Bragg Frequency Research Articles

Frozen and unfrozen moisture content estimation in coral calcareous sand during artificial freezing

In tropical areas where coral calcareous sands are prevalent, artificial freezing techniques are frequently employed during construction. However, the fundamental thermodynamic behaviors and moisture dynamics of calcareous sands under freezing conditions are poorly understood. Therefore, we conducted laboratory tests and developed a numerical model to capture the total moisture, liquid water, and ice contents of calcareous sand during artificial freezing. The thermal fiber Bragg grating (T − FBG) and frequency − domain reflectometry methods were used in the study. Freezing characteristic curves were quantitatively analyzed with taking into account the initial moisture content and ambient temperature. The results indicate that T − FBG effectively estimates the total moisture content in unfrozen and frozen calcareous sand, as well as ice content in frozen soil, with less than 0.029 m3/m3 error. Ice melting induced by T − FBG heating is affected by the initial moisture content, heating duration, power, and ambient temperature. However, the maximum change is below 0.008 m3/m3, which is negligible. The van Genuchten model accurately describes the liquid moisture–temperature relationship of unsaturated calcareous sand, with an R2 exceeding 0.98. The residual–initial moisture content relationship follows a quadratic function. During freezing, the temperature reduction aligns with the Kozlowski model, and the liquid moisture–temperature relationship follows a cubic polynomial function.

Strong Laser Emission Modulation by Coherent Perfect Absorption Inside Complex-Coupled Distributed Feedback Laser Diodes.

The proof-of-concept of the exploitation of Coherent Perfect Absorption (CPA) in electrically-injected distributed-feedback laser sources is reported. Capitalizing on the essence of CPA as "light extinction by light", an integrated laser-modulator scheme emerges. The key ingredient compared to conventional single-frequency laser diodes is a careful periodic in-phase modulation of both real and imaginary parts of the complex grating index profile that enables both single-frequency operation and 40dB line purity at the Bragg frequency. It is shown that this combination is most apt for the operation of CPA as a modulation mechanism that respects the laser spectral purity. The specific proof-of-concept is based on an ultra-short external cavity formed by a metallic micro-mirror, whose role is to generate the second beam of more conventional CPA interferometric approaches. The implemented complex-coupled grating is compatible with existing industrial technologies and promising for real-life laser source applications. Furthermore, the concept can be directly transferred to other material platforms and other wavelengths ranging from terahertz to ultraviolet.

Tuning the acoustic characteristics of bottle plant garden

This work presents a finite element study to compute the acoustic performance of a garden that consists of periodically arranged bottle plants. The purpose of this work is to achieve low- and high-frequency noise reduction using plastic waste bottles. A harmonic excitation is given at one end, and transmission loss is calculated as a function of frequency between the two ends of the computational domain. The result shows that bottle plants produce a high-frequency wide band (1300–2100 Hz) around Bragg's frequency as well as a low-frequency sharp narrow band at the resonance frequency (∼305 Hz) of the bottle. To achieve a wider band gap at low frequencies, water level in the bottles is kept at different levels. The result showed that placing bottles with various water levels next to one another causes a bigger band gap at low frequencies (255–360 Hz). As a result, the low-frequency band gap can be tuned by changing the water level in the bottles. Plants' ability to absorb sound further dampens it at specific frequencies. Utilizing the plastic bottle to form such a garden has a major environmental impact and can function as a visually pleasing area while promoting calm and tranquilly.

An equivalent lattice-modified model of interfering Bragg bandgaps and Locally Resonant Stop Bands for phononic crystal made from Locally Resonant elements

The design and development of advanced devices based on metamaterials to control the transmission of acoustic waves is a hot topic. An important class of these metamaterials is based on phononic crystals with Locally Resonant Structure, included in those commonly known as Locally Resonant Sonic Materials. In these metamaterials, wave control is basically performed by two mechanisms: internal (or local) resonances in the scatterers that form the phononic crystal, and Bragg bandgaps due to structural periodicity. Their main control feature is the resonance peaks forming additional stop-bands away from the Bragg frequency, mainly in the low frequency regime. For some applications, coupling of the two phenomena is necessary to create a broad transmission gap. However, when both are located in close frequency ranges, some destructive interferences can occur. In this paper, the authors develop a comprehensive numerical model of periodic arrays of Hemholtz resonators, which explains in detail the physical mechanisms of this destructive interference and, simultaneously, allows the reproduction of the consequences of the interference. The numerical results are supported by experimental tests.

Wave interaction with a rigid porous structure under the combined effect of refraction–diffraction

Water wave scattering, trapping, and generation by a thin porous structure placed on an uneven bottom are studied approximately. The classical eigenfunction expansion method is employed for the uniform bottom, whereas the Galerkin-Eigenfunction expansion method is employed for the uneven bottom. The solution is derived in the form of a system of algebraic equations through matching procedure. Wave reflection, transmission, and wave-induced forces on the porous structure and backwall are investigated under the combined refraction–diffraction effect. In the scattering problem with periodic bottom, Bragg resonance is minimized at the Bragg frequency, whereas the porous structure at other frequencies maximizes wave reflection. Consequently, wave transmission is very low for all wave frequencies in this situation. In the wave trapping problem, forces on the porous structure and the backwall are reduced substantially when the water depth ratio decreases. Feasible locations of the porous structure are shown at which wave reflection and forces on it and backwall are optimized. In the wave generation problem, the comparison of results for flat and uneven bottoms reveals that waves have a smaller amplitude at infinity as uniform bottom levels become farther for relatively small values of the reciprocal of the wave-effect parameter (C).

Vibro-Acoustic Performance of a Fluid-Loaded Periodic Locally Resonant Plate

The vibro-acoustic performance of a fluid-loaded periodic locally resonant (LR) plate was examined in this research, with a specific focus on the effect of water fluid on the vibration and sound radiation of the LR structure. The analytical models of the fluid-loaded LR plate’s band gap, vibration, and acoustic radiation were theoretically derived with closed-form solutions, which can be used to predict the general vibro-acoustic rules of underwater LR structure. The results show that the LR band-gap width and Bragg frequency are significantly reduced when water fluid is considered. Besides, the frequency range that can be tuned to control the vibration and sound radiation for the LR plate with fluid is much narrower than that without fluid. The reason for inducing the above effects was also given in this research, which can be physically explained by the attached mass caused by the water fluid. In addition, the reason for the enhanced radiation efficiency close above the band gap was also discussed, which is caused by the change of radiation mode from corner or edge radiation to monopole radiation. Furthermore, adding small damping into the resonator could reduce the vibration and sound radiation in the frequency range above or close below the band gap, inducing the attenuation zone to be significantly broadened. Thus, designing the periodic resonators with proper damping could be an efficient method to make the LR plate more beneficial for vibration and noise reduction in water-surrounding applications.

Resonance and Bragg band frequencies coupling of lossy sonic crystals based on nonuniform resonators and lattice

Traditional sonic crystals (SCs) are based on scatterers or resonators distributed periodically that attenuate sound waves in different bands of frequencies. Recently, we adopted an improved design of SC, namely, the gradient-based sonic crystal (GBSC), designed with a gradient in the geometric parameters using nonuniform resonators and lattices. The finite element (FE) study on the GBSCs indicated that the gradient induces better attenuation than the traditional periodic SCs. This work is an extension of our previous work with experimental and new FE studies on different parameters of the GBSC in an attempt to improve the attenuation of the GBSCs. It was found that the gradient of the geometry enhances the Bragg scattering and resonance in the array, creating a large number of band frequencies. When designed properly by manipulating their gradient, GBSCs were found to target particular frequency bands over other GBSCs of identical filling ratios by coupling the resonant frequencies or the resonant and Bragg frequencies. It was also found that the thickness of the GBSC can be reduced by removing some specific columns from the array without significantly affecting the attenuation by the GBSC. Similarly, it was also found that the relative position of the resonator columns in a GBSC affects the band frequencies, and swapping the columns may also improve the attenuation by the GBSC.

Implementation of Traveling Wave Models of Grating-Based Integrated Optical Devices for Circuit Simulation

This paper presents the development and use of a traveling wave model of waveguide-based grating devices for use as a compact model in a circuit-level simulator. Both passive and active devices are modeled, with the grating being characterized by coupling coefficients for the two counter-propagating waves. It is shown how the implicit carrier frequency can be offset from the Bragg frequency using two possible methods: either by the static detuning of the model or introducing a phase modulation into the coupling coefficients. Other physical aspects of the model are addressed such as dispersion and energy conservation. A comparison to a 1D Yee-cell model is used to verify the applicability of the traveling wave model. As an example of the circuit simulation of a passive device, an optical code generating application is used and it is noted that for a passive device the interface between the compact model and the circuit simulator is not a concern. Using traveling-wave-based laser simulations of grating-based laser structures it is demonstrated that the model captures the complex behaviour of the devices. In particular the lasing frequency is naturally produced from the model and the introduction of delay elements into the structure can be used to restrict the laser to single mode operation. A number of examples are used to illustrate important aspects of its use as a compact model. Firstly, it is shown that an operating point for either of the laser configurations can be constructed and detuning used to produce an un-modulated output, allowing for much more efficient simulations. A final example uses a directly modulated laser to illustrate the effect of back reflection on the stability of the laser simulation.

Wave transport in stealth hyperuniform materials: The diffusive regime and beyond

By varying the degree of correlation in stealthy hyperuniform (SHU) materials, the continuous evolution from uncorrelated disorder to periodic media is possible and allows us, as such, to study the fate of the bimodal distribution, the characteristic of a diffusive transport. Considering the wave transport through a SHU distribution of a given number of scatterers and at a given frequency, the transition from a diffusive to a transparent medium is clearly observed only below the Bragg frequency. This transition is characterized by a threshold value of the stealthiness at the vicinity of which the material abruptly changes from diffusive to transparent. In contrast, no such clear transition is observed at or above the Bragg frequency and, surprisingly, a seemingly bimodal distribution of the transmission eigenvalues still characterizes the SHU materials, even when strongly correlated.

On the acoustic effects of sonic crystals in heat exchanger arrangements

Heat exchangers can be found in a large number of technical systems and installations. They are usually operated in combination with other machines, such as axial fans, in order to remove or supply heat to a system. The heat exchanger can influence the existing flow field and thus lead to increased noise emission from fans located downstream of the heat exchanger. This can be observed, for example, in air conditioning units in which axial fans operate in combination with heat exchangers. Even though this mechanism is known, it is not yet understood how the heat exchanger affects the sound propagation of the sound produced by the downstream machine. For example, the heat exchanger may lead to a change in directional characteristics or specific frequencies may be attenuated. In order to better understand the interaction of the heat exchanger with the sound field, sound power measurements were carried out on various heat exchangers and the sound propagation was simulated numerically. It was shown that the sound attenuation due to the interaction with the periodic tube array is detectable in heat exchangers and that this leads to a sound reduction at the Bragg frequency. Based on its filling factor, the heat exchanger can reduce the sound propagation in certain frequency bands by up to 10 dB if the geometrirical properties are selected suitably. The simulations of a single unit cell confirm in very good agreement with the experimental results. This allows the conclusion that the approach presented in this paper is a cost-effective way to model acoustic effects of heat exchangers. Furthermore, sound attenuation effects by the heat exchanger were caused by thermoviscous effects on the cooling fins and dimensions of the heat exchanger housing.

Quasi-optical theory of relativistic Cherenkov surface-wave oscillators with oversized cylindrical waveguides

Within the quasi-optical approach, we investigate the propagation of azimuthally symmetric TM waves in periodically corrugated cylindrical waveguides and their excitation by relativistic electron beams. Presenting the field as two, forward and backward, quasi-optical wavebeams coupled at the shallow corrugation, we obtain a dispersion equation for normal waves and thus a criterion of existence of the surface wave. For a finite-length corrugation section, the spectrum of axial evanescent eigenmodes is estimated; the spatial structure and the quality factor of the fundamental mode are found at an eigenfrequency close to the Bragg frequency. A self-consistent system of equations describing the interaction of electromagnetic waves with a rectilinear electron beam injected into the system is derived. Based on this model, we recognize two oscillation regimes, namely, the π-mode excitation regime and the regime of backward surface wave oscillator. We demonstrate the viability of practical implementation of relativistic surface wave oscillators with a power level of up to 140 MW in the sub-millimeter wavelength band.

Low observable group targets detection based on space–frequency cascaded adaptive processing for MIMO OTH radar

The spread of sea clutter in the Doppler domain caused by multi-mode propagation would make it difficult for over-the-horizon radar to detect low observable group targets. To solve this problem, a multi-mode clutter suppression method based on space–frequency cascaded adaptive processing is proposed by utilizing the spatial diversity characteristics of multiple-input–multiple-output radar. Then, the low observable group targets can be detected. The proposed method adopts the idea of space–frequency stepwise processing. First, the multi-stage Wiener filter ESPRIT algorithm is exploited to estimate the direction of departure and direction of arrival of different path signals, and then the multi-mode clutter is adaptively transformed into single-mode clutter based on the estimated angles. Second, the atomic norm minimization algorithm and Vandermonde decomposition are used to estimate the steering vector of the first order Bragg frequency of sea clutter, whose path contains the group targets. Finally, the least squares algorithm is used to realize the adaptive suppression of sea clutter in the frequency domain. Because there is no need to estimate the signal covariance matrix and its eigenvalue decomposition and conduct spectral peak searching, the proposed method greatly reduces the computation of the angle estimation. Compared with the existing second-order blind identification and spatial smoothing SOBI algorithms, the proposed algorithm does not need the information of the first order Bragg frequency of sea clutter, so it can improve the output signal-to-clutter-noise ratio when there is an error in the first order Bragg frequency of sea clutter. Therefore, the proposed algorithm is more conducive to the detection of low observable group targets. Simulation results are given to demonstrate the effectiveness of the proposed method.

Water wave scattering by a sinusoidal bed in the presence of vertically sheared current

Wave scattering over a sinusoidal bottom in the presence of a vertically sheared current is investigated experimentally and compared with a modified-mild slope model taking into account a linear vertical shear. Waves were generated in a flume without or with an opposing sheared current and propagated over a patch of 10 sinusoidal bars before dissipating on a beach. The shear of the current was controlled using a perforated screen. The Doppler shift of resonant Bragg frequency in the presence of a sheared current is observed. The modified mild-slope model taking into account both incoming and reflected waves predicts both the location and the amplitude of the peak of reflection at the Bragg resonance conditions, which are found to be sensitive to both the surface current intensity and the vertical shear.

Anomalous transmission through periodic resistive sheets

This work investigates anomalous transmission effects in periodic dissipative media, which is identified as an acoustic analogue of the Borrmann effect. For this, the scattering of acoustic waves on a set of equidistant resistive sheets is considered. It is shown both theoretically and experimentally that at the Bragg frequency of the system, the transmission coefficient is significantly higher than at other frequencies. The optimal conditions are identified: one needs a large number of sheets, which induce a very narrow peak, and the resistive sheets must be very thin compared to the wavelength, which gives the highest maximal transmission. Using the transfer matrix formalism, it is shown that this effect occurs when the two eigenvalues of the transfer matrix coalesce (i.e., at an exceptional point). Exploiting this algebraic condition, it is possible to obtain similar anomalous transmission peaks in more general periodic media. In particular, the system can be tuned to show a peak at an arbitrary long wavelength.

Double topological edge states investigation in sonic metamaterials

We investigate the topologically protected sound propagation in sonic metamaterials, analogous to quantum spin hall effect (QSHE). The sonic metamaterials consist of circular rods and meta-molecules arranged in air with a honeycomb-lattice. The on-demand inversion in topological phase can be achieved by two ways of scatterer controls at locally resonant frequency and Bragg frequency. The Helmholtz resonators in the structure are contributed to the formation of subwavelength double Dirac cones which are more likely to appear due to local resonance enhancement with more number of resonators. By combining two sonic metamaterials with different topological invariants, we demonstrate the robust sound propagation and pseudospin-dependent one-way acoustic propagation at the interface. Experimental measurement of the topologically protected acoustic wave transmission matches well with the simulation result.

Terahertz Bragg Resonator Based on a Mechanical Assembly of Metal Grating and Metal Waveguide

Terahertz (THz) waveguide Bragg grating is experimentally demonstrated on the basis of a mechanical assembly that includes a metal-grating sheet and a metal parallel-plate waveguide (PPWG). Bragg frequency waves, which are also called phase-matching waves, are efficiently coupled from the free space on the basis of the designed parameters of the air gap width, the metal grating thickness, and the PPWG taper angle. These optimal parameters are critical to reflect the Bragg frequency waves with a transversely resonant field across the metal grating. For other THz waves that are not phase matching to the meal grating vector, transverse resonance simultaneously occurs. However, these waves propagate forward as waveguide modes. PPWG-assembled metal grating thus performs highly distinct Bragg frequencies in the waveguide transmittance when the noisy resonance field, not following Bragg law, is exactly constrained by the waveguide scheme. Given that all components of the waveguide Bragg grating are made of metal without dielectric perturbation, a series of strong Bragg resonant modes in a wide bandwidth and a hollow-core chamber is potentially facilitated for THz gas sensors.

Многоадресные волоконные брэгговские структуры в радиофотонных сенсорных системах

The article describes the transition concept from addressable fiber Bragg structures and microwave-photonics sensor systems based on them to multicast fiber Bragg structures. The difference between multicast structures and address structures is that in the fiber Bragg structure formes three or more super narrow-band frequency components, spaced from each other by the microwave frequency. The central frequencies shift of multicast Bragg structures is determined by the result of processing the signal of optical frequencies beats at the photodetector, which parameters judge the applied physical fields. We see the solved problem of uniquely determining the central (Bragg) frequency shift of the multicast fiber Bragg structure, with a unique set of address frequencies.

Static pure strain sensing using dual-comb spectroscopy with FBG sensors.

We propose a method to precisely characterize the optical response of a fiber Bragg grating (FBG) sensor for static strain sensing by using dual-comb spectroscopy (DCS). By digitally post-correcting the mutual noise between the two combs, a robust and pure strain sensing system is achieved by compensating for the temperature-induced frequency shift of the FBG sensor. The comb-resolved radio-frequency (RF) spectra generated by DCS are obtained. Meanwhile, the stability of the central Bragg frequency of the FBG-reflected spectrum in the RF domain is 0.315 kHz, which is better than the difference of repetition rate (1 kHz). A reference FBG is used for detecting and compensating for the temperature-induced central frequency drift. Finally, a spectral sensitivity of 0.85 pm/µɛ with 0.8 µɛ static strain resolution is achieved.

The control system elements of the new generation optical switching cell

In this paper calculation of new optical switch parameters that allows us to create next generation all-optical non-blocking switching system without external control devices is carried out for the first time. In particular, switching cell control device is studied in detail. The presented one includes Bragg filter, frequency detector, optical isolator, and former of a control signal. Here we also present the detail description and the numerical calculations of these devices for third transparent window (1550nm). The reflection and transmission coefficients are obtained, the passbands of the Bragg filter are presented, and the amplitude characteristic of the frequency detector is calculated.

Wave-Height Mapping From Second-Order Harmonic Peaks of Wide-Beam HF Radar Backscatter Spectra

Compact high-frequency surface wave radar has been widely applied to the measurement of sea surface current, but its accuracy and direction resolution of wave parameter estimation are always limited due to the wide beam of the antenna. In this article, a novel wave-height mapping method based on the second-order harmonic peak (SHP) of radar Doppler spectra is proposed to address this concern. The characteristic of the SHP at the Doppler frequency of $\sqrt {2}$ times the Bragg frequency is studied through the theoretical derivation and numerical simulation. A relationship between the ratio ( $R$ ) of the SHP power to the Bragg peak power and significant wave height ( $H_{s}$ ) is derived. Furthermore, the $R$ – $H_{s}$ model is improved by incorporating influences, such as background noise and antenna beamwidth. With this improved model, a wave-height mapping algorithm based on the direction finding technique is presented. This approach enables the significant wave-height map extraction using a broad-beam radar. Finally, wave-height maps obtained at different sea states are depicted and analyzed, and the wave heights appearing on the maps are compared with buoy data over a one-month experiment to verify the validity and robustness of the algorithm. During this period, the significant wave height varies from about 0.5 to 4.5 m, and the radar measured wave heights at different range/distance bins show an overall root-mean-square error (RMSE) of 0.33–0.77 m and a correlation coefficient (CC) of 0.78–0.94, with respect to the buoy measurements.