Pulse Characteristics Research Articles

Research on SAR Imaging Simulation Based on Time-Domain Shooting and Bouncing Ray Algorithm

The signal source of synthetic aperture radar (SAR) usually adopts the linear frequency modulation (LFM) signal, which exhibits the characteristics of a wide pulse. Hence, in the case of usage of the LFM signal by time-domain shooting and bouncing ray (TDSBR) to simulate the SAR echo signal, numerous time sampling points are generated, resulting in huge computational efforts; thereby, it is hard to exploit the TDSBR algorithm for simulating the SAR echo. In order to unlock this dilemma, the hybrid approach, the transfer function in conjunction with the range frequency-domain pulse coherence, is developed, in which the transfer function is stated by the radar cross-section. The proposed methodology is capable of enhancing the computational efficiency through avoiding the massive time sampling, so that the suggested TDSBR approach could be more conveniently applied to the SAR echo simulations. Furthermore, because of the efficiency advantage of the TDSBR in the calculation of wideband scattering field, the proposed methodology exhibits higher computational efficiency than the frequency-domain shooting and bouncing ray in the SAR image simulation. Finally, the key equation of the TDSBR for the transient scattering field of dielectric targets based on a closed-form integration formula is analytically derived, which is the basis of SAR imaging simulation for the composite scene of ships above the sea surface.

1.3-μm 2.8-ns electro-optical cavity damping Nd:YVO<sub>4</sub> laser

<sec>1.3-μm Nd laser has significant practical applications in various fields, such as fiber communication, medical treatment, frequency conversion, and scientific research. Many applications of a 1.3-μm laser, particularly frequency conversion, benefit greatly from a short pulse width with high peak power. In the paper, an electro-optical cavity dumping Nd:YVO<sub>4</sub> laser at 1342 nm wavelength is studied theoretically and experimentally.</sec><sec>The pulse width for an electro-optical cavity dumping laser is determined by the optical length of the cavity. A narrower pulse width is obtained by reducing the length of the cavity and the round trip time of the laser in the cavity. However, when the round trip time in the cavity approaches to the falling edge time of the electro-optical switch, shortening the length of the cavity will not obtain a narrower pulse width, and the falling edge time of the electro-optical switch will influence the laser pulse width. The temporal characteristics of the laser pulse are simulated when the falling edge time of the electro-optical switch is close to the round trip time in the cavity.</sec><sec>The influence of the falling edge time of the electro-optical switch on the laser pulse duration is analyzed theoretically. The modified rate equation is used to study the relationship between the falling edge time and the laser pulse width.</sec><sec>We demonstrate an electro-optical cavity dumping Nd:YVO<sub>4</sub> laser. The atom percent of 0.3% Nd:YVO<sub>4</sub> placed in a short Plano-concave cavity is in-band pumped by an 880 nm quasi-continuous-wave diode. A fiber-coupled diode laser module (<i>NA</i> = 0.22) with a power of 30 W is used. An LiNbO<sub>3</sub> electro-optical switch is employed for the cavity-dumping. The 1342-nm cavity-dumping laser operates at a repetition rate of 1 kHz, single-pulse energy of 0.21 mJ, and pulse width of 2.8 ns. Near-diffraction-limited beam quality with an <inline-formula><tex-math id="Z-20230713064624-1">\begin{document}$ M^2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230014_Z-20230713064624-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20230014_Z-20230713064624-1.png"/></alternatives></inline-formula>value of < l.2 is achieved. The setup uses MgO:PPLN crystal to generate efficient second harmonic at 671 nm, with a pulse width of 1.8 ns. To the best of our knowledge, this is the shortest pulse duration ever obtained from 1.3 μm actively <i>Q</i>-switched Nd-doped laser.</sec>

1342 nm 2.8ns electro-optical cavity damping Nd: YVO<sub>4</sub> laser

1.3 μm Nd laser has significant practical applications in various fields, such as fiber communication, medical treatment, frequency conversion, and scientific research, etc. Many applications of a 1.3 μm laser, especially like frequency conversion, benefit much from a short pulse width with high peak power. In the paper, an electro-optical cavity dumping Nd:YVO<sub>4</sub> laser at 1342 nm wavelength has been studied theoretically and experimentally. The pulse width for an electro-optical cavity dumping laser is determined by the optical length of the cavity. A narrower pulse width can be obtained by reducing the length of the cavity and the round trip time of the laser in the cavity. However, when the round trip time in the cavity approaches to the falling edge time of the electro-optical switch, shortening the length of the cavity will not get a narrower pulse width, and the falling edge time of the electro-optical switch will influence the laser pulse width. The temporal characteristics of the laser pulse are simulated when the falling edge time of the electro-optical switch is close to the round trip time in the cavity. Influences of the falling edge time of the electro-optical switch on the laser pulse duration are analyzed theoretically. The modified rate equation is used to study the relationship between the falling edge time and the laser pulse width. We demonstrate an electro-optical cavity dumping Nd:YVO<sub>4</sub> laser. The 0.3 at.% Nd:YVO<sub>4</sub> placed in a short Plano-concave cavity is in-band pumped by an 880 nm quasi-continuous-wave diode. A fiber-coupled diode laser module (NA=0.22) with a power of 30 W was used. A LiNbO<sub>3</sub> electro-optical switch was employed for the cavity-dumping. The 1342 nm cavity-dumping laser operates at a repetition rate of 1kHz, and a single-pulse energy of 0.21 mJ is obtained with a pulse width of 2.8 ns. Near-diffraction-limited beam quality with an M<sup>2</sup> value of < l.2 is achieved. The setup provides efficient second harmonic generation at 671 nm using a MgO:PPLN crystal, and the pulse width is 1.8 ns. To the best of our knowledge, this is the shortest pulse duration obtained from 1.3μm actively Q-switched Nd-doped laser.

Using Spin-labeled MR Imaging to Depict Slow Pancreatic Juice Flow.

This study evaluated the dependence of the signal characteristics of time-spatial labeling inversion pulse (time-SLIP) on flow velocity and tag thickness to depict the pancreatic juice flow by analyzing signal profile using a tube phantom study. The tag edge property was evaluated by edge rise distance (ERD). For various slow flow velocities and tag thicknesses, the signal profile characteristics were evaluated using two indices: the tag center value (RTCV) reduction rate and the total signal value along the tube (TSVT). ERD, which was about 10% of the tag thickness, was higher for thicker tags, making slow flow detection difficult. TSVT was proportional to the thickness of the tag and was preserved irrespective of the flow velocity. RTCV became lower with higher flow velocity and decreased significantly with thinner tags. These results suggest that the visualization of pancreatic juice flow might improve stability by considering the appropriate tag thickness.

Featurization of Ultrafast Expansion and Geometrical Properties of Heterogeneous Colliding Plasmas

Numerous fields of research and industry have undergone revolutionary change because of the unique characteristics of ultrashort laser pulses. Moreover, the ultrafast imaging sensors, such as ICCD technique, can help to understand the ionization features and expansion properties of colliding laser-induced plasma (CLPP) and related stagnation layer (S.L.) geometry. In this work, the effort will be focused on CLPP experiments from two seeds of heterogeneous elements. The research’s goal is to analyse the geometrical development of the colliding plasma, the temporal evolution of plume composition features and its associated characteristics. The expansion velocity and forward propagation range (FPR) of the stagnation layer in a nanosecond scale—both of which have been discovered. The ultrafast imaging results give the sight and explain the possibilities of extant technologies that can help to re-engineer the plasma characteristics for the next generation of lithography applications or new selective physical concepts.

Pulse Characteristics of Quasi Continuous-Wave Pumped Yb∶KYW/Cr4+∶YAG Q-Switched Laser

Pulse Characteristics of Quasi Continuous-Wave Pumped Yb∶KYW/Cr4+∶YAG Q-Switched Laser

Effect of Temperature on Current Pulse Characteristics of Negative Corona Discharge Based on Numerical Model

Temperature is an important factor affecting the dynamic processes of corona discharge, since the ionization, attachment, and mobility of charged particles are directly related to temperature. Most existing research has investigated the temperature effect on corona discharge through experimental works. However, they are hard to acquire the corona discharge microphysical processes, which are related to the formation of the current pulse and electromagnetic (EM) wave signals. This article proposes a numerical model with temperature-related transport parameters to study the temperature influence on microphysical processes and current pulses for negative corona. By comparing microphysical quantities such as the effective ionization coefficient, density, and velocity of space charges, the effect of temperature on the current pulse parameters including the rise and fall times, amplitude, and pulsewidth is systematically explained. It shows that as the temperature rises, the current pulse amplitude increases, while the other three parameters decrease. The rise time is dependent on the effective ionization coefficient. The fall time is affected by the velocities of negative and positive ions. The experimental results indicate the simulated current pulses are consistent with the measured ones, proving the effectiveness of the proposed numerical model with temperature variation.

Theoretical Analysis and Experimental Study of Pulse Characteristics of Fe2+∶ZnSe Passively Q-switched 2.794 μm Laser with High Repetition Rate

Theoretical Analysis and Experimental Study of Pulse Characteristics of Fe2+∶ZnSe Passively Q-switched 2.794 μm Laser with High Repetition Rate

Хрупкое разрушение проводника в сильном импульсном магнитном поле

The main factors resulting in conductor failure under the action of a strong pulsed magnetic field are analyzed. The theoretical model describes the geometry of a cylindrical thick-walled solenoid and considers magnetic field diffusion, ohmic heating of the material and mechanical stresses arising in it. The magnetic field amplitude at which induced stresses in the material reach the von Mises yield criterion is used as the Bth threshold field separating the areas of safe (non-destructive) and dangerous fields. In the case of an initially uniform material, the maximum heating temperature corresponding to this limit, which predetermines the thermomechanical stress, has been derived analytically. In the general case, based on the analysis of the calculated threshold field, the influence of various parameters (magnetic pulse characteristics, elastic moduli of the material, etc.) on the conductor resistance in the pulsed magnetic field is studied and ways of increasing the threshold field are proposed, in particular, by using different spatial profiles of the initial resistivity. It is shown that in comparison with a uniform material, a modified layer with increased resistivity formed on the surface allows to significantly increase the amplitude of the magnetic pulse withstood by the material without fracture.

Study on Output Characteristics of 20 kV/100 ns Nonlinear GaAs Photoconductive Semiconductor Switch

In this paper, the output electric pulse characteristics of gallium arsenide photoconductive semiconductor switch (GaAs PCSS) with bias voltage of 20 kV and pulse width of about 100 ns are studied. By designing the reflection and transmission optical paths, the absorption rate of the trigger optical of the GaAs PCSS at a wavelength of 1064 nm was measured to be 24.8%. When the bias voltage is 16 kV-20 kV, the avalanche multiplication rates of carriers are calculated respectively. The relationship between the output current waveform and the residual charge in the energy storage capacitor is analyzed when the bias voltage is 20 kV. And the trend of GaAs PCSS electric field intensity and current waveform change with time. This research will be helpful for the application design of GaAs PCSS under the nonlinear mode with special requirements, and has important significance for the application of frontier science.

Influence of high-order dispersion on output pulse characteristics in passively mode-locked fiber laser

Influence of high-order dispersion on output pulse characteristics in passively mode-locked fiber laser

Brittle fracture of a conductor in a strong pulsed magnetic field

The main factors resulting in conductor failure under the action of a strong pulsed magnetic field are analyzed. The theoretical model describes the geometry of a cylindrical thick-walled solenoid and considers magnetic field diffusion, ohmic heating of the material and mechanical stresses arising in it. The magnetic field amplitude at which induced stresses in the material reach the von Mises yield criterion is used as the Bth threshold field separating the areas of safe (non-destructive) and dangerous fields. In the case of an initially uniform material, the maximum heating temperature corresponding to this limit, which predetermines the thermomechanical stress, has been derived analytically. In the general case, based on the analysis of the calculated threshold field, the influence of various parameters (magnetic pulse characteristics, elastic moduli of the material, etc.) on the conductor resistance in the pulsed magnetic field is studied and ways of increasing the threshold field are proposed, in particular, by using different spatial profiles of the initial resistivity. It is shown that in comparison with a uniform material, a modified layer with increased resistivity formed on the surface allows to significantly increase the amplitude of the magnetic pulse withstood by the material without fracture. Keywords: Magnetic field diffusion, plastic deformation, thermomechanical stress, yield strength, von Mises yield criterion.

Research progress of ultra-short laser pulse characterization

Research progress of ultra-short laser pulse characterization

SAR Imaging in Frequency Scan Mode: System Optimization and Potentials for Data Volume Reduction

Frequency scanning (FScan) is an innovative acquisition mode for synthetic aperture radar (SAR) systems. The method is based on the frequency-dependent beam pointing capabilities of phased array antennas, artificially increased via the combined use of true time delays and phase shifters within the array antenna. By this, typical limitations of conventional SAR systems in terms of achievable swath width and azimuth resolution can be mitigated, and so a wide swath can be imaged maintaining a fine azimuthal resolution. In the first part of the article, we introduce the theoretical concept, which is necessary to evaluate the reduced echo window length (EWL) with respect to equivalent stripmap data and the implications for the transmit pulse characterization. An FScan sensor flying in a TerraSAR-X-like orbit is shown to be capable of imaging an 80-km wide swath with 1-m azimuth resolution. The resulting time–frequency properties of the recorded raw data make the traditional SAR data compression algorithms such as block-adaptive quantization (BAQ) highly inefficient in this case. Therefore, the second part of the article investigates dedicated quantization methods for efficient data volume reduction in FScan systems. Different solutions are investigated and evaluated through simulations. Various transformations of the raw data have been exploited to optimize the encoding process, including deramping, fast Fourier transform (FFT), and blockwise approaches. Compared with standard BAQ in the time domain, the suggested data compression methods significantly improve the resulting signal-to-quantization noise ratio, allowing for the reduction in the overall data volume by about 60% for the considered system scenario, while maintaining robustness in the presence of inhomogeneous scene characteristics at the cost of a modest complexity increase for its on-board implementation.

Concentration-Dependent Viscoelasticity of Poloxamer-Shelled Microbubbles.

The oscillation of shelled microbubbles during exposure to ultrasound is influenced by the mechanical properties of the shell components. The oscillation behavior of bubbles coated with various phospholipids and other amphiphiles has been studied. However, there have been few investigations of how the adsorption conditions of the shell molecules relate to the viscoelastic properties of the shell and influence the oscillation behavior of the bubbles. In the present study, we investigated the oscillation characteristics of microbubbles coated with a poloxamer surfactant, that is, Pluronic F-68, at several concentrations after the adsorption kinetics of the surfactant at the gas-water interface had reached equilibrium. The dilatational viscoelasticity of the shell during exposure to ultrasound was analyzed in the frequency domain from the attenuation characteristics of the acoustic pulses propagated in the bubble suspension. At Pluronic F-68 concentrations lower than 2.0 × 10-2 mol L-1, the attenuation characteristics typically exhibited a sharp peak. At concentrations higher than 2.0 × 10-2 mol L-1, the peak flattened. The dilatational elasticity and viscosity of the shell were estimated by fitting the theoretical model to the experimental values, which revealed that both the elasticity and viscosity increased markedly at approximately 2.0 × 10-2 mol L-1. This suggests that the adsorption properties of Pluronic F-68 strongly affect the oscillation characteristics of microbubbles of a size suitable for medical ultrasound diagnostics.

Study on the spectral characteristics of noise-like pulses in Erbium-doped fiber lasers

We demonstrate an Erbium-doped fiber laser (EDFL) based on nonlinear polarization rotation (NPR) mode-locking. The operating characteristics of noise-like pulse (NLP) with different cavity lengths and pump powers has been studied. For the first time, we experimentally demonstrate that the maximum spectral width of NLP narrows with the increase of cavity length under the same pump power, the maximum spectral width of NLP increases with the increase of pump power with the same cavity length.

Investigation of stable pulse mode-locking regimes in a NALM figure-9 Er-doped fiber laser.

We demonstrate three typical mode-locking processes of a nonlinear amplifying loop mirror (NALM) fiber laser via a general nonlinear Schrödinger equation-based (GNLSE) simulation model. First, the pulse evolutions in the NALM cavity were separately simulated under asymmetric and weakly asymmetric conditions. We found that the splitting ratio and positions of the gain fiber can result in a suitable phase bias between clockwise and counter-clockwise beams, enabling the realization of a self-starting low-threshold operating condition. To assess the roles of the splitting ratio and gain in the mode-locking process, we simulated three pulse formation processes: in the soliton, stretched-pulse, and dissipative soliton mode-locking regimes. The simulation results show that the splitting ratio, gain, and dispersion directly influence the mode-locking condition and pulse characteristics, thereby providing effective quantified guidance for high-quality pulse generation. Finally, an experimental NALM oscillation operating under stretched pulse conditions was established to investigate the impact of the splitting ratio and pump power on the pulse characteristics. The experimental results prove that the splitting ratio, gain, and dispersion can be used to manipulate the mode-locking threshold, self-starting threshold, nonlinear effects, and pulse characteristics.

Seismic performance of post-tensioned self-centering concrete frames under near-fault pulse-like ground motions

Post-tensioned self-centering (PTSC) frames have been identified as a class of high-performance structures with capabilities of mitigating damage and minimizing residual deformations. However, the seismic behavior of PTSC frames subjected to near-fault pulse-like ground motions (NPGMs) is not fully understood yet. This study provides a comprehensive assessment on the seismic performance of PTSC frames under NPGMs. 4-story and 12-story PTSC frames with different energy dissipation capacities are numerically modeled in the OpenSees platform. Various degrees of structural damage of the PTSC frames are properly described by particular limit states. Incremental dynamic analysis (IDA) is performed using a set of 91 near-fault ground motions with a wide range of pulse periods. Fragility surfaces for different damage states are developed for the exemplar PTSC frames with respect to both seismic intensity and pulse period. The numerical results indicate that the impacts of pulse period on the PTSC frames of different stories are quite different. Besides, the most unfavorable pulse period changes with the severity of structural damage. The effects of energy dissipation ratio and pulse period on the exceedance probabilities of different damage states are compared, and the limitation of the commonly adopted friction dampers is discussed. Based on the results and observations of structural performance with respect to the pulse characteristics, guides and recommendations for the design of PTSC frames are provided.

Remote acoustic sizing of tethered fish using broadband acoustics

Remote fish sizing is desirable in fisheries (e.g., pre-catch) and research (e.g., platforms without biological sampling capacity) applications. In those contexts, the high spatial resolution of pulse compressed broadband echoes combined with narrow beamwidth transducers makes it feasible to resolve the scattering from different parts of the fish body and hence can be used to measure the body size. A motorized apparatus was used to suspend individual fish in the acoustic beam of two laterally oriented transducers (45–90 kHz, 160–260 kHz, 12.2 m range) with precise control of rotation angle. Broadband scattering was measured from tethered Atlantic mackerel (Scomber scombrus), saithe (Pollachius virens), and pollack (Pollachius pollachius) ranging in standard length from 239 to 491 mm as a function of orientation angle to validate sizing based on the acoustic resolution of fish body parts. Under these controlled conditions, fish size was underestimated by 11–19 mm, varying with broadband pulse characteristics, orientation angle, species, and fish size. The best remote acoustic sizing results were obtained using 160–260 kHz pulses with a slow rise and fall of pulse amplitude (aka, taper).

Adaptive Approach to Time-Frequency Analysis of AE Signals of Rocks

The paper describes a new adaptive approach to the investigation of acoustic emission of rocks, the anomalies of which may serve as short-term precursors of strong earthquakes. The basis of the approach is complex methods for monitoring acoustic emission and for analysis of its time-frequency content. Piezoceramic hydrophones and vector receivers, installed at the bottom of natural and artificial water bodies, as well as in boreholes with water, are used as acoustic emission sensors. To perform a time-frequency analysis of geoacoustic signals, we use a sparse approximation based on the developed Adaptive Matching Pursuit algorithm. The application of this algorithm in the analysis makes it possible to adapt to the concrete characteristics of each geoacoustic pulse. Results of the application of the developed approach for the investigation of acoustic emission anomalies, occurring before earthquakes, are presented. We analyzed the earthquakes, that occurred from 2011 to 2016 in the seismically active region of the Kamchatka peninsula, which is a part of the circum-Pacific orogenic belt also known as the "Ring of Fire". It was discovered that geoacoustic pulse frequency content changes before a seismic event and returns to the initial state after an earthquake. That allows us to make a conclusion on the transformation of acoustic emission source scales before earthquakes. The obtained results may be useful for the development of the systems for environmental monitoring and detection of earthquake occurrences.