Chirp Excitation Research Articles

Generation of carrier-envelope phase-controlled isolated attosecond pulses using chirped femtosecond excitation lasers

ABSTRACTWe present an efficient approach for producing a carrier-envelope phase controlled isolated attosecond pulse by an optimized intense driving laser pulse. High-order harmonics are produced by numerically solving the time-dependent Schrödinger equation for the one-dimensional hydrogen atom in an ultrashort laser pulse. We define an efficient cost function to optimize the laser pulse by a genetic algorithm scheme. Our approach produces single attosecond pulses with desired properties, including the carrier-envelope phase, central frequency, and duration. Also, we analyze the time–frequency profiles of the attosecond emissions to gain a deeper insight into the underlying physical mechanism.

Identification of the nonlinearity in mass-spring system via experimental method

Engineering structures operate with a nonlinear dynamics behaviour at certain amplitude of input range which will produce noticeable changes and unpredictable effects. In this paper, the nonlinear property of a spring is investigated. The simplified mass-spring system is developed which consists of two springs attached on a thin plate structure. The translational degree of freedom is considered in the experimental and a preferential direction on the length direction of the spring is investigated. The linear property is first investigated via impact testing for the acceleration measurement. Subsequently, the spectral testing with different amplitude of periodic chirp excitations is carried out to identify the nonlinear property of the springs. The nonlinear dynamics behaviour of the springs are identified from the significant changes of natural frequencies from the frequency response function of the system.

Contextual Application of Pulse-Compression and Multi-frequency Distance-Gain Size Analysis in Ultrasonic Inspection of Forging

Ultrasonic pulse-echo non-destructive testing, combined with Distance Gain Size (DGS) analysis, is still the main method used for the inspection of forgings such as shafts or discs. This method allows the inspection to be carried out, assuring in turns the necessary sensitivity and defect detection capability in most of the cases. However, when testing large or highly attenuating samples with standard pulse-echo, the maximum achievable signal-to-noise ratio is limited by both the beam energy physical attenuation during the propagation and by the inherent divergence of any ultrasound beam emitted by a finite geometrical aperture. To face this issue, the application of the pulse-compression technique to the ultrasonic inspection of forgings was proposed by some of the present authors, in combination with the use of broadband ultrasonic transducers and broadband chirp excitation signals. Here, the method is extended by applying DGS analysis to the pulse-compression output signal. Both standard single-frequency/narrowband DGS and multi-frequency/broadband DGS analyses applied on pulse-compression data acquired on a forging with known defects are tested and compared. It is shown that the DGS analysis works properly with pulse-compression data collected by using a separate transmitter and receiver transducers. Narrowband analysis and broadband analyses provide almost identical results, but the latter exhibits advantages over the traditional method: it allows the inspection frequency to be optimized by using a single pair of transducers and with a single measurement. In addition, the range resolution achieved is higher than the one achievable for the narrowband case.

Comparison of optimisation strategies for the improvement of depth detection capability of Pulse-Compression Thermography

ABSTRACT In Pulse-Compression Thermography, the impulse response of the Sample Under Test is retrieved pixelwise by applying a proper matched filter on the set of acquired thermal images obtained by stimulating the system with a heat source amplitude – modulated by a proper coded signal. Linear frequency-modulated chirp signals and binary codes are the most employed coded excitations, and to improve the detection capability of the technique, a non-linear frequency-modulated chirp signal can be used to deliver more energy to the sample in a frequency range of interest. In this work, we report the application of an exponential chirp to modulate the heating source and we compare it with a standard linear chirp excitation. To do a fair comparison, various windowing functions have been applied on the matched filters to reduce range side lobes, thus enhancing the retrieved impulse response quality. It is shown that the combined use of an exponential chirp and an appropriate matched filter obtained by exploiting the Reactance Transform window provides a faithful reconstruction of the sample impulse response and an enhanced signal-to-noise ratio with respect to the use of linear chirp. This has been demonstrated on a 3D-printed polymethyl methacrylate (PMMA) sample containing 16 flat-bottom holes of different depths.

G-SERF Editing in Two-Dimensional Pure-Shift Total Correlation Spectroscopy: Scalar Coupling Measurements for a Group of Spins in Organic Molecules.

A novel G-SERF-PSYCHE-TOCSY (gradient encoded selective refocusing in pure shift yielded by chirp excitation version of total correlation spectroscopy) NMR pulse scheme has been proposed, which produces TOCSY chemical shift correlations, on one hand, and scalar coupling values for the spins scalarly coupled to irradiated resonances, by showing them as doublets along the indirect dimension, on the other. Therefore, recording such an experiment, for a group of spins with overlapping chemical shifts, in organic molecules can adequately provide scalar coupling information in a G-SERF manner along the indirect dimensions, and they can be assigned to particular spin pairs. Such COSY chemical shift correlations (which appear as doublets for the scalarly coupled spins) can be readily discriminated from the TOCSY peaks (which do not show such splitting) in the G-SERF-PSYCHE-TOCSY spectrum.

Application of pure shift and diffusion NMR for the characterisation of crude and processed pyrolysis oil

By combining diffusion-ordered NMR spectroscopy (DOSY) and pure shift NMR spectroscopy ((Pure Shift Yielded By Chirp Excitation (PSYCHE)) with 1D NMR metabolite assignment, we demonstrate an improved method for in situ analysis of pre and post-processed pyrolysis oil to quickly establish the most effective upgrading procedure. These experiments use molecular mass estimations from DOSY and single component verification using PSYCHE to confirm the identity of metabolites and show how the mass pattern for pyrolysis oils varies depending on how it is upgraded.With the use of a semi-automatic approach for metabolite assignment, we have verified and quantified individual components, giving rise to a collection of potential ‘marker compounds’; their changes in concentration being correlated to the upgrading process a pyrolysis oil undergoes.

Improved ultra-broadband chirp excitation.

The design and application of ultra-broadband excitation pulses have been among the most interesting and timely areas in NMR and EPR methodology in recent years, due especially to advances in hardware design in EPR, the advent and popularity of high- and ultrahigh-field NMR, and the application of numerical methods like optimal control theory to the design and optimization of radiofrequency pulses and pulse sequences. In this communication, we present a short, robust, and flexible version of the CHORUS family of constant-phase, very broadband excitation sequences. We demonstrate that more than 0.5 MHz excitation with uniform amplitudes and phases can be achieved with this excitation sequence.

GPR Imaging for Deeply Buried Objects: A Comparative Study Based on Compositing of Scanning Frequencies and a Chirp Excitation Function

Compositing of ground penetrating radar (GPR) scans of differing frequencies have been found to produce cleaner images at depth using the Gaussian mixture model (GMM) feature of the expectation-maximization (EM) algorithm. GPR scans at various heights (“Stand Off”), as well as ground-based scans, have been studied. In this paper, we compare the GPR response from a chirp excitation function-based radar with the response from the EM GMM algorithm compositing process, using the same mix of frequencies. A chirp excitation pulse was found to be effective in delineating the defined buried object, but the resulting image is less sharp than the GMM EM method.

Classification of Sonar Targets in Air: A Neural Network Approach.

Ultrasonic sonar sensors are commonly used for contactless distance measurements in application areas such as automotive and mobile robotics. They can also be exploited to identify and classify sound-reflecting objects (targets), which may then be used as landmarks for navigation. In the presented work, sonar targets of different geometric shapes and sizes are classified with custom-engineered features. Artificial neural networks (ANNs) with multiple hidden layers are applied as classifiers and different features are tested as well as compared. We concentrate on features that are related to target strength estimates derived from pulse-compressed echoes. In doing so, one is able to distinguish different target geometries with a high rate of success and to perform tests with ANNs regarding their capabilities for size discrimination of targets with the same geometric shape. A comparison of achievable classifier performance with wideband and narrowband chirp excitation signals was conducted as well. The research indicates that our engineered features and excitation signals are suitable for the target classification task.

A Wideband Electrical Impedance Tomography System Based on Sensitive Bioimpedance Spectrum Bandwidth

A wideband electrical impedance tomography (EIT) system based on bioimpedance spectrum (BIS) analysis was proposed to image biological objects. The system integrated 16 electrodes to realize driving and measurement over 1 kHz-1.1 MHz. A modified Howland current source and voltage measurement channels were designed to obtain complex impedance. Peripheral component interconnect interface-based serial data acquisition module transferred the measurement data to a host PC. The system has the least signal-to-noise ratio of 40 dB in each channel and good consistency. The system utilized a two-step strategy, based on short-time Fourier transform and Hilbert transform, the wideband chirp excitation signal was employed to analyze the BIS first, from which the sensitive bandwidth, i.e., the frequency range in which the amplitude (and phase angle) of the object changes rapidly, is obtained. Then, the discrete frequency points among the sensitive bandwidth were selected to compose a multisinusoidal signal for impedance tomography with time-difference and frequency-difference (FD) methods. An impedance model was tested to evaluate the performance of BIS analysis. Experiments on phantoms consisting of carrot cylinder and nylon rod were designed to verify the working strategy of the system. The results indicated that the developed system can distinguish the objects and reconstruct medium distribution with high image contrast and frequency resolution. Compared with the traditional multifrequency EIT, the proposed method and system show advantages in imaging the local electrical properties with high-frequency resolution and obtaining the FD images in the sensitive BIS bandwidth.

Propagation Characteristics and Defect Sensitivity Analysis of Guided Wave From Single Excitation Source in Elbows

Ultrasonic guided wave has received significant attention in the field of nondestructive testing because of their advantages in detecting the entire wall thickness. Since pipe elbows are an important component of the piping system, it is desirable to study the propagation behavior of the guided wave in the elbows. In this paper, instead of using the traditional method of a ring of transducers to generate guided wave at the same time, the propagation behavior of the guided wave in the elbows under the action of a single excitation source is simulated. Using the chirp excitation signal, the transmission signals of the guided wave at different excitation central frequencies are extracted with high efficiency, and the behavior of the guided wave in the elbow under different excitation frequencies is studied. In addition, it is proposed to use the flight time quantity to characterize the complete information of the structure, and the influence of the defects on the flight time is also studied. The results prove that the flight time feature has good detection sensitivity to the defects with varying depth in different locations of the elbow.

Non-destructive characterization of surfaces and thin coatings using a large-bandwidth interdigital transducer.

This paper deals with non-destructive testing of thin layer structures using Rayleigh-type waves over a broad frequency range (25-125 MHz). The dispersion phenomenon was used to characterize a layer-on-substrate-type sample comprising a thin layer of platinum 100 nm thick on a silicon substrate. The originality of this paper lies in the investigation of different ways of generating surface acoustic waves (SAWs) with large bandwidth interdigital transducers (IDTs) as well as the development of a measuring device to accurately estimate the SAW phase velocity. In particular, this study focuses on comparing the performance (in terms of SAW amplitude and bandwidth) of different excitations imposed on IDTs. The three types of excitations are burst, impulse, and chirp. The interest of chirp excitation compared to the other two types was clearly demonstrated in terms of the SAW bandwidth and amplitude of displacement. With these IDT transducers, measurements could be performed over a wide frequency band (20-125 MHz), and consequently, dispersion curves could be obtained over a wide frequency band with a range of velocity variations in the order of 100 m/s. Under these conditions, an extremely accurate estimate of the phase velocity as a function of the frequency could be obtained using a Slant Stack transformation. Finally, from these experimental dispersion curves and theoretical dispersion curves, an accurate estimate of the thickness of the layer could be obtained by inversion. This estimated thickness was then confirmed using profilometer measurements.

High Resolution for Chemical Shifts and Scalar Coupling Constants: The 2D Real‐Time J‐Upscaled PSYCHE‐DIAG

The facile determination of chemical shift and scalar coupling constants in NMR spectra is often prevented by spectral overlap and limited resolution. Here, we present a high-resolution NMR experiment for the simultaneous detection of both resonance frequencies and coupling patterns even with small J-values. A PSYCHE-decoupled DIAG (Pure Shift Yielded by Chirp Excitation- DIAGonal) experiment, which resolves chemical shift in the indirect dimension of a 2D experiment is combined with real-time J-upscaling in order to visualize small coupling constants that would otherwise be hidden in the linewidth of a regular proton or DIAG spectrum.

Nonlinear Aeroelastic System Identification Based on Neural Network

This paper focuses on the nonlinear aeroelastic system identification method based on an artificial neural network (ANN) that uses time-delay and feedback elements. A typical two-dimensional wing section with control surface is modelled to illustrate the proposed identification algorithm. The response of the system, which applies a sine-chirp input signal on the control surface, is computed by time-marching-integration. A time-delay recurrent neural network (TDRNN) is employed and trained to predict the pitch angle of the system. The chirp and sine excitation signals are used to verify the identified system. Estimation results of the trained neural network are compared with numerical simulation values. Two types of structural nonlinearity are studied, cubic-spring and friction. The results indicate that the TDRNN can approach the nonlinear aeroelastic system exactly.

A piezoelectric spring pendulum oscillator used for multi-directional and ultra-low frequency vibration energy harvesting

Low frequency vibration is a ubiquitous energy existing in our environment, but a large efficient harvesting of which remains challenging. This paper presents a simple piezoelectric spring architecture based on a common binder clip structure. The harvester with pendulum spring allows the energy of the dynamic mass to be converted into electrical energy in the piezoelectric transducer. Due to the basic characteristics of spring pendulums, the proposed harvester can efficiently scavenge not only ultra-low frequency but also multi-directional vibrational energies. Modeling and design are conducted and a normalized expression of the harvester behavior is given. Chirp and human motion excitations are used to evaluate the proposed harvester’s performances. Simulation and experimental results are in good agreement. The proposed device could generate a high output power (13.29 mW) at a low operating frequency (2.03 Hz), which shows great application prospects in the power supply of wearable products, ocean buoys, etc.

A chirp excitation for focussing flexural waves

In this paper, the dispersive nature of flexural waves is exploited to generate a shock response at an arbitrary location on a waveguide. The input waveform is an up-chirp whose instantaneous frequency is chosen to ensure synchronous arrival at an arbitrary focal point. An analytical expression is derived for the required chirp waveform as a function of bandwidth and focal point location given prior knowledge of the dispersion relation.The principle is illustrated for an analytical model of a uniform beam. Simulated results show that it is possible, in theory, to achieve peak responses that are at least an order of magnitude larger than steady state response due to harmonic excitation. Further, the peak response increases with approximately the square root of distance from the point of excitation when damping is negligible. Velocity, acceleration, normal strain and shear stress exhibit qualitatively similar results which differ quantitatively owing to their different frequency responses with respect to the input.A single degree-of-freedom model of an electrodynamic shaker is coupled to the analytical beam model in order to predict peak mechanical responses per peak input voltage of the chirp waveform. The coupled electromechanical model is then validated experimentally through both frequency response and transient measurements. The technique is potentially applicable to situations where a large and reasonably localised transient response is required on a beam or plate-like structure using minimal instrumentation.

PSYCHE Pure Shift NMR Spectroscopy.

Broadband homodecoupling techniques in NMR, also known as "pure shift" methods, aim to enhance spectral resolution by suppressing the effects of homonuclear coupling interactions to turn multiplet signals into singlets. Such techniques typically work by selecting a subset of "active" nuclear spins to observe, and selectively inverting the remaining, "passive", spins to reverse the effects of coupling. Pure Shift Yielded by Chirp Excitation (PSYCHE) is one such method; it is relatively recent, but has already been successfully implemented in a range of different NMR experiments. Paradoxically, PSYCHE is one of the trickiest of pure shift NMR techniques to understand but one of the easiest to use. Here we offer some insights into theoretical and practical aspects of the method, and into the effects and importance of the experimental parameters. Some recent improvements that enhance the spectral purity of PSYCHE spectra will be presented, and some experimental frameworks, including examples in 1D and 2D NMR spectroscopy, for the implementation of PSYCHE will be introduced.

High-intensity focused ultrasound (HIFU) ablation by frequency chirp excitation: reduction of the grating lobe in axial focus shifting

High-intensity focused ultrasound (HIFU) ablation has gained popularity in tumor treatment. However, the pre-focal heating due to the presence of the grating lobe may generate unintended heating in clinical applications and limit the steering abilities of the phased array design. In order to solve this problem, frequency chirp excitation was proposed and evaluated both numerically and experimentally. First, propagation of an HIFU burst through multi-layer media was simulated in a nonlinear acoustic model using the angular spectrum approach. Influences of axial focus shifting on the acoustic intensity at the both grating and main lobes at different excitation frequencies were investigated. It was found that different excitation frequencies result in variations in the position and magnitude of the grating lobe in the pre-focal region, but have little influence on the main lobe. Thus, the frequency chirp excitation can spread the acoustic energy in the pre-focal region, and subsequently reduce the temperature elevation. The performances of downward and upward frequency chirps in different sweeping times were compared to those of conventional excitation at the constant frequency. Then, HIFU plane ablation was evaluated. Even without the cooling time between treatment spots when using the frequency chirps, the pre-focal heating was still less than that at the constant frequency. Finally, the thermocouple measurement in the gel phantom showed an ~40% reduction in pre-focal temperature elevations produced by a single-element concave HIFU transducer using the frequency chirps. Altogether, the frequency chirp excitation could effectively reduce the pre-focal grating lobe at a large axial focus shifting distance, which may enhance both the efficacy and safety of clinical HIFU ablation.

Use of chirp excitations for frequency domain spectroscopy measurement of oil-paper insulation

Frequency domain spectroscopy is a non-invasive, offline test performed to estimate moisture content, conductivity and aging status of the composite oil-paper insulation of a transformer. In conventional frequency domain spectroscopy test, sinusoidal excitation voltage is applied on the insulation under test for two cycles starting from 0.1 mHz to 1 kHz. One important aspect of conventional dielectric response measurement is the choice of excitation voltage. The insulation in real life operation is often stressed by voltage waveforms, which are non-stationary in nature. Therefore, modification of excitation voltage waveform is required for reliable assessment of insulation condition. Another important factor is the measurement time. Conventional frequency sweep from 0.1 mHz to 1 kHz, makes frequency domain spectroscopy measurement inadvertently lengthy, hence development of new measuring technique is necessary to reduce measurement duration for diagnosis of insulation condition. Considering these two facts, this contribution reports the feasibility of conducting dielectric response measurement in frequency domain using chirp excitation voltage waveforms. Instead of applying sinusoidal excitation at discrete measurement frequencies, chirp signals (band limited from 0.1 mHz to 1 Hz) were used to conduct frequency domain spectroscopy measurement on three laboratory prepared test samples and on two transformers. The dielectric dissipation factors evaluated using chirp waveforms showed a very good agreement when compared to that obtained by using sinusoidal excitation. Besides, investigations also revealed that the proposed method offered least amount of measurement time in comparison with conventional measurement as well as with some existing state of the art methods.

Chirping and Sudden Excitation of Energetic-Particle-Driven Geodesic Acoustic Modes in a Large Helical Device Experiment.

Energetic-particle-driven geodesic acoustic modes (EGAMs) observed in a Large Helical Device experiment are investigated using a hybrid simulation code for energetic particles interacting with a magnetohydrodynamic (MHD) fluid. The frequency chirping of the primary mode and the sudden excitation of the half-frequency secondary mode are reproduced for the first time with the hybrid simulation using the realistic physical condition and the three-dimensional equilibrium. Both EGAMs have global spatial profiles which are consistent with the experimental measurements. For the secondary mode, the bulk pressure perturbation and the energetic particle pressure perturbation cancel each other out, and thus the frequency is lower than the primary mode. It is found that the excitation of the secondary mode does not depend on the nonlinear MHD coupling. The secondary mode is excited by energetic particles that satisfy the linear and nonlinear resonance conditions, respectively, for the primary and secondary modes.