Traveling-wave Modulator Research Articles

Traveling wave mode analysis of a coherence collapse regime semiconductor laser with optical feedback

A highly developed traveling wave model for a semiconductor laser system supports sophisticated mode analysis of the coherence collapse regime in semiconductor lasers with delayed optical feedback. The concept of instantaneous optical modes is used. Time-frequency representations of chaotic trajectories are constructed and interpreted. This involves synthesizing the calculated optical modes with their corresponding steady states, analysis of the mode driving and coupling sources, and field expansion into modal components. The results support detailed physical interpretation of the optical and radiofrequency spectra throughout the coherence collapse regime. This includes simulation results for the transition out of coherence collapse at high optical feedback. Also key frequencies observed in the radiofrequency spectrum of the chaotic output are linked to the modes that mix to generate them.

RF crosstalk mitigation via floating shields in parallel silicon traveling-wave modulators

High-density integration of electro-optic (E-O) elements in photonic integrated circuits (PICs) offers compact, low-cost high-capacity transceiver modules. Tighter component spacing increases compactness, but leads to crosstalk that can degrade performance. We investigate and characterize the crosstalk between silicon traveling-wave Mach-Zehnder modulators (TW-MZMs) laid out in parallel in PICs. We propose the use of floating shield strips to reduce E-O crosstalk between these TW-MZMs. We show via simulation and experiment that these strips can increase shoreline bandwidth density by nearly 50%. The improvement in E-O crosstalk covers frequencies as high as 60 GHz.

Focus control of a concave–convex ultrasonic gel lens in the radial direction

Optical image stabilization (OIS) systems maintain the three-dimensional focal position of a lens through mechanical actuation systems. This paper examines an optical lens for OIS that utilizes ultrasound vibration to alter the focal position, not only in the depth direction but also in the radial direction. The lens has a simple structure with no mechanical moving parts and consists of an ultrasound transducer divided into four pieces, a glass disk, and a transparent viscoelastic gel film that functions as a lens. The acoustic radiation force generated by the resonant flexural vibration of the glass disk can alter the surface profile of the gel film, allowing for a variable-focus function. The concave and convex lenses can be interchanged using two resonant vibration modes: the standing-wave mode, in which the vibration loop appears at the center, and the traveling-wave mode, in which the vibration node appears at the center. The positions of ultrasound vibrations on the lens can be controlled in a two-dimensional plane by adjusting the driving amplitudes of each channel, thereby achieving focus control in the radial direction. The focusing characteristics of the lens are evaluated through ray-tracing simulation.

The broad study of blade cascade under controlled torsional flutter: Dynamics of the flow and stability analysis

The experimental and numerical investigation of the flow instabilities acting on rigid blades and vice versa was conducted for both compressor and turbine configuration. The blade cascade consisted of five rectangular NACA 0010 blades, with three middle blades capable of performing harmonic motion with one degree of freedom (pitching) using force excitation. The base case (all blades fixed) and excited regime were examined. The influence of various angles of attack, harmonic frequency values, amplitude values, inter-blade phase angles and Reynolds numbers (Re) were tested. The mean flow properties as well as the fluid - structure interaction (FSI) were studied using Particle Image Velocimetry (PIV), Reynolds-averaged Navier-Stokes (RANS) CFD methods and using force measurement. Additionally, two different approaches, namely traveling wave mode (TWM) and aerodynamic influence coefficient (AIC), were adopted to estimate the aeroelastic stability of the blade cascade, and the results were compared. The results show significant aeroelastic coupling between the blades in both compressor and turbine configuration. However, the aerodynamic coupling effect for torsional flutter is more prominent in turbine configuration.

Non-reciprocity in a silicon photonic ring resonator with time-modulated regions.

Non-reciprocity and breaking of the time-reversal symmetry is conventionally achieved using magneto-optic materials. However, the integration of these materials with complementary metal-oxide semiconductor (CMOS)-compatible platforms is challenging. Temporal modulation is a well-suited approach for achieving non-reciprocity in integrated photonics. However, existing experimental implementations based on this method in silicon uses traveling-wave modulation in the whole structure or tandem ring or waveguide modulators, and they lead to high insertion loss and large footprint. In this work we achieve, to the best of our knowledge, the first experimental demonstration of non-reciprocity in a compact single silicon photonic ring resonator with time-modulated regions, fabricated with a CMOS-compatible commercial foundry. We demonstrate symmetry breaking of counter-rotating modes in an active silicon photonic ring resonator by applying phase-shifted RF signals to only two small p-i-n junctions on the ring, without employing traveling-wave modulation in the whole structure. The non-reciprocity is caused by the cross-coupling between the counter-rotating modes of the ring, which breaks their degeneracy. By reversing the polarity of the RF phase difference (e.g. (45°,-45°) asymmetric phases) opposite resonance wavelengths are obtained, with a 16-dB contrast between the transmissions of the asymmetric phases and a low insertion loss of 0.6 dB under 27 dBm RF power. We achieve the highest ratio of the asymmetric transmission to the insertion loss, among the state-of-the-art silicon non-reciprocal integrated optical structures based on time varying modulation. The non-reciprocal ring can be used as a magnetic-free, low-loss, compact, and CMOS-compatible integrated optical isolator.

Modal decomposition of the shedding mechanism of partial cavitation in a Venturi Tube

Abstract The present paper studies the shedding mechanism of partial cavitation in a Venturi Tube, dominated by re-entrant jet and bubbly shock mechanisms, by using two data-driven modal decomposition methods: proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). According to the snapshot data series obtained by high-speed photography, the modal decomposition and reconstruction of the grey image are carried out. The POD results indicate that the main frequency of the re-entrant jet is higher than that of the shock under the sixth-order mode, and the energy amplitude of the latter is about 20 times that of the former. Furthermore, as the cavitation number increases, the condensation shock mechanism eventually replaces the re-entrant jet mechanism. The DMD results show that the shock behaves obvious traveling wave mode, because the frequency is higher and the phase of the spatial distribution changes evenly under the fourth-order mode. POD and DMD can help to understand the shedding mechanism of partial cavitation.

Particle-hole instabilities in photonic time-varying systems

Photonic systems with time-varying modulations have attracted considerable attention as they allow for the design of non-reciprocal devices without the need for an external magnetic bias. Unlike time-invariant systems, such modulations couple modes with different frequencies. Here, we discuss how this coupling and particle-hole symmetry may lead to the resonant interaction of positive and negative frequency oscillators. To illustrate this idea, we analyze a dispersive spacetime crystal described by a Drude-Lorentz model with a traveling-wave modulation. Our findings demonstrate that the interaction between positive and negative frequency bands can induce parametric instabilities under certain conditions, stemming from the interplay between dispersion and spacetime modulations. In particular, we find that material dispersion creates the conditions for the formation of instabilities for arbitrarily small modulations speeds in the absence of dissipation mechanisms.

Imperfectly conducting eigenflows in a vertical fluid layer

The stability of buoyant flow in an infinite vertical fluid layer bounded by imperfectly conducting rigid walls, called imperfectly conducting eigenflows, is discussed. The third kind boundary conditions describing heat transfer to the external environment are applied to perturbations in temperature. The linear stability analysis is carried out numerically by employing the Chebyshev collocation method. Instability arises when the Grashof number G exceeds its critical value, which depends on the Prandtl number Pr and the Biot number Bi. It is found that the onset of instability changes dramatically depending on the magnitude of Prandtl and Biot numbers particularly when the instability is through the traveling-wave mode. The numerical results show that the Biot number plays a pivotal role in determining the transition Prandtl number PrT at which the instability switches over from one mode to another mode. The novel outcomes suggest the presence of a single PrT for Bi<2.1739 while three distinct values of PrT for Bi≥2.1739. The departure from the conventional single value typically observed at isothermal boundaries signifies the complexity of the instability mechanism.

Fixed-flux Rayleigh–Bénard convection in doubly periodic domains: generation of large-scale shear

This work studies two-dimensional fixed-flux Rayleigh–Bénard convection with periodic boundary conditions in both horizontal and vertical directions and analyses its dynamics using numerical continuation, secondary instability analysis and direct numerical simulation. The fixed-flux constraint leads to time-independent elevator modes with a well-defined amplitude. Secondary instability of these modes leads to tilted elevator modes accompanied by horizontal shear flow. For $Pr=1$ , where $Pr$ is the Prandtl number, a subsequent subcritical Hopf bifurcation leads to hysteresis behaviour between this state and a time-dependent direction-reversing state, followed by a global bifurcation leading to modulated travelling waves without flow reversal. Single-mode equations reproduce this moderate Rayleigh number behaviour well. At high Rayleigh numbers, chaotic behaviour dominated by modulated travelling waves appears. These transitions are characteristic of high wavenumber elevator modes since the vertical wavenumber of the secondary instability is linearly proportional to the horizontal wavenumber of the elevator mode. At a low $Pr$ , relaxation oscillations between the conduction state and the elevator mode appear, followed by quasi-periodic and chaotic behaviour as the Rayleigh number increases. In the high $Pr$ regime, the large-scale shear weakens, and the flow shows bursting behaviour that can lead to significantly increased heat transport or even intermittent stable stratification.

Abundant exact traveling wave solutions and modulation instability analysis to the generalized Hirota–Satsuma–Ito equation

Abundant exact traveling wave solutions and modulation instability analysis to the generalized Hirota–Satsuma–Ito equation

A switching method for traveling/standing wave transportation modes in two-dimensional acoustic fields using a dual-transducer support structure

The dual-transducer support structure discussed has the advantages of a simple structure and low cost, as well as allowing for the use of both Traveling-Wave (TW) and Standing-Wave (SW) acoustic transportation, supporting its use in pharmaceutical and biochemical analysis, for example. By adjusting the distance between the vibrating plate and the reflector which forms SW field in the y direction, the control of the position of the SW nodes or the TW component along the x direction allows the formation of a Two-Dimensional Standing Wave (2D-SW) or a Traveling Wave (TW) acoustic field, and these could be used for transportation in the x direction. It has been found that the x position of the SW nodes can be adjusted through changing the temporal phase shift, θ, which permits multiple objects to be transported using the 2D-SW mode. By comparison, TWs in the opposite direction could be generated at a pair of specific temporal phase shifts, allowing fast transportation using the TW mode. In this research work, an experiment has been carried out to transport polystyrene spheres using the two modes by programming the temporal phase shift, θ, this illustrating that precise position control of the multiple objects transported was possible using the 2D-SW mode, while high-speed transportation (up to 540 mm/s) was realized using the TW mode, showing that the dual-transducer support structure could be used effectively for accurate and fast transportation. As a fully non-contact method, the dual-transducer support structure can be seen to work in the 2D-SW mode for reaction synthesis or detection applications, and also in TW mode for rapid sample transportation applications.

Instability of a viscoelastic film with insoluble surfactants on an oscillating plane

The linear instability of viscoelastic film with insoluble surfactants on an oscillating plane for disturbances with arbitrary wavenumbers is investigated. The combined effects of viscoelastic and insoluble surfactants on the instability are described using Floquet theory. For long-wavelength instability, the solution in the limit of long wave perturbations is obtained by the asymptotic expansion method. The results show that the presence of viscoelastic film shifts the stability boundaries to the low-frequency region in the absence of gravity when the imposed frequency is less than 6. The U-shaped neutral curves with separation bandwidth appear in the presence of gravity. The finite-wavelength instability is solved numerically based on the Chebyshev spectral collocation method. Different from the previous results, a new branch point with special structure of a neutral curve is detected for clean-surface film. Results show that the presence of the surfactants will decrease the unstable frequency bandwidth and increase the critical Reynolds number. Both the travelling-wave mode and standing-wave mode are found due to the existence of surface surfactants. For high-frequency oscillation, the viscoelastic parameter may significantly destabilize the flow and the instability is determined by the finite-wavelength mode over a relatively large frequency range.

Traveling wave modes of flexible structures with distributed gyros

With the development of space structures, flexible structures are widely studied. To achieve attitude and vibration control of flexible structures, flywheels or control moment gyros can be introduced. However, the critical challenge in studying gyro flexible structures is establishing a systematic framework to manage its physical properties and calculate the control moment precisely. This study aims to construct a theoretical framework for a two-dimensional flexible structure with distributed gyros using classical methods and explore the contribution of the gyro moment on the dynamic mode shapes. Both eigenvalue and traveling wave mode shape analysis are applied to investigate the dynamical behaviors of the system. It is found that dynamic modes with nodal lines showing whirling phenomena have arisen instead of the traditional modes featured with static nodal lines triggered by the gyro moment, and the dynamic mode shapes can be displayed by the direction and frequency of the mode nodal lines motion. The discovery and research of gyro flexible structure traveling wave modes can provide a solid base for the design of large space structures, improve their stability, and avoid harmful vibrations.

Thermocapillary effects during the melting of phase change materials subjected to lateral heat flux in microgravity

We numerically investigate the melting dynamics of n-octadecane in a rectangular container that is subjected to a constant lateral flux and is open to air in microgravity. As the melting progresses, the temperature gradient existing at the open boundary drives thermocapillary flow within the liquid phase and modifies the phase change dynamics. We conduct a parametric study varying key dimensionless parameters — the Marangoni and Stefan numbers, and the aspect ratio — and compare the associated temporal dynamics with those observed in the (so-called) reference case, where heat is transferred solely by conduction, and with those driven by isothermal boundary conditions [1,2]. Compared to the isothermal case, where the overall effect of thermocapillary flows is to accelerate melting by a factor up to 20, the analogous contribution with lateral flux is about one order of magnitude smaller, with values up to 2.8. Finally, we perform spectral analyses of the oscillatory standing wave and hydrothermal traveling wave modes that eventually appear during melting for large applied flux. The critical Marangoni number (Macr) is estimated considering the characteristic dependence ∝(Ma−Macr)0.25 of the (so-called) oscillatory mode contribution.

Numerical Investigation of Friction Induced Interaction of Flutter Modes in a Realistic Low Pressure Turbine Rotor

Abstract Flutter response of bladed disks is a quite an important problem in modern designs of low pressure turbines, but, even for tuned configurations, it is not fully understood yet. Some research work suggests that, in a tuned rotor, the flutter induced vibration state that sets in consists of just a single traveling wave mode, while others show flutter states composed of several traveling waves active at the same time. Many of these studies were performed using conceptual models, such as mass-spring systems or asymptotically reduced models. In this work, a high fidelity bladed disk model is integrated into the time domain for the case of a realistic aerodynamically unstable low pressure turbine. In this configuration, the only nonlinear effect is the friction present at the blade disk attachment, which is responsible for the saturation of the flutter growth, and it is also the only mechanism that allows the interaction among the different unstable traveling wave modes. A case with multiple unstable modes of the same modal family is studied first. After that, the study is extended to a case where modes of different modal families are unstable and have similar aerodynamic instability levels. The interaction of unstable modes from different modal families is studied, we believe, for the first time, as previous flutter analyses were restricted to the interaction of unstable modes from the same family.

A Sub-Terahertz High-Speed Traveling-Wave Switch Modulator Based on Dynamically Tunable Double-Resonant Coupling Units

High-speed direct modulation devices are of great significance for the development of high-speed communication systems with large bandwidth, miniaturization, and low complexity. However, an increase in the carrier frequency in the sub-terahertz band and above will negatively impact high-performance direct modulation devices due to the large insertion loss and low on–off ratio. Based on the traveling-wave modulation mechanism, this article adopts the zero-phase-difference amplitude modulator design method. More specifically, a 1- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> m small anode Schottky diode is combined with a Z-type matching structure to form a dual-resonant coupling dynamic tunable unit. Interunit coupling is found to further improve the on–off ratio. In this way, high-speed dynamic modulation is performed for the transmission traveling-wave amplitude with low insertion loss and a high on–off ratio. On this basis, a 0.14-THz on–off keying (OOK) high-speed traveling-wave modulator is proposed, and our results showed that a on–off ratio of 25.4 dB could be achieved at the zero-phase-difference point by only using two dynamic structural units. Meanwhile, the control voltage could be adjusted to achieve high-speed modulation with a phase difference of only 3.66 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^\circ$</tex-math> </inline-formula> during the entire control voltage transition, a data rate of 24 Gb/s, and 33.99% sidebands occupation.

Focus control of a concave ultrasonic gel lens

Optical image stabilization is a powerful tool for camera devices. This paper investigates a tunable lens using ultrasound and a transparent viscoelastic gel in which the focal point can be controlled in the radial direction. The lens has a simple structure and consists of four piezoelectric ultrasonic transducers, a glass disc, and a transparent silicone gel film. A four-phase drive of the lens at the resonant frequency generates the flexural traveling-wave mode, resulting in changes of the surface profile of the gel by acoustic radiation force. At a resonant frequency of 60.8 kHz, a concave shape was observed on the gel surface, indicating that a concave lens can be fabricated by controlling the driving frequency and the phase difference between the transducers. Optical characteristics of the lens were evaluated by ray tracing. The deformation displacement of the lens surface and the change in focal length increased with the voltage amplitude. The voltage ratio between the four transducers was changed, allowing the center position of the gel deformation and its focal position to be shifted in the radial direction, thus realizing image stabilization functions.

Flutter analysis of compressor blades under travelling wave modes using an efficient fluid–structure interaction method

Flutter is a self-sustained vibration which could create serious damage to compressor blades. Improving the efficiency and accuracy of Fluid-Structure Interaction (FSI) method is crucial to flutter analysis. An efficient FSI method which combines a fast mesh deformation technology and Double-Passage Shape Correction (DPSC) method is proposed to predict blades flutter under traveling wave modes. Firstly, regarding the fluid domain as a pseudo elastic solid, the flow mesh deformation and blade vibration response can be quickly obtained by solving the governing equations of the holistic system composed of blade and pseudo elastic solid. Then, by storing and updating the Fourier coefficients on the circumferential boundary, the phase-lagged boundary condition is introduced into the computational domain. Finally, the aerodynamic stability for the blades of an axial compressor under various Inter-Blade Phase Angle (IBPA) is analyzed. The results show that the proposed method can effectively predict the characteristics of aerodynamic damping, aerodynamic force and blade displacement. And a conceptual model is proposed to describe the motion behavior of the shock wave. Compared with the multi-passage method, the proposed method obtains almost the same unstable IBPA interval and the blade displacement error is less than 3.4%. But the calculation time is significantly shortened especially in small IBPA cases.

Broadband multi-source excited meander embedding sector antenna for series superconducting Josephson junction THz mixer

Series Josephson junction (JJ) structure has been presented to enhance the impedance of a superconducting terahertz mixer to match the coupled antenna. And the impedance matching between a single antenna and low-impedance JJs embedded in the antenna has been a research hotspot recently. However, the reported antennas all work in resonant modes, resulting in a strict requirement for the positions and the number of the JJs in series. In this paper, we propose a meander embedding sector antenna operated in a traveling-wave mode for series JJs. Multi-source excitation in an antenna is applied to the numerical simulation. The experiment shows that up to seven JJs in series have been successfully inserted into the antenna, and the antenna still maintains good performance. Furthermore, up to 48 % of the 10-dB relative bandwidth of this antenna is achieved, which has a great advantage for wideband superconducting series JJ devices.

Insulating traveling-wave electrophoresis.

Traveling-wave electrophoresis (TWE) is a method for transporting charged colloidal particles used in many microfluidic techniques for particle manipulation and fractionation. This method exploits the traveling-wave components of the electric field generated by an array of electrodes subjected to ac voltages with a phase delay between neighboring electrodes. In this article, we propose an alternative way of generating traveling-wave electric fields in microchannels. We apply a rotating electric field around a cylindrical insulating micropillar and the resulting traveling-wave modes induce particle drift around the cylinder. We term this phenomenon insulating traveling-wave electrophoresis (i-TWE) to distinguish it from standard TWE performed with arrays of microelectrodes. We characterized the particle drift experimentally and show a quantitative comparison of the particle velocity with theoretical predictions. Excellent agreement is found when the influence of electro-osmosis on the channel walls is also considered.