interactions. However, plasmonic sensors based on intensity detection provide limited resolution with long acquisition time owing to their high vulnerability to environmental and instrumental noises. Here, we demonstrate fast and precise detection of noble gas dynamics at single molecular resolution via frequency-comb-referenced plasmonic phase spectroscopy. The photon-sample interaction was enhanced by a factor of 3,852 than the physical sample thickness owing to plasmon resonance and thermophoresis-assisted optical confinement effects. By utilizing a sharp plasmonic phase slope and a high heterodyne information carrier, a small atomic-density modulation was clearly resolved at 5 Hz with a resolution of 0.06 Ar atoms per nano-hole (in 10–11 RIU) in Allan deviation at 0.2 s; a faster motion up to 200 Hz was clearly resolved. This fast and precise sensing technique can enable the in-depth analysis of fast fluid dynamics with the utmost resolution for a better understanding of biomedical, chemical, and physical events and interactions. [ABSTRACT FROM AUTHOR]

We theoretically investigate the frequency shift of a free-running semiconductor laser when optically injected by an optical frequency comb. By numerical integration of the laser rate equations, we have identified two types of frequency shifts, frequency pulling, and frequency pushing. The free-running slave laser seems to switch between pulling and pushing in a regular manner. We have systematically determined the dependence of these shifts on comb parameters, including comb spacing and number of lines. We have found that the occurring frequency pushing seems to be almost independent of comb signals and comb spacing, while in contrast, the frequency pulling shows weak dependence on comb signals but largely affected by comb spacing. We have also shown that carrier density dynamic is the origine of these phenomena.

Since the beginning of this millennium, frequency comb generators have reshaped frequency metrology and related areas. After more than two decades since their first realization, several other ways to generate frequency combs, in any spectral region, have been demonstrated, each way with its peculiar features. This trend has triggered the need to quantitatively assess how close the new comb realizations are to an ideal comb, a feature that will be called combness throughout this paper. We will briefly review the very dynamic area of novel frequency comb sources and we will describe the techniques that have been recently developed to quantitatively assess the key parameters of old and new frequency combs, in view of the specific applications. Finally, we will try to sketch future steps in this recently born research area.

Subject and Purpose. Narrow-band filters are among the basic components of modern communication systems, instruments for spectroscopy, high-sensitivity sensors, etc. Photonic crystal structures open up broad possibilities for creating compact-sized, narrow-band filters in the optical and terahertz ranges. Tuning of spectral characteristics of photonic crystal filters is usually carried out through introduction of certain elements into their structure that are sensitive to external electric and magnetic fields. This work has been aimed at investigating electrodynamic characteristics of one-dimensional magnetophotonic crystals with structural layers characterized by "hyperbolic" dispersion, and suggesting a multichannel, narrow-band filter on their base. Methods and Methodology. The dispersion equation for excitations in an infinite magnetophotonic crystal has been obtained within the framework of the Floquet-Bloch theory, with the use of fundamental solutions of Hill’s equation. The transfer matrix approach has been used to obtain an analytical expression for the transmission coefficient. Results. The band diagram of the one-dimensional magnetophotonic crystal has been analyzed for the case where one of the layers on the structure’s spatial period is characterized by a hyperbolic dispersion law. The areas of existence of surface wave regimes have been found for such layers for the case of normal incidence of the wave upon the finite-seized magnetophotonic crystal. Frequency dependences of the transmission coefficient are characterized by a set of high-Q resonant peaks relating to Fabry-Perot resonances in a periodic structure of finite length. Conclusions. Application of a finite-seized, one-dimensional magnetophotonic crystal is considered as of a means forachieving multichannel optical filtering and formation of a frequency comb. Expressions for the dispersion equation and transmission coefficient have been obtained within the framework of the Floquet-Bloch theory and with the use of the transfer matrix. The feasibility of surface mode excitation has been shown for gyrotropic layers of the periodic structure characterized by a hyperbolic dispersion law, for the case of normal incidence upon the magnetophotonic crystal. The spectral response of the filter contains narrow-band peaks with a high transmission efficiency. By increasing the number of the structure’s periods it is possible to form a frequency comb, which effect can be useful for applications in metrology and modern optical communication systems.

In research and engineering, short laser pulses are fundamental for metrology and communication. The generation of pulses by passive mode-locking is especially desirable due to the compact setup dimensions, without the need for active modulation requiring dedicated external circuitry. However, well-established models do not cover regular self-pulsing in gain media that recover faster than the cavity round trip time. For quantum cascade lasers (QCLs), this marked a significant limitation in their operation, as they exhibit picosecond gain dynamics associated with intersubband transitions. We present a model that gives detailed insights into the pulse dynamics of the first passively mode-locked QCL that was recently demonstrated. The presence of an incoherent saturable absorber, exemplarily realized by multilayer graphene distributed along the cavity, drives the laser into a pulsed state by exhibiting a similarly fast recovery time as the gain medium. This previously unstudied state of laser operation reveals a remarkable response of the gain medium on unevenly distributed intracavity intensity. We show that in presence of strong spatial hole burning in the laser gain medium, the pulse stabilizes itself by suppressing counter-propagating light and getting shortened again at the cavity facets. Finally, we study the robustness of passive mode-locking with respect to the saturable absorber properties and identify strategies for generating even shorter pulses. The obtained results may also have implications for other nanostructured mode-locked laser sources, for example, based on quantum dots. [ABSTRACT FROM AUTHOR]

Next-generation 6G communication holds the potential to revolutionize data transfer, enabling the realization of eXtended Reality (XR) with enhanced sensory experiences. To achieve this, advanced components such as high-performance intensity/phase modulators, waveguides, multiplexers, splitters, combiners, and filters operating in terahertz (THz) regime, specifically within the frequency range of 0.1–1 THz, are essential. However, existing microwave equipment and vector network analyzers designed for this frequency range suffer from limitations in resolution, stability, and accuracy when evaluating the intensity and phase responses of critical 6G THz devices. In this comprehensive review, we delve into the critical device requirements and emerging trends in next-generation 6G communication, essential performance evaluation parameters, comparisons between microwave and nano/microphotonic devices for testing, and the application of high-resolution THz sensors in 6G Internet-of-Things (IoT) scenarios. Notably, a frequency comb in the photonic regime emerges as the prime candidate for achieving precision evaluations of 6G networks and devices. Consequently, this review highlights the latest research in frequency comb measurements in the 6G THz frequency regime, with a particular emphasis on nano/microphotonic devices and metamaterials. The integration of frequency comb measurements into 6G and THz photonic devices and networks promises to accelerate the realization of high-density next-generation 6G communication. [ABSTRACT FROM AUTHOR]

This review paper deals with length measurement based on a spectral-domain interferometer (SDI) using an optical comb. The SDI can measure the optical path difference in real time by acquiring the interference spectrum in a wide spectral bandwidth. A typical SDI using a broadband light source has a limited measurement range of only several millimeters due to its short coherence length. The characteristics of the frequency comb (wide spectral bandwidth and narrow linewidth of each frequency mode) dramatically enhance the measurement range of the SDI to more than several kilometers while also enabling highly reliable measurements based on length standards. In this paper, research on a SDI using a frequency comb with a mode-locked laser, a SDI using a frequency comb with a high repetition rate, and other applications of SDIs using frequency combs are summarized. With the development of photonic technology in the future, SDIs that use a frequency comb can be expected to be more widely used in various industrial and scientific areas and more aptly to realize the 'meter' as a length standard. [ABSTRACT FROM AUTHOR]

This work explores the frequency comb of magnetic vortex in a square disk and a circular disk created by an out-of-plane and in-plane oscillating field using micromagnetic modeling. The broadest frequency comb ever observed in a magnetic system is displayed. When driven by an out-of-plane field, the robustness of the frequency comb of a vortex in a square disk is significantly enhanced due to a reduction in rotational symmetry when compared to a vortex in a circular disk. In addition, comb lines increase as driven amplitude increases, and frequency spacing could be adjusted in the square disk, so facilitating its application. In a situation where the vortex is driven by in-plane field, the square disk exhibits a significantly neater frequency comb than the circular disk. The conclusion is that magnetic vortex in a square disk is better suited for frequency comb applications. [ABSTRACT FROM AUTHOR]

For the applications of the frequency comb in microresonators, it is essential to obtain a fully frequency-stabilized microcomb laser source. In this study, we present a system for generating a fully atom-referenced stabilized soliton microcomb. The pump light around 1560.48 nm is locked to an ultra-low-expansion (ULE) cavity. This pump light is then frequency-doubled and referenced to the atomic transition of 87Rb. The repetition rate of the soliton microcomb is injection-locked to an atomic-clock-stabilized radio frequency (RF) source, leading to mHz stabilization at 1 s. As a result, all comb lines have been frequency-stabilized based on the atomic reference and the ULE cavity, achieving a very high precision of approximately 18 Hz at 1 s, corresponding to the frequency stability of 9.5 × 10−14. Our approach provides a fully stabilized microcomb experiment scheme with no requirement of f-2f technique, which could be easily implemented and generalized to various photonic platforms, thus paving the way towards the ultraprecise optical sources for high precision spectroscopy. [ABSTRACT FROM AUTHOR]

Chip‐scale optical frequency combs have attracted significant research interest and can be used in applications ranging from precision spectroscopy to telecom channel generators and lidar systems. In the time domain, microresonator based frequency combs correspond to self‐stabilized soliton pulses. In two distinct regimes, microresonators are shown to emit either bright solitons in the anomalous dispersion regime or dark solitons (a short time of darkness in a bright background signal) in the normal dispersion regime. Here, the dynamics of continuous‐wave‐laser‐driven soliton generation is investigated in the zero‐group‐velocity‐dispersion regime as well as the generation of solitons that are spectrally crossing different dispersion regimes. In the measurements, zero‐dispersion solitons with multipeak structures (soliton molecules) are observed with distinct and predictable spectral envelopes that are a result of fifth‐order dispersion of the resonators. Numerical simulations and the analysis of bifurcation structures agree well with the observed soliton states. This is the first observation of soliton generation that is governed by fifth‐order dispersion, which can have applications in ultrafast optics, telecom systems, and optical spectroscopy. [ABSTRACT FROM AUTHOR]

Digital radar waveforms such as orthogonal frequency-division multiplexing (OFDM) often have the disadvantage that they require high sampling rates if fine range resolutions have to be achieved. The frequency comb OFDM radar scheme offers a possibility to overcome this drawback and to improve the range resolution without increasing the sampling rate. Simultaneously, the high unambiguous velocity, which is one of the advantages of digital radar waveforms, is retained and due to the simple generation of orthogonal transmit signals, it is well suited for multiple-input multiple-output (MIMO) applications. To prove all these features of the frequency comb OFDM radar scheme, a suitable 4 × 4 MIMO demonstrator including frequency comb generation as well as up- and downconversion with these combs has been set up. Its functionality has been validated with real measurements in an anechoic chamber in conjunction with a radar target simulator to emulate very high velocities. [ABSTRACT FROM AUTHOR]

With increasing demands on resolution and flexibility in current and future radar applications, the focus is moving to digital radar systems such as orthogonal frequency-division multiplexing (OFDM) radars. To achieve high bandwidths and consequently a high range resolution, high sampling rates are needed. To overcome this constraint, an approach called frequency comb OFDM radar has been developed. This paper presents a novel, hardware efficient implementation of such a frequency comb OFDM radar including a novel way of comb generation. Special attention is put on the suppression of unwanted frequency components. Measurements which demonstrate the functionality of the hardware efficient radar system in combination with the frequency comb OFDM technique are presented. [ABSTRACT FROM AUTHOR]

The discovery of stable and broad frequency combs in monochromatically pumped high-Q optical Kerr microresonators caused by the generation of temporal solitons can be regarded as one of the major breakthroughs in nonlinear optics during the last two decades. The transfer of the soliton–comb concept to χ (2) microresonators promises lowering of the pump power, new operation regimes, and entering of new spectral ranges; scientifically, it is a big challenge. Here we represent an overview of stable and accessible soliton–comb regimes in monochromatically pumped χ (2) microresonators discovered during the last several years. The main stress is made on lithium niobate-based resonators. This overview pretends to be rather simple, complete, and comprehensive: it incorporates the main factors affecting the soliton–comb generation, such as the choice of the pumping scheme (pumping to the first or second harmonic), the choice of the phase matching scheme (natural or artificial), the effects of the temporal walk off and dispersion coefficients, and also the influence of frequency detunings and Q-factors. Most of the discovered nonlinear regimes are self-starting—they can be accessed from noise upon a not very abrupt increase in the pump power. The soliton–comb generation scenarios are not universal—they can be realized only under proper combinations of the above-mentioned factors. We indicate what kind of restrictions on the experimental conditions have to be imposed to obtain the soliton–comb generation. [ABSTRACT FROM AUTHOR]

A Kerr microresonator frequency comb, combined with an ultrafast photodiode, has been utilized to generate millimeter- and terahertz-waves with low-phase-noise. This new light source shows promise for wireless communication above the 100 GHz band, where a high signal-to-noise ratio carrier signal is necessary to achieve high data rates. In this study, two efficient wireless link architectures based on a microresonator comb are demonstrated. The experiment shows that simultaneous modulation and detection of multiple comb lines leads to more than 10 times stronger modulation signal strength compared to two-line detection at the receiver. The successful transmission of complex modulation formats up to 64 quadrature amplitude modulation confirms that the microresonator comb and the proposed modulation method are effective for modern wireless communication.

As a novel portable and robust broadband coherent light source, mid-infrared (MIR) Kerr microresonator-based frequency combs (microcombs) have prospective applications in the precision spectroscopy of molecules and biochemical sensing. The mature integrated silicon photonics platform is well suited for the MIR microcombs study because silicon has both large linear and nonlinear refractive index, but the transparency window of the platform is limited by the cladding material. Here, we numerically demonstrate the generation of a broadband MIR comb in a silicon microring resonator, harnessing the large-cross-section air-cladding waveguide to alleviate the absorption loss. The effects of higher order nonlinearities are also investigated, which show that the effect of five-photon absorption around the pump wavelength (4.78 $\mu$m) is negligible while an octave-spanning (3.5-8 $\mu$m) Raman-Kerr comb line can be obtained with the assistance of Raman effect and a quite pure Kerr frequency soliton comb can also be achieved at large detuning. The proposed structure can be compatible with the CMOS technology, thus can be a very promising solution to the MIR integrated photonics.

The fabrication processes of silicon nitride (Si3N4) photonic devices used in foundries require low temperature deposition, which typically leads to high propagation losses. Here, it is shown that propagation loss as low as 0.42 dB cm−1 can be achieved using foundry compatible processes by solely reducing waveguide surface roughness. By postprocessing the fabricated devices using rapid thermal anneal (RTA) and furnace anneal, propagation losses down to 0.28 dB cm−1 and 0.06 dB cm−1, respectively, are achieved. These low losses are comparable to the conventional devices using high temperature, high‐stress LPCVD films. The dispersion of the devices is also tuned, and it is proved that these devices can be used for linear and nonlinear applications. Low threshold parametric oscillation, broadband frequency combs, and narrow‐linewidth laser are demonstrated. This work demonstrates the feasibility of scalable photonic systems based on foundries. [ABSTRACT FROM AUTHOR]

A frequency comb generator (FCG) based on dual-cavity Brillouin random fiber lasing oscillation in the 1.5 μm telecon spectral window is established and experimentally demonstrated. In the half-open main cavity of the dual cavity, the stimulated Brillouin scattering in highly nonlinear fiber (HNLF) and Rayleigh scattering in single-mode fiber are employed to provide sufficient Brillouin gain and the randomly distributed feedback, respectively, for random mode resonance. The sub-cavity includes an Er-doped fiber amplifier to couple back and boost lower-order Stokes and anti-Stokes light for the cascade of stimulated Brillouin scattering to generate multiple higher-order Stokes and anti-Stokes light. Meanwhile, efficient four-wave mixing is stimulated in the HNLF-based main cavity, further enhancing the number and intensity of the resonant Stokes and anti-Stokes light. By taking advantages of the unique transmission characteristics of nonlinear optical loop mirrors, the power deviation between Stokes and anti-Stokes lines is further optimized with 17 orders of stable Stokes lines and 15 orders of stable anti-Stokes lines achieved within the 10 dB power deviation, with maximum optical signal-to-noise ratio (OSNR) of ~22 dB and ~17 dB and minimum OSNR of ~10 dB and ~7.5 dB for Stokes and anti-Stokes lines, respectively. In addition, the dynamic characteristics of the proposed FCG have been experimentally investigated. Such an FCG with fixed frequency spacing will find promising applications in fields of optical communication, microwave, optical sensing, etc. [ABSTRACT FROM AUTHOR]

We demonstrate gain-sharing polarization-duplex dual-comb laser. Here, a nonlinear amplifying loop mirror (NALM) with a single gain fiber is used to achieve simultaneous mode-locking for the generation of the two frequency combs with slightly different repetition rates. Our system provides stable mode-locking and a good overlap of frequency comb spectra. The repetition rate difference is about 100 kHz and has high stability: we determined a standard deviation of 0.75 Hz, and the Allan deviation is 0.073 Hz at an integration time of 1 s for this free-running system. In the case of simultaneous mode-locking, our system shows a relative intensity noise (RIN) characteristic that has, to the best our knowledge, not yet been reported for dual-comb lasers. On the other hand, the phase noise characteristic of our mode-locked dual-comb laser is similar to the characteristic obtained when one beam path is blocked and the laser is used for single frequency-comb generation.

To meet the high demand of data transmission capacity, optical communications systems have been developed. In order to increase the channel numbers for larger communication bandwidth, multi-mode lasers and laser arrays can be used. As an alternative coherent light source, optical frequency comb (OFC) contains multi-wavelength signal, and hence enables communication with data stream of tens of terabit/s. Fully integrated electrically driven OFCs are expected as a compact, robust, and low-cost light source for data communication. In this review article, the recent development progress on fully integrated electrically driven OFC generators are reviewed, with focus on the demonstrations in the past five years. Based on comb generation approaches, the works are categorized into two main types: one is OFC generators based on four-wave mixing in high-Q resonator, and the other is OFC generators based on mode-locked laser. At the end, a summary and future outlook are provided.

This paper is devoted to a detailed analysis of the appearance of frequency combs in the dynamics of a micro-electro-mechanical systems (MEMS) resonator featuring 1:2 internal resonance. To that purpose, both experiments and numerical predictions are reported and analysed to predict and follow the appearance of the phononic frequency comb arising as a quasi-periodic regime between two Neimark-Sacker bifurcations. Numerical predictions are based on a reduced-order model built thanks to an implicit condensation method, where both mechanical nonlinearities and electrostatic forces are taken into account. The reduced order model is able to predict a priori, i.e. without the need of experimental calibration of parameters, and in real time, i.e. by solving one or two degrees-of-freedom system of equations, the nonlinear behaviour of the MEMS resonator. Numerical predictions show a good agreement with experiments under different operating conditions, thus proving the great potentiality of the proposed simulation tool. In particular, the bifurcation points and frequency content of the frequency comb are carefully predicted by the model, and the main features of the periodic and quasi-periodic regimes are given with accuracy, underlining that the complex dynamics of such MEMS device is effectively driven by the characteristics of the 1:2 internal resonance. [ABSTRACT FROM AUTHOR]

Radars can be used as sensors to detect the breathing of victims trapped under layers of building materials in catastrophes like earthquakes or gas explosions. In this contribution, we present the implementation of a novel frequency comb continuous wave (FCCW) bioradar module using a commercial software-defined radio (SDR). The FCCW radar transmits multiple equally spaced frequency components simultaneously. The data acquisition of the received combs is frequency domain-based. Hence, it does not require synchronization between the transmit and receive channels, as time domain-based broadband radars, such as ultra wideband (UWB) pulse radar and frequency-modulated CW (FMCW) radar, do. Since a frequency comb has an instantaneous wide bandwidth, the effective scan rate is much higher than that of a step frequency CW (SFCW) radar. This FCCW radar is particularly suitable for small motion detection. Using inverse fast Fourier transform (IFFT), we can decompose the received frequency comb into different ranges and remove ghost signals and interference of further range intervals. The frequency comb we use in this report has a bandwidth of only 60 MHz, resulting in a range resolution of up to 2.5 m, much larger than respiration-induced chest wall motions. However, we demonstrate that in the centimeter range, motions can be detected and evaluated by processing the received comb signals. We want to integrate the bioradar into an unmanned aircraft system for fast and safe search and rescue operations. As a trade-off between ground penetrability and the size and weight of the antenna and the radar module, we use 1.3 GHz as the center frequency. Field measurements show that the proposed FCCW bioradar can detect an alive person through different nonmetallic building materials. [ABSTRACT FROM AUTHOR]

The generation of a quadratic soliton in a pulse-pumped microresonator has attracted significant interest in recent years. The strong second-order nonlinearity and high peak power of pumps offer a straightforward way to increase efficiency. In this case, the influence of the third-order nonlinearity effect becomes significant and cannot be ignored. In this paper, we study the quadratic soliton in a degenerate optical parametric oscillator driven synchronously by the pulse pump with third-order nonlinearity. Our simulations verify that the robustness of quadratic soliton generation is enhanced when the system experiences a perturbation from pump power, cavity detuning, and pump pulse width. These results represent a new way of manipulating frequency comb in resonant microphotonic structures. [ABSTRACT FROM AUTHOR]

The present study proposes a terahertz quantum cascade laser frequency comb (THz QCL FC) with a semi-insulated surface plasma waveguide characterized by a low threshold current density, high power and a wide current dynamic range. The gain dispersion value and the nonlinear susceptibility were optimized based on the combination of a hybrid bound-to-continuum active region with a semi-insulated surface plasmon waveguide. Without any extra dispersion compensator, stable frequency comb operation within a current dynamic range of more than 97% of the total was revealed by the intermode beat note map. Additionally, a total comb spectral emission of about 300 GHz centered around 4.6 THz was achieved for a 3 mm long and 150 µm wide device. At 10 K, a maximum output power of 22 mW was obtained with an ultra-low threshold current density of 64.4 A·cm−2. [ABSTRACT FROM AUTHOR]

We have numerically investigated the dynamics of frequency shifts in semiconductor lasers under the injection of a frequency comb. We have studied the effect of comb spacing on the locking bandwidth. Frequency comb spacing was found to play an important role in the boundaries of the locking bandwidth as well as in the frequency shift of the SL peak. [ABSTRACT FROM AUTHOR]

Dispersion and dissipation are the key parameters of the optical components. Generally, dissipation is an unfavorable factor. In this article, we designed a dispersion-dissipation hybird coupling optomechanical cavity and achieved dissipation-assisted frequency comb and chaos generation. We studied the bistable state and the spectrum of optomechanical cavity. We found that chaos and frequency comb can be effectively adjusted by adjusting the dissipation coupling. We given out the formation mechanism of the two types of frequency combs of the optomechanical cavity. Our result provides an effective way to achieving low threshold, adjustable frequency comb and chaos in an optical microcavity.

Three-dimensional integrated circuits (3D-ICs) are becoming more significant in portable devices, autonomous vehicles, and data centers. As the demand for highly integrated and high-performance semiconductor devices grows, recent 3D integration technologies focus on lowering the size of the micro-structures on such devices for high density. In order to inspect the 3D semiconductor devices, it is critical to measure the heights, depths, and overall surface profiles of the micro-structures made with silicon materials. Here, we demonstrate precise surface imaging for silicon devices by using a femtosecond mode-locked laser centered at 785 nm wavelength and an electro-optic sampling-based time-of-flight detection method with sub-10-nanometer axial precision. We could successfully measure the surface profiles as well as the step heights of silicon wafer stacks and micro-scale structures on silicon substrates.

In this paper, we report the frequency comb response experimentally and analytically in a rhombic micro-resonator with parametrical modulation. When the electrostatically actuated rhombic micro-resonator is modulated axially by a low-frequency periodic excitation, a comb-like vibration response with few equidistant positioned fingers in the frequency domain is observed. The finger spacing of frequency comb response is exactly consistent with modulation frequency and the number and amplitude of the fingers can be tuned by modulation strength. A mixed frequency comb with extra comb fingers is further generated when the resonator is modulated simultaneously by two different low-frequency excitation signals. By adjusting the relation of the two modulation frequencies, unequal spacing frequency combs are achieved for the first time, which leads to a more flexible tunability of the comb spacing for different applications. Theoretical analysis based on the dynamic model well explains the corresponding observations. [ABSTRACT FROM AUTHOR]

Three-dimensional integrated circuits (3D-ICs) are becoming more significant in portable devices, autonomous vehicles, and data centers. As the demand for highly integrated and high-performance semiconductor devices grows, recent 3D integration technologies focus on lowering the size of the micro-structures on such devices for high density. In order to inspect the 3D semiconductor devices, it is critical to measure the heights, depths, and overall surface profiles of the micro-structures made with silicon materials. Here, we demonstrate precise surface imaging for silicon devices by using a femtosecond mode-locked laser centered at 785 nm wavelength and an electro-optic sampling-based time-of-flight detection method with sub-10-nanometer axial precision. We could successfully measure the surface profiles as well as the step heights of silicon wafer stacks and micro-scale structures on silicon substrates. [ABSTRACT FROM AUTHOR]

Generation of nonlinear frequency combs in χ(3) optical microresonators has attracted tremendous research interest during the last decade. Recently, realization of the microcomb owing to χ(2) optical nonlinearity in the microresonator promises new breakthroughs and is a big scientific challenge. Moreover, it is of high scientific interest that the presence of both second- and third-order nonlinearities results in complex cavity dynamics. In particular, the role of χ(3) nonlinearity in the generation of the quadratic microcomb is still far from being well understood. Here, we demonstrate the interaction between the second- and third-order nonlinearity in the lithium niobate microresonator, which can provide a new way of phase matching to control the mode-locking condition and pulse number for the quadratic microcomb. Our results verify that the Kerr nonlinearity can benefit the quadratic microcomb. The principle can be further extended to other material platforms to provide more manipulation methods for comb generation based on χ(2) nonlinearity at mid-infrared.

A novel study on harmonic frequency combs emitted by quantum cascade lasers (QCLs) is presented here, demonstrating the presence of intensity correlations between twin modes characterizing the emission spectra. These originate from a four‐wave mixing process driven by the active medium's third‐order nonlinearity. The study of such correlations is essential for the engineering of a new generation of semiconductor devices with the potential of becoming integrated emitters of light with quantum properties, such as squeezing and entanglement. Starting from experimental results, the limits of state‐of‐the‐art technology are discussed as well as the possible methodologies that could lead to the detection of nonclassical phenomena, or alternatively improve the design of QCLs, in the compelling perspective of generating quantum correlations in mid‐infrared light.

The discovery of stable and broad frequency combs in monochromatically pumped high-Q optical Kerr microresonators caused by the generation of temporal solitons can be regarded as one of the major breakthroughs in nonlinear optics during the last two decades. The transfer of the soliton–comb concept to χ(2) microresonators promises lowering of the pump power, new operation regimes, and entering of new spectral ranges; scientifically, it is a big challenge. Here we represent an overview of stable and accessible soliton–comb regimes in monochromatically pumped χ(2) microresonators discovered during the last several years. The main stress is made on lithium niobate-based resonators. This overview pretends to be rather simple, complete, and comprehensive: it incorporates the main factors affecting the soliton–comb generation, such as the choice of the pumping scheme (pumping to the first or second harmonic), the choice of the phase matching scheme (natural or artificial), the effects of the temporal walk off and dispersion coefficients, and also the influence of frequency detunings and Q-factors. Most of the discovered nonlinear regimes are self-starting—they can be accessed from noise upon a not very abrupt increase in the pump power. The soliton–comb generation scenarios are not universal—they can be realized only under proper combinations of the above-mentioned factors. We indicate what kind of restrictions on the experimental conditions have to be imposed to obtain the soliton–comb generation.

Coherent reception becomes an interesting option when data rates in time-division-multiplexed (TDM) passive optical networks (PONs) grow beyond 50 Gbit/s. Controlling the wavelength, i.e., the optical frequency, and the phase of the laser acting as local oscillator (LO) is one of the main technical challenges in the design of coherent TDM PONs. In the optical network units (ONUs), low-cost lasers are required, which come at the expense of wavelength variations and drifts over multiple nanometers due to fabrication imperfections, and temperature variations. This contradicts the requirement of wavelength-stable LOs in coherent receivers. The use of a wavelength locker circuit and a temperature controller is considered as too complex for applications in access networks. In this work, we propose a novel colorless coherent architecture with high resilience to ONU laser wavelength drifts of up to $\pm$ 4 nm ($\pm$ 0.5 THz) for future 100 Gbit/s PON. It allows the use of distributed feedback lasers at the ONU side. This is rendered possible by generating a frequency comb with carefully chosen free spectral range in a quantum-dash mode-locked laser diode at the optical line terminal. In upstream operation, the frequency comb serves as an LO, whereas the same information is modulated onto all comb lines for the case of downstream. As a result, the ONU laser can drift over the entire comb bandwidth without substantial performance penalty. We experimentally demonstrate downstream and upstream operation with an aggregated raw data rate of 96 Gbit/s, respectively. We further introduce advanced digital signal processing (DSP) methods including a coarse frequency offset compensation (CFOC) and a multiple-input multiple-output (MIMO) equalizer to improve the performance of our concept. We show that the receiver sensitivity can be increased by 3 dB for a high-bandwidth receiver when using a 6 × 2 MIMO equalizer scheme. A 4 × 2 MIMO equalizer scheme enables colorless reception even with a limited-bandwidth receiver. [ABSTRACT FROM AUTHOR]

Structures carrying moving loads present rich vibration phenomena and colorful spectrum characteristics. This study analytically investigates the moving-inertial-load-pumped frequency comb free vibration behavior of a single-span rectangular linear elastic Kirchhoff thin plate in the low-speed region. Multiple-time-scales expansion is made and the asymptotic solution describing the non-resonance modal oscillation is derived. Case studies on a single-span rectangular isotropic plate with the clamped opposite boundaries dominated by the first two transverse flexural modes are presented. Numerical computation based on finite element method structure modeling and response numerical integration verifies the modal frequency comb characteristics as predicted by the proposed theoretical model. The results also present the weak modal interaction for the studied single-span structure in the observed light-mass low moving speed region. The outcomes of this study will benefit the related structural dynamic system design and intervention. [ABSTRACT FROM AUTHOR]

The nonlinear modal coupling in a T-shaped piezoelectric resonator, when the former two natural frequencies are away from 1:2, is studied. Experimentally sweeping up the exciting frequency shows that the horizontal beam exhibits a nonlinear hardening behavior. The first primary resonance of the vertical beam, owing to modal coupling, exhibits an abrupt amplitude increase, namely the Hopf bifurcation. The frequency comb phenomenon induced by modal coupling is measured experimentally. A Duffing-Mathieu coupled model is theoretically introduced to derive the conditions of the modal coupling and frequency comb phenomenon. The results demonstrate that the modal coupling results from nonlinear stiffness hardening and is strictly dependent on the loading range and sweeping form of the driving voltage and the frequency of the piezoelectric patches. [ABSTRACT FROM AUTHOR]

A scheme is proposed to generate multi-color quantum steering frequency comb in a two-dimensional hexagonal-poled LiTaO3crystal through enhanced Raman scattering. The quantum steering correlations among the Stokes and anti-Stokes Raman beams are demonstrated by applying the criteria for genuine multipartite steering. Multi-color quantum steering frequency comb generated in present scheme has potential application prospects in quantum information processing.

Advances in laser spectroscopy have enabled many scientific breakthroughs in physics, chemistry, biology, and astronomy. Optical frequency combs pushed measurement limits with ultrahigh-frequency accuracy and fast-measurement speed, while tunable-diode-laser spectroscopy is used in scenarios that require high power and continuous spectral coverage. Despite these advantages of tunable-diode-laser spectroscopy, it is challenging to precisely determine the instantaneous laser frequency because of fluctuations in the scan speed. Here, we demonstrate a simple spectroscopy scheme with a frequency-modulated diode laser that references the laser on-the-fly to a fiber cavity. The fiber cavity's free spectral range is on-the-fly calibrated with sub-10-Hz frequency precision. We achieve a relative precision of the laser frequency of 2×10−8 for an 11-THz frequency range at a measurement speed of 1 THz/s. This is an improvement of more than 2 orders of magnitude compared to existing diode-laser-spectroscopy methods. Our scheme provides precise frequency calibration markers, while simultaneously tracking the instantaneous scan speed of the laser. We demonstrate the versatility of our method through various applications, including dispersion measurement of a fiber, ultrahigh- Q microresonators, and spectroscopy of a hydrogen fluoride gas cell. The simplicity, robustness, and low cost of this spectroscopy scheme are valuable for out-of-the-lab applications like lidar and environmental monitoring. [ABSTRACT FROM AUTHOR]

A frequency comb generator (FCG) based on dual-cavity Brillouin random fiber lasing oscillation in the 1.5 μm telecon spectral window is established and experimentally demonstrated. In the half-open main cavity of the dual cavity, the stimulated Brillouin scattering in highly nonlinear fiber (HNLF) and Rayleigh scattering in single-mode fiber are employed to provide sufficient Brillouin gain and the randomly distributed feedback, respectively, for random mode resonance. The sub-cavity includes an Er-doped fiber amplifier to couple back and boost lower-order Stokes and anti-Stokes light for the cascade of stimulated Brillouin scattering to generate multiple higher-order Stokes and anti-Stokes light. Meanwhile, efficient four-wave mixing is stimulated in the HNLF-based main cavity, further enhancing the number and intensity of the resonant Stokes and anti-Stokes light. By taking advantages of the unique transmission characteristics of nonlinear optical loop mirrors, the power deviation between Stokes and anti-Stokes lines is further optimized with 17 orders of stable Stokes lines and 15 orders of stable anti-Stokes lines achieved within the 10 dB power deviation, with maximum optical signal-to-noise ratio (OSNR) of ~22 dB and ~17 dB and minimum OSNR of ~10 dB and ~7.5 dB for Stokes and anti-Stokes lines, respectively. In addition, the dynamic characteristics of the proposed FCG have been experimentally investigated. Such an FCG with fixed frequency spacing will find promising applications in fields of optical communication, microwave, optical sensing, etc.

Featured Application: Plasma science, Fusion reactor. A new Doppler radar using millimeter-waves in the Ka-band, named the "dual-comb Doppler reflectometer", has been developed to measure the turbulence intensity and its velocity in high-temperature plasmas. For the realization of a fusion power generation, it is very important to know the spatial structure of turbulence, which is the cause of plasma confinement degradation. As a non-invasive and high spatial resolution measurement method for this purpose, we apply a multi-frequency Doppler radar especially with simultaneous multi-point measurement using a frequency comb. The newly developed method of synchronizing two frequency combs allows a lower intermediate frequency (IF) than the previously developed frequency comb radar, lowering the bandwidth of the data acquisition system and enabling low-cost, long-duration plasma measurements. In the current dual-comb radar system, IF bandwidth is less than 0.5 GHz; it used to be 8 GHz for the entire Ka-band probing. We applied this system to the high-temperature plasma experimental device, the Large Helical Device (LHD), and, to demonstrate this system, verified that it shows time variation similar to that of the existing Doppler radar measurements. [ABSTRACT FROM AUTHOR]

The magnonic frequency comb (MFC) induced by magnon–skyrmion interaction not only exhibits interesting nonlinear physics but also can be used to detect magnetic solitons and defects in ferromagnetic materials. Modulating the tooth spacing of MFC is crucial for its application in magnon‐based devices. Herein, it is demonstrated from micromagnetic simulations that the MFC tooth spacing of ferromagnetic thin film can be effectively modulated by a biaxial in‐plane strain via magnetoelastic coupling. It is found that the MFC tooth spacing increases with the increase of compressive strain and decreases with the increase of tensile strain. The strain modulation of MFC tooth spacing stems from the strain‐dependent magnetocrystalline anisotropy. The detailed energy analysis shows that the larger the tooth spacing is, the higher the exchange energy is, and the lower the magnetoelastic and anisotropy energies are. In addition, when the biaxial in‐plane strain becomes nonequally axial (or anisotropic), the tooth spacing becomes smaller, accompanied by a decrease in the magnetoelastic energy. The results of this study suggest an effective way to modulate the tooth spacing of MFC by strain, which may hold a promise for application in tunable magnon‐based devices. [ABSTRACT FROM AUTHOR]

Current fibre optic communication systems owe their high-capacity abilities to the wavelength-division multiplexing (WDM) technique, which combines data channels running on different wavelengths, and most often requires many individual lasers. Optical frequency combs, with equally spaced coherent comb lines derived from a single source, have recently emerged as a potential substitute for parallel lasers in WDM systems. Benefits include the stable spacing and broadband phase coherence of the comb lines, enabling improved spectral efficiency of transmission systems, as well as potential energy savings in the WDM transmitters. In this paper, we discuss the requirements to a frequency comb for use in a high-capacity optical communication system in terms of optical linewidth, per comb line power and optical carrier-to-noise ratio, and look at the scaling of a comb source for ultra-high capacity systems. Then, we review the latest advances of various chip-based optical frequency comb generation schemes and their applications in optical communications, including mode-locked laser combs, spectral broadening of frequency combs, microresonator-based Kerr frequency combs and electro-optic frequency combs.

Developing compact and stable 2 $\mu \text{m}$ ultrafast laser sources is important for generating mid-inferred frequency combs. Here, we report on a simple, compact and reliable thulium-doped fiber laser based on figure-of-9 mode-locking. Self-starting and stable mode-locked operation is achieved with 0.30 nJ pulse energy and 128 fs pulse width under 121.3 MHz repetition rate. The generated pulses centered at 1990 nm have a 3 dB bandwidth of 39.4 nm. The laser has been assembled into a $197\times 78\times 20$ mm3 fiber box under temperature-control. The repetition rate was phase locked over 20 h with a fluctuation standard deviation of 192.5 $\mu $ Hz. Such compact and highly-reliable mid-inferred laser could be used beyond laboratory and promote numerous critical applications including dual-comb spectroscopy, ranging and frequency calibration. [ABSTRACT FROM AUTHOR]