Peak Spectral Density Research Articles

Research on LD-pumped single-frequency laser based on twisted-mode-cavity and RVBG

Abstract Single-frequency lasers have the advantages of good spectral purity, high peak spectral density, long coherence distance and low phase noise. They are widely used in fields such as laser spectroscopy, laser lidar, sodium guide stars, coherent optical communication and high-resolution measurement. We designed an all-solid-state single-frequency laser based on twisted-mode-cavity (TMC) structure and used a reflective volume Bragg grating (RVBG) to replace the ordinary output mirror. Through the composite mode selection effect of the twisted-mode-cavity and RVBG, the occurrence of multi-longitudinal mode oscillation is eliminated successfully, and precise optical frequency selection is achieved. When the pump power reached 88.0 W, the output power was 1.6 W, allowing for stable single-frequency laser operation at 1064.44 nm, with an output laser linewidth of 9.77 × 10−3 pm, the corresponding slope efficiency was 10.00%, and the light-to-light conversion efficiency was 1.82%. The power instability was less than 4.44% and the beam quality factor were M x 2 = 1.35 , M y 2 = 1.38 . Experimental results show that compared with using a RVBG or twisted-mode-cavity individually, the combination of the two methods can significantly compress the laser linewidth and achieve single-frequency laser output.

Computations of energetic nearshore waves: Are weakly dispersive phase-resolving models telling the same story?

Three phase-resolving weakly dispersive wave models are used for 2DH (2D depth-integrated) computations of large-scale wave-by-wave processes induced by highly energetic sea/swell (SS) forcing near Haleʻiwa on the North Shore of Oʻahu, Hawaiʻi. The computed model results are compared to observations obtained over a nearshore cross-reef transect and from the basin of a small boat harbor. The level of agreement between the model results and observations in complex coastal environments under highly energetic wave forcing, along with the qualitative consistency among the three models, makes these models good candidates for operational applications in nearshore environments exposed to energetic wave forcing conditions.Spectral analyses inside the harbor and over the reef indicate that all three models generally account for infragravity (IG) spatial modal structures that are consistent with observations and the theory of edge and leaky waves. Over the reef, auto- and cross-spectral analyses reveal that the dominant waveforms are qualitatively reproduced by all three models, as indicated through: (i) the growth of IG wave amplitudes from deeper water to the shallow reef sites; (ii) the agreement of power spectral density peaks at the nearshore locations; and (iii) the remarkable similarity of spatial coherence functions among the models and between the models and observations. The computations of swell entering the small boat harbor at Haleʻiwa demonstrate that the models can successfully reproduce the variability in the narrow IG frequency bands that are spatially dependent and often subject to resonant amplifications.

Photo-induced dynamics with continuous and discrete quantum baths.

The ultrafast quantum dynamics of photophysical processes in complex molecules is an extremely challenging computational problem with a broad variety of fascinating applications in quantum chemistry and biology. Inspired by recent developments in open quantum systems, we introduce a pure-state unraveled hybrid-bath method that describes a continuous environment via a set of discrete, effective bosonic degrees of freedom using a Markovian embedding. Our method is capable of describing both, a continuous spectral density and sharp peaks embedded into it. Thereby, we overcome the limitations of previous methods, which either capture long-time memory effects using the unitary dynamics of a set of discrete vibrational modes or use memoryless Markovian environments employing a Lindblad or Redfield master equation. We benchmark our method against two paradigmatic problems from quantum chemistry and biology. We demonstrate that compared to unitary descriptions, a significantly smaller number of bosonic modes suffices to describe the excitonic dynamics accurately, yielding a computational speed-up of nearly an order of magnitude. Furthermore, we take into account explicitly the effect of a δ-peak in the spectral density of a light-harvesting complex, demonstrating the strong impact of the long-time memory of the environment on the dynamics.

A mixer architecture using GaN-based split-gate nanowire transistor

Frequency mixer is an essential block in radio-frequency signal processing for frequency translation and phase comparison. The most common mixers are fabricated using passive elements which suffer from significant conversion loss and low isolation. Mixers using active devices are used less frequently and rather less matured on GaN technology. Here, we demonstrate a mixer based on GaN split-gate nanowire transistor, allowing low conversion loss and high isolation. A constriction is formed by electrostatic modulation of the effective gate width. The threshold voltage of the transistor is modified by one of the gate voltages through the width variation, while the other gate voltage biases the transistor in the saturation region. The nonlinear dependency of the transistor characteristics on the two gate voltages facilitates frequency translation. The mixing characteristics of this architecture are verified both experimentally and theoretically. The output power spectral density peaks at the difference frequency with a minimal conversion loss. Extremely high isolation is measured using three-port S-parameter measurements. The proposed architecture shows multiple benefits, additionally facilitating monolithic mixers on the GaN platform.

Effects of windbreak types on aerodynamics of high-speed trains traversing from flat ground to semi-cutting and semi-embankment under crosswinds

Under the operation of strong crosswinds, the aerodynamic performance of high-speed trains (HSTs) will be seriously deteriorated when the transition section of flat ground and semi-cutting and semi-embankment (FGSCSE) is traversed, and the setting of windbreaks will help to slow down the impact of strong crosswinds on the trains. In this study, a three-dimensional coupled computational fluid dynamics numerical model to assess the aerodynamic performance of train–windbreak–FGSCSE–air system is developed. A comparative assessment is carried out to identify the variations in aerodynamic performance on the train carriage: no windbreak (NW), 50% ventilation windbreak (VW), and solid windbreak (SW), and the reasons for these variations are elucidated by examining the flow field structure's evolution. Furthermore, the operational safety of the train is discussed based on the indicator of wheel unloading ratio (fΔQ). Across the three distinct conditions, significant abrupt changes in aerodynamic load coefficients (ALCs) and the shedding of vortex structures are experienced by HSTs traversing the FGSCSE transition sections. Compared to the VW condition, the NW and SW conditions result in a greater number of shedding vortices on the leeward side and the tail of the train, and the VW condition results in the smallest magnitude of ALCs fluctuation. The power spectral density peak values of the aerodynamic loads follow the order: SW > NW > VW. Upon the train fully enters the subsequent operational environment, the VW condition has the smallest standard deviation of these coefficients. The standard deviations of CFy, CFz, CMx, CMy, and CMz for the head train in the VW condition are only 57.17% (46.81%), 55.85% (54.15%), 72.74% (34.62%), 57.99% (51.92%), and 44.60% (43.82%) of the corresponding values in the NW (SW) condition, respectively. In the NW, VW, and SW conditions, the fΔQ exceeds 0.9 when the wind speeds reach 30, 40, and 35 m/s, respectively. The windbreak with a ventilation rate of 30% performs the best, providing the most effective safety and stability for train operation.

Cascaded hard X-ray self-seeded free-electron laser at megahertz repetition rate

AbstractHigh-resolution X-ray spectroscopy in the sub-nanosecond to femtosecond time range requires ultrashort X-ray pulses and a spectral X-ray flux considerably larger than that presently available. X-ray free-electron laser (XFEL) radiation from hard X-ray self-seeding (HXRSS) setups has been demonstrated in the past and offers the necessary peak flux properties. So far, these systems could not provide high repetition rates enabling a high average flux. We report the results for a cascaded HXRSS system installed at the European XFEL, currently the only operating high-repetition-rate hard X-ray XFEL facility worldwide. A high repetition rate, combined with HXRSS, allows the generation of millijoule-level pulses in the photon energy range of 6–14 keV with a bandwidth of around 1 eV (corresponding to about 1 mJ eV–1 peak spectral density) at the rate of ten trains per second, each train including hundreds of pulses arriving at a megahertz repetition rate. At 2.25 MHz repetition rate and photon energies in the 6–7 keV range, we observed and characterized the heat-load effects on the HXRSS crystals, substantially altering the spectra of subsequent X-ray pulses. We demonstrated that our cascaded self-seeding scheme reduces this detrimental effect to below the detection level. This opens up exciting new possibilities in a wide range of scientific fields employing ultrafast X-ray spectroscopy, scattering and imaging techniques.

Statistical characteristics of sea surface waves derived from buoy measurements in the eastern Guangdong offshore water

The wave characteristics of the eastern Guangdong offshore water were quantified based on 1-year wave observations at a water depth of 60 m. This study statistically analyzed the observed wave characteristics and quantified the variations of the wave parameters and spectrum due to Super Typhoon Mangkhut. Results showed that the dominant and strong wave directions in the northeastern South China Sea were north-eastern and eastern, respectively. The average significant wave height was in the range of 0.7–2.4 m. During Super Typhoon Mangkhut, the maximum wave height and period were 11.3 m and 10–12 s, respectively, and peak spectral density mainly varied between 0.06 and 0.1 Hz. Results of this study can provide a basis for offshore engineering design and are of great significance for marine forecasting and disaster prevention and reduction.

OrcaFlex predictions for a multi-float hinged WEC with nonlinear mooring systems: Elastic mooring force and dynamic motion

The mooring system can affect the motion and energy output characteristics of a WEC. This study investigated the effects of an SPM mooring system in a M4 6-float WEC. The scaled model of M4 was modelled via both Orcaflex and experimental approaches. Compared with experimental results, the accuracy of the numerical results was found to be sensitive to the simulation methods in OrcaFlex, and the filter method supplied a better agreement with experimental results in this study. Following the numerical method selection, another two mooring cables were introduced to compare different stiffness properties with respect to tension reduction. Compared with the other two mooring cables, cable 3 with negative bending has the lowest peak spectral density of tension in all wave conditions; however, its stiffness needs to be optimised to reduce the peak tension in some large wave conditions. According to this, the stiffness of cable 3 is initially adjusted, and the new cable 3 can reduce up to 30% offset distance and 50% peak tension. The results also found that the relative rotational motion of the M4 is not sensitive to stiffness properties, which means the future PTO selection could be independent of the mooring cable.

Особенности волнения в районе м. Свободный (юго-восточная часть о. Сахалин) во время прохождения циклонов

The study of marine wave processes was carried out according to field observations using two autonomous wave recorders, temperature and weather station installed near Cape Svobodny, south-east coast of Sakhalin (Russia). Spectral and cross-spectral analysis showed the existence of edge waves with a period of about 10.7 min. Measurements in 2021 showed that the edge wave existing from Cape Ostry to Cape Svobodny, just beyond the cape Svobodny significantly weakens and does not spread further. The analysis of temperature fluctuations for the period range 1-80 hours showed that since the periods of spectral density peaks of water temperature fluctuations for periods longer than 5 hours do not coincide with the periods of peaks of sea level fluctuations, these peaks are determined by internal waves. Temperature fluctuations with a period of 25.5 hours detected by peaks in the spectra can be excited by shelf waves with the same period because of their interaction with islands, coastal currents and baroclinic instability. The analysis of cyclone wakes based on the time course of temperature fluctuations made it possible to establish that cyclone wakes are formed when the water temperature of the upper mixed layer exceeds 10 °C, and internal waves with a period of about 13 hours are also present when cyclones do not move near the point of installation of devices and the water temperature is below 10 °C. The Burger number is determined, which makes it possible to correct the range of existence near inertial internal waves and determine this range periods of 12.1-18.2 hours. Using the results of a simple linear Phillips model, the possibility of baroclinic instability for periods of shelf waves is estimated. It is shown that baroclinic instability is possible for waves with a period of 13.1 hours, and even more so for shelf waves with a significantly longer wavelength.

The Use of Empirical Mode Decomposition on Heart Rate Variability Signals to Assess Autonomic Neuropathy Progression in Type 2 Diabetes

In this study, we investigated the use of empirical mode decomposition (EMD)-based features extracted from electrocardiogram (ECG) RR interval signals to differentiate between different levels of cardiovascular autonomic neuropathy (CAN) in patients with type 2 diabetes mellitus (T2DM). This study involved 60 participants divided into three groups: no CAN, subclinical CAN, and established CAN. Six EMD features (area of analytic signal representation—ASRarea; area of the ellipse evaluated from the second-order difference plot—SODParea; central tendency measure of SODP—SODPCTM; power spectral density (PSD) peak amplitude—PSDpkamp; PSD band power—PSDbpow; and PSD mean frequency—PSDmfreq) were extracted from the RR interval signals and compared between groups. The results revealed significant differences between the noCAN and estCAN individuals for all EMD features and their components, except for the PSDmfreq. However, only some EMD components of each feature showed significant differences between individuals with noCAN or estCAN and those with subCAN. This study found a pattern of decreasing ASRarea and SODParea values, an increasing SODPCTM value, and a reduction in PSDbpow and PSDpkamp values as the CAN progressed. These findings suggest that the EMD outcome measures could contribute to characterizing changes associated with CAN manifestation in individuals with T2DM.

Experimental investigation of strut effects on slanted cylinder afterbody aerodynamics using magnetic suspension and balance system

In this study, strut support effects on a slanted cylinder afterbody were investigated by a magnetic suspension and balance system installed in a wind tunnel. The results showed that the aerodynamic characteristics with respect to the critical Reynolds number where the flow field and aerodynamic forces change significantly. In the cases with the dummy strut, the range of the critical Reynolds number decreases, and the variation depends on the location of the strut. The size of the recirculation region and the separation bubble on the wake center plane also changes depending on the location of the strut. Moreover, additional weak vortices are observed to be formed behind the strut, which changes the wake structure. That change affects the variation of the vortex core wandering, and is a factor in the power spectral density peaks observed in previous studies. It is suggested that the strut support strongly interferes with the flow around the test model and should be carefully considered.

Wind Interference Effect of Neighbouring Square Buildings in Rhombic Arrangement on an Octagonal Building

Four square plan shaped tall buildings of the same height and lateral dimension are positioned at the vertices of a rhombus. The octagonal plan shaped tall building of same height is placed at the centroid of the rhombus. The wind-induced responses of the octagonal plan shaped tall building for specified interference cases are compared with that of the isolated cases. The octagonal plan shaped principal building is exposed to atmospheric boundary layer wind flow. The wind angle varies from 0° to 90° at an interval of 15° for both isolated and interfering conditions. Computational fluid dynamics is implemented to simulate the effect of wind-structure interaction on the principal and neighbouring buildings in the boundary layer wind flow. The wind-induced static responses like drag and lift coefficients, pressure coefficients for isolated and interference cases are evaluated and compared graphically. The responses of isolated and interference cases vary significantly with the varying wind incidence angle. The dynamic behaviour of wind is also thoroughly investigated to identify the adverse local effect on the different faces of the octagonal building. As the characterisation of wind-induced dynamic responses, peak pressure coefficients and power spectral densities are evaluated and plotted accordingly. The peak pressure coefficients are plotted against mean pressure coefficients. The power spectral densities for different interfering cases are compared with the isolated case. Significant variation in power spectral densities is observed in some cases, which portrays the different characteristics for formation and shedding of the vortices in the wake of the octagonal building. The comparison of various dynamic responses between isolated and interfering cases highlights the interference by the square plan shaped buildings.

Measurement and Analysis of Crowdsourced Vehicle Vibration Levels during Last Mile Delivery Segments for Parcel Shipments

Crowdsourced logistics has emerged as a delivery channel for many single-parcel packages. As a result, this logistics network has introduced personal passenger vehicles as a means to transport parcels during last mile delivery segments. To understand this network’s vibration levels and cargo capacity restraints, four vehicle types (a sedan, sports sedan, compact SUV and full-size SUV) commonly used in crowdsourced logistics deliveries were selected for measurement and analysis. This study shows that the vibration levels were significantly higher in the vertical axis and that the overall vibration energy increased as vehicle speed increased, except in the sedan. The sedan and SUV vehicles showed power spectral density peak frequencies in the low-frequency range, occurring at approximately 2 Hz, matching previous studies using similar vehicles. The vibration levels were greatest in the sports sedan and lowest in the sedan. The recorded vibration events showed a right-skewed heavy-tailed distribution and were non-Gaussian.

Evaluation of effect of micro-vortex generator on dynamic characteristics of unsteady partial cavitating flow over hydrofoil

The unsteady characteristics of cavitation induce a series of destructive effects, such as cavitation erosion, vibration, and noise. Passive control of hydrofoil cavitation involves changing the structure or material of the hydrofoil surface, thereby inhibiting cavitation damage, and vortex generators (VGs) are the most popular means of passive control because of their effectiveness, simplicity, and lower production and installation costs. In this study, a new VG structure that can improve the cavitating flow state of a hydrofoil is proposed. A multiphase volume-of-fluid model is used to capture the interface between different phases, the Schnerr–Sauer model is used to describe the cavitation process, and turbulence is modeled by large-eddy simulation. The evolution characteristics of the cavitating flow field around a National Advisory Committee for Aeronautics (NACA) 0015 hydrofoil are investigated numerically both with and without the VG to illustrate its control effect. It is found that the VG can change the hydrodynamic characteristics of the cavitating flow field around the hydrofoil, such as the cavity morphology, the vortex structures, and the peak and power spectral density of the pressure. Upon using Lagrangian coherent structures, the motion state of the fluid particles and the distance between adjacent fluid particles are found to change dramatically with time under the effect of the VG. Upon using dynamic mode decomposition, the VG is found to decrease the energy in the first three modes, whereas that in mode 4 is larger without the VG, thereby suggesting that the VG might help to reduce flow noise. Finally, it is found that the VG converts laminar flow to turbulence, thereby increasing the turbulence intensity of the wake flow field and decreasing the maximum value of the turbulence integral scale.

Statistical Properties of Whistler‐Mode Hiss Waves in the Inner Radiation Belt

AbstractUsing the Van Allen Probes A and B observations from 01 January 2013 to 28 February 2018, we surveyed statistically the occurrence rate, intensity, and propagation properties of hiss waves in the inner radiation belt (1.1 < L ≤ 2). Like the outer plasmaspheric hiss (L > 2), the occurrence rate and amplitude of lower‐band hiss (<600 Hz) are higher in the dayside high‐L region (L > 1.3 and magnetic local time [MLT] = 6–20 hr) than the nightside (MLT ∼ 20–6 hr) and increase with enhanced substorm activities (AE increases). Furthermore, their peak power spectral densities are located nearly in the same band (∼200–500 Hz). The equatorward propagation of middle‐latitude hiss suggests that the lower‐band hiss waves in the inner radiation belt mostly originate from the outer plasmaspheric hiss at high latitudes. Although the outer plasmaspheric hiss is also a likely source of weak upper‐band hiss (≥600 Hz) in the dayside high‐L region (L > 1.3 and MLT = 6–20 hr), the intense upper‐band hiss waves mostly appear in the dayside low‐L region (L < 1.3) and most nightside regions. In the low‐L region, the amplitude of the upper‐band hiss has no obvious substorm dependence, and the average of its wave normal angles is comparable to that of lightning‐generated whistlers reported in the past.

Propeller–duct interaction on the wake dynamics of a ducted propeller

The propeller–duct interaction on the wake dynamics of a ducted propeller is numerically investigated via detached eddy simulations. The blade–blade interference and blade–duct interaction are analyzed through different configurations under non-ducted and ducted conditions. It is found that the blade–blade interference benefits the loading stability, and the duct leads to a faster efficiency decrease in a single blade with the increasing blade number. The short-wave instability dominates the wake as the unstable secondary vortices accelerates the vortex evolution. The multi-induction effect stabilizes the two tip vortices system in a two-bladed configuration, while the tip vortex grouping occurs early in a four-bladed propeller due to the combined effect of the duct retardation and smaller spiral-to-spiral distance. Additionally, the enhanced wake instability leads to the fast decline of the power spectral density peaks of kinetic energy at blade passing frequency and shaft frequency harmonics toward the far field under ducted conditions.

Effect of aerodynamic modifications on the surface pressure patterns of buildings using proper orthogonal decomposition

This study analyzed the pressure patterns and local pressure of tall buildings with corner modifications (recessed and chamfered corner) using wind tunnel tests and proper orthogonal decomposition (POD). POD can distinguish pressure patterns by POD mode and more dominant pressure patterns can be found according to the order of POD modes. Results show that both recessed and chamfered corners effectively reduced wind-induced responses. Additionally, unique effects were observed depending on the ratio of corner modification. Tall building models with recessed corners showed fluctuations in the approaching wind flow in the first POD mode and vortex shedding effects in the second POD mode. With large corner modification, energy distribution became small in the first POD mode, which shows that the effect of the first POD mode reduced. Among building models with chamfered corners, vortex shedding effects appeared in the first POD mode, except for the model with the highest ratio of corner modifications. The POD confirmed that both recessed and chamfered corners play a role in reducing vortex shedding effects, and the normalized power spectral density peak value of modes showing vortex shedding was smaller than that of the building model with a square section. Vortex shedding effects were observed on the front corner surfaces resulting from corner modification, as with the side surface. For buildings with recessed corners, the local pressure on corner surfaces was larger than that of side surfaces. Moreover, the average wind pressure was effectively reduced to 88.42% and 92.40% in RE1 on the windward surface and CH1 on the side surface, respectively.

Spatial coherence effects in second-harmonic generation of scalar light fields

We consider the spectral spatial coherence characteristics of scalar light fields in second-harmonic generation in an optically non-linear medium. Specifically, we take the fundamental-frequency (incident) field to be a Gaussian Schell-model (GSM) beam with variable peak spectral density and different coherence properties. We show that with increasing intensity the overall degree of coherence of both the fundamental and the second-harmonic field in general decreases on passage through the non-linear medium. In addition, the spectral density distributions and the two-point degree of coherence may, for both beams, deviate significantly from those of the GSM, especially at high intensities. Propagation in the non-linear medium is numerically analyzed with the Runge–Kutta and the beam-propagation methods, of which the latter is found to be considerably faster. The results of this work provide means to synthesize, via non-linear material interaction, random optical beams with desired coherence characteristics.

Source separation from single-channel abdominal phonocardiographic signals based on independent component analysis.

Purpose: Continuous monitoring of fetal heart rate (FHR) is essential to diagnose heart abnormalities. Therefore, FHR measurement is considered as the most important parameter to evaluate heart function. One method of FHR extraction is done by using fetal phonocardiogram (fPCG) signal, which is obtained directly from the mother abdominal surface with a medical stethoscope. A variety of high-amplitude interference such as maternal heart sound and environmental noise cause a low SNR fPCG signal. In addition, the signal is nonstationary because of changes in features that are highly dependent on pregnancy age, fetal position, maternal obesity, bandwidth of the recording system and nonlinear transmission environment. Methods: In this paper, a sources separation process from the recorded fPCG signal is proposed. Independent component analysis (ICA) has always been one of the most efficient methods for extracting background noise from multichannel data. In order to extract the source signals from the single-channel fPCG data using ICA algorithm, it is necessary to first decompose the signal into multivariate data using a proper decomposition technique. In this paper, we implemented three combined methods of SSA-ICA, Wavelet-ICA and EEMD-ICA. Results: In order to validate the performance of the methods, we used simulated and real fPCG signals. The results indicated that SSA-ICA recovers sources of single-channel signals with different SNRs. Conclusion: The performance criteria such as power spectral density (PSD) peak and cross correlation value show that the SSA-ICA method has been more successful in extracting independent sources.

Density Gain-Rate Peaks for Spectral Clustering

Clustering has been troubled by varying shapes of sample distributions, such as line and spiral shapes. Spectral clustering and density peak clustering are two feasible techniques to address this problem, and have attracted much attention from academic community. However, spectral clustering still cannot well handle some shapes of sample distributions in the space of extracted features, and density peak clustering encounters performance problems because it cannot mine the local structures of data and well deal with non-uniform distributions. In order to solve above problems, we propose the density gain-rate peak clustering (DGPC), a new type of density peak clustering method, and then embed it in spectral clustering for performance promotion. Firstly, in order to well handle non-uniform sample distributions, we propose density gain-rate for density peak clustering. Density gain-rate is based on the assumption that the density of a clustering center will be higher with the reduce of the radius. Even under non-uniform distributions, the cluster center in low density region will still have a significant density gain-rate thus can be detected. We combine density gain-rate in density peak clustering to construct DGPC method. Then in the framework of spectral clustering, we use our new density peak clustering to cluster the samples by their extracted features from a similarity graph of these samples, such as the neighbor-based similarity graph or the self-expressiveness similarity graph. Compared with the previous spectral clustering and density peak clustering, our method leads to better clustering performances on varying shapes of sample distributions. The experiment measures the performances of our clustering method and existing clustering methods by NMI and ACC on seven real-world datasets to illustrate the effectiveness of our method.