Frequency Chirp Research Articles

Frequency-Modulated Continuous-Wave Chirp Frequency Error and Phase Noise Measurement: How to Characterize Frequency-Modulated Continuous-Wave Chirp Using Commercial Oscilloscope

Frequency-Modulated Continuous-Wave Chirp Frequency Error and Phase Noise Measurement: How to Characterize Frequency-Modulated Continuous-Wave Chirp Using Commercial Oscilloscope

Dispersion response broadband tunable underwater FMCW blue chirped laser source

Frequency-modulated continuous-wave (FMCW) narrow linewidth lasers have served as the cornerstone behind applications such as autonomous driving, wearable technology, virtual reality, and remote sensing mapping. Strongly coherent lasers are typically used for these studies, with a clear demand for linear fast response and wide frequency tuning range. In this paper, profiting from the ultrahigh-quality factor of the crystalline whispering-gallery-mode resonator, by using a self-injection locking mechanism to suppress spontaneous emission noise and improve coherence, sub-kHz linewidth at 450 nm is obtained. Furthermore, based on the dispersive response principle, fast electrical tuning is realized by using the strain-influenced resonator, and the experimental test result reaches 81 pm/V. More importantly, we demonstrate the comprehensive performance of this type of FMCW laser in underwater detection, with a sensitivity of 319 MHz/m at a chirp frequency of 1 kHz.

Energy-managed soliton fiber laser

Ultrafast fiber lasers constitute a flexible platform to investigate new solitary wave concepts. To surpass the low energy limitation of the conventional solitons generated in standard telecom fibers, successive breakthroughs have promoted the usage of an important frequency chirping within fiber oscillators. This lead to original solitary wave regimes such as stretched-pulse, all-normal-dispersion, and self-similar dynamics. We here revisit ultrafast fiber lasers built from standard optical fibers featuring solely anomalous dispersion. We propose a new cavity design enhancing key dissipative effects with contained frequency chirping and demonstrate the generation of high energy pulses in the few-picoseconds regime. The involved intracavity dynamics blends conventional and dissipative soliton features in an unseen way with low- and high-energy propagation regions, allowing an increased flexibility and novel scalability prospects.

Experimental investigation of kinetic instabilities driven by runaway electrons in the EXL-50 spherical torus

Abstract In this study, high-frequency instabilities driven by runaway electrons in the EXL-50 spherical torus have been reported using a high-frequency magnetic pickup coil. The frequency of these instabilities is found to be power function dependent on the plasma density, similar to the dispersion relation of the whistler wave. The observed instability seems to exhibit a fluctuating pattern, resembling frequency chirping behavior, which appears to align with the expected outcomes predicted by the Berk-Breizman model. Theoretically, the excitation threshold of the instability driven by runaway electrons is related to the ratio of the runaway electron density to the background plasma density, and the stability criterion is first demonstrated qualitatively in this work. The instability can be stabilized by the spontaneous rise of plasma density, consistent with the wave-particle resonance mechanism. This investigation demonstrates the excitation characteristic of chirping instabilities in a tokamak plasma and reveals new features of these instabilities, thereby advancing the understanding of the mechanisms for controlling and mitigating runaway electrons.

Directly modulated FMCW tunable laser with highly linear frequency chirp and narrow linewidth

Frequency-modulated continuous-wave light detection and ranging (FMCW LiDAR) is a promising technology for long-range, high-accuracy, stray-light-immune distance and velocimetry sensing. To achieve this, a precisely chirped and highly coherent laser source is required. In this work, we propose to generate a highly linear frequency chirp by utilizing an intracavity phase modulator with high linearity and demonstrate a proof-of-concept fully integrated monolithic FMCW laser source based on an InP generic platform. By electrically modulating the intracavity phase modulator at a repetition rate of 100 kHz, a 1.85 GHz chirp range with root mean square (RMS) frequency nonlinearity down to 1.8 MHz is achieved without the need of external feedback loop or predistortion. Meanwhile, the laser shows a 51 nm tuning range with a linewidth down to 16 kHz. By taking advantage of the high-speed electro-optical effects, fast frequency modulation with repetition rate of up to 1 MHz is realized with a frequency chirp range of ∼1.65 GHz and RMS frequency nonlinearity of ∼12 MHz. We demonstrate its feasibility for LiDAR through in-fiber ranging at a distance of 50 m, where a 51 cm resolution is directly achieved.

Narrow linewidth III-V-on-SiN laser with extended frequency chirp capability based on micro-transfer printing for high resolution distributed acoustic sensing

Narrow linewidth III-V-on-SiN laser with extended frequency chirp capability based on micro-transfer printing for high resolution distributed acoustic sensing

A perturbative multi-mode model with finite parallel electric field for fast-ion-driven Alfvén eigenmodes

Abstract Alfvén eigenmodes are of great interest in any fusion device as they can be excited by fast ions in the plasma. If the modes grow to large amplitudes, they can cause transport and redistribution of the fast ions, thus limiting fusion performance. To save computational resources, the resonant kinetic interaction between the fast-particle species and the modes is often modelled by MHD-kinetic hybrid codes. Here, we present such a hybrid model which is applicable to three-dimensional magnetic fields, accounts for a finite parallel electric field and multiple MHD modes present at the same time. The model extends the one previously implemented in the CKA-EUTERPE code allowing for a better estimate of the damping due to the parallel electric field and nonlinear mode-mode interaction. The capabilities of our model are illustrated by applying the code to model nonlinear frequency chirping and fast-ion profile flattening.

Analysis of high-quality ultrashort pulse generation based on a dual-drive Mach-Zehnder modulator and chirp compensation

This study reports on high-quality picosecond pulse generation using a single-stage dual-drive Mach-Zehnder modulator (DDMZM) and chirp compensation. Sinusoidal microwave signals with different amplitudes are sent to two RF ports of the DDMZM to form a pulse train and pulse compression is achieved by compensating for the linear frequency chirp with a single-mode fiber (SMF). Three parameters encompassing the power difference between two RF signals, the power of RF signals, and the bias point of the DDMZM that affect the pulse formation have been numerically studied. The optimum length of SMF used for chirp compensation is obtained by simulating the temporal propagation and evolution of the pulse in SMF and 3.56-ps chirp-compensated ultrashort pulses are realized at 1.55 µm with a 25-GHz repetition rate experimentally. Modulator-based flexible ultrashort pulse generation can be achieved easily by tuning the RF generator and light source, and customized high-quality pulses according to practical applications can be expected.

A hybrid Eulerian-Lagrangian Vlasov method for nonlinear wave-particle interaction in weakly inhomogeneous magnetic field

We present a hybrid Eulerian-Lagrangian (HEL) Vlasov method for nonlinear resonant wave-particle interactions in weakly inhomogeneous magnetic field. The governing Vlasov equation is derived from a recently proposed resonance tracking Hamiltonian theory. It gives the evolution of the distribution function with a scale-separated Hamiltonian that contains the fast-varying coherent wave-particle interaction and slowly-varying motion about the resonance frame of reference. The hybrid scheme solves the fast-varying phase space evolution on Eulerian grid with an adaptive time step and then advances the slowly-varying dynamics by Lagrangian method along the resonance trajectory. We apply the HEL method to study the frequency chirping of whistler-mode chorus wave in the magnetosphere and the self-consistent simulations reproduce the chirping chorus wave and give high-resolution phase space dynamics of energetic particles at low computational cost. The scale-separated HEL approach could provide additional insights of the wave instabilities and wave-particle nonlinear coherence compared to the conventional Vlasov and particle-in-cell methods.

Identifying the electromagnetic counterparts of LISA massive black hole binaries in archival LSST data

ABSTRACT LSST will catalogue the light curves of up to 100 million quasars. Among these there can be $\sim$100 ultra-compact massive black hole (MBH) binaries, whose gravitational waves (GWs) can be detected 5–15 yr later by LISA. Here, we assume such a LISA detection occurred, and assess whether or not its electromagnetic (EM) counterpart can be identified as a periodic quasar in archival LSST data. We use the binary’s properties derived from the LISA waveform, including the evolution of its orbital frequency, its total mass, distance, and sky localization, to predict the redshift, magnitude, and historical periodicity of the quasar expected in the LSST data. We then use Monte Carlo simulations to compute the false alarm probability (FAP), i.e. the number of quasars in the LSST catalogue matching these properties by chance, based on the (extrapolated) quasar luminosity function, the cadence of LSST, and intrinsic ‘damped random walk’ quasar variability. We analyse four fiducial LISA binaries, with masses and redshifts of $(M_{\rm bin}/{\rm M_{\odot }},z) = (3\times 10^5,0.3)$, $(3\times 10^6,0.3)$, $(10^7,0.3)$, and $(10^7,1)$. While noise and aliasing due to LSST’s cadence produces false periodicities by chance, we find that the frequency chirp of the LISA source during the LSST observations washes out these noise peaks and allows the genuine source to stand out in appropriately scaled Lomb–Scargle periodograms. We find that all four fiducial binaries can be uniquely identified, with ${\rm FAP}\lt 10^{-5}$, a week or more before merger. This should enable follow-up EM observations targeting individual EM counterparts during their inspiral stage.

THz radiation generation in the unmagnetized plasma due to beating of two Hermite Cosh Gaussian lasers in the presence of frequency chirp

THz radiation generation in the unmagnetized plasma due to beating of two Hermite Cosh Gaussian lasers in the presence of frequency chirp

Temporal reflection from short pump pulses inside a dispersive nonlinear medium: the impact of pump parameters

We have studied, through a series of experiments and numerical simulations, how temporal reflection from an intense pump pulse inside a photonic crystal fiber is affected by parameters of the pump pulse used to form a moving high-index boundary. We used femtosecond pump pulses, which slow down inside the fiber as their spectrum red-shifts because of intrapulse Raman scattering. Temporal reflection of probe pulses occurs from such decelerating pump pulses. We changed the width and chirp of our pump pulses with a 4f pulse shaper capable of providing both spectral filtering and frequency chirping. We found that temporal refection exhibited novel features, to our knowledge, when pump pulses were made wider or chirped. In both cases, two or more reflected pulses were produced at different wavelengths in a specific range of the initial pump-probe delays. Numerical simulations reveal that the origin of such novel features is related to the complex nonlinear evolution of pump pulses inside optical fibers.

Cricket: A Self-Powered Chirping Pixel

We present a sensor that can measure light and wirelessly communicate the measurement, without the need for an external power source or a battery. Our sensor, called cricket, harvests energy from incident light. It is asleep for most of the time and transmits a short and strong radio frequency chirp when its harvested energy reaches a specific level. The carrier frequency of each cricket is fixed and reveals its identity, and the duration between consecutive chirps is a measure of the incident light level. We have characterized the radiometric response function, signal-to-noise ratio and dynamic range of cricket. We have experimentally verified that cricket can be miniaturized at the expense of increasing the duration between chirps. We show that a cube with a cricket on each of its sides can be used to estimate the centroid of any complex illumination, which has value in applications such as solar tracking. We also demonstrate the use of crickets for creating untethered sensor arrays that can produce video and control lighting for energy conservation. Finally, we modified cricket's circuit to develop battery-free electronic sunglasses that can instantly adapt to environmental illumination.

Bit weight-aware high-fidelity digital RoF fronthaul based on dual-drive MZM and chirp-dispersion Interaction.

Digital radio-over-fiber (D-RoF) quantizes the wireless waveform to improve the noise tolerance in fronthaul links. Unlike conventional data transmission, the quantization bits exhibit different weights, offering a new strategy to protect the high-weight bits. By introducing a dual-drive MZM (DD-MZM)-based optical transmitter, the interaction between frequency chirp and chromatic dispersion (CD) results in eye closure/open. Such an effect would potentially achieve higher fidelity of the reconstructed wireless signal in the SNR-limited region. In this paper, we first establish a theoretical model for chirp and dispersion-induced eye closure/open, characterizing the influence of fiber length and baud rate on the constellation interval. We then propose two protection schemes for 4-level pulse amplitude modulation (PAM-4), in which the high- and low-weight bits are interleaved and loaded onto the upper or lower arms of DD-MZM to leverage the negative or positive chirp. Furthermore, we extend to high-order modulation formats and propose two uneven optical PAM-8 schemes with different frequency chirps, which protect one or two of the three bits in each PAM-8 symbol, respectively. The chirp-dispersion interaction-enabled uneven optical PAM-4/8 schemes are experimentally demonstrated in intensity modulation and direct detection (IM-DD) D-RoF system. We compare the performance with chirp-free PAM-4/8 schemes generated by differential-driven mode. Then, we adopt the µ-law quantizer to verify the effectiveness of the proposed schemes and the Lloyd-Max algorithm-based quantizer to maximize the signal-to-noise ratio (SNR). With the help of chirp-dispersion interaction-enabled uneven optical PAM-4/8 schemes, the experimental results show 11.3-dB and 4.8-dB SNR gains of the wireless signal after 5-km fiber transmission, respectively.

Adiabatic theory of resonant particle dynamics with frequency chirping chorus waves in magnetospheric plasmas

We present the adiabatic regime for the particles interacting with the frequency chirping waves in the inhomogeneous magnetic field. Despite the rapid change of the parameters during the interaction, we can construct an adiabatic invariant with new canonical coordinates, which is shown to be conserved as long as the particles stay trapped in the reference frame moving with the resonance. Assuming the trapped particle distribution as a function of the adiabatic invariant and the water-bag approximation, we derive an analytic form of the nonlinear current as a function of the inhomogeneous parameter that describes the frequency chirping and inhomogeneities in the background magnetic field. The nonlinear current expression is also examined in the Vlasov hybrid simulations, and the simulation results show that the nonlinear current can be well described by the adiabatic water-bag approximation, except for the chirping onset stage and the source region where the adiabatic approximation is invalid.

Resonant Electron Signatures in the Formation of Chorus Wave Subpackets

AbstractA 2‐D GCPIC simulation in a dipole field system has been conducted to explore the excitation of oblique whistler mode chorus waves driven by energetic electrons with temperature anisotropy. The rising tone chorus waves are initially generated near the magnetic equator, consisting of a series of subpackets, and become oblique during their propagation. It is found that electron holes in the wave phase space, which are formed due to the nonlinear cyclotron resonance, oscillate in size with time during subpacket formation. The associated inhomogeneity factor varies accordingly, giving rise to various frequency chirping in different phases of subpackets. Distinct nongyrotropic electron distributions are detected in both wave gyrophase and stationary gyrophase. Landau resonance is found to coexist with cyclotron resonance. This study provides multidimensional electron distributions involved in subpacket formation, enabling us to comprehensively understand the nonlinear physics in chorus wave evolution.

Asymmetric pulse effects on pair production in chirped electric fields

We investigate the effects of the asymmetric pulse shapes on electron-positron pair production in a field with three different cases of chirping via the real-time Dirac-Heisenberg-Wigner formalism. Our findings reveal the disappearance of interference effects with shorter falling pulse length, and the peak is concentrated on the left side of the momentum spectrum. As the falling pulse length extends, an incomplete multiring structure appears in the momentum spectrum. The number density of particles are very sensitive to the asymmetry of the pulse. With a long falling pulse, the number density can be significantly enhanced by over four orders of magnitude when certain frequency chirps are utilized. These results highlight the impact of the effective dynamically assisted mechanism and the frequency chirp on pair creation. Published by the American Physical Society 2024

Spacetime quasicrystals in Bose-Einstein condensates

An autoresonant approach for exciting spacetime quasicrystals in Bose-Einstein condensates is proposed by employing two-component chirped frequency parametric driving or modulation of the interaction strength within Gross-Pitaevskii equation. A weakly nonlinear theory of the process is developed using Whitham's averaged variational principle yielding reduction to a two-degrees-of-freedom dynamical system in action-angle variables. Additionally, the theory also delineates permissible driving parameters and establishes thresholds on the driving amplitudes required for autoresonant excitation. Published by the American Physical Society 2024

A Hamiltonian theory for nonlinear resonant wave–particle interaction in weakly inhomogeneous magnetic field

We develop a Hamiltonian theory for the nonlinear resonant interactions between energetic particles and nonlinear frequency chirping waves in the weakly inhomogeneous magnetic field. A canonical transformation is constructed to separate the fast and slowly varying scales, and the Hamiltonian of the resonant particle is transformed to the local resonance reference frames. The Vlasov equation of the local distribution function moving at the local resonance velocity is obtained using Liouville's theorem. The evolution for the slowly varying wave envelope is derived from the Ampère's law with both cold plasma and energetic particle currents. The Vlasov equation coupled with the wave envelope equation self-consistently describes the dynamics of the deeply trapped resonant particles and the slowly varying coherent wave envelope. The application of the theory to the frequency chirping chorus wave in magnetospheric plasmas is also discussed.

Combining 77–81 GHz MIMO FMCW radar with frequency-steered antennas: a case study for 3D target localization

Abstract In this paper, we introduce a compact 6 × 8 channel multiple-input multiple-output frequency-modulated continuous-wave radar system capable of determining the three-dimensional positions of targets despite utilizing a linear virtual array. The compact system, containing two cascaded radar transceiver ICs, has 48 virtual channels. We conduct a direction of arrival estimation with these virtual channels to determine the azimuth angle. To overcome the spatial limitation of the linear array, we use frequency-steered transmit antennas, which vary their main lobe direction during the frequency chirp, allowing the elevation angle to be determined by using a sliding window fast Fourier transform algorithm. In this study, we present the system’s concept along with the associated signal processing. By taking measurements in different scenarios, each with differently placed corner reflectors, we investigate the capability of the system to separate adjacent targets concerning range, azimuth, and elevation. These measurements are additionally employed to point out the design trade-offs inherent to the system.