Frequency Comb Generation Research Articles

Linear and nonlinear coupling of light in twin-resonators with Kerr nonlinearity

Nonlinear effects in microresonators are efficient building blocks for all-optical computing and telecom systems. With the latest advances in microfabrication, coupled microresonators are used in a rapidly growing number of applications. In this work, we investigate the coupling between twin-resonators in the presence of Kerr nonlinearity. We use an experimental setup with controllable coupling between two high-Q resonators and discuss the effects caused by the simultaneous presence of linear and nonlinear coupling between the optical fields. Linear-coupling-induced mode splitting is observed at low input powers, with the controllable coupling leading to a tunable mode splitting. At high input powers, the hybridized resonances show spontaneous symmetry breaking (SSB) effects, in which the optical power is unevenly distributed between the resonators. Our experimental results are supported by a detailed theoretical model of nonlinear twin-resonators. With the recent interest in coupled resonator systems for neuromorphic computing, quantum systems, and optical frequency comb generation, our work provides important insights into the behavior of these systems at high circulating powers.

Quadratic-soliton-enhanced mid-IR molecular sensing

Optical solitons have long been of interest both from a fundamental perspective and because of their application potential. Both cubic (Kerr) and quadratic nonlinearities can lead to soliton formation, but quadratic solitons can practically benefit from stronger nonlinearity and achieve substantial wavelength conversion. However, despite their rich physics, quadratic cavity solitons have been used only for broadband frequency comb generation, especially in the mid-infrared. Here, we show that the formation dynamics of mid-infrared quadratic cavity solitons, specifically temporal simultons in optical parametric oscillators, can be effectively leveraged to enhance molecular sensing. We demonstrate significant sensitivity enhancement while circumventing constraints of traditional cavity enhancement mechanisms. We perform experiments sensing CO2 using cavity simultons around 4 μm and achieve an enhancement of 6000. Additionally, we demonstrate large sensitivity at high concentrations of CO2, beyond what can be achieved using an equivalent high-finesse linear cavity by orders of magnitude. Our results highlight a path for utilizing quadratic cavity nonlinear dynamics and solitons for molecular sensing beyond what can be achieved using linear methods.

Integrated chirped photonic-crystal cavities in gallium phosphide for broadband soliton generation

Chirped mirrors have underpinned advances in ultra-fast lasers based on bulk optics but have yet to be fully exploited in integrated photonics, where they could provide a means to engineer otherwise unattainable dispersion profiles for a range of nonlinear optical applications, including soliton frequency comb generation. The vast majority of integrated resonators for frequency combs make use of microring geometries, in which only waveguide width and height are varied to engineer dispersion. Here, we present an integrated photonic-crystal Fabry–Pérot resonator made of gallium phosphide (GaP), a material exhibiting a Kerr nonlinearity 200 times larger than that of silicon nitride and a high refractive index that permits the creation of strongly chirped photonic-crystal mirrors. Leveraging the additional degrees of freedom provided by integrated chirped mirrors, we disentangle optical losses from dispersion. We obtain an overall dispersion that is more anomalous than that achievable in both silicon nitride and gallium phosphide ring resonators with the same free-spectral range (FSR), while simultaneously obtaining higher quality factors than those of GaP ring resonators. With subharmonic pulsed pumping at an average power of 23.6 mW, we are able to access stable dissipative Kerr frequency combs in a device with a FSR of 55.9 GHz. We demonstrate soliton formation with a 3-dB bandwidth of 3.0 THz, corresponding to a pulse duration of 60 fs. This approach to cavity design based on photonic-crystal reflectors offers nearly arbitrary dispersion engineering over the optical transparency window of the nonlinear material.

Continuously and Precisely Tunable Microwave Frequency Comb Generation Based on Actively Mode-Locked OEO

Continuously and Precisely Tunable Microwave Frequency Comb Generation Based on Actively Mode-Locked OEO

Tunable microwave and optical frequency comb generator with the aid of cascaded MZM structure

Tunable microwave and optical frequency comb generator with the aid of cascaded MZM structure

High signal-to-noise ratio optical frequency comb generation based on a Brillouin amplified recirculating frequency shifter loop

A novel approach to generate optical frequency combs (OFCs) with high signal-to-noise ratio (SNR) based on a Brillouin amplified recirculating frequency shifter loop (RFSL) is proposed and experimentally demonstrated. Different from the conventional RFSL-based OFC generator using an EDFA, a multi-channel Brillouin gain fiber is introduced in the scheme for optical power amplification to compensate for the optical losses inside the loop. The frequency positions of the Brillouin pump laser lines and their corresponding Brillouin gain profiles are precisely characterized by employing the laser scanning spectroscopy method. The SNR of the OFC is significantly enhanced from 12 dB to 30 dB. The proposed OFC generation with high-SNR performance plays an essential role in the development of optical communications and high-precision spectroscopy.

Electro-optic frequency comb generation via cascaded modulators driven at lower frequency harmonics

Electro-optical modulation of a continuous wave laser is a highly stable way to generate frequency combs, gaining popularity in telecommunication and spectroscopic applications. These combs are generated by modulating non-linear electro-optic crystals with radio frequencies, creating equally spaced side-bands centered around the single-frequency seed laser. Electro-optic frequency comb architectures often choose between optical bandwidth (cascaded GHz combs) or higher mode density (chirped RF generation). This work demonstrates an electro-optic frequency comb with > 120 GHz of bandwidth and an 80 MHz repetition rate. The comb has three cascaded electro-optic modulators driven at sequentially lower harmonics, the last megahertz modulation dictating the repetition rate. This architecture can modulate at any individual harmonic and repetition rate without changes to the components. This comb can be used in any applications where a stable and tunable repetition rate is needed.

An approach to generate cross-polarization modulation-enabled optical frequency comb with enhanced spectral flatness in traveling-wave semiconductor optical amplifiers

Abstract This paper proposes a method for generating an optical frequency comb (OFC) using the cross-polarization modulation (XPolM) effect in a traveling-wave semiconductor optical amplifier (TW-SOA). The TW-SOA acted as a lumped amplifier, enabling pulse generation within the comb. The proposed method of OFC generation achieved a comb spectrum with 51 lines and a maximum power deviation of 1.8 dB, indicating excellent spectral flatness. The proposed approach effectively utilized XPolM in a TW-SOA to create broad flat OFCs. The effectiveness of XPolM depends on various SOA parameters, such as pump and signal powers, and the confinement factor. To understand the XPolM-enabled OFC behavior, various parameters, such as input polarization, bias current, SOA confinement factor, and carrier density, have been optimized. These parameters significantly influenced the generated comb spectrum. This study also provides valuable insights into optimizing comb characteristics through theoretical analysis. The proposed OFC generation was simulated using an OptiSystem simulator. This has paved the way for the potential use of the comb as a multichannel source in various optical devices.

Nonlinear response of very high frequency contour mode resonators

We explore the source of nonlinearities in Aluminum Nitride (AlN) Contour Mode Resonators (CMRs) operating in the Very High Frequency (VHF) range. We demonstrate that the red-shift of the resonance frequency found in VHF CMRs when the input RF power increases is due to nonlinear stiffness appearing from self-heating, and variable damping due to geometric nonlinearities. Moreover, we find a linear relationship between the variable damping coefficient and the resonator quality factor (Q). Such nonlinear mechanisms are modeled using a spring-mass-damper physical system and, in the electrical domain, a modified Butterworth-Van Dyke (MBVD) circuit where the nonlinear stiffness and variable damping are captured by a charge-dependent motional capacitor and a charge-dependent motional resistor, respectively. Detailed guidelines are provided to accurately analyze nonlinear CMRs using full-wave numerical simulations based on a finite-element method. Such simulations allow us to isolate the influence of each independent nonlinear mechanism and establish a relation between variable damping and geometric nonlinearities. Circuit and full-wave numerical simulations are in good agreement with measured data from fabricated 225 MHz CMRs exhibiting different Q. Finally, we exploit nonlinearities in high-Q CMRs to generate frequency combs at the MHz range opening the door to new exciting applications in telecommunication and sensing.

Full-Scale Readout Electronics for the ECHo Experiment

Recent advances in the development of cryogenic particle detectors such as magnetic microcalorimeters allow the fabrication of sensor arrays with an increasing number of pixels. Since these detectors must be operated at the lowest temperatures, the readout of large detector arrays is still quite challenging. This is especially true for the ECHo experiment, which presently aims to simultaneously run 6,000 two pixel detectors to investigate the electron neutrino mass. For this reason, we developed a readout system based on a microwave SQUID multiplexer (μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}MUX) that is operated by a custom software-defined radio (SDR) at room-temperature. The SDR readout electronics consist of three distinct hardware units: a data processing board with a Xilinx ZynqUS+ MPSoC; a converter board that features DACs, ADCs, and a coherent clock distribution network; and a radio frequency front-end board to translate the signals between the baseband and the microwave domains. Here, we describe the characteristics of the full-scale SDR system. First, the generated frequency comb for driving the μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}MUX was evaluated. Subsequently, by operating the SDR in direct loopback, the crosstalk of the individual channels after frequency demultiplexing was investigated. Finally, the system was used with a 16-channel μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}MUX to evaluate the linearity of the SDR, and the noise contributed to the overall readout setup.

Generation of Soliton Frequency Combs in NEMS.

In this study, we present an innovative approach leveraging combination internal resonances within a NEMS platform to generate mechanical soliton frequency combs (FCs) spanning a broad spectrum. In the time domain, the FCs take the form of a periodic train of narrow pulses, a highly coveted phenomenon within the realm of nonlinear wave-matter interactions. Our method relies on an intricate interaction among multiple vibration modes of a bracket-nanocantilever enabled by the strong nonlinearity of the electrostatic field. Through numerical simulation and experimental validation, we demonstrate that by amplifying the motions of the NEMS with the external electrostatic forcing tuned to excite the superharmonic resonance of order-n of the fundamental mode and exploiting combination internal resonances, we can generate multiple stable localized mechanical wave packets with different lobe sizes embodying soliton states I and II. This represents a significant breakthrough with profound implications for quantum computing and metrology.

Hybrid Patterns and Solitonic Frequency Combs in Non-Hermitian Kerr Cavities.

We unveil a new scenario for the formation of dissipative localized structures in nonlinear systems. Commonly, the formation of such structures arises from the connection of a homogeneous steady state with either another homogeneous solution or a pattern. Both scenarios, typically found in cavities with normal and anomalous dispersion, respectively, exhibit unique fingerprints and particular features that characterize their behavior. However, we show that the introduction of a periodic non-Hermitian modulation in Kerr cavities hybridizes the two established soliton formation mechanisms, embodying the particular fingerprints of both. In the resulting novel scenario, the stationary states acquire a dual behavior, playing the role that was unambiguously attributed to either homogeneous states or patterns. These fundamental findings have profound practical implications for frequency comb generation, introducing unprecedented reversible mechanisms for real-time manipulation.

Frequency comb generation from the ultraviolet to mid-infrared region based on a three-stage cascaded PPLN chain

Optical frequency combs (OFCs) have enabled significant opportunities for high-precision frequency metrology and high-resolution broadband spectroscopy. Although nonlinear photonics chips have the capacity of frequency expansion for OFCs, most of them can only access the limited bandwidths in the partial infrared region, and it is still hard to satisfy many measurement applications in the ultraviolet-to-visible region. Here, we demonstrate a compact broadband OFC scheme via the combination of three χ(2) nonlinearities in a three-stage periodically poled lithium niobate (PPLN) chain. With a supercontinuum spectrum OFC delivered into the PPLN chain, the intra-pulse diffidence frequency generation, optical parametric amplification, and high-order harmonic generation were carried out in sequence. It is crucial that the harmonics of the 1st–10th orders are simultaneously obtained with an offset-free OFC spectrum from 0.35 to 4.0 μm. In view of the great potential for integration and spectral expansion, this wideband frequency comb source will open a new insight for the valuable applications of two-dimensional material analysis, biofluorescence microscopy, and nonlinear amplitude-phase metrology.

Optical frequency comb significantly spanned to broadband by an optomechanical resonance

An optical frequency comb, as a spectrum made of discrete and equally spaced spectral lines, is a light source with essential applications in modern technology. Cavity optomechanical systems were found to be a feasible candidate for realizing an on-chip frequency comb with low repetition rate. However, it was difficult to increase the comb line numbers of this type of frequency combs because the mechanical oscillation amplitude of such a system, which determines the frequency comb bandwidth, cannot quickly increase with pump laser power. Here, we develop a new approach to generate a broadband optomechanical frequency comb by employing a different mechanism to enhance the mechanical oscillation. Two pump tones with their frequency difference matching the mechanical frequency will drive the system into a self-organized nonlinear resonance and thus tremendously transfer the energy to the mechanical resonator. As a result, more than 10,000 or even more comb lines become available under the pump laser power of the order of milliwatts. A unique feature of the self-organized resonance is the mechanical frequency locking so that, within a certain range of the frequency difference between two drive tones, the distance between comb teeth can be locked by the two drive tones and becomes independent of any change of pump power. This property guarantees a stable repetition rate of the generated frequency comb.

Giant Kerr nonlinearity of terahertz waves mediated by stimulated phonon polaritons in a microcavity chip

Optical Kerr effect, in which input light intensity linearly alters the refractive index, has enabled the generation of optical solitons, supercontinuum spectra, and frequency combs, playing vital roles in the on-chip devices, fiber communications, and quantum manipulations. Especially, terahertz Kerr effect, featuring fascinating prospects in future high-rate computing, artificial intelligence, and cloud-based technologies, encounters a great challenge due to the rather low power density and feeble Kerr response. Here, we demonstrate a giant terahertz frequency Kerr nonlinearity mediated by stimulated phonon polaritons. Under the influences of the giant Kerr nonlinearity, the power-dependent refractive index change would result in a frequency shift in the microcavity, which was experimentally demonstrated via the measurement of the resonant mode of a chip-scale lithium niobate Fabry-Pérot microcavity. Attributed to the existence of stimulated phonon polaritons, the nonlinear coefficient extracted from the frequency shifts is orders of magnitude larger than that of visible and infrared light, which is also theoretically demonstrated by nonlinear Huang equations. This work opens an avenue for many rich and fruitful terahertz Kerr effect based physical, chemical, and biological systems that have terahertz fingerprints.

A synthetic moving-envelope metasurface antenna for independent control of arbitrary harmonic orders

Flexible frequency controls are crucial in many photonic and electronic applications, ranging from communications systems, spectroscopy, and metrology to quantum information processing. However, the state-of-the-art solutions based on nonlinear bulk media, electro-optic effect, and nonlinear metasurfaces incur very limited spectral controllability, and merely a couple of harmonic orders can be independently manipulated. Here, we theoretically propose and experimentally demonstrate synthetic moving-envelope metasurface antennas capable of simultaneously generating arbitrary harmonic orders and independently manipulating their wave properties in a software-defined manner. As proof-of-principle examples, we demonstrate unidirectional frequency transition, frequency comb generation, arbitrary harmonic orders independent control, and their applications in frequency-division multiplexing communications. All these complicated functionalities are achieved by the 1-bit spatiotemporally ON-OFF switching of meta-atoms of the waveguide-integrated metasurface antenna. Our proposed synthetic metasurface antenna solution greatly expands the frontiers of wave engineering and information manipulation, showing promising potential in wireless communications, spectroscopy, metrology, and quantum science.

Parity-time-symmetry-enabled broadband quantum frequency-comb generation

Parity-time-symmetry-enabled broadband quantum frequency-comb generation

Harnessing tin phthalocyanine for ultrafast mode-locking in an erbium-doped fiber laser

We present the first demonstration, to the best of our knowledge, of a mode-locked erbium-doped fiber laser (EDFL) employing tin phthalocyanines (SnPc) as a saturable absorber (SA). The investigated SA exhibits a notable modulation depth of 18.6%, fabricated via a simple, cost-effective method, wherein the SnPc compound is embedded into a polyvinyl alcohol (PVA) film. Integration of this thin film into the EDFL cavity facilitates the generation of output pulses with a repetition rate of 0.96 MHz and a pulse duration of 2.68 ps. The frequency spectrum showcases a signal-to-noise ratio of up to 73.2 dB, affirming the stability of the mode-locking operation. Operating at a wavelength of 1531.7 nm, this picosecond pulse train holds significant promise for diverse applications, including high-precision optical metrology, frequency-comb generation, and broadband absorption spectroscopy.

Direct-modulation optoelectronic oscillator for optical pulse and frequency comb generation

We report the generation of an optical pulse train with a 10 GHz repetition rate in a dual loop direct-modulation optoelectronic oscillator (OEO). Pulse generation is achieved using nonlinear compression in the OEO 5-km-long optical delay line. 3 ps pulses with a timing jitter of 13 fs are reported, while the OEO maintains a phase noise of −133dBc/Hz at 10 kHz from the carrier. Two architectures are compared experimentally and theoretically. A frequency comb with a 1.2 THz linewidth at −30dB is generated.

Wavelength tuning of VCSELs via controlled strain.

Besides major advantages for telecommunication applications, vertical-cavity surface-emitting lasers (VCSELs) have attracted interest for their potential for neuro-inspired computing, frequency comb generation, or high-frequency spin oscillations. In the meantime, strain applied to the laser structure has been shown to have a significant impact on the laser emission properties such as the polarization dynamics or birefringence. In this work, we further explore the influence of strain on VCSELs and how this effect could be used to fine-tune the laser wavelength. Through a comprehensive investigation, we demonstrate a consistent wavelength shift up to 1 nm and report a sensitivity between 0.12 and 0.18 nm/millistrain. We also record birefringence values up to 292 GHz. Our results show that a controlled strain level could be considered for fine wavelength tuning and possibly alleviate the selection of VCSEL for precise wavelength requirements.