Frequency Chirp Research Articles

A PhC-SOA based cancerous cell detection biosensor

In this paper, we present a novel method to design an ultra-small photonic integrated biosensor to detect cancerous cells. The proposed biosensor is based on the self-phase modulation in PhC-SOA, inducing a frequency shift on a pulse traveling through the device. The amount of the frequency chirp depends on the group velocity of the active medium waveguide being determined by the refractive index of the microfluidic infiltrating the holes around the waveguide. The refractive index of the microfluidic is also determined by the cell type that can be normal or cancerous. Since the refractive index of a cancerous cell is higher than that of a normal one, the group index of the waveguide and the output chirp will decrease. By measuring the amount of the output chirp, we can detect the cell type. The Simulation results showed that for a 0.02 change in the refractive index of the cell, a 3.71 nm central wavelength shift occurred for a 10-ps 7-mW gaussian pulse input with a central wavelength of 1533.53 nm. In terms of the wavelength shift, the sensitivity and figure of merit are 185.5 and 530, respectively. To detect the cell type, we integrated a PhC channel drop filter to drop the chirped signal due to the cancerous cell infiltration. Designing an appropriate PhC-CDF leads to achieving an ultra-small cancerous detection cell biosensor with more than 97% precision.

Simulation of Downward Frequency Chirping in the Rising Tone Chorus Element

AbstractThe frequency chirping of chorus waves is commonly observed in the Earth’s inner magnetosphere, but its generation remains an open question. Recently, Liu et al. (2021), https://doi.org/10.1029/2021JA029258 reported two unusual rising‐tone (upward chirping) chorus elements. Although the central frequency of constituent subpackets rises, the frequency of a single subpacket is surprisingly downward chirping. With a gcPIC‐ simulation in the dipole field, we successfully reproduce this kind of substructure, which contains alternating signs of chirping. Interestingly, both hole and hill structures are formed around the theoretical resonant velocities in the electron phase space, no matter whether the chirping is upward or downward. However, during each chirping interval, only one structure (either a hole or a hill) is associated with wave excitation: the upward chirping is related to the hole, while the hill contributes to the downward chirping. Our study provides a fresh perspective on the theory of frequency chirping in chorus waves.

Generating ultrashort visible light pulses based on multidimensional solitary states in gas-filled hollow core fiber

Multidimensional solitary states (MDSS) are self-sustaining light wave packets confined in multiple dimensions in multimode fibers. In this work, we experimentally demonstrate the generation of MDSS, driven by a few hundreds of femtoseconds (fs) of long frequency doubled pulses from a Titanium:Sapphire chirped pulsed amplifier in a nitrous oxide-filled hollow core fiber (HCF). The MDSS output, resulting from intermodal interactions in a Raman-active gas-filled large core diameter HCF, features a broadband, red-shifted spectrum in the visible spectral region with a characteristic negative quadratic spectral phase. Therefore, the output with broadband spectra and negative chirp results in the generation of sub-30 fs pulses upon propagation through glass windows and a spectral filter. Backed with experimental observations and multidimensional simulations, we demonstrate that the sign of the frequency chirp of input pulses influences the spectral broadening in the HCF in the high gas-dispersion regime. We observed that the MDSS red-shifted pulses have a clean spatial profile. Therefore, the experimental requirements on the input beam size and quality to achieve a clean MDSS beam profile at the output of large core HCFs can be relaxed. Hence, this work extends the validation of the MDSS phenomenon toward the ultraviolet-visible region of the electromagnetic spectrum, thus providing an alternate source with a clean spatial beam profile for various applications in the field of ultrafast spectroscopy.

Propagation of chirped gray solitons in weakly nonlocal media with parabolic law nonlinearity and spatio-temporal dispersion

In our research, we have examined the existence and stability of chirped periodic and solitary waves in a weakly nonlocal nonlinear medium, which exhibits various types of effects, including inter-modal dispersion, nonlinear dispersion, detuning, spatio-temporal, and parabolic law nonlinearity. By studying the nonlinear Schrödinger equation that describes the field dynamics in this system, a class of nonlinearly chirped periodic waves is derived in the presence of all physical processes. In addition, we have obtained solitary waves of the gray type in the long-wave limit of these nonlinear waveforms. We have found that the frequency chirp associated with these optical waves depends on their intensity and its magnitude can be controlled by manipulating the nonlinear dispersion parameter. Furthermore, we have numerically studied the stability of the gray soliton solution under finite initial perturbations. Our results indicate that the nonlinear waves we have identified represent new types of extremely robust chirped localized structures in weakly nonlocal nonlinear parabolic law media.

Chirped nonlinear waves in the cubic-quintic distributed nonlinear Schrödinger equation with external trap, self-steepening and self-frequency shift

A cubic-quintic inhomogeneous nonautonomous nonlinear Schrödinger (NLS) equation with an external trap, self-steepening, and self-frequency shift that models the transmission of nonlinear waves in nonautonomous inhomogeneous systems is considered. The integrability conditions under which the equation is converted into the standard NLS equation are obtained and a variety of nonlinear wave solutions with nonlinear chirping such as one-soliton solution, two-soliton solution, Akhmediev breather (AB) solution, Ma breather (MB) solution, as well as first- and second-order rogue wave (RW) solution are reported. We show how various parameters of either the model equation or the solutions may affect the nature of nonlinear waves with their frequency chirps propagating in nonautonomous inhomogeneous systems modeled by the NLS equation under consideration. The density profiles of nonlinear waves obtained from all these solutions are analyzed. These solutions are used for investigating analytically the dynamics of nonlinear waves in Bose-Einstein condensates (BECs) with both two- and three-body interatomic interactions when the gain/loss of atoms is taken into consideration. Considering BEC systems with time-independent trap frequency, we show that under a constant gain/loss parameter, the trap parameter can be used to convert triplet second-order RWs into either doublet second-order RWs or a composite of one bright solitary wave and a doublet second-order RWs. Also, our results reveal that for BECs with constant trap frequency under kink-like gain/loss parameter, the trap parameter can be used to convert (i) triplet second-order RWs into single second-order RWs, and (ii) two-soliton into doublet rogue wave. The family of chirped nonlinear wave solutions presented in this work may prove significance for designing the manipulation and transmission of nonlinear waves. Also, the found here results may provide possibilities to manipulate experimentally some nonlinear waves such as rogue waves or two-soliton in BEC systems.

Probing non-linear MHD stability of the EDA H-mode in ASDEX Upgrade

Regimes of operation in tokamaks that are devoid of large edge localised modes have to be better understood to extrapolate their applicability to reactor-relevant devices. This paper describes non-linear extended magnetohydrodynamic (MHD) simulations that use an experimental equilibrium from an enhanced D-alpha (EDA) H-mode in ASDEX Upgrade. Linear ideal MHD analysis indicates that the operational point lies slightly inside of the stable region. The non-linear simulations with the visco-resistive extended MHD code, JOREK, sustain non-axisymmetric perturbations that are linearly most unstable with toroidal mode numbers of , but non-linearly higher and lower n become driven and the low-n become dominant. The poloidal mode velocity during the linear phase is found to correspond to the expected velocity for resistive ballooning modes. The perturbations that exist in the simulations have somewhat smaller poloidal wavenumbers (–) than the experimental expectations for the quasi-coherent mode in EDA, and cause non-negligible transport in both the heat and particle channels. In the transition from linear to non-linear phase, the mode frequency chirps down from approximately 35 kHz to 13 kHz, which corresponds approximately to the lower end of frequencies that are typically observed in EDA H-modes in ASDEX Upgrade.

Chirping instabilities produced by a runaway electron beam at a spherical tokamak

Two different types of MHD instabilities with rapidly chirping frequency were found to arise in the Globus-M2 spherical tokamak in substantially different frequency ranges. The first type arises at frequencies of an order of 1 MHz in ohmic plasmas at relatively low density in a wide range of toroidal magnetic fields and plasma currents. This type of instability was identified as compressional Alfvén waves, driven by electrons, accelerated during a sawtooth crush. It was found that the mode frequency is sweeping in time, according to the Berk–Breizman hole–clump nonlinear chirping model. The second type of wave arises in a specific single-swing regime of the central solenoid current with a very narrow plasma column, when the plasma tends to decay at extremely low density and, in fact, is an instability of the runaway electron beam. The exited modes cover the whole observed frequency range and are divided into several (two or three) frequency regions: approximately 0–30 MHz, 60–120 MHz and sometimes 30–60 MHz. Reconnection of the branches was also observed. Single chirps are more rapid than for 1 MHz Alfvén instability and follow an exponential law. This paper, to our knowledge, is the first report of frequency chirping instabilities excited by accelerated electrons at a spherical tokamak.

Supercontinuum generation by frequency chirp in photonic crystal fibers

Supercontinuum generation by frequency chirp in photonic crystal fibers

Chirped spatial solitons on a continuous-wave background in weak nonlocal media with polynomial law of nonlinearity

Spatial soliton propagation in a cubic-quintic-septimal optical material with a weakly nonlocal response is examined. We demonstrate that bright and gray solitons on a continuous-wave background exist for the envelope equation describing their dynamis in the physical media. In addition, optical kink and anti-kink type solitons are also identified. We find that these waveforms exhibit a frequency chirping property which is inversely proportional to the beam intensity. A noteworthy characteristic is that the chirped soliton solutions include no free parameters and their properties such as their amplitude, spatial width and propagation constant are determined solely by the material parameters. The numerical examples are structured for illustrating the propagation dynamics of these solitons in the optical system. The role of weak nonlocality on the modulation instability gain has also been discussed.

Creating pair plasmas with observable collective effects

Although existing technology cannot yet directly produce fields at the Schwinger level, experimental facilities can already explore strong-field quantum electrodynamics (QED) phenomena by taking advantage of the Lorentz boost of energetic electron beams. Recent studies show that QED cascades can create electron–positron pairs at sufficiently high density to exhibit collective plasma effects. Signatures of collective pair plasma effects can appear in exquisite detail through plasma-induced frequency upshifts and chirps in the laser spectrum. Maximizing the magnitude of the QED plasma signature demands high pair density and low pair energy, which suits the configuration of colliding an over energy-density electron beam with a 1022– intensity laser pulse. The collision creates pairs that have a large plasma frequency, made even larger as they slow down or reverse direction due to both the radiation reaction and laser pressure. This paper explains at a tutorial level the key properties of the QED cascades and laser frequency upshift, and at the same time finds the minimum parameters that can be used to produce observable QED plasma.

Asynchronous Upconversion Sampling of Frequency Modulated Combs

AbstractIn recent years, optical frequency combs with a frequency modulated output have attracted considerable attention. While first observed decades ago in externally modulated lasers, a whole class of semiconductor lasers has since been identified to spontaneously enter such a regime of self frequency modulation. However, the temporal properties of these sources could so far only be measured indirectly via phase‐resolved frequency domain measurements. In this work, an asynchronous optical sampling technique which operates directly in time domain is demonstrated. On the basis of a mid‐infrared quantum cascade laser frequency comb, both its instantaneous intensity and optical frequency are measured. The results confirm a quasi‐constant intensity output alongside a close to linear frequency chirp, in accordance with phase‐resolved spectral measurements. Contrary to previous works, stable phase‐locking is achieved in a regime of positive intracavity dispersion resulting in an inversion of the observed frequency modulation.

Radio-frequency measurements of energetic-electron-driven emissions using high-frequency magnetic probe on XuanLong-50 spherical torus

A high-frequency magnetic probe is designed and developed on the XuanLong-50 (EXL-50) spherical torus to measure high-frequency magnetic field fluctuation. The magnetic loop, radio filters, radio-frequency limiter, and data acquisition system of the probe are comprehensively examined. The fluctuation data from the EXL-50 plasma are analyzed in the time–frequency domain using fast Fourier transforms. Moreover, distinct high-frequency instabilities are detected using this diagnostic system. In particular, significant frequency chirping is observed, which is consistent with the bump-on-tail drive instability predicted using the Berk–Breizman model.

Analytical and numerical study of chirped optical solitons in a spatially inhomogeneous polynomial law fiber with parity-time symmetry potential

This work studies the dynamical transmission of chirped optical solitons in a spatially inhomogeneous nonlinear fiber with cubic-quintic-septic nonlinearity, weak nonlocal nonlinearity, self-frequency shift and parity-time () symmetry potential. A generalized variable-coefficient nonlinear Schrödinger equation that models the dynamical evolution of solitons has been investigated by the analytical method of similarity transformation and the numerical mixed method of split-step Fourier method and Runge–Kutta method. The analytical self-similar bright and kink solitons, as well as their associated frequency chirps, are derived for the first time. We found that the amplitude of the bright and kink solitons can be controlled by adjusting the imaginary part of the -symmetric potential. Moreover, the influence of the initial chirp parameter on the soliton pulse widths is quantitatively analyzed. It is worth emphasizing that we could control the chirp whether it is linear or nonlinear by adjusting optical fiber parameters. The simulation results of bright and kink solitons fit perfectly with the analytical ones, and the stabilities of these soliton solutions against noises are checked by numerical simulation.

Wavelength- and Time-Division Multiplexing via Pump Current Variation of a Pulsed Semiconductor Laser—A Method of Synchronization for Quantum Key Distribution

The dependence of the semiconductor laser wavelength on the pump current is a well-known phenomenon, which is generally attributed to a change in the refractive index of the active layer due to carrier injection. This effect is usually considered to be a drawback as it causes frequency chirping of the pulses produced via direct current modulation. Here, we show that at high values of thermal resistance of a laser diode, the lasing wavelength is red-shifted due to the change of the refractive index caused by a thermal effect and this shift may significantly exceed (in absolute value) the blue shift related to a refractive index change induced by the carrier injection. We propose to benefit from the ability to tune wavelength by the pump current variation and use the same laser to generate qubits and synchronization optical pulses for quantum key distribution at different wavelengths. To demonstrate the proposed method of synchronization, we perform numerical simulations as well as proof-of-principle experiments.

Hybrid simulations of m/n = 1/1 instability driven by energetic counter-passing particles in tokamak plasmas

A systematic simulation study of the $n/m=1/1$ instability driven by energetic counter-passing particles in tokamak plasmas has been carried out using the kinetic-MHD (Magnetohydrodynamics) hybrid code M3D-K. The safety factor's radial profile is monotonically increasing with central value $q_0$ less than unity. The linear simulation results show that the instability is either a $m/n=1/1$ energetic particle mode or a $m/n=1/1$ global Alfvén eigenmode depending on the value of the central safety factor. The mode frequencies are close to the tip of Alfvén continuum spectrum at the magnetic axis. The excited modes are radially localized near the magnetic axis well within the safety factor $q=1$ surface. The main wave particle resonance is found to be $\omega _\phi +2\omega _\theta =\omega$ , where ω is the mode frequency. The nonlinear simulation results show that there is a long period of quasi-steady-state saturation phase with frequency chirping up after initial saturation. Correspondingly, the energetic particle distribution with low energies is flattened in the core of the plasma. After this quasi-steady phase, the mode amplitude grows again and frequency jumps down to a low value corresponding to a new mode similar to the energetic co-passing particle-driven low-frequency fishbone while the energetic particle distribution is flattened for higher energies in the core of plasma.

Concept of enhanced frequency chirping for multi-pass cells to improve the pulse contrast

The technique of enhanced frequency chirping can be implemented in multi-pass cells to significantly improve the temporal contrast without a penalty in efficiency. In contrast to waveguides, multi-pass cells offer the unique opportunity to tailor the nonlinear interaction, as spectral broadening and dispersion can mostly be spatially separated. By including additional dispersion through the multi-pass cell mirrors, the pulse form can be controlled, which leads to a decrease of the spectral modulations. A pulse possessing this smoother spectrum can be compressed to reach higher peak powers and have more energy in the main feature compared to classical multi-pass cells.

Beta-induced Alfvén eigenmodes with frequency chirping driven by energetic ions in the HL-2A Tokamak

The beta-induced Alfvén eigenmodes (BAEs) with frequency chirping, observed in the HL-2A Tokamak, are analysed by a MHD-kinetic hybrid code MEGA. Realistic parameters are applied to the code, such as equilibrium, electron density and temperature, ion temperature, which is different from the kinetic Berk-Breizman theory. The BAEs are observed by Mirnov probes and soft X-ray arrays. Toroidal and porloidal mode number are confirmed to be <i>n</i>/<i>m</i> = 2/3 by using the phase shift method with toroidal filtered Mirnov signal arrays. And the soft X-ray arrays’ signal shows that BAEs are located at the core of the plasma and they have a relatively broad mode structure. The BAEs with up- and down-chirping are reproduced with MEGA code. The simulation results of mode structure accord well with experimental observations. Compared with up-chirping BAEs, the down-chirping BAEs are excited with higher plasma parameters and beta value, thus the energetic ion distribution in pitch angle has a broader width, and the beta value of energetic ions in the core of plasma and diffusion value are higher in the down-chirping simulation. The simulation results show that the phase space distribution of energetic ions affects the wave chirping direction. The energetic ions parallel to the magnetic field drive the up-chirping behavior. When the down-chirping behavior dominates, the density of energetic ions perpendicular to the magnetic field increases significantly. It shows that the down-chirping BAEs require higher beta and energetic ion density, which is consistent with the previous simulation result.

Nonlinear frequency chirps from a stabilized injected phase-modulated fiber laser loop

A phase-modulated frequency-shifting loop is injected by a single-frequency laser at 1.5 μm. In so-called Talbot conditions, i.e., when the modulation frequency is an integer multiple of the inverse of the cavity round-trip time, the loop generates a frequency comb whose temporal trace consists in a train of pulse doublets whose positions in time depend on the frequency of the injection laser. When the modulation frequency is slightly detuned from the Talbot condition, nonlinear frequency chirps are predicted and observed in the output pulse train. We demonstrate that these nonlinear chirps are not restricted to sinusoidal shapes, and also that the loop can be stabilized by exploiting the intracavity phase modulation.

Banded chorus generation by an electron shell distribution in an inhomogeneous magnetic field: 1D PIC simulations

Although many theoretical models have been proposed over several decades, the origin of banded chorus with a gap in intensity at Ωe/2 (Ωe being the angular electron cyclotron frequency) is still debated. In one of those models, the chorus gap formation is attributed to two anisotropic electron populations separated by an isotropized population (called the parallel plateau) at energy resonant with waves of frequency ∼Ωe/2. Here, we simplify the plateau population as an electron shell distribution in velocity space and investigate its role in the gap formation using a particle-in-cell code in a non-uniform magnetic field. The base plasma is configured to generate rising chorus elements spanning in frequency from below to above Ωe/2. Then, multiple simulations are run with an additional shell distribution with different density values to investigate the gap formation. The simulation results show that even a relatively small fraction of shell is quite effective in arresting the frequency chirping of lower-band chorus at ∼Ωe/2, resulting in a power gap there. Phase space analysis indicates that the resonant current contributed by the phase-trapped shell electrons (forming a phase space hill) can counteract nonlinear chorus growth driven by the phase space hole of energetic, anisotropic electrons. This process occurs in combination with the linear cyclotron damping suggested in earlier studies, and the resulting nonlinear damping may exceed the latter effect.

Optical beat notes assisted attosecond soft X-ray pulse generation in high-gain free electron lasers

Abstract Attosecond soft X-ray pulses are of great importance for the study of ultrafast electronic phenomena. In this paper, a feasible method is proposed to generate isolated fully coherent attosecond soft X-ray free electron laser via optical frequency beating. Two optical lasers with the opposite frequency chirps are used to induce a gradient frequency energy modulation, which helps to generate a gradually varied spacing electron pulse train. Subsequently, the undulator sections with electron beam delay lines are used to amplify the target ultra-short radiation. Numerical start-to-end simulations have been performed and the results demonstrate that an isolated soft X-ray pulse with the peak power of 330 GW and pulse duration of 620 as can be achieved by the proposed technique.