Дифракционная селекция мод в гетеролазерах с планарными брэгговскими структурами

Within the coupled waves method complemented by quasi-optical approach, we investigate a possibility of diffraction mode selection with respect to the transverse mode index in lasers with planar Bragg resonators. Allowable values of the Fresnel parameter are determined which provide for establishment of stationary single-mode oscillation regime with narrow angle spectrum due to diffraction losses from the edges of the Bragg structure. We show that at larger values of the Fresnel parameter, oscillation stability can be improved by shifting the operating transition frequency to the eigenfrequency of the highest-Q longitudinal mode of the Bragg resonator.

About the Oscillatory Flow Phenomenon within 3D Cylindrical Annulus: Critical Buoyancy and Annulus’ Aspect Ratio for Oscillation Stability

The main purpose of our investigation is to providethe impact of some pertinent parameters,asthe thermal buoyancy and the geometryratio,on the oscillatory flow’stability of a Newtonian fluid which occurs within an annulus, found between a cold outer circular cylinder and a hot inner one, to come out at the end with critical conditions that couldpredict this phenomenon intosuch an industrial geometry. To do so, aphysical model is developed using the Lattice-Boltzmann approachside by side with the finite difference one. The validity of the latter is ascertained after comparison between our primary predictionsand various experimental & theoretical ones. By usinganunsteady-state regime, both Isotherms and velocity profiles of ourconvective fluidare widely inspected. Going far with its value, the use of a critical aspect ratio could die-out the impact of any investigated parameter on the oscillatoryflow. Then, only a conductive regime will take control intothe annulus. It is tonote that athree dimensions D3Q19model was adopted based on a cubical Lattice.

Online Evaluation Method for Low Frequency Oscillation Stability in a Power System Based on Improved XGboost

Low frequency oscillation in an interconnected power system is becoming an increasingly serious problem. It is of great practical significance to make online evaluation of actual power grid’s stability. To evaluate the stability of the power system quickly and accurately, a low frequency oscillation stability evaluation method based on an improved XGboost algorithm and power system random response data is proposed in this paper. Firstly, the original input feature set describing the dynamic characteristics of the power system is established by analyzing the substance of low frequency oscillation. Taking the random response data of power system including the disturbance end time feature and the dynamic feature of power system as the input sample set, the wavelet threshold is applied to improve its effectiveness. Secondly, using the eigenvalue analysis method, different damping ratios are selected as threshold values to judge the stability of the system low-frequency oscillation. Then, the supervised training with improved XGboost algorithm is performed on the characteristics of stability. On this basis, the training model is obtained and applied to online low frequency oscillation stability evaluation of a power system. Finally, the simulation results of the eight-machine 36-node test system and Hebei southern power grid show that the proposed low frequency oscillation online evaluation method has the features of high evaluation accuracy, fast evaluation speed, low error rate of unstable sample evaluation, and strong anti-noise ability.