Mode Competition Research Articles

Multimode Physics in the Mode Locking of Semiconductor Quantum Dot Lasers

Quantum dot lasers are an attractive option for light sources in silicon photonic integrated circuits. Thanks to the three-dimensional charge carrier confinement in quantum dots, high material gain, low noise and large temperature stability can be achieved. This paper discusses, both theoretically and experimentally, the advantages of silicon-based quantum dot lasers for passive mode-locking applications. Using a frequency domain approach, i.e., with the laser electric field described in terms of a superposition of passive cavity eigenmodes, a precise quantitative description of the conditions for frequency comb and pulse train formation is supported, along with a concise explanation of the progression to mode locking via Adler’s equation. The path to transform-limited performance is discussed and compared to the experimental beat-note spectrum and mode-locked pulse generation. A theory/experiment comparison is also used to extract the experimental group velocity dispersion, which is a key obstacle to transform-limited performance. Finally, the linewidth enhancement contribution to the group velocity dispersion is investigated. For passively mode-locked quantum dot lasers directly grown on silicon, our experimental and theoretical investigations provide a self-consistent accounting of the multimode interactions giving rise to the locking mechanism, gain saturation, mode competition and carrier-induced refractive index.

Analysis of the Spontaneous Emission Limited Linewidth of an Integrated III–V/SiN Laser

AbstractThis article describes a calculation of the spontaneous emission limited linewidth of a semiconductor laser consisting of hybrid or heterogeneously integrated, silicon and III–V intracavity components. Central to the approach are a) description of the multi‐element laser cavity in terms of composite laser/free‐space eigenmodes, b) use of multimode laser theory to treat mode competition and multiwave mixing, and c) incorporation of quantum‐optical contributions to account for spontaneous emission effects. Application of the model is illustrated for the case of linewidth narrowing in an InAs quantum‐dot laser coupled to a high‐ SiN cavity.

Privatisation policy with different oligopolistic competition in the public utilities market

This study constructs an oligopoly model in public utilities sector to explore the optimal privatisation policy and the factors affecting equilibrium outcomes and explores the optimal proportion of state-owned shares. We also offer empirical evidence of China’s public utilities from 1985 to 2019 to prove the applicability of model results. The results show that, depending on product differentiation, cost variance, technical level, nationalisation, partial or full privatisation can be optimal. Improving capital efficiency increases social welfare in Model PP, but not in Model PS. Product differentiation improves social welfare at the expense of profits in SS model. In Model PM, technical improvements boost private enterprise profits but induce a decrement in social welfare. A high proportion of state-owned shares fail to improve social welfare in Model SM. In a word, the value range of parameters and competition modes in public utilities sector affect market players’ welfare distribution, which identifies with the empirical analysis of China’s public utilities development.

Compound Cavity Passively Q-Switched Single-Longitudinal-Mode Diode-Pumped Laser

A compact solid-state single-longitudinal-mode (SLM) Q-switched laser based on a compound cavity is demonstrated. SLM operation is achieved through the enhancement of mode competition (via manipulation of the effective reflectivity of the resonator mirrors and intracavity field accumulation time) and does not require the use of a feedback loop. In this work, SLM emission with a linewidth of 254.3 MHz is demonstrated, and a high SLM ratio of up to 99.6% is achieved. The laser operates at a repetition rate of 10 Hz, producing 10 ns pulses with a pulse energy of 14.81 mJ.

Realistic Design Studies on a 300-GHz, 1-MW, DEMO-Class Conventional-Cavity Gyrotron

This article presents the realistic initial design studies of a 300-GHz, 1-MW, conventional-cavity gyrotron for its probable application in the next-generation thermonuclear fusion reactors. Keeping the design goals, parameters, and constraints in view, the very high-order TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">49,18</sub> mode is chosen as the operating mode after a careful mode-selection calculation considering realistic ohmic cavity losses. After mode selection and mode competition studies, the cold-cavity design and initial design of a triode-type magnetron injection gun (T-MIG) and a gyrotron magnet are carried out and an electron beam radius of 8.11 mm is obtained with 2.4% velocity spread. Furthermore, investigation on RF behavior of the cavity is performed with the T-MIG beam parameters. By varying the nominal beam parameters, single-mode self-consistent calculations are conducted and achieved the desired output power. Then, multimode time-dependent self-consistent calculations are carried out before and after space-charge neutralization (SCN) with realistic velocity spread (up to 6%) and different beam radii for the assessment of the start-up scenario. Before SCN without velocity spread, the beam voltage is depressed to 70.08 kV and 0.72-MW output power is obtained, whereas with velocity spread (6%), 0.69-MW output power is obtained with 8.11 mm of beam radius. After 60% of SCN in the start-up scenario with velocity spread (6%), the beam voltage increases to 74.83 kV, and thereby, an output power of 0.91 MW is obtained.

Particle-in-cell model of a relativistic magnetron system driven by a pulse forming network Marx generator

A three dimensional particle-in-cell (PIC) model of a relativistic magnetron (RM) is established using CST Studio Suite. As we adopted a novel technique to imitate pulse forming action of pulser, simulation domain is now fully extended to include a pulser, a transmission line, and a magnetron tube. This feature enables the model to describe a dynamic power coupling process between the pulser and RM in a self-consistent manner. It is demonstrated that model accuracy is strongly dependent on user-defined parameters for an explosive electron emission model. Particularly, emission rise time for space charge formation influences significantly on an initial mode competition phase. Compared to a previous excitation method using a step voltage pulse, the new model gives more reliable results as the intrinsic impedance of the pulser is now considered. Our model is able to estimate the major RM characteristics over a wide range of an external magnetic field. As a result of dynamic power coupling, the RM impedance, the gap voltage, the excitation frequency, and the output power tend to increase at a higher external magnetic field condition. It is suitable to adopt a proposed modeling technique for the virtual assemble and simulation of the system. Hence, it will suggest a practical way to implement and verify complicated structures or innovative designs that could not be solved by existing PIC codes.

Lactose Biosensor Development and Deployment in Dairy Product Analysis

Herein, development of an enzymatic biosensor for rapid quantitation of the disaccharide lactose in dairy samples (whey permeates and milk protein isolates (MPI)) is presented. Biosensor fabrication involved a chitosan/enzyme/crosslinker configuration with enzymes glucose oxidase and β-galactosidase at platinum and glassy carbon electrodes. Solution phase mediation was used to lower the operating potential (Eapp = 0.3 V vs Ag/AgCl), minimising any contribution from endogenous background electroactive species. Potential sweep and potentiostatic experiments realised analytical data for the lactose sensor with linear range 5.83 × 10−3 to 1.65 × 10−2 M, sensitivity 9.41 × 10−4 C cm−2 mM−1 and LOD of 1.38 mM. Scanning electrochemical microscopy realised surface characterisation of the enzyme layers with approach curves and redox competition mode imaging achieved over the active enzyme bilayer. Both glucose and lactose sensing was realised in whey permeate sample measuring 23.7 mM lactose, correcting for free glucose contributing signals, with 92.2% correlation with results obtained from the sample certificate of analysis. Solution phase mediation at glassy carbon enzyme electrodes resulted in lactose quantitation in milk protein isolates, measuring 1.16 mM for the MPI 1 (low lactose) and 1.54 mM for the MPI 10 (standard) sample with validation by HPLC analysis.

Optimal trade policy and welfare in a differentiated duopoly

This study considers a differentiated duopoly, including domestic and foreign enterprises, in trade, analyzes the impacts of product differentiation and productivity variance on equilibrium results, and explores the optimal trade policy in different competition modes. We find that differentiated products can boost the supply of foreign enterprises in a Cournot competition. In a home‐leading Stackelberg duopoly, increasing tariffs decreases consumer surplus but improves the home country's social welfare. The optimum‐welfare tariff of a home‐leading Stackelberg duopoly cannot exceed that of the foreign‐leading Stackelberg duopoly. An easy or tight tariff policy can be optimal, depending on the parameters and duopoly modes.

Two Polarization Comb Dynamics in VCSELs Subject to Optical Injection

Optical frequency comb technologies have received intense attention due to their numerous promising applications ranging from optical communications to optical comb spectroscopy. In this study, we experimentally demonstrate a new approach of broadband comb generation based on the polarization mode competition in single-mode VCSELs. More specifically, we analyze nonlinear dynamics and polarization properties in VCSELs when subject of optical injection from a frequency comb. When varying injection parameters (injection strength and detuning frequency) and comb properties (comb spacing), we unveil several bifurcation sequences enabling the excitation of free-running depressed polarization mode. Interestingly, for some injection parameters, the polarization mode competition induces a single or a two polarization comb with controllable properties (repetition rate and power per line). We also show that the performance of the two polarization combs depends crucially on the injection current and on the injected comb spacing. We explain our experimental findings by utilizing the spin-flip VCSEL model (SFM) supplemented with terms for parallel optical injection of frequency comb. We provide a comparison between parallel and orthogonal optical injection in the VCSEL when varying injection parameters and SFM parameters. We show that orthogonal comb dynamics can be observed in a wide range of parameters, as for example dichroism linear dichroism (γa=−0.1 ns−1 to γa=−0.8 ns−1), injection current (μ=2.29 to μ=5.29) and spin-flip relaxation rate (γs=50 ns−1 to γs=2300 ns−1).

Conversion Efficiency in Kerr-Microresonator Optical Parametric Oscillators: From Three Modes to Many Modes.

Optical parametric oscillation in a Kerr nonlinear microresonator can generate coherent laser light with frequencies that are widely separated from the pump frequency, allowing, for example, visible light to be generated using a near-infrared pump. To be practically useful, the pump-to-signal conversion efficiency must be far higher than what has been demonstrated in microresonator-based oscillators with widely-separated output frequencies. To address this challenge, here we theoretically and numerically study parametric oscillations in Kerr nonlinear microresonators, revealing an intricate solution space that arises from an interplay of nonlinear processes. As a start, we use a three-mode approximation to derive an efficiency-maximizing relation between pump power and frequency mismatch. However, realistic devices, such as integrated microring resonators, support far more than three modes. Hence, a more accurate model that includes the entire modal landscape is necessary to determine potential inefficiencies arising from unwanted competing nonlinear processes. To this end, we numerically simulate the Lugiato-Lefever Equation that accounts for the full spectrum of nonlinearly-coupled resonator modes. We observe and characterize two nonlinear phenomena linked to parametric oscillations in multi-mode resonators: Mode competition and cross phase modulation-induced modulation instability. Both processes may impact conversion efficiency. Finally, we show how to increase the conversion efficiency to ≈ 25 % by tuning the microresonator loss rates. Our analysis will guide microresonator designs that aim for high conversion efficiency and output power.

Localized Electrosynthesis and Subsequent Electrochemical Mapping of Catalytically Active Metal–Organic Frameworks

AbstractIn recent years, metal–organic frameworks (MOFs) have shown great potential to be used as porous, high surface area catalytic materials capable of driving electrochemical energy conversion reactions. However, further improvement in their electrocatalytic performance necessitates methods to couple high‐throughput MOF synthesis and their subsequent electrochemical activity characterization. In this work, scanning electrochemical microscopy (SECM) is employed to perform a localized, micron‐scale electrosynthesis of two types of MOFs, Al2(OH)2‐TCPP, and HKUST‐1. SECM is also utilized to analyze the electrocatalytic hydrogen evolution reaction activity of the as‐prepared MOF micropatterns, via i) substrate‐generation tip‐collection mode to map the MOF's electrochemical reactivity, and ii) redox competition mode, to accurately extract the MOF's catalytic onset potential. Thus, the presented method provides a means to shed light on the operation principles of electroactive MOFs, toward their future incorporation in alternative fuel‐production schemes.

A Multi-Wavelength Thulium-Doped Fiber Laser Using a Photonic Crystal Fiber-Based Sagnac Loop

A multi-wavelength thulium-doped fiber laser incorporating a photonic crystal fiber (PCF) Sagnac loop was presented and experimentally demonstrated. In the laser cavity, the Sagnac loop mirror used a 0.5 m PCF as a comb filter. A nonlinear polarization rotation (NPR)-based 600 m single-mode fiber (SMF) was used to suppress the mode competition. By adjusting the polarization controllers (PCs), up to nine stable wavelength outputs near the 2 μm spectrum were obtained at room temperature with an optical signal-to-noise ratio (OSNR) of up to 30 dB. The channel spacing of this fiber laser is 4.88 nm, and the 10 dB bandwidth encompasses 1950.39 to 1989.43 nm.

Mode Excitation in Gyrotrons With Triode-Type Electron Guns

Gyrotrons operating in high-order modes often suffer from severe mode competition. For the selective excitation of the desired mode, triode-type electron guns are often used. In the case of triode-type guns, where the beam voltage and the mod-anode voltage can be varied independently, the mode excitation during the startup process is studied in this article by using a generalized approach. The term “generalized approach” means that the obtained results can be valid for gyrotrons operating at arbitrary voltages and in any mode. The conditions for excitation of the modes are analyzed for different types of startup scenarios. The goal of the study is to find such relations between the beam and mod-anode voltages during the gyrotron startup that only the desired mode will be excited. It is shown that by using a specific triode-type startup scenario with a proper timing for the rise of the mod-anode voltage with respect to the beam voltage, the initial excitation of the desired mode can be realized in the cases of practically any mode density. The mode interaction in such a case is studied; also, the dependences of the results on the timing relation between the two voltages and on the voltage rise speed are considered. This article also contains the numerical analysis of a specific megawatt (MW) gyrotron with a triode-type gun.

Cournot–Bertrand comparisons under R&amp;D competition: Output versus R&amp;D subsidies

AbstractWe compare Cournot and Bertrand competitions with R&amp;D investment under output versus R&amp;D subsidy policies. We demonstrate that Cournot firms invest more (less) in R&amp;D and the government grants more (less) subsidies than for Bertrand firms with output (R&amp;D) subsidies. We also find that both competition modes yield the same welfare with output subsidy, while Bertrand yields higher welfare than Cournot with R&amp;D subsidy. Finally, firms' profits and welfare in Cournot are higher under output subsidies, while they can be higher in Bertrand under R&amp;D subsidies if the product substitutability is high and the firm's R&amp;D investment is efficient.

Research and application of power market trading index system for power generation enterprises

With the continuous opening of China’s electricity market, power generation enterprises have shifted from planned operation mode to market competition mode. Guided by the idea of index, the power market trading index system of power generation enterprises is established by comprehensively considering the four aspects of electricity trading price, market activity, settlement timeliness and market benefit. This paper selects A company as an example to demonstrate the scientificity and practicability of the index system. Finally, according to the result of the case analysis, some suggestions are given to improve the market competitiveness of power generation enterprises.

Emission tax and strategic environmental corporate social responsibility in a Cournot–Bertrand comparison

This study considers the strategic relations between emission tax and environmental corporate social responsibility (ECSR) in a Cournot–Bertrand comparison and analyzes two different timings of the games between a tax-then-ECSR (T game) and an ECSR-then-tax (E game). We show that the T game always yields higher emission tax than the E game irrespective of competition modes, but lower ECSR under Cournot competition and higher ECSR when the marginal damage is high under Bertrand competition. We also show that compared with Bertrand competition, Cournot competition yields lower (higher) ECSR in the T (E) game but lower emission tax in the E game and higher emission tax when the product substitutability is low in the T game. We finally show that firms endogenously choose Cournot competition with the commitment of the E game irrespective of marginal damage and product substitutability.

The subcritical transition to turbulence of Faraday waves in miscible fluids

We study the development and the breaking process of standing waves at the interface between two miscible fluids of small density contrast. In our experiment, a subharmonic wave is generated by a time-periodic vertical acceleration via the Faraday instability. It is shown that its wavelength may be selected not only by the linear process predicted by the Floquet theory and favouring the most unstable modes allowed by the tank geometry, but also by a nonlinear mode competition mechanism giving the preference to subcritical modes. Subsequently, as the standing wave amplitude grows, a secondary destabilization process occurs at smaller scales and produces turbulent mixing at the nodes. We explain this phenomenon as a subcritical parametric resonance instability. Different approaches derived from local and global stability analysis are proposed to predict the critical wave steepness. These theories are then assessed against various numerical and experimental data varying the frequencies and amplitudes of the forcing acceleration.

Research on the Integration of Subject Competition and Professional Course Teaching Mode Based on OBE Concept

Based on the demand of training applied innovative talents in mechanical specialty and OBE educational idea, this paper discusses the effect of the integration of discipline competition and professional course teaching mode on improving students' innovative practical ability. At the same time, this paper also puts forward the reform of professional teaching mode based on subject competition, which provides more practical measures for students to transform subject competition and professional knowledge into ability.

Influence of reflections on mode-competition processes in a high-power multimode gyrotron

Reflection of radiation from an output window may strongly affect operation of a gyrotron. In recent years, the control of gyrotron radiation by reflection has become a subject of active study. In this paper, we study the effect of reflection on mode competition in a multimode gyrotron. We consider the interaction of five modes with neighboring azimuthal indexes in a gyrotron with a fixed Gaussian axial field profile. We assume the mode spectrum to be nearly equidistant and set the spectral density parameter value as in high-power gyrotrons used for plasma heating. We analyze the domain of stability of the operating mode on the cyclotron frequency mismatch–normalized beam current parameter plane with and without the reflections. The possibility of suppressing spurious sideband modes and expanding the single-mode oscillation zone of the operating mode by injecting the reflected signal is demonstrated by numerical simulation. The most advantageous situation is when the designed mode is matched with the output window while the reflections of the parasitic modes are rather strong and the reflected waves return into the gyrotron resonator in the adverse phase reducing the Q-factors of the parasitic modes.

A mode competition and jump of HeNe laser with monoistopic Ne

A mode competition and jump of HeNe laser with monoistopic Ne