Mode Coupling Effects Research Articles

Comparison of tandem pumping and direct pumping on high-power linearly polarized fiber laser.

In this article, a thorough model of linearly polarized fiber laser considering polarization coupling, mode coupling, SBS, and SRS effects is established. The output results of direct pumping and tandem pumping linearly polarized fiber laser under different SBS and SRS intensity settings are simulated. The results show that direct pumping is a better pumping scheme at present, and if the doping concentration of gain fiber can be further increased and the mode field quality of corresponding passive fiber can be optimized, the disadvantages of tandem pumping can be suppressed. To explore the potential of tandem pumping, a backward tandem pumped linearly polarized fiber amplifier is built and 875 W over 13 dB linearly polarized laser output is obtained.

Vibration and bifurcation analysis of rotor systems with air ring bearings including ring tilting

Air ring bearings may be considered as a further development of classical air bearings. A drawback of classical rigid air bearings is their reduced damping behavior, which usually prevents a secure operation above the linear threshold speed of instability. Considering air ring bearings, the ring-shaped bearing bushing is visco-elastically mounted in the housing so that external damping is provided. Alternatively, the bushing ring may also be mounted in the housing by a foil structure (beam or bump foil design) so that dissipation is generated by dry friction. Due to the external damping/friction provided by the ring mounting, rotor systems with air ring bearings can be operated above the threshold speed of instability so that stable self-excited vibrations with moderate amplitudes are observed. For certain system parameters, however, the rotor system can bifurcate into a rather dangerous vibration mode, where bearing ring modes are excited by the whirl/whip frequency of the air films. To accurately predict the bifurcation into the problematic ring modes, ring tilting has to be taken into account.Here, a detailed analysis for rotor systems with air ring bearings is presented, which fully incorporates ring tilting effects. The influence of ring tilting on the stability and bifurcation behavior is discussed in detail with the help of nonlinear rotor/bearing models. In connection with the bifurcation into the dangerous ring mode, an interesting nonlinear mode coupling effect (1:2 mode synchronization) is detected. A detailed analysis and a clear physical explanation of this mode coupling phenomenon is given.

Mode Coupling in Mode Division Multiplexing techniques for futuristic high speed optical networks and exploring optical fiber parameters to control mode coupling

Fiber optic communications are inevitable to achieve higher data rates of modern telecom networks. After utilization of Wavelength division multiplexing, higher order modulations and polarization multiplexing, mode division multiplexing is a new dimension to achieve higher transmission capacity for optical fiber communication links. Different spatial distributions of optical energy along cross sectional area of optical fiber allows simultaneous transmission of data by considering each mode as an independent channel. During such simultaneous transmissions, possibility of mixing of signals amongst modes causes signal degradations and acts as limiting factor for bandwidth – distance product of the link. This effect of mode coupling has been explored in this article by presenting its mathematical formulations. A simulation has been performed to study the impact of fiber constructional parameters on mode coupling using optical wavelengths used for telecommunication systems. The observations help to develop fibers for reduced mode coupling for particular group of modes and operating wavelengths. This article paves the way forward for study of mode coupling in micro and macro bending conditions for forthcoming research endeavours.

Determination of diffusivities in binary fluid mixtures with various Lewis numbers as well as positive and negative Soret coefficients by the shadowgraph method.

The present work demonstrates the accessibility of the Fick diffusion coefficient D11 and/or the thermal diffusivity a of the binary mixtures dicyclohexylmethane/diphenylmethane, n-hexane/carbon dioxide, 1-hexanol/carbon dioxide, and methane/propane by the analysis of the dynamics of non-equilibrium fluctuations using the shadowgraph method. It is evidenced that D11 and a can be simultaneously determined for binary mixtures with Lewis numbers Le = a/D11 ranging over two orders of magnitude down to Le ≈ 5 or in the presence of minor advection for binary mixtures possessing a negative Soret coefficient in the investigated temperature and pressure ranges from (298.15 to 473.15) K and from about (0.5 to 25) MPa. The determined diffusivities are compared with those measured by heterodyne dynamic light scattering or obtained from the literature, with a focus on achievable uncertainties. By this comparison, it is shown that the determination of a by the shadowgraph method was hindered by mode-coupling effects for Le ≈ 5, whereas a determination of D11 was always possible for mixtures with Le ≥ 5. Furthermore, it is demonstrated that even in the presence of solutal advection, the description of the purely diffusive behavior of non-equilibrium fluctuations in concentration remains valid.

The impact of anisotropic redshift distributions on angular clustering

A leading way to constrain physical theories from cosmological observations is to test their predictions for the angular clustering statistics of matter tracers, a technique that is set to become ever more central with the next generation of large imaging surveys. Interpretation of this clustering requires knowledge of the projection kernel, or the redshift distribution of the sources, and the typical assumption is an isotropic redshift distribution for the objects. However, variations in the kernel are expected across the survey footprint due to photometric variations and residual observational systematic effects. We develop the formalism for anisotropic projection and present several limiting cases that elucidate the key aspects. We quantify the impact of anisotropies in the redshift distribution on a general class of angular two-point statistics. In particular, we identify a mode-coupling effect that can add power to auto-correlations, including galaxy clustering and cosmic shear, and remove it from certain cross-correlations. If the projection anisotropy is primarily at large scales, the mode-coupling depends upon its variance as a function of redshift; furthermore, it is often of similar shape to the signal. In contrast, the cross-correlation of a field whose selection function is anisotropic with another one featuring no such variations — such as CMB lensing — is immune to these effects. We discuss explicitly several special cases of the general formalism including galaxy clustering, galaxy-galaxy lensing, cosmic shear and cross-correlations with CMB lensing, and publicly release a code to compute the biases.

Investigation of wheel squeal noise under mode coupling using two-disk testrig experiments

Wheel squeal noise is a common problem that occurs in railway systems especially on curved tracks. These tonal noises are usually found at high frequency, typically over 1 kHz. The cause of the squeal noise remains unclear and could be different under varied conditions. Several mechanisms that could cause wheel squeal, including falling friction and mode coupling, have been proposed in literature. Most existing lab experimental investigations were focused on the squeal noise caused by falling friction with vertical contacts. As a result, the squeal noise generated by mode coupling could be missed due to the absence of a non-vertical contact angle. In this work, a two-wheel test rig has been designed with curved wheel profiles to achieve adjustable contact angles with lateral components. Experimental and numerical modal analysis of the two-disk system was conducted to determine the dynamic characteristics for exploration of mode coupling effects. Using a set of benchmark parameters predicted to be the cause of significant mode-coupled instability, squeal noise was found at frequencies higher than the existing falling friction experimental tests as reported in the literature. The frequency and amplitude measured in the experiments agreed with the predictions of the analytical mode coupling model. Further experiments varying control parameters, contact angle, angle of attack and direction of rotation were performed, confirming that the new squeal noise was caused by mode coupling.

Investigation of the low-frequency chatter in robotic milling

In robotic milling with large allowance process, low-frequency chatter (LFC) is an important factor observed in high-speed and low-speed milling, affecting the processing efficiency and quality. Previous research has used the regenerative chatter theory, ignoring modulated tool-workpiece engagement conditions, or mode coupling theory under the assumption of threading operations to explain the LFC mechanism and predict the stability boundary. However, these models overlook or inaccurately characterize the modulation effect, leading to inaccurate modeling of dynamic chip thickness changes during milling, making it difficult to understand the mechanism of LFC. Here, we propose an LFC stability model that considers modulated tool-workpiece engagement conditions and the mode coupling effect of the robotic structure for robotic milling. This approach allows us to reveal the mechanism of LFC and identify the characteristic signal of low-frequency vibration, which is the sideband frequency signal. Initially, the evolution of LFC is analyzed, and its characteristics are summarized. Further, a surface renewal (SR) model is proposed to accurately calculate the dynamic cutting force caused by modulated tool-workpiece engagement conditions in LFC. Furthermore, the LFC stability model, considering the modulated tool-workpiece engagement conditions and mode coupling effect, is established based on impulse response function (IRF) method. Finally, we verify the accuracy of our model through milling experiments and compare it with that of the classical stability prediction model. Our results show that LFC is highly dependent on speed, and our stability model can effectively predict the stability boundary of LFC in robotic milling with large allowance process.

Convergent Richtmyer–Meshkov instability on two-dimensional dual-mode interfaces

We report the first shock-tube experiments on two-dimensional dual-mode air–SF $_6$ interfaces with different initial spectra subjected to a convergent shock wave. The convergent shock tube is specially designed with a tail opening to highlight the Bell–Plesset (BP) and mode-coupling effects on amplitude development of fundamental mode (FM). The results show that the BP effect promotes the occurrence of mode coupling, and the feedback of high-order modes to the FM also arises earlier in convergent geometry than that in its planar counterpart. Relatively, the amplitude growth of the FM with a higher mode number is inhibited by the feedback, and saturates earlier. The FM with a lower mode number is affected more heavily by the BP effect, and finally dominates the flow. A new model is proposed to well predict the amplitude growths of the FM and high-order modes in convergent geometry. In particular, for FM that reaches its saturation amplitude, the post-saturation relation is introduced in the model to achieve a better prediction.

Damping vibration in three-dimensional helically tapered rod with power-law thickness

Tapered structure with power-law thickness has been regarded as an efficient approach for passive vibration control. In this paper, a three-dimensional (3D) helically tapered structure with power-law feature is proposed to reduce the vibration in rod structure. Compared with the traditional one-dimensional tapered rod, this novel helix type can compress the length of tapered region by the coiling configuration and transform the flexural wave modes into various complex modes. Drawing on its folded characteristic, the helix structure can generate highly efficient space utilization and achieve a superior vibration attenuation effect in the low frequency band. Numerical investigations on the dynamics and reflection coefficient analyses of different rod types are conducted and the reduction mechanism of 3D helically tapered rod is revealed. It is found that the larger helical radius can contribute to the reduction in low-frequency vibration, but it deteriorates the vibration performance in broadband. To overcome this “sudden collapse” phenomenon in the damping performance caused by the low stiffness, the novel step helically tapered rod is presented. According to the introduced multimodal and multidirectional mode-coupling effects, this step 3D helix structure not only makes up for the shortcoming of vibration control in low frequency range, but also further improves the innate advantage of reduction effect in the high frequency band. Meanwhile, parametric studies of step 3D helically tapered rod are performed and the effect of different constituent materials is discussed to expand the utility. To validate these analyses, experimental tests are also conducted. The results demonstrate its remarkable vibration reduction in broadband frequency.

Hydrodynamic instabilities of a dual-mode air–SF6 interface induced by a cylindrically convergent shock

Shock-tube experiments are performed on the convergent Richtmyer–Meshkov (RM) instability of a multimode interface. The temporal growth of each Fourier mode perturbation is measured. The hydrodynamic instabilities, including the RM instability and the additional Rayleigh–Taylor (RT) effect, imposed by the convergent shock wave on the dual-mode interface, are investigated. The mode-coupling effect on the convergent RM instability coupled with the RT effect is quantified. It is evident that the amplitude growths of all first-order modes and second-order harmonics and their couplings depend on the variance of the interface radius, and are influenced by the mode-coupling from the very beginning. It is confirmed that the mode-coupling mechanism is closely related to the initial spectrum, including azimuthal wavenumbers, relative phases and initial amplitudes of the constituent modes. Different from the conclusion in previous studies on the convergent single-mode RM instability that the additional RT effect always suppresses the perturbation growth, the mode-coupling might result in the additional RT effect promoting the instability of the constituent Fourier mode. By considering the geometry convergence, the mode-coupling effect and other physical mechanisms, second-order nonlinear solutions are established to predict the RM instability and the additional RT effect in the cylindrical geometry, reasonably quantifying the amplitude growths of each mode, harmonic and coupling. The nonlinear solutions are further validated by simulations considering various initial spectra. Last, the temporal evolutions of the mixed mass and normalized mixed mass of a shocked multimode interface are calculated numerically to quantify the mixing of two fluids in the cylindrical geometry.

Characterization of the material and electrical properties depending on the Mg:Ag ratio as a cathode for TEOLED

In this study, Mg:Ag mixed alloy films were comprehensively analyzed as a potential transparent cathode, depending on the deposition ratio of Mg:Ag atoms. It was determined that the work function, refractive index, and film formation characteristics varied depending on the deposition ratio of Mg:Ag, and affected electro-optical characteristics such as transmittance and sheet resistance. In addition, the applicability of the proposed transparent cathode for top emitting organic light-emitting diodes (TEOLEDs) was analyzed by comprehensively considering the TEOLED performance in relation to the Mg:Ag deposition ratio. The TEOLED performance was optimized by adjusting the Mg:Ag transmittance, sheet resistance and appropriate work function, and the emission spectrum could be controlled by localized surface plasmon resonance (LSPR) and cavity mode coupling effect.

Minuscule Corrections to Near-surface Solar Internal Rotation Using Mode Coupling

The observed solar oscillation spectrum is influenced by internal perturbations such as flows and structural asphericities. These features induce splitting of characteristic frequencies and distort the resonant-mode eigenfunctions. Global axisymmertric flow—differential rotation—is a very prominent perturbation. Tightly constrained rotation profiles as a function of latitude and radius are products of established helioseismic pipelines that use observed Dopplergrams to generate frequency-splitting measurements at high precision. However, the inference of rotation using frequency splittings does not consider the effect of mode coupling. This approximation worsens for modes with high angular degrees, as they become increasingly proximal in frequency. Since modes with high angular degrees probe the near-surface layers of the Sun, inversions considering coupled modes could potentially lead to more accurate estimates of rotation very close to the surface. In order to investigate if this is indeed the case, we perform inversions for solar differential rotation, considering coupling of modes for angular degrees 160 ≤ ℓ ≤ 300 in the surface gravity f-branch and first-overtone p modes. In keeping with the character of mode coupling, we carry out a nonlinear inversion using an eigenvalue solver. Differences in inverted profiles for frequency-splitting measurements from MDI and HMI are compared and discussed. We find that the corrections to the near-surface differential rotation profile, when accounting for mode-coupling effects, are smaller than 0.003 nHz and hence are insignificant. These minuscule corrections are found to be correlated with the solar cycle. We also present corrections to even-order splitting coefficients, which could consequently impact inversions for structure and magnetic fields.

Multi-mode instability interactions in twin jet configurations

Twin jet configurations are prone to a wide range of interactions due to flow instabilities driven by jet self-excitation and cross-excitation of one jet on another. These instability modes and associated phases are sensitive to many parameters chief among which is the exit profile of the nozzle. Extensive studies on Twin Circular nozzles and Large Aspect Ratio (AR) Twin rectangular nozzles have shown various mode coupling but, not much is known about twin jets with moderate aspect ratios (AR < 3). This study aims to understand the effect of the exit nozzle geometry on the generation of flow instabilities by comparing a Twin Rectangular nozzle (AR 2) to a Twin Square nozzle (AR 1) with identical design parameters through experimental analysis. High speed Schlieren imaging is coupled with Particle Image Velocimetry (PIV) to explore both qualitative (internal shock cell structure) and quantitative aspects (flow vorticity and turbulence) of these jets. Near field and far field acoustics are also studied to provide a wholistic understanding of the effect of mode coupling exhibited by these jets. Generation and propagation mechanisms of multi-mode instabilities which effect both nozzle orientations are also studied through Spatial and Temporal Fourier analysis.

Coupled model analysis of the ablative Rayleigh–Taylor instability

The weakly nonlinear stage of the ablative Rayleigh–Taylor instability (ARTI) is investigated by expanded hydrodynamic equations in which the third-order corrections of the two-mode perturbations are considered. In the present coupling model, two linear perturbations are simultaneously added near the ablation front at the initial moment, and we have derived the first three coupling harmonics. Furthermore, the coupling model analysis is studied via direct numerical simulation as well. When the original two fundamental modes are always dominant over other modes, the time evolution of the density amplitudes for these modes agrees well with the results of direct numerical simulation. It is found that the harmonics are stabilized by the mode coupling effects, and the long wavelength mode of the fundamental modes tends to dominate the growth of the ARTI. Two-mode coupling is one of the restriction factors for the realization of controlled inertial confinement fusion. Therefore, the coupling harmonics excited by two-mode perturbations have good application potential and are worth further study.

Polarization-maintaining large-mode-area solid-core anti-resonant fiber for high-power fiber lasers

A polarization-maintaining large-mode-area solid-core anti-resonant fiber is investigated in this work with double-layer anti-resonant elements for improved capability on fundamental mode guidance and higher order mode suppression, and structural and material asymmetry on inner-layer cladding tubes for high birefringence. Mode coupling between fundamental core mode in one polarization direction and mode on two high-index inner cladding tubes is used to enhance the fiber birefringence under large core diameter. Eventually, an optimized design of polarization-maintaining large-mode-area solid-core anti-resonant fiber with the currently largest mode field area of 3026 μm2 and birefringence of 1.20 × 10−5 can be anticipated with the confinement losses for all the higher order modes exceeding 10 dB/m. Moreover, confinement loss below 0.1 dB/m can still be obtained for the fundamental core mode in both polarization direction even under the mode-coupling effect. Active polarization-maintaining anti-resonant fiber is analyzed and the refractive index of the central rare-earth-doped region is optimized for the first time for both well guided signal and pump laser. Large pump absorption coefficient can thus be anticipated from the multimode laser core-pumped active large-mode-area fiber, which facilitates short active fiber length needed for sufficient laser gain. The fiber proposed here offers great potential for the development of high-performance fiber laser sources including high-power single-frequency fiber amplifiers and high-peak-power ultrashort pulsed fiber amplifiers.

Prediction and Analysis of Milling Stability Under Multi-effect Synergy

In the milling process is often accompanied by a lot of vibration, these vibrations will lead to the instability of the processing process resulting in the occurrence of chatter, chatter in the process will seriously affect the processing efficiency, reduce the quality of processing, so the mechanism of chatter and influencing factors to study, and through the analysis of stable milling processing parameters, to achieve efficient precision machining is very important. In this paper, based on the traditional milling model, the regeneration effect, process damping effect and modal coupling effect are taken into account, and the friction effect of the front cutter face is also taken into account for frictional chatter to make the prediction range of the model more accurate. Then, a milling dynamic model was established combining various effects, and the stability lobe diagram was solved by the fully discrete method. The influence of each effect on milling stability was analyzed and the influence of friction effect was emphatically discussed. Finally, the accuracy of the model was verified by the milling stability experiment.

Nonlinear Fast Magnetosonic Waves in Solar Prominence Pillars

We investigate the properties of nonlinear fast magnetosonic (NFM) waves in a solar prominence, motivated by recent high-resolution and high-cadence Hinode/Solar Optical Telescope (SOT) observations of small-scale oscillations in a prominence pillar. As an example, we analyze the details of the 2012 February 14 Hinode/SOT observations of quasi-periodic propagating features consistent with NFM waves, imaged in emission in Ca ii and in the far blue wing of Hα. We perform wavelet analysis and find oscillations in the 1–3 minutes period range. Guided by these observations, we model the NFM waves with a three-dimensional magnetohydrodynamics (3D MHD) model, extending previous 2.5D MHD studies. The new model includes the structure of the high-density, low-temperature material of the prominence pillar embedded in the hot corona, in both potential and non-force-free sheared magnetic field configurations. The nonlinear model demonstrates the effects of mode coupling and the propagating density compressions associated with linear and NFM waves. The guided fast magnetosonic waves, together with density compressions and currents, are reproduced in the 3D pillar structure. We demonstrate for the first time the dynamic effects of the Lorentz force due to the magnetic shear in the non-force-free field on the pillar structure and on the propagation of the waves. The insights gained from the 3D MHD modeling are useful for improving the coronal seismology of prominence structures that exhibit fast MHD wave activity.

Dispersion-engineered broadband diffractive optical elements with multilayer subwavelength structures.

Diffractive optical elements (DOEs) play an important role in modern optical applications such as spectral and imaging systems, but it is challenging to balance the diffraction efficiency with the working bandwidth. The core issue is controlling the broadband dispersion of all phase units to achieve achromatic 2π-phase modulation in the broadband domain. Here, we demonstrate broadband DOEs utilizing multilayer subwavelength structures with different materials, making it possible to freely control the phase and phase dispersion of the structural units on a much larger scale than monolayer structures. The desired dispersion-control abilities arose due to a dispersion-cooperation mechanism and vertical mode-coupling effects between the top and bottom layers. An infrared design comprised of two vertically concatenated T i O 2 and Si nanoantennas separated by a S i O 2 dielectric spacer layer was demonstrated. It showed an average efficiency of over 70% in the three-octave bandwidth. This work shows enormous value for broadband optical systems with DOEs such as spectral imaging and augmented reality.

Modeling of the thermal-lens induced mode coupling in fiber amplifiers

In this paper, a novel model for describing the thermal-lens induced mode coupling (TLMC) effect in the high power fiber amplifier is proposed firstly, to the best of our knowledge. Based on the rate equation and couple-mode equation, the model is built with both consideration of the thermal-lens induced mode field variation and the gain effect. With this model, the mode coupling behaviors with various pump schemes are numerically investigated and the impact of the TLMC is quantitatively discussed in a finite-length single-frequency fiber amplifier for paradigm. It is found that the higher-order mode (HOM) can be induced by the TLMC even with pure single-mode input signal and this effect is most severe with the co-pumped scheme while weakest with the counter-pumped scheme. It is also revealed that the TLMC will enhance the average HOM content and will keep predominant until the incident HOM is larger than a certain content. Such an enhancement of HOM content can play an important role as the seed triggering the transverse mode instability (TMI) and the pertinent study is helpful on investigating the relationship between the thermal-lens effect and the TMI.

Longest relaxation time versus maximum loss peak in the field-dependent longitudinal dynamics of suspended magnetic nanoparticles

Magnetic nanoparticles in suspensions provide fascinating model systems to study field-induced effects. Their response to external fields also opens up promising new applications, e.g., in hyperthermia. Despite significant research efforts, several basic questions regarding the influence of external fields on the magnetization dynamics are still open. Here we revisit the classical model of a suspended magnetic nanoparticle with combined internal and Brownian dynamics in the presence of an external field and discuss the field-dependent longitudinal relaxation. While internal and Brownian dynamics are independent in the field-free case, the coupling of both processes when an external field is present leads to richer and more complicated behavior. Using a highly efficient and accurate solver to the underlying Fokker-Planck equation allows us to study a broad parameter range. We identify different dynamical regimes and study their respective properties. In particular, we discuss corrections to the popular rigid-dipole approximation which are captured in terms of a simplified diffusion-jump model in the Brownian-dominated regime with rare N\'eel relaxation events. In addition, we discover a regime with surprising mode-coupling effects for magnetically soft nanoparticles. We explain our findings with the help of a perturbation theory, showing that in this regime the magnetization relaxation at late times is slaved by the slow Brownian motion of the nanoparticle. We discuss consequences of these findings such as the discrepancy of the longest relaxation time and the inverse frequency of the loss peak of the magnetic susceptibility.