Comb Generation Research Articles

Optimizing auxiliary laser heating for Kerr soliton microcomb generation.

Auxiliary laser heating has become a widely adopted method for Kerr soliton frequency comb generation in optical microcavities, thanks to its reliable and easy-to-achieve merits for solving the thermal instability during the formation of dissipative Kerr solitons. Here, we conduct optimization of auxiliary laser heating by leveraging the distinct loss and absorption characteristics of different longitudinal and polarization cavity modes. We show that even if the auxiliary and pump lasers enter orthogonal polarization modes, their mutual photothermal balance can be efficient enough to maintain a cavity thermal equilibrium as the pump laser enters the red-detuning soliton regime, and by choosing the most suitable resonance for the auxiliary and pump lasers, the auxiliary laser power can be reduced to 20% of the pump laser and still be capable of warranting soliton generation. Moreover, we demonstrate soliton comb generation using integrated laser modules with a few milliwatt on-chip pump and auxiliary powers, showcasing the potential for further chip integration of the auxiliary laser heating method.

Fast-reconfigurable frequency comb generation based on AlGaAsOI waveguide with electro-optic time lens

Tunable optical frequency combs offer a flexible solution for specific applications such as dual-comb spectroscopy, optical communications and microwave photonics, delivering improved precision, compatibility, and performance. However, previously, there has been a trade-off between reconfigurability and system simplicity in comb generation. Here, we present a fast-switched repetition rate frequency comb system that utilizes an electro-optic modulation time-lens technique with a high third-order nonlinear AlGaAsOI waveguide. Only one stage of modulator is used in the time-lens system which significantly reduces the complexity of the overall system. Our system allows for tuning of the center wavelength from 1542 nm to 1556 nm, as well as independent adjustment of the repetition rates from 18 GHz to 26.5 GHz, enabling fast-switching capabilities. Additionally, our system exhibits a high pump-to-comb conversion efficiency of up to 67.9%. It also demonstrates robustness to temperature changes and environmental instability. All the involved devices can be integrated onto a single chip, making this comb suitable for various applications.

Coherent Optical Comb Generation in the Joint Space of Frequency and Topological Charge

AbstractCoherent photonic states in the frequency domain or optical frequency combs are investigated widely over the past decade. Meanwhile, photonic states based on spatial coherence, in particular optical vortex states, have also been studied extensively. Further exploitation of coherent photonics state in hitherto unexplored higher dimensional spaces can reveal new realms of physics. The on‐chip generation of coherent optical combs in the 2D parameter space of frequency and topological charge is reported. Using a micro‐ring resonator embedded with angular gratings, optical frequency combs are generated by Kerr nonlinearity. The coherent comb lines, with their frequencies inherently matching the cavity resonances, are immediately diffracted by the angular grating to emit an ensemble of coherent spatial photonic states with each state carrying distinctive spin and orbital angular momenta. The mapping between the frequency and the topological charge dimensions leads to the construction of coherent combs in the high‐dimensional frequency‐topological charge (f‐l) space. Operating the device in the normal‐dispersion regime enables the adjustment of comb line spacing from 1 to 11 free spectral ranges by selectively pumping different cavity resonances with high pump‐to‐comb conversion efficiency. Such on‐chip reconfigurable high‐dimensional coherent optical combs open avenues for innovative exploration of physics.

Active mid-infrared ring resonators

High-quality optical ring resonators can confine light in a small volume and store it for millions of roundtrips. They have enabled the dramatic size reduction from laboratory scale to chip level of optical filters, modulators, frequency converters, and frequency comb generators in the visible and the near-infrared. The mid-infrared spectral region (3−12 μm), as important as it is for molecular gas sensing and spectroscopy, lags behind in development of integrated photonic components. Here we demonstrate the integration of mid-infrared ring resonators and directional couplers, incorporating a quantum cascade active region in the waveguide core. It enables electrical control of the resonant frequency, its quality factor, the coupling regime and the coupling coefficient. We show that one device, depending on its operating point, can act as a tunable filter, a nonlinear frequency converter, or a frequency comb generator. These concepts extend to the integration of multiple active resonators and waveguides in arbitrary configurations, thus allowing the implementation of purpose-specific mid-infrared active photonic integrated circuits for spectroscopy, communication, and microwave generation.

Dissipative coupling induced UWB magnonic frequency comb generation

Magnonic frequency combs have recently attracted particular attention due to their potential impact on spin-wave science. Here, we demonstrate theoretically the generation of ultra-wideband magnonic frequency combs induced by dissipative coupling in an open cavity magnomechanical system. A broadband comb with gigahertz repetition rates is obtained in the magnonic spectrum, and a typical non-perturbation frequency-comb structure is also observed. The total width of the magnonic comb in the robust plateau region can be up to ∼400 comb lines, which is much broader and flatter than that reported in the previous works. Furthermore, when the dissipative coupling strength is further increased, the chaotic motion is predicted in the magnonic spectrum. Our results provide an in-depth understanding of nonlinear magnomechanic dynamics in open quantum systems and fundamentally broaden the research range of magnon in wider spectral regimes.

Visible-to-ultraviolet frequency comb generation in lithium niobate nanophotonic waveguides

Visible-to-ultraviolet frequency comb generation in lithium niobate nanophotonic waveguides

Spectral Engineering of Optical Microresonators in Anisotropic Lithium Niobate Crystal.

On-chip optical microresonators are essential building blocks in integrated optics. The ability to arbitrarily engineer their resonant frequencies is crucial for exploring novel physics in synthetic frequency dimensions and practical applications like nonlinear optical parametric processes and dispersion-engineered frequency comb generation. Photonic crystal ring (PhCR) resonators are a versatile tool for such arbitrary frequency engineering, by controllably creating mode splitting at selected resonances. To date, these PhCRs have mostly been demonstrated in isotropic photonic materials, while such engineering can be significantly more complicated in anisotropic platforms that often offer more fruitful optical properties. Here, the spectral engineering of chip-scale optical microresonators is realized in the anisotropic lithium niobate (LN) crystal by a gradient design that precisely compensates for variations in both refractive index and perturbation strength. Controllable frequency splitting is experimentally demonstrated at single and multiple selected resonances in LN PhCR resonators with different sizes, while maintaining high quality-factors up to 1 × 106. Moreover, a sharp boundary is experimentally constructed in the synthetic frequency dimension based on an actively modulated x-cut LN gradient-PhCR, opening up new paths toward the arbitrary control of electro-optic comb spectral shapes and exploration of novel physics in the frequency degree of freedom.

Flexible broadband optical frequency comb generation assisted by injection locking of semiconductor laser

A broadband optical frequency comb (OFC) generator is proposed and experimentally demonstrated. Based on the injection locking process in a semiconductor laser diode, multiple coherent carriers with large intervals can be generated, which are further combined to give a much broader OFC through the electro-optical modulation (EOM) technique. The corresponding theoretical analysis and simulation have been given in detail. In the experimental demonstration, an OFC with 9 comb lines can be generated on a single carrier by the cascaded MZMs with a multi-harmonic RF drive signal. After tripling the number of carriers, the number of the comb lines becomes 27. By properly adjusting the carrier’s interval and drive signal frequency, the generated OFC shows great flexibility on spacing and the number of comb lines, where a 27-line OFC with FSR of 5 GHz and 6.5 GHz have been demonstrated respectively. In addition, by increasing the number of RF sources, the OFC with up to 43 comb lines can be generated in this system. Compared with the traditional OFC generation implemented by the EOM technique on a single optical carrier, three times optical spectrum extension or the number of comb lines increasing can be achieved. The Multi-carrier broaden spectrum method retains all the advantages of EOM-based OFC generation and overcomes its disadvantages, such as limited spectrum width and the number of comb lines. It provides a new idea for broadband OFC generation.

Kerr frequency comb generation in fiber Fabry-Pérot resonator: Technological locks and leveraging fiber properties

We explore fiber Fabry-Pérot (FFP) resonators, a new platform for frequency comb generation We experimentally identified mirror diffraction losses dependent on the effective area of the fiber and simulated them via Fourier optics. In the nonlinear regime, a linear stability analysis of a generalized Lugiato-Lefever equation revealed optimization of reflectivity and detuning, leading to a significant reduction in the required power threshold for comb generation compared to linear regime, and to improved energy frugality. Furthermore, controlled or exploited birefringence in various experimental settings enabled the generation of Kerr frequency combs and stimulated Brillouin lasers. In this communication we propose an overview of the practical characteristics related to the fabrication and use of these resonators.

Generation of spatial combs digitized by orbital angular momentum

Shaping the transverse spatial domain of light has been experiencing recent growth in attention for its applications in optical tweezing, microscopy, communications, and quantum information sciences. The orbital angular momentum (OAM) of light is a transverse physical property that functions as a viable basis for many of such applications. While fields containing a single OAM mode order have extensively been used, fields containing a broad, discrete, and equally spaced OAM spectrum—an OAM comb—has largely been unexplored due to a lack of viable creation methods. Much like how frequency combs enabled myriad applications, it is conceivable that OAM combs would likewise enable further opportunities or expand upon single-OAM applications. For instance, an OAM comb may pave the way for more sophisticated particle manipulation, object detection, and pump shaping for high-dimensional spatial-mode entanglement. Here, we create four OAM combs with rectangular, sinusoidal, sinc, and Gaussian OAM spectral distributions using multiplane light conversion, which only involves phase modulations (hence is inherently lossless) and allows for improved tailoring of the spatial shape. The verification of such combs is done qualitatively by analyzing their propagation evolutions and quantitatively by employing a mode sorting technique—both exhibiting good agreement with user specifications and simulations. The scattering losses of all four combs were measured to be minimal, ranging from 0.66 to 1.04 dB.

Generation of microwave frequency combs based on dual-frequency injected optoelectronic oscillator

Generation of microwave frequency combs based on dual-frequency injected optoelectronic oscillator

Continuous and deterministic generation of chip-based frequency combs with a computer program

Continuous and deterministic generation of chip-based frequency combs with a computer program

Quantum cascade disk and ring lasers

Quantum cascade lasers (QCLs) are a prominent semiconductor laser source operating in the mid-infrared and terahertz regimes. As is typical with semiconductor lasers, QCLs usually monolithically integrate the active gain material and the resonator. Hence, over nearly 30 years of QCL development, resonator geometries have developed alongside active region designs. Disk and ring geometries, in particular, have long been recognized for their unique attributes, which have, in turn, contributed to the demonstration of ultra-small cavities as well as surface emission from QCLs. In recent years, ring geometries have witnessed a resurgence as promising platforms for frequency comb and soliton generation as well as mid-infrared photonic integration. In this Perspective, we describe the attributes that make ring and disk QCLs unique by discussing key demonstrations. We present recent results, which indicate that these devices are poised to become building blocks of highly integrated, next-generation spectrometers operating in the mid-infrared. We discuss promising avenues for future research centered around monolithic ring and disk-type QCLs in applications ranging from gas sensing and spectroscopy to quantum optics and non-Hermitian photonics.

Time-expanded ΦOTDR based on Orthogonal Polarization Frequency Comb generation

Time-expanded ΦOTDR based on Orthogonal Polarization Frequency Comb generation

Frequency comb generation dynamics in χ(2) + χ(3) AlGaAs microresonators

We propose an AlGaAs microring resonator design for the generation of an optical frequency comb by means of the interplay between harmonic generation and Kerr effect. Modal phase matching imposes specific waveguide geometries and, consequently, it impacts the nonlinear efficiency of the system. We show the dynamics of χ(2) + χ(3) comb generation resulting from type-I modal phase matching.

O-U Band Flat Optical Frequency Comb Generation Based on Dispersion-controlled Fluorotellurite Glass Fiber

O-U Band Flat Optical Frequency Comb Generation Based on Dispersion-controlled Fluorotellurite Glass Fiber

Broadband near-visible frequency comb generation via high-order mode dispersion engineering in a microbubble resonator

Broadband near-visible frequency comb generation via high-order mode dispersion engineering in a microbubble resonator

First demonstration of ultrafast-electron diffraction using a 3-GHz electro-optical comb generator as a UV photoinjector laser

We demonstrate ultrafast-electron diffraction using an electro-optic comb-generator, emitting a 3 GHz train of ps pulses at 257 nm. This new operation mode improves electron coherence and reduces space-charge effects.

Analysis and optimization of optical frequency comb spectra of magnesium fluoride microbottle resonator

<sec>Optical frequency comb has shown great potential applications in many areas including molecular spectroscopy, RF photonics, millimeter wave generation, frequency metrology, atomic clock, and dense/ultra-dense wavelength division multiplexed high speed optical communications. Optical frequency comb in the microresonator supporting whispering-gallery mode has attracted widespread interest because of its advantages such as flexible repetition rate, wide bandwidth, and compact size. The exceptionally long photon lifetime and small modal volume enhance light-matter interaction, which enables us to realize intracavity nonlinear frequency conversions with low pump threshold. With the advantages of small size, low power consumption, wide spectral coverage and adjustable dispersion, the magnesium fluoride microresonator optical frequency comb has potential applications in optical communication and mid-infrared spectroscopy.</sec><sec>In this work, the spectral characteristics of the optical frequency comb generated by a magnesium fluoride whispering-gallery mode microbottle resonator platform are investigated. In order to optimize the spectral distribution of the optical frequency comb of the magnesium fluoride microbottle resonator, the second-order dispersion and higher-order dispersion of the bottle resonator structure under different curvatures and axial modes are solved iteratively by the finite element method, and the spectral evolutions of the optical frequency comb under different axial mode excitations are simulated by solving the nonlinear Schrödinger equation through the split-step Fourier method. The results show that near-zero anomalous dispersion tuning can be achieved in a wide bandwidth range by exciting low-order axial mode at an optimal radius of curvature, while the high-order axial mode will lead the microbottle resonator to present the weak normal dispersion. The weaker anomalous dispersion in the lower-order axial mode broadens the bandwidth of the optical comb, demonstrating that the third-order dispersion and the negative fourth-order dispersion can broaden the Kerr soliton optical comb; the weak normal dispersion in the higher-order axial mode suppresses the generation of the Kerr optical comb, and the Raman optical comb dominates. The selective excitation of Kerr soliton combs and Raman combs can be achieved by modulating the axial mode of the microbottle resonator under suitable pumping conditions. The present work provides guidance for designing the dispersion in magnesium fluoride microresonator and the experimental tuning of broadband Kerr soliton optical combs and Raman optical combs.</sec>

Low‐Temperature Sputtered Ultralow‐Loss Silicon Nitride for Hybrid Photonic Integration

AbstractSilicon‐nitride‐on‐insulator (Si3N4) photonic circuits have seen tremendous advances in many applications, such as on‐chip frequency combs, Lidar, telecommunications, and spectroscopy. So far, the best film quality has been achieved with low pressure chemical vapor deposition (LPCVD) and high‐temperature annealing (1200°C). However, high processing temperatures pose challenges to the cointegration of Si3N4 with pre‐processed silicon electronic and photonic devices, lithium niobate on insulator (LNOI), and Ge‐on‐Si photodiodes. This limits LPCVD as a front‐end‐of‐line process. Here, ultralow‐loss Si3N4 photonics based on room‐temperature reactive sputtering is demonstrated. Propagation losses as low as 5.4 dB m−1 after 400°C annealing and 3.5 dB m−1 after 800°C annealing are achieved, enabling ring resonators with highest optical quality factors of > 10 million and an average quality factor of 7.5 million. To the best of the knowledge, these are the lowest propagation losses achieved with low temperature Si3N4. This ultralow loss enables the generation of microresonator soliton frequency combs with threshold powers of 1.1 mW. The introduced sputtering process offers full complementary metal oxide semiconductor (CMOS) compatibility with front‐end silicon electronics and photonics. This could enable hybrid 3D integration of low loss waveguides with integrated lasers and lithium niobate on insulator.