Mode Competition Phenomena Research Articles

Vortex-Induced Vibration of Flexible Cylinders in Cross-Flow

This review provides a comprehensive analysis of the literature on vortex-induced vibration (VIV) of flexible circular cylinders in cross-flow. It delves into the details of the underlying physics governing the VIV dynamics of cylinders characterized by low mass damping and high aspect ratio, subject to both uniform and shear flows. It compiles decades of experimental investigations, modeling efforts, and numerical simulations and describes the fundamental findings in the field. Key focal points include but are not limited to amplitude–frequency response behavior, the relationship between the distributed loading acting on the cylinder and the trajectories and the near wake structures around the cylinder, the existence of traveling waves, the identification of power-in/power-out regions, and the modal overlapping and mode competition phenomena.

Influence of multi-modes contest on the dynamics of a quantum dot light emitting diode

Abstract The effects of quick mode-competition phenomena on the dynamics of semiconductor quantum dot light-emitting diode (QDLED) are the subject of theoretical investigations in this study. The analyses are based on numerical simulations of the superposed multi-mode rate equations, which take into account the inherent fluctuations related to the spontaneous emission. The modal photon number and the injected electron number are reserved correlated during the multimode sources’ numerical generating process. Based on a self-consistent model, the optical frequency w effects which result in competition phenomena among lighting modes are introduced. The phenomenon of mode-competition is used to explain how noise sources affect it. Mode-competition processes cause instantaneous coupling between variations in mode intensity, which leads to instability in the dynamics of the modes and changes the state of operation. Over a broad range of carriers and photon rates (including the capture rate of the witting layer (WL) into the dot, the photon output rate in the optical mode, and the nonradiative decay rates of the number of carriers in the QD and WL), the dynamics of modes and the characteristics of the output spectrum are investigated. Three types of operation are distinguished: jittering single mode, stable single mode, and stable multimode.

Numerical investigation of mode competition and cooperation on the combustion instability in a non-premixed combustor

The phenomena of combustion instabilities beyond limit cycles caused by the nonlinear interaction of various instability modes have recently received much attention. This paper numerically studied the mode competition and cooperation phenomena in the liquid rocket engines when combustion instabilities occur. The liquid rocket engine with the single-element structure is the focus of the present work, and the numerical model is developed employing the Large Eddy Simulation technology, with the complete conservation Navier-Stokes equations taken into account. The results first reveal the relationship between the acoustic mode and the intrinsic vorticity mode: the competition and cooperation between the two will result in the complex dynamic process, such as semi-stable oscillation and intermittency. What's more, a more detailed coupling process between intrinsic vortex shedding and acoustic oscillation is analyzed, indicating existence of four stability regimes in the liquid rocket engines and an important insight into combustion instability in non-premixed dump combustors is provided.

An experiment study of vortex induced vibration of a steel catenary riser under steady current

The vortex induced vibration (VIV) of marine risers has been investigated by many researchers in experimental studies of a straight flexible riser model as well as a rigid cylinder to reveal the dynamic response characteristic and the mechanics behind it. However, due to the limitation of experimental apparatus, very few studies are about the VIV of a steel catenary riser (SCR) which is with a complex geometry. To investigate the VIV features and to further develop the corresponding numerical predictions of a SCR under steady current, a large-scale model test of a SCR was towed in an ocean basin at various speeds. Fiber Bragg grating strain sensors are instrumented on the riser model to measure both in-plane and out-of-plane responses. The characteristics of oscillating amplitude and dominating frequency response, the phenomenon of mode competition and travelling wave and the fatigue damage of the steel catenary riser in inline and cross-flow direction under steady current are analyzed.

Fully nonlinear mode competition in magnetised Taylor–Couette flow

We study the nonlinear mode competition of various spiral instabilities in magnetised Taylor-Couette flow. The resulting finite-amplitude mixed-mode solution branches are tracked using the annular-parallelogram periodic domain approach developed by Deguchi & Altmeyer (2013). Mode competition phenomena are studied in both the anti-cyclonic and cyclonic Rayleigh-stable regimes. In the anti-cyclonic sub-rotation regime, with the inner cylinder rotating faster than the outer, Hollerbach, Teeluck & Rudiger (2010) found competing axisymmetric and non-axisymmetric magneto-rotational linearly unstable modes within the parameter range where experimental investigation is feasible. Here we confirm the existence of mode competition and compute the nonlinear mixed-mode solutions that result from it. In the cyclonic super-rotating regime, with the inner cylinder rotating slower than the outer, Deguchi (2017) recently found a non-axisymmetric purely hydrodynamic linear instability that coexists with the non-axisymmetric magneto-rotational instability discovered a little earlier by Rudiger, Schultz, Gellert & Stefani (2016). We show that nonlinear interactions of these instabilities give rise to rich pattern-formation phenomena leading to drastic angular momentum transport enhancement/reduction.

Current induced mode competition in microdisk lasers

A phenomenon of mode competition in microdisk lasers under various injection currents was studied. The increase of the injection current causes rise of the internal temperature. The modeling has shown that this increase leads to the shift of the gain spectrum. That may be enough to cause drastic redistribution of emitted power between microdisk modes. This effect has been studied in the experiment and by the numerical simulation.

Modeling mode competition in laser diodes

We present an advanced model for the simulation of laser dynamics based on the microscopic many-particle equations of motion and going beyond the often used rate equation model. Our model is used to simulate (Al,In)GaN based edge-emitting laser diodes showing longitudinal mode-competition phenomena which are responsible for the effect of mode hopping, i.e. the modes are switched on and off causing mode rolling from higher to lower frequencies. We show that the effect is caused by beating vibrations of the carrier density which result in an asymmetric coupling term between neighboring modes. Here, the results agree with the case where we assume that the coupling of two modes is independent of all the other modes, as long as the mode frequencies are not equidistant. If the mode frequencies are equidistant, the mode dynamics depend strongly on the initial phases and the initial noise.

Retraction Note to: Mode-competition phenomena among longitudinal modes in semiconductor lasers under the effect of external optical feedback

The authors have retracted the original article because the article shows significant overlap.

RETRACTED ARTICLE: Mode-competition phenomena among longitudinal modes in semiconductor lasers under the effect of external optical feedback

RETRACTED ARTICLE: Mode-competition phenomena among longitudinal modes in semiconductor lasers under the effect of external optical feedback

Effect of a Roughness Element on the Receptivity of a Hypersonic Boundary Layer over a Blunt Cone Due to Pulse Entropy Disturbance with a Single Frequency.

A high-order finite difference method was used to simulate the hypersonic flow field over a blunt cone with different height roughness elements. The unsteady flow field induced by pulse disturbances was analyzed and compared with that under continuous disturbances. The temporal and spatial evolution characteristics of disturbances in the boundary layer were investigated and the propagation of different disturbance modes in the boundary layer was researched through the fast Fourier transform (FFT) method. The effect of the roughness element on the receptivity characteristic of the hypersonic boundary layer under pulse entropy disturbances was explored. The results showed that the different mode disturbances near roughness in the boundary layer were enlarged in the upstream half of the roughness element and suppressed in the downstream half. However, the effect of roughness weakened gradually as the disturbance frequency increased in the boundary layer. A phenomenon of mode competition in the downstream region of the roughness element exited. As the disturbances propagated downstream, the fundamental mode gradually became the dominant mode. A certain promotion effect on the mode competition was induced by the roughness element and the effect was enhanced with the increase in the roughness element height.

Nonstationary behavior in a delayed feedback traveling wave tube folded waveguide oscillator

Folded waveguide traveling-wave tubes (FW TWT) are among the most promising candidates for powerful compact amplifiers and oscillators in millimeter and submillimeter wave bands. In this paper, the nonstationary behavior of a FW TWT oscillator with delayed feedback is investigated. Starting conditions of the oscillations are derived analytically. Results of numerical simulation of single-frequency, self-modulation (multifrequency) and chaotic generation regimes are presented. Mode competition phenomena, multistability and hysteresis are discussed.

Time-dependent measurement of the mode-competition phenomena among longitudinal modes in long-wavelength lasers

Modal behaviors of an InGaAsP-InP Fabry-Perot laser emitting in a long-wavelength region were experimentally examined. The authors had predicted theoretically that the mode-competition phenomena induce quasi-periodic hopping among several longitudinal modes, which reveal multimode-like output spectra as the time-averaged spectra in long-wavelength lasers. In this paper, experimental measured data of the time variations of photon number and their frequency spectra in addition to the longitudinal mode spectra are reported together with theoretically simulated results. The previous theoretical predictions were well proved by these experimental measurements.

Influence of instantaneous mode competition on the dynamics of semiconductor lasers

Comprehensive theoretical investigations of the influence of instantaneous mode-competition phenomena on the dynamics of semiconductor lasers are introduced. The analyzes are based on numerical simulations of the multimode rate equations superposed by Langevin noise sources that account for the intrinsic fluctuations associated with the spontaneous emission. Numerical generation of the Langevin noise sources is performed in such a way as to keep the correlation of the modal photon number with the injected electron number. The gain saturation effects, which cause competition phenomena among lasing modes, are introduced based on a self-consistent model. The effect of the noise sources on the mode-competition phenomena is illustrated. The mode-competition phenomena induce instantaneous coupling among fluctuations in the intensity of modes, which induce instabilities in the mode dynamics and affect the state of operation. The dynamics of modes and the characteristics of the output spectrum are investigated over wide ranges of the injection current and the linewidth enhancement factor in both AlGaAs-GaAs and InGaAsP-InP laser systems. Operation is classified into stable single mode, stable multimode, hopping multimode, and jittering single mode. Based on the present results, the experimental observations of multimode oscillation in InGaAsP-InP lasers are explained as results of the large value of the linewidth enhancement factor.

Stabilization of Semiconductor Lasers by Suppressing the Optical Feedback Noise.

Generating mechanism of the optical feedback noise in semiconductor lasers and whose suppressing manner by utilizing the self-pulsation laser or the high frequency superposition method are theoretically reviewed. Those mechanisms are characterized in terms of the mode competition phenomena among the internal cavity modes of the solitary laser and the external cavity modes which are built up by the optical feedback. Suppression of the feedback noise is understood as an effect to shift the operating point to more stable state by help of the modulated photons and electron densities in the laser.

Mode competition in a system of two parametrically driven pendulums with nonlinear coupling

This paper is part three in a series on the dynamics of two coupled, parametrically driven pendulums. In the previous parts Banning and van der Weele (1995) and Banning et al. (1997) studied the case of linear coupling; the present paper deals with the changes brought on by the inclusion of a nonlinear (third-order) term in the coupling. Special attention will be given to the phenomenon of mode competition. The nonlinear coupling is seen to introduce a new kind of threshold into the system, namely a lower limit to the frequency at which certain motions can exist. Another consequence is that the mode interaction between 1α and 2β (two of the normal motions of the system) is less degenerate, causing the intermediary mixed motion known as MP to manifest itself more strongly.

Characteristics of mode-hopping noise and its suppression with the help of electric negative feedback in semiconductor lasers

Some detailed properties of mode-hopping noise in semiconductor injection lasers are reported, and the validity of electric negative feedback as a method of suppressing the noise is demonstrated. Mode-hopping noise was found to be an intrinsic property of the laser, independent of optical feedback (reinjection of the output light) reflected at the surface of an external device. The optical feedback works to shift the operating point of the laser from a stable condition having single-mode operation to an unstable condition revealing the modehopping phenomena. Characteristics of the mode-hopping noise support well the theoretical analysis in which the source of the noise is assumed to be fluctuations of the emission and the electron wave accompanied by spontaneous emission and in which the noise is amplified through mechanism of the mode competition phenomena. Mode-hopping noise was suppressed with the help of the electric negative feedback from the optical detector to the injection current source more effectively than by another reducing method of superposition of the high-frequency current. The suppressed noise level was lower than that at stable single-mode operation.

Theory of mode competition noise in semiconductor injection lasers

A theoretical analysis of the mode-competition noise in semiconductor injection lasers is given. The source of the noise is supposed to be fluctuations of the number of photons and electrons on optical emission, and is amplified by optical gain in which the mode-competition phenomena are taken into account. When a lasing mode jumps to another mode, noise becomes largest and shows <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/f^{\alpha}</tex> characteristics on its frequency spectrum, where the index of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha = 2</tex> is theoretically obtained based on a perturbation analysis of the noise. The noise of an individual mode is in general larger than that of the overall lasing modes. The noise reduces to the level of the quantum noise when the laser operates either in single mode or in stable multimodes.

Transverse and longitudinal mode control in semiconductor injection lasers

Mechanisms which determine the oscillating transverse and longitudinal modes in semiconductor injection lasers are discussed in this paper. The analysis is based on the semiclassical method in which the optical field is represented by Maxwell equations and the lasing phenomenon is analyzed quantum mechanically using the density matrix formalism. Guided modes are classified by the relation between refractive index and gain-loss differences at the boundaries of the active region as normal guided mode (index guiding), active-guided mode (gain guiding), and leaky mode (anti-index guiding). The guiding loss and cutoff conditions are given for these modes. The optimum range to obtain stable fundamental transverse mode operation is discussed with respect to several guiding factors, such as width of active region, the refractive index difference, and gain-loss differences at the boundaries of the active region. Longitudinal mode behavior is discussed in terms of electron transition mechanism in semiconductor crystals. The relaxation effect of the electron wave is introduced in this model. Profiles of the saturated gain and the spatial diffusion of the electron are related to this relaxation effect. Mode competition phenomena are analyzed, and a strong gain suppression among the longitudinal modes is shown to be as an intrinsic property of semiconductor lasers. The possibility of obtaining single longitudinal mode operation is postulated. Physical influences for stable single longitudinal mode operation are discussed in terms of transverse mode control (or stripe structure), spontaneous emission, threshold current level, impurity concentration in the active region, and direct modulation. Some experimental results are also given to support these analyses.

The control of polarization of laser diodes

The mode-competition phenomena are studied through three experiments. At first, in double-heterostructure laser diodes with thick active layers, TE and TM modes tend to coexist and the mode competition between these two modes is observed from the correlation of the light outputs of these two modes where the increase of light output of one mode tends to decrease the light output of other mode. In a laser diode operating with an external reflector, the increase of light output for the TE mode leads to the decrease of light output for the TM mode. In case the external reflector is set to deviate from the parallel plane to the cleaved surface of the laser diode, the TM-mode component tends to its magnitude, even if the inherent mode of this diode is TE. In the experiment of external light-beam irradiation onto a laser diode, the light intensity of one mode is decreased by an incidence of light beam having another mode from the other laser diode. In this experiment, the significance of a gap distance between the two laser diodes is confirmed clearly, where the two laser diodes interact with each other through the light beam.

High-power single-mode CO&amp;lt;inf&amp;gt;2&amp;lt;/inf&amp;gt;laser

The performance of a single-mode single-frequency CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser operating on a single rotational-vibrational transition is described. The laser is sealed off with a length of 152 cm, and it has a continuous output power of 60 watts. Mode-competition phenomena are of great importance in obtaining this high output power of high spectral purity. In-order to maintain the high output power over a long period of time, we constructed our laser with a water vapor replenisher and a bypass to reduce gas segregation in the discharge. The use of a plane parallel germanium out-coupling window creates conditions for a single rotational-vibrational transition. The experimental procedure and results are discussed.