Smooth Spectrum Research Articles

Generation of terahertz radiation with a specified spectral profile from a mosaic combined organic crystal

We present a method for the generation of terahertz (THz) pulses with a high optical-to-THz conversion efficiency and a smooth spectrum. The method is based on the optical rectification of near-infrared femtosecond laser pulses with a wavelength of 1240 nm in a mosaic combined crystal, consisting of two pieces of nonlinear organic crystals, OH1 and DSTMS. The mosaic combined crystal produces a relatively smooth spectrum in the 0.3–4.5 THz range with no pronounced absorption dips and with a maximum in a spectral amplitude at about 2 THz. The peak field strength is 9.6 MV/cm for an effective crystal diameter of 4.3 mm.

A Damping Modification Factor Prediction Model for Horizontal Pseudoacceleration Spectrum for Subduction Slab Earthquakes Using Offshore Ground Motions in the Japan Trench Area

ABSTRACT This study develops a damping modification factor (DMF) model for offshore ground motion from subduction slab earthquakes that is a function of the damping ratio, spectral period, moment magnitude, focal depth, and rupture distance. The DMF values for 10 damping ratios between 1% and 30% and 34 spectral periods ranging from 0.03 to 5.0 s are calculated using 2803 offshore ground-motion records obtained from the S-net in Japan. An examination of the residuals from a median model, which considers deployment methods, indicates that DMF is influenced by parameters such as magnitude, depth, and distance. The DMF model includes the following components: a median model considering deployment method, a magnitude linear term, a magnitude square term, a focal depth term, an ln(R) scaling, and a constant term. The influence of damping ratio on model coefficients can be simulated by a quadratic function of the damping ratio. In addition, we established a DMF standard deviation model, for which the influence of damping ratio and spectral period on the standard deviation can be simulated by a quadratic function of the damping ratio and a quartic polynomial of the spectral period, respectively. The uncertainty related to the path and site effects is the main source of the deviation of the DMF model. We have conducted a comparison between the offshore DMF model and onshore model proposed by Rezaeian, Al Atik, et al. (2021), revealing that for lower damping ratios, the offshore DMF is generally lower at periods less than 0.2 s and larger at periods greater than 2.0 s compared to its onshore counterparts. The offshore DMF model adjusts the acceleration spectrum proposed by Tan and Hu (2024) to generate smooth pseudoacceleration spectra for most spectral periods.

A tunable high-order micro-perforated panel metamaterial with low-frequency broadband acoustic absorption

We proposed a tunable high-order micro-perforated panel metamaterial for broadband absorption, in which the energy dissipation modes is reconstructed by the cavity partition plates. Owing to the more degrees of freedom in impedance design, the metamaterial not only obtains multiple perfect absorption peaks with broader bandwidth, but also allows for flexible frequency regulations. By coupling 12 high-order cells, the metamaterial finally achieves a smooth spectrum with 94% average absorption across the range of 300–3200 Hz, which is verified by the simulation and experiment. This metamaterial could have great applications in noise control engineering owing to the extraordinary performance.

Design and analysis of the C6H6-filled circular photonic crystal fiber with ultra-flat dispersion for broadband supercontinuum generation with peak power of 130 W and 250 W

This article introduces a new model of a circular silica-based photonic crystal fiber with a hollow core filled with C6H6. The difference in the air hole size and the distance between them in the first ring around the core has a profound effect on the dispersion, leading to ultra-flat dispersion with values as low as ±0.996 ps nm−1· km in wavelength range 0.74 µm. The high nonlinear coefficient of several 1000 W−1 · km−1 and the low confinement loss of a few tens of dB m−1 suggest proposing three fibers with dispersion and nonlinear properties suitable for broadband supercontinuum generation at low peak power. The influence of peak power on the broadening of the supercontinuum spectrum is also investigated. Fibers with a flat all-normal dispersion profile provide a smooth spectrum with bandwidths of 1.215 and 1.626 µm at 30 dB with a peak power of 250 W. A fiber with an anomalous dispersion regime generates a supercontinuous spectrum, broadening to 3.868 µm in the mid-infrared region (2.467 µm bandwidth at 30 dB) under laser pulse excitation with 130 W peak power. Our results provide further insights into the generation of broadband mid-infrared supercontinuum using liquid-core silica-fibers, which have great potential for applications in the fields of optical communications and optical sensing.

Research on bark-frequency spectral coefficients heart sound classification algorithm based on multiple window time-frequency reassignment

The multi-window time-frequency reassignment helps to improve the time-frequency resolution of bark-frequency spectral coefficient (BFSC) analysis of heart sounds. For this purpose, a new heart sound classification algorithm combining feature extraction based on multi-window time-frequency reassignment BFSC with deep learning was proposed in this paper. Firstly, the randomly intercepted heart sound segments are preprocessed with amplitude normalization, the heart sounds were framed and time-frequency rearrangement based on short-time Fourier transforms were computed using multiple orthogonal windows. A smooth spectrum estimate is calculated by arithmetic averaging each of the obtained independent spectra. Finally, the BFSC of reassignment spectrum is extracted as a feature by the Bark filter bank. In this paper, convolutional network and recurrent neural network are used as classifiers for model comparison and performance evaluation of the extracted features. Eventually, the multi-window time-frequency rearrangement improved BFSC method extracts more discriminative features, with a binary classification accuracy of 0.936, a sensitivity of 0.946, and a specificity of 0.922. These results present that the algorithm proposed in this paper does not need to segment the heart sounds and randomly intercepts the heart sound segments, which greatly simplifies the computational process and is expected to be used for screening of congenital heart disease.

Time-lapse attenuation variations using distributed acoustic sensing vertical seismic profile data during CO2 injection at CaMI Field Research Station, Alberta, Canada

For seismic monitoring of geologic [Formula: see text] storage, it is useful to measure time-lapse (TL) variations of seismic attenuation. Seismic attenuation directly connects to different petrophysical parameters within the [Formula: see text] storage complex. We have developed an approach to derive smooth time-variant amplitude spectra from seismic signals by using the sparse strongest signal peaks, and then measuring two different attributes (conventional Q factor and its “geometric” counterpart) characterizing the path effects of seismic attenuation from the smooth spectra. This approach is straightforward and does not require sophisticated algorithms or parameterization schemes. We apply this approach to TL distributed acoustic sensing (DAS) vertical seismic profile (VSP) data from the Field Research Station (FRS) [Formula: see text] injection project in southern Alberta, Canada. High-quality stacked reflection records are obtained from the baseline and monitor DAS VSP surveys at the FRS and TL attenuation-attribute differences are derived from these reflection records. TL variations of attenuation are observed within the injection zone at the FRS, which are interpreted as being related to the injected [Formula: see text]. Although there is always a significant trade-off between the accuracy and temporal resolution of the measured attenuation parameters, reliable attenuation measurements around the injection zone are still achieved with an in-use reflection signal of a sufficient length and bandwidth. Attenuation attributes measured from this approach can be an advantageous tool for monitoring the distribution and migration of [Formula: see text] plumes.

An effective DOA estimation method for low SIR in small-size hydrophone array

The estimation ability of traditional direction of arrival (DOA) estimation methods is relatively fragile in small-size hydrophone arrays with limited space. Especially in low signal to interference ratio (SIR), the strong interference signals may submerge some weak signals of interest (SOI) and make DOA estimation difficult in response to this issue. This paper introduces an improved sparse DOA estimation method for practical multi-objective DOA estimation in complex scenarios. The main work is to introduce a noise weight constraint in the sparse iterative covariance process. It leads the algorithm to output sparse peaks and smooth spatial energy spectra and achieve faster fitting while reducing the probability of false peaks. The algorithm can complete DOA estimation of the multi-target reliably without prior information of sources. Then, we propose a fast region grid refinement method based on allocation reconstruction to increase angle resolution. The method increases the accuracy of multi-objective DOA estimation while reducing computational costs. Finally, simulation and experiment have verified the method's effectiveness.

The first demonstration of wavelength-tunable Yb3+-doped low silica calcium aluminosilicate (LSCAS) glass lasers

We report on the first demonstration of diode-pumped wavelength-tunable Yb3+-doped low silica calcium aluminosilicate (LSCAS) glass lasers. Using a 976 nm diode laser as pump source, a Yb:LSCAS laser has been successfully achieved with a maximum output power of about 0.28 W. Using an MgF2 thin plate as birefringence filter, we have found that the lasing wavelength can be tuned with a range of about 60 nm. We have also analyzed the intracavity round-trip loss of the current laser cavity, and related it to the optical quality of the used Yb glass. This sheds light on the path of the development of upcoming Yb3+ glass lasers in the future since it has many advantages, and after improving the optical quality, the wide and smooth emission spectrum of Yb:LSCAS will be very conducive to obtaining femtosecond pulse lasers.

Detecting Cosmic 21 cm global signal using an improved polynomial fitting algorithm

ABSTRACT Detecting the cosmic 21 cm signal from epoch of reionization has always been a difficult task. Although the Galactic foreground can be regarded as a smooth power-law spectrum, due to the chromaticity of the antenna, additional structure will be introduced into the global spectrum, making the polynomial fitting algorithm perform poorly. In this paper, we introduce an improved polynomial fitting algorithm – the Vari-Zeroth-Order Polynomial (VZOP) fitting and use it to fit the simulation data. This algorithm is developed for the upcoming low-frequency anechoic chamber experiment, yet it is a general method suitable for application in any single antenna-based global 21 cm signal experiment. VZOP defines a 24-h averaged beam model that brings information about the antenna beam into the polynomial model. Assuming that the beam can be measured, VZOP can successfully recover the 21 cm absorption feature, even if the beam is extremely frequency-dependent. In real observations, due to various systematics, the corrected measured beam contains residual errors that are not completely random. Assuming the errors are frequency-dependent, VZOP is capable of recovering the 21 cm absorption feature even when the error reaches 10 per cent. Even in the most extreme scenario where the errors are completely random, VZOP can at least give a fitting result that is not worse than the common polynomial fitting. In conclusion, the fitting effect of VZOP depends on the structure of the error and the accuracy of the beam measurement.

Wide Range Si3N4 Optical Sensor Using Cascaded Mach-Zehnder Interferometer Coupled Ring Resonator

Abstract A cascaded MZI-coupled ring resonator (CMCR) optical sensor based on silicon nitride waveguide is proposed. The sensor is composed of a large size microring and a bus waveguide coupled with the microring for four times. The three-segment bus and the microring waveguide between the four coupling regions form three MZI of the same size, forming a three-stage cascaded MZI. The resonant effect of the cascaded MZI transforms the smooth comb spectrum of the microring into a V-shaped comb spectrum, expands the effective free spectral range of the ring resonator, and realizes the cascaded MZI-coupled microring sensor with a large measurement range. For the cascade MZI-coupled microring with a radius of 100μm, the effective free spectrum range is 53.7nm, which is more than 24 times larger than that of the all-pass microring of the same size. The refractive index sensitivity is 339.49 nm RIU/and the detection limit is 2.95 × 10−5 RIU. these excellent properties prove the feasibility of cascaded MZI-coupled microring structure for large measurement range sensing, and are expected to be used in large refractive index range sensing applications in the future.

Binding dynamics of cavity solitons in a Kerr resonator with high order dispersion.

Cavity solitons are persistent light pulses arising from the externally driven Kerr resonators. Thanks to the passive parametric gain, cavity soliton has been endowed with the natural advantage of the chip-scaled integration since it was first experimentally generated in the fiber-based platform. Deterministic single soliton with smooth spectrum is a preferred state for numerous applications. However, multiple solitons are more common in the resonators with anomalous dispersion. In this condition, adjacent solitons are easily perturbed to attract and collide with each other. Some experimental observations deviated from the aforementioned description have recorded the stable soliton intervals that can last for a long time scale. This phenomenon is known as soliton binding and is attributed to the presence of narrow resonant sidebands in the spectrum. While the stationary configuration of two binding solitons has been investigated, the dynamical evolution remains an area for further exploration. In this paper, we discuss the binding dynamics of the cavity solitons in the presence of high-order dispersion. The proposed theoretical predictions match well with the numerical results, encompassing both the stationary stable intervals and dynamic trajectories. Our research will provide a comprehensive insight into the soliton motion induced by the internal perturbations.

A chemistry-based explainable machine learning model based on NIR spectra for predicting wood properties and understanding wavelength selection

ABSTRACT A chemistry-based explainable machine learning (ML) approach was used to predict wood properties using near infrared (NIR) spectral data collected from rough and smooth surfaces, and to provide better understanding of the role of important NIR wavelengths (features) in the performance of ML models. NIR spectra collected from western hemlock (Tsuga heterophylla) and coastal Douglas-fir (Pseudotsuga menziesii) boards with rough and smooth surfaces were fed into random forest and TreeNet; a gradient boosting machine algorithm, for predicting wood density, modulus of elasticity (MOE) and modulus of rupture (MOR). The TreeNet model could predict the MOE, MOR, and density with R2 of 0.66, 0.64, and 0.64 using spectra collected from rough surface and R2 of 0.54, 0.46, and 0.46 using spectra collected from smooth surface. TreeNet outperformed the random forest, and for both ML algorithms higher R2 and lower error were obtained using NIR data collected from rough surfaces. This suggested that for Douglass fir and western hemlock, NIR spectra could be collected on a sawn surface prior to surface planing. However, it is difficult to generalize the impact of surface roughness on the performance of predictive model as different factors (e.g. what constitutes a smooth or rough surface, variability of data set in terms of wood properties) impact the success of predictive models. NIR features having the greatest influence on TreeNet models were examined and consistently had wood chemistry specific band assignments. The most important features occurred in the O-H first overtone, and C–H second overtone regions and a narrow zone (approximately 2400–2500 nm) of the C–H stretch C–C stretch combination region. Important features also differed by property and with surface roughness. Explaining ML model performance using the relative importance of the NIR features showed the importance of wood chemistry related information when developing models, however MOE and MOR TreeNet models based on smooth surface NIR spectra showed an increased importance of water related features. Overall, the chemistry-based explainable machine learning model approach allows for identification of important NIR features, and regions, and aids in understanding how they contribute to the performance of NIR-based wood property predictive models.

Optimization of dispersion in hollow-core photonic crystal fibers filled with toluene

In this work, we thoroughly investigated the dispersion in SiO2-based photonic crystal fibers with a C7H8-infiltrated hollow core. By cleverly modifying the air hole diameters and lattice constants in the structural design, we achieved ultra-flat near-zero dispersion as small as 0.462 ps·nm–1·km–1 and diverse dispersion properties of PCFs, very beneficial for supercontinuum generation. Based on the simulation results, we propose three optimal structures with small and flat dispersion capable of generating a broad and smooth supercontinuum spectrum. The results of our study can be advantageous for fabricating fibers in low-cost all-fiber laser systems.

An algorithm for spectrally matching bi-directional ground motions to a target RotD100 spectrum while maintaining the radial spectral acceleration pattern

An integrated algorithm to spectrally match two seed-recorded horizontal bi-directional ground motion components to a maximum-direction response spectrum (RotD100) while maintaining the radial spectral acceleration pattern (RadSAP) at target natural periods is presented. The algorithm is based on the complex-discrete-Fourier-transform (CDFT) and iterative manipulation of the seed-recorded bi-directional ground motions. Initially, a population of candidate bi-directional ground motion pairs is generated, and the candidate resulting in the minimum error in RadSAP is picked. Subsequently, the CDFT coefficients of the picked bi-directional ground motion near the target natural periods are rotated to modify the orientation angle, which contributes to the modification of RadSAP. Then, a double iterative method is introduced to modify the bi-directional ground motion after CDFT coefficients rotation to be spectral and envelope shape compatible in the frequency and time domain. In order to show the effectiveness of the proposed algorithm, two target response spectra are determined following the current ASCE/SEI 7-16 and CJJ 166-2011 specifications, and 10 bi-directional ground motions are selected based on the resemblance in response spectrum for each target spectrum. It is shown that with the proposed algorithm, the RotD100 response spectra estimated from the modified bi-directional ground motions closely match the smooth target RotD100 response spectrum, and the RadSAPs at these target natural periods show an improved match with the seed-recorded bi-directional ground motions than the previous version method. Furthermore, the waveforms of the results obtained using the proposed method are similar to that of the scaled seed records in terms of acceleration, velocity, and displacement. Thus, the generated bi-directional ground motions can be used for future seismic risk assessment of buildings considering the directionality effects.

The 2016 Menyuan Earthquake: The Largest Self‐Arrested Crustal Earthquake Ever Observed

AbstractThe 2016 Mw 5.9 Menyuan earthquake occurred near the Haiyuan fault system in the northeastern Tibetan plateau. Although some kinematic properties of this event have been investigated, more dynamic properties still need to be clarified. First, we use the empirical Green's function method to obtain this event's apparent source time functions (ASTFs), which present a bell shape with a duration of 3s and corresponding smooth spectra. We then conduct dynamic rupture simulations, which are well constrained by the seismic and geodetic observations, to explore more in‐depth source properties. The results show that the characteristics of ASTFs can be explained by the self‐arresting rupture model, which implies that the rupture propagation of this event may spontaneously terminate before reaching the barriers. This result suggests that the initial conditions when this earthquake nucleated may have controlled the rupture process. Our work contributes to understanding the stopping mechanism of some moderate earthquake ruptures.

Comparative Study of Intze Type of Water Tank for Different Bracing Pattern under Wind and Seismic Load

Abstract Overhead water tank is a water storage facility supported by a tower and constructed at an elevation to provide useful storage and pressure for a water distribution system. Safety as well as working of such structures is considered as very crucial during earthquakes, since they put up for necessities like drinking water, firefighting during fire accidents, etc. Such structures should stay in operating condition even after several earthquakes. This RC overhead water tanks involves huge water mass supported on the top of tank supporting system known as staging. This investigation is basically to discover the seismic behavior of RC overhead water tanks. In this study, a FEM based model is used for finding out the nonlinear seismic performance of RC overhead water tanks with several staging variations. The staging that is different bracing patterns considered in this study in lateral and vertical direction, which relatively increases the strength and stiffness of tank in supporting system. Staad models are prepared and analyze for different seismic zones as well as for cyclonic regions and observed the parameters such as base shear, base moment and lateral displacement. The commercial software STAAD.Pro is used for structural analysis. Since the seismic response of water tank structure depends on its dynamic properties and frequency of ground motion. This seismic analysis of overhead water tank structure cannot be carried out on the basis of maximum value of ground acceleration because of nonavailability of ground acceleration data at every location. Hence for seismic analysis of overhead water tank, earthquake response spectrum analysis is widely used. While using this method, smooth design spectra is used to determine the value of displacement and forces in members at every mode of vibration. Octagonal, Radial, Cross, Alternate tie , Diagonal and X- type bracings are used to analyze the structure under all the seismic zones viz. II, III, IV & V. Also observed the behavior of tank for cyclonic region where wind load is increased by 30%. Analysed the overhead water tanks of same capacity for different bottom dome deviation angles and concluded.

MODEL OF A CHAOTIC ULTRA-WIDEBAND INFORMATION TRANSMISSION SYSTEM FOR WIRELESS SENSOR NETWORKS

Distributer measuring systems, consisting of interconnected wireless sensor nodes with autonomous supply, have plenty of applications in different areas of engineering end technology. Therefore, wireless sensor network design and optimization is a vital problem in informational technologies and computer engineering. Regardless of particular application and architecture, a wireless sensor network consists of several small sensor nodes somehow distributed in an area of inspection. Such a design ensures a distributed parameter measuring, according to which, each node performs preprocessing procedures for the data, which are to be aggregated by a so-called sink node and sent via other networks, e.g., wired or wireless local area network. For healthcare solutions, it is important to ensure a stable connection with the network and server back-end to provide a high-level analysis based on predicting methods of machine learning algorithms. This paper describes an application of an ultra-wideband communication model based on chaos synchronization as a low-power and efficient alternative solution for building wireless sensor networks. Thus, using chaotic signals as information carriers in wireless communication has several advantages, including a wide smooth spectrum, high information capability, cybersecurity, and low power consumption. The synchronization problem is one of the most vital tasks to be solved to design a chaos application for ultra-wide-band communication. Being well-studied for periodic signals, modern synchronization theory contains plenty of solutions for classical telecommunication and radio engineering systems, however, it is not developed enough for chaotic systems. Hence, a one-directional dissipative synchronization between two Chen systems is studied in the first section. The second section is devoted to the computer simulation of the model, described in the previous section. All the models built and the simulations performed have been done using MATLAB/Simulink software. The negative impact of channel noise and inequality of system parameters is considered. The possible way how to improve technical characteristics is also provided. Proposed models are to be used to design and develop low-cost wireless sensor networks for multi-channel healthcare solutions.

Transient absorption spectroscopy based on uncompressed hollow core fiber white light proves pre-association between a radical ion photocatalyst and substrate.

We present a hollow-core fiber (HCF) based transient absorption experiment, with capabilities beyond common titanium:sapphire based setups. By spectral filtering of the HCF spectrum, we provide pump pulses centered at 425nm with several hundred nJ of pulse energy at the sample position. By employing the red edge of the HCF output for seeding CaF2, we obtain smooth probing spectra in the range between 320 and 900nm. We demonstrate the capabilities of our experiment by following the ultrafast relaxation dynamics of a radical cationic photocatalyst to prove its pre-association with an arene substrate, a phenomenon that was not detectable previously by steady-state spectroscopic techniques. The detected preassembly rationalizes the successful participation of radical ionic photocatalysts in single electron transfer reactions, a notion that has been subject to controversy in recent years.

Benefits of cascaded nonlinear dynamics in hybrid fibers for low-noise supercontinuum generation.

The recent development of fiber supercontinuum (SC) sources with ultra-low noise levels has been instrumental in advancing the state-of-the-art in a wide range of research topics. However, simultaneously satisfying the application demands of maximizing spectral bandwidth and minimizing noise is a major challenge that so far has been addressed with compromise, found by fine-tuning the characteristics of a single nonlinear fiber transforming the injected laser pulses into a broadband SC. In this work, we investigate a hybrid approach that splits the nonlinear dynamics into two discrete fibers optimized for nonlinear temporal compression and spectral broadening, respectively. This introduces new design degrees of freedom, making it possible to select the best fiber for each stage of the SC generation process. With experiments and simulations we study the benefits of this hybrid approach for three common and commercially available highly nonlinear fiber (HNLF) designs, focusing on flatness, bandwidth and relative intensity noise of the generated SC. In our results, hybrid all-normal dispersion (ANDi) HNLF stand out as they combine the broad spectral bandwidths associated with soliton dynamics with extremely low noise and smooth spectra known from normal dispersion nonlinearities. Hybrid ANDi HNLF are a simple and low-cost route for implementing ultra-low noise SC sources and scaling their repetition rate for various applications such as biophotonic imaging, coherent optical communications, or ultrafast photonics.

Observing signals of spectral features in the cosmic-ray positrons and electrons from Milky Way pulsars

The Alpha Magnetic Spectrometer (AMS-02) has provided unprecedented precision measurements of the electron and positron cosmic-ray fluxes and the positron fraction spectrum. At the higher energies, sources as energetic local pulsars, may contribute to both cosmic-ray species. The discreteness of the source population, can result in features both on the positron fraction measurement and in the respective electron and positron spectra. For the latter, those would coincide in energy and would contrast predictions of smooth spectra as from particle dark matter. In this work, using a library of pulsar population models for the local part of the Milky Way, we perform a power-spectrum analysis on the cosmic-ray positron fraction. We also develop a technique to cross-correlate the electron and positron fluxes. We show that both such analyses, can be used to search statistically for the presence of spectral wiggles in the cosmic-ray data. For a significant fraction of our pulsar simulations, those techniques are already sensitive enough to give a signal for the presence of those features above the regular noise, with forthcoming observations making them even more sensitive. Finally, by cross-correlating the AMS-02 electron and positron spectra, we find an intriguing first hint for a positive correlation between them, of the kind expected by a population of local pulsars.