Mode Competition Research Articles

The Nonlinear Design and Experiment of a 0.68-THz Second Harmonic Gyrotron

One of biggest obstacles for the development of terahertz (THz) science and technology is the lack of the THz sources, especially, when the requirement of the power reaches to kilowatts. Among all the existing sources, the gyrotron is the most promising one which can generate high-power THz radiation. Recently, a THz detection system is under development in the THz Research Center, University of Electronic Science and Technology of China, Chengdu, China, which is applied for detecting radioactive material. Thus, a 0.68-THz second harmonic gyrotron has been simulated and tested in the center. In this article, we present the theoretical study on the designed gyrotron based on the linear and nonlinear theories. The influences of the different electron beam parameters on the beam–wave interaction are analyzed. The mode competition based on the time-dependent multimode nonlinear theory has also been investigated in the designed gyrotron; it is shown that the operating mode can dominate the mode competition when the gyrotron is working at the optimized operating point. The experiment results show that the output power can reach 3.25 kW with the efficiency 9.26%, at frequency 0.679 THz, when the beam voltage is 39 kV, the beam current is 0.9 A, and the magnetic field is 12.84 T.

Side-mode suppression in ultraviolet quasi-semicircle microlaser cavity

We report a quasi-semicircle GaN ultraviolet microcavity laser. Poincaré map of the quasi-semicircle cavity shows periodic, quasi-periodic and chaotic optical trajectories. Finite-element-method optical near-field distribution patterns of the periodic optical modes agree with the optical loops in the quasi-semicircle cavity. Based on the numerical results, optically pumped lasing experiment shows that ultraviolet multimode lasing action from different optical resonance modes can be effectively stimulated at low pumping intensity in the quasi-semicircle GaN cavity, and the side-mode suppression phenomenon can be obtained at high pumping intensity. Because the possible period optical loops are quasi-periodic and unstable, the quasi-whispering-gallery mode lasing with higher quality factor prefer to dominate the lasing action under high pumping. The experimental and simulated lasing modes suggest that side-mode suppression phenomenon can be caused by the mode competition between quasi-whispering-gallery modes and other quasi-periodic modes.

Flow-induced vibrations of an equilateral triangular prism at various angles of attack

Flow-induced vibrations (FIV) of an equilateral triangular prism are numerically studied using the immersed boundary method in a parametric space of α=0°–60° and U∗=U∕fnD=1–20, with α=0°(60°) representing the configuration of one vertex facing upstream (downstream). The Reynolds number based on the effective edge length perpendicular to the incoming flow is Re = 200 and the mass ratio is m∗=4m∕3ρD2=2.0. In above, U is the incoming flow velocity, fn is the natural frequency of the prism, D is the edge length of the prism, m is the prism mass and ρ is the fluid density. Three different vibration regimes, i.e., VIV at α=0°–25°, combined VIV and galloping at α=30°–40°, and galloping at α=45°–60°, are confirmed. In the VIV regime, the vibration amplitude is relatively small and the vibration frequency linearly increases with U∗, signifying the non-locked response. In the combined VIV and galloping regime, the amplitude is determined by the competition of the VIV and galloping modes, with large amplitude appearing when the galloping mode dominates. In the galloping regime, the vibration amplitude monotonically increases with U∗, with a lower-than-unity vibration frequency. Moreover, seven wake modes are observed in the examined α and U∗ ranges: 2S mode, 2S(P) mode, P+S mode, P+S/2S mode, the competition mode (CM), and the galloping modes (miS+niS and mS+nS). In CM, the VIV and galloping modes compete each other. In the galloping modes, the vortices are arranged in a ‘V’ shape. The effects of the streamwise vibration on the transverse responses is significant only in the combined VIV and galloping regime.

Stable multiwavelength semiconductor amplifier fiber laser based on nonlinear fiber loop mirror

A stable multiwavelength semiconductor amplifier (SOA) fiber laser using a nonlinear fiber loop mirror (NOLM) and a Lyot–Sagnac loop mirror is presented and experimentally demonstrated. The Lyot–Sagnac loop mirror serves as a wavelength selection device, and the wavelength spacing is controlled by the length of polarization-maintained fiber (PMF). The NOLM serves as a power equalizer to achieve power equalization and improve stability. Moreover, the NOLM can also suppress the mode competition effect within homogenous broadening linewidth of SOA. Multiwavelength output with power equality has been achieved at room temperature, with no obvious power fluctuation. The number of lasing lines is 31 within the 10-dB bandwidth, and the wavelength spacing is 0.63 nm. In addition, the average optical signal-to-noise ratio is up to 25.15 dB. Finally, narrower wavelength spacing is obtained by increasing the length of the PMF.

Analysis of Coaxial Gyrotron Cavity With a Misaligned Insert

The coaxial cavity can rarefy the mode spectrum and restrain the mode competition, which is beneficial to the mode selection of high-frequency and high-power gyrotron. However, the misaligned insert would result in frequency shift and a decrease in interaction efficiency. Besides, the ohmic loss on the cavity walls would become uneven, which deteriorates the long pulse operation of the gyrotron. In this article, the effects of the insert structural parameters on the characteristics of the cavity, the beam-wave interaction, and the ohmic loss distribution on the cavity walls in a coaxial cavity with a small misalignment of insert are discussed. An optimal design of coaxial resonator is presented to reduce the influence of a slightly misaligned insert on the output characteristics of the resonator.

Training a Multi-Layer Photonic Spiking Neural Network With Modified Supervised Learning Algorithm Based on Photonic STDP

We propose a framework for hardware architecture and learning algorithm co-design of multi-layer photonic spiking neural network (SNN). The vertical-cavity surface-emitting laser with an embedded saturable absorber (VCSEL-SA) which contains two polarization-resolved modes is employed as a spiking neuron. The connection between two identical polarization modes is considered as the excitatory synapse, whereas the connection between two orthogonal polarization modes is regarded as the inhibitory synapse. The physical model of the photonic spiking neuron is derived based on the combination of spin-flip model and Yamada model. The photonic spike timing dependent plasticity (STDP) is applied to design a hardware-friendly biologically plausible supervised learning algorithm for a multi-layer photonic SNN. Thanks to the polarization mode competition effect in the VCSEL-SA, the proposed neuromorphic network is capable of solving the classical XOR problem. The effect of physical parameters of photonic neuron on the training convergence is also considered. We further extend the multi-layer photonic SNN to realize other logic learning tasks. To the best of our knowledge, such a modified supervised learning algorithm dedicated for a multi-layer photonic SNN has not yet been reported, which is interesting for spiking learning of neuromorphic photonics.

Study of the beam–wave interaction of a 220 GHz coaxial-cavity gyrotron with two electron beams

In this paper, the mode selection, ohmic loss quality factor, mode competition, and beam–wave interaction of a 220 GHz coaxial-cavity gyrotron with two electron beams are studied by using the linear and nonlinear theories of a gyrotron. Research shows that a coaxial-cavity gyrotron with two electron beams has more advantages than a coaxial-cavity gyrotron with one electron beam in mode selection, suppressing mode competition and increasing output power, and the eigenvalue and the ohmic loss quality factor of modes have similar monotonicity in the direction of the gyrotron axis. Results find that (1) the current ratio K of the gyrotron has a greater influence on the beam–wave interaction efficiency of the two electron beams, (2) under B = 8.90 T, I = 136 A, α = 1.40, and K = 0.45, the coaxial-cavity gyrotron with two electron beams can operate steadily in the TE51,32− mode, which has an efficiency of 37.67% and an output power of 4.53 MW at f = 220.04 GHz, and (3) the total ohmic loss power of the gyrotron accounts for 1.3% of the output power.

Real-time observation of vortex mode switching in a narrow-linewidth mode-locked fiber laser

Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation. In ultrafast fiber lasers, enormous progress has been made toward controlling the interactions of many longitudinal modes, which results in temporally mode-locked pulses. Recently, optical vortex beams have been extensively investigated due to their quantized orbital angular momentum, spatially donut-like intensity, and spiral phase front. In this paper, we have demonstrated the first to our knowledge observation of optical vortex mode switching and their corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser. The spatial mode switching is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber laser. The vortex mode-switching dynamics have four stages, including quiet-down, relaxation oscillation, quasi mode-locking, and energy recovery prior to the stable mode-locking of another vortex mode. The evolution dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion Fourier transform method. The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses, which plays an essential role in the mode-switching dynamic process. The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser applications.

Dynamic signatures of mode competition in optically injected high-β lasers

In this paper we theoretically investigate the statistical light-emission properties of an optically injected bimodal quantum-dot micropillar laser with high spontaneous emission rates. The nanostructured device is described in terms of a stochastic, semiclassically derived rate equation model. We focus on the stochastic switching dynamics between the two fundamental modes and correlate the results with an in-depth bifurcation analysis of the underlying deterministic dynamics. By analyzing different statistical measures, e.g. average intensity, auto- and cross-correlation functions, as well as dwell-time distributions, we give a road map on how to unravel the different dynamic regimes in the presence of large noise from experimentally accessible quantities.

Experimental Study on a Moderately Overmoded Ka-Band Cherenkov Oscillator Operating With Low Magnetic Field

This article presents the results of the numerical and experimental study of a Ka-band Cherenkov oscillator with the average diameter of the slow wave structure (SWS) D = 2.4λ, where λ is the wavelength. The operating point on TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> is set close to the π-point to enhance the electric field in SWS. Meanwhile, by optimizing the structural parameters, the influence of TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> on the mixed output mode is minimized. Then in the optimized structure with a straight output waveguide, the proportion of TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> in the mixed output mode reaches as high as 97%, achieving the target of mode selection. The experimental researches are carried out with a high-current SINUS881 accelerator. A solenoid magnet provides the guiding magnetic field of 0.9T. In the range of voltages 552-585 kV and currents of 7.3-7.85 kA, the generation of 7-10ns pulse at the frequency 29.93-29.96 GHz and 277-312 MW output power was realized. The radiation mode is investigated by the neon lamp luminescence and the radiation pattern of power density in far-field, which both verified that the output mode is mainly composed of TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> , demonstrating the practicability of the proposed method in solving the mode competition problem.

RF Behavior of a Coaxial Interaction Structure for 0.24-THz, 2-MW Gyrotron

The coaxial interaction structure for a 0.24-THz, 2-MW gyrotron is designed, and its RF behavior is analyzed for the beam current of 60 A and the beam voltage of 90 kV. The searching of a suitable, very high-order TE mode, which satisfies all the technical conditions required to generate 2-MW RF power, is an important and critical step in the development of a coaxial-cavity (COCA) gyrotron. The detailed analysis of mode selection is described in this article. The geometry of the axially corrugated insert is optimized using an in-house code COCA. Based on the optimized geometry of the coaxial insert and the cylindrical cavity, the coupling coefficient and the start oscillation current are computed to analyze the mode competition for the operating mode TE42,28. The particle-in-cell (PIC) algorithm-based simulation tool is used for the beam–wave interaction computation. Very fine meshing is implemented in the simulations, considering the high frequency and fine geometry of the coaxial insert. The electron beam parameters and the cavity magnetic field are optimized first aiming at the RF power ≥2 MW and frequency 240 GHz. The final results of the cavity design confirm 2.2-MW RF power and 239.975-GHz frequency at 9.74-T magnetic field.

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.

Determination of the transition mass ratio for onset of galloping of a square cylinder at the least permissible Reynolds number of 150

The Den Hartog stability criterion tests for galloping of an oscillator. A square cylinder satisfies this and is susceptible to galloping. This criterion being necessary for occurrence of galloping appears insufficient as certain parameters, i.e., angle of incidence, α; mass ratio, m*; damping ratio, ζ; reduced speed, U*; and Reynolds number, Re; also assume key roles in determining if the oscillator motion is vortex-induced vibrations (VIV) or galloping. At Re ≈ 150 and U* ≈ 10 or smaller, a square cylinder does not gallop despite satisfying the Den Hartog criterion. By coupling U* and Re, K. Sourav and S. Sen [“Transition of VIV-only motion of a square cylinder to combined VIV and galloping at low Reynolds numbers,” Ocean Eng. 187, 106208-1–106208-19 (2019)] two-degrees-of-freedom motion over Re ≤ 250, the minimum m* or mtr* as 3.4 below which galloping cannot develop. For transverse-only motion, X. Li et al. [“Mode competition in galloping of a square cylinder at low Reynolds number,” J. Fluid Mech. 867, 516–555 (2019)] considered U* = 40 only and determined mtr*≈4 at the least permissible Re of 150. For α = 0° and ζ = 0, we determine the mtr* numerically at Re = 150. By analyzing the transverse response and oscillation frequency over an extended U* range of 10–60, a novel “VIV-galloping transition map” is generated in the m*–U* plane. From this map, the value of mtr* converges to 3.4. The mtr* decays as U*−2.36. The conditions leading to “VIV forever” of a square cylinder are also identified.

Influences of plasma on a relativistic backward wave oscillator with a resonant reflector and an extracting cavity

Using the particle-in-cell method, the influences of the plasma produced in the resonant reflector and extracting cavity of an X-band relativistic backward oscillator are investigated. For the reflector case, the dominant rule of the microwave emission termination of the TM01 mode is the disturbance of the initial phase relationship between the RF fields of the modulated electron and the synchronous harmonic, which increases the generation frequency, excites a high-order mode competition, and then leads to microwave pulse shortening. For the extracting cavity case, the influence of plasma on the beam–wave interaction process is relatively negligible. A notable reduction in the output power is principally caused by the absorption of plasma electrons, which has been preliminarily validated in experiments.

Third harmonic working based on the Smith–Purcell radiation in a closed structure

Smith–Purcell radiation is well analyzed in an open structure; however, it is seldom studied in a closed structure. In a high order mode resonant system, the harmonics of the spoof surface plasmon in the subwavelength hole array will be transformed into Smith–Purcell radiation and the radiation energy will be embedded in a high order mode of this resonant system. The spoof surface plasmon and the high order mode have the same field distribution in the beam channel by a special design, and then, the electron beam will interact with them simultaneously. Based on the phenomenon and mechanism, an effective method that works on the higher order harmonic is proposed. It is possible to mitigate the frequency limits as well as the start current limits in vacuum electronic devices, and the mode competition issue is also improved because of the frequency-locked characteristic.

Mode evolution mechanism of Pr3+:YLF single longitudinal mode laser

We develop a theoretical model of multimode rate equation that accurately describes mode evolution mechanism of prelase Q-switched Pr3+:YLF single longitudinal mode (SLM). We find that it is important to consider the effect of gain difference when calculating the mode competition time accurately. We identify that there is an optimal parameter for the two step signal of acoustic-optic modulator which determines the stability of SLM output. Then, a visible light direct oscillation Q-switched SLM Pr3+:YLF laser at 639.5 nm is obtained, and the experimental results is agree with the simulation very well.

A study of the electrochemical reactivity of titanium under cathodic polarization by means of combined feedback and redox competition modes of scanning electrochemical microscopy

The effect of cathodic polarization on the electrochemical behavior of the thin titanium dioxide film formed by anodic pretreatment over pure commercial titanium metal for biomaterial application was investigated in situ using scanning electrochemical microscopy (SECM). Quantitative information on the electron transfer rates (keff) at the titanium surface was obtained using the feedback operation of SECM with ferrocene-methanol (FcMeOH) as electrochemical mediator. An increase of keff values with the increase of the negative polarization was detected, a feature that correlates well with the decrease of titanium oxide resistance with increasing cathodic polarization observed using electrochemical impedance spectroscopy (EIS). In addition, SECM operation in the redox competition mode proved that hydrogen was absorbed in the surface oxide film leading to changes in conductivity and electrochemical reactivity.

Strategic corporate social responsibility of high-speed rail in China

This paper investigates the strategic corporate social responsibility (CSR) of a high-speed rail (HSR) firm when competing with an airline firm in a differentiated transportation market and the interactions with government environmental policies in the context of a Cournot–Bertrand comparison. We find that a standard CSR chosen by the government is lower under Cournot than that under Bertrand when both the marginal cost of HSR and product substitutability are low enough, while a voluntary CSR chosen by the HSR operator is always higher under Cournot than that under Bertrand. We also find that the voluntary CSR could be higher or lower than the standard CSR, depending on marginal costs, product substitutability, and competition modes. We then show that when the government imposes an environmental tax together with CSR activities, both the strategic CSR and environmental tax are always lower under Cournot than those under Bertrand. Finally, we show that equilibrium traffic volumes, ticket fares, consumer welfare, and social welfare are independent of competition modes with environmental tax.

Switchable dual-wavelength Q-switched fiber laser based on sputtered indium tin oxide as saturable absorber

This paper demonstrate experimentally the generation of a stable switchable dual-wavelength Q-switched fiber laser. The pulsed fiber lasers is generated from deposited Indium Tin Oxide (ITO) saturable absorber (SA) on fiber ferrule, which is prepared by using DC Magnetron Sputtering. The deposited ITO SA is then being sandwiched in between the two fiber ferrules. A stable switchable dual-wavelength is generated by the aid of two cascaded fiber Bragg gratings (FBGs). To achieve a flexible switched in individual wavelength of 1532 or 1533 nm and simultaneously dual-wavelength fiber laser, the in line polarization controller is utilized to control the mode competition in the fiber laser cavity. The proposed fiber laser is capable of generating output at the pump power of 118.10 mW, centered at wavelength 1532 nm. The proposed design is having a repetition rate of 48.30 kHz and a pulse width of 5.22 µs. Similarly, at 1533 nm, it generates a repetition rate of 25.61 kHz and a pulse width of 7.72 µs. The output is also observed to remain unchanged over time, indicating high stability with the signal to noise ratio of 52.70 dB and 52.00 dB at wavelength 1532 nm and 1533 nm, respectively. The combination between ITO and cascaded FBG can offer a feasible and straightforward candidate for a multiwavelength fiber laser according to these results. Laser radar and terahertz generation may become a few of the potential fields that related to this work.

Theoretical modelling of single-mode lasing in microcavity lasers via optical interference injection.

The effective manipulation of mode oscillation and competition is of fundamental importance for controlling light emission in semiconductor lasers. Here we develop a rate equation model which considers the spatially modulated gain and spontaneous emission, which are inherently governed by the ripple of the vacuum electromagnetic field in a Fabry-Pérot (FP) microcavity. By manipulating the interplay between the spatial oscillation of the vacuum field and external optical injection via dual-beam laser interference, single longitudinal mode operation is observed in a FP-type microcavity with a side mode suppression ratio exceeding 40 dB. An exploration of this extended rate equation model bridges the gap between the classical model of multimode competition in semiconductor lasers and a quantum-optics understanding of radiative processes in microcavities.