2D Bragg Structures Research Articles

Двумерные брэгговские резонаторы на основе планарных диэлектрических волноводов (от теории к модельному тестированию)

Using analytical approaches and CST Microwave Studio 3D simulations, we have undertaken a theoretical analysis of electrodynamical characteristics of 2D Bragg structures based on planar oversized dielectric waveguides with double-periodical corrugation. Such structures are of significant interest for obtaining directed narrow-band radiation in heterolasers with large dimensions of an active area. Modeling electrodynamical experiments on 'cold' testing of such structures in millimeter wavelength band were conducted. Fine agreement between the experimental results and the simulation results was demonstrated which includes the presence of the highest-quality mode inside the Bragg reflection band in absence of the periodicity defects.

Synchronization of semiconductor laser arrays with 2D Bragg structures

A model of a planar semiconductor multi-channel laser is developed. In this model two-dimensional (2D) Bragg mirror structures are used for synchronizing radiation of multiple laser channels. Coupling of longitudinal and transverse waves can be mentioned as the distinguishing feature of these structures. Synchronization of 20 laser channels is demonstrated with a semi-classical approach based on Maxwell-Bloch equations.

Generation of a spatially coherent field structure in free-electron masers with 2D distributed feedback

Nonlinear dynamics of free-electron masers (FEMs) is studied in the planar geometry with 2D distributed feedback (DFB). As is distinct from previous works, the field structure is not fixed with respect to the three spatial coordinates including the coordinate that is orthogonal to surfaces of the plates of the 2D Bragg resonator. Conditions on the allowed oversize parameter (ratio of the gap between the resonator plates to wavelength) under which the steady-state generation remains stable upon variation in electron-beam parameters are derived. It is demonstrated that, at a relatively large gap, variations in the mismatch lead to the jumps of oscillation frequency that correspond to the excitation of bunches of modes with different transverse indices of partial waves. The results of simulation using a particle-in-cell method are presented for a FEM prototype with 2D DFB that is created using an ELMI accelerator at the Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. The simulated results show that narrow-band spatially coherent radiation can be generated at experimental parameters of the electron beam and electrodynamic system. The advantages of 2D Bragg structures in comparison with conventional 1D structures are demonstrated for FEMs.

Nonlinear theory of coaxial free-electron masers with 2D distributed feedback (quasi-optical approximation)

The nonlinear dynamics of coaxial free-electron masers with 2D distributed feedback, which is realizable in 2D Bragg structures, is analyzed in terms of a quasi-optical approximation. It is shown that feedback with the spatial synchronization of radiations from tubular electron beams with a perimeter exceeding 1000 wavelengths can be provided under such conditions. The objects of investigation are the one-section design of a free-electron maser with 2D distributed feedback and a design with a combined two-mirror resonator. In the latter, an entrance 2D Bragg mirror provides the spatial synchronization of radiation and weak reflections from a conventional exit Bragg mirror are sufficient for the self-excitation of the oscillator. The advantage of the two-mirror design is a decrease in ohmic losses. The adequacy of the geometric optics approximation used earlier to describe the dynamics of such self-excited oscillators is demonstrated under various boundary conditions for transverse (azimuthal) energy fluxes at the edges of a Bragg structure.

Peculiarities of mode spectrum in two-dimensional Bragg structures

The fundamental difference in the eigenmode spectrum for traditional one-dimensional (1D) periodical Bragg structures and novel two-dimensional (2D) periodical Bragg structures of planar and coaxial geometry is discussed. In the case of 1D Bragg structures, the eigenmode frequencies are located outside the Bragg reflection zone while for 2D Bragg structures the highest Q-factor modes are located just near to the frequency of Bragg resonance and the mechanism for the formation of these modes is related to the coupling of the longitudinal and transverse propagating wave fluxes. The main eigenmode possesses a Q-factor which substantially exceeds the Q-factor of the other modes even in the case when the system transverse size is much greater than the wavelength that encourage the use of 2D Bragg structures for mode selection in powerful oscillators with oversized microwave systems.

Experimental and computational studies of novel coaxial 2D Bragg structures for a high-power FEM

Two-dimensional (2D) coaxial Bragg structures have been suggested for use in high-power Free Electron Masers (FEM) to synchronize radiation from different parts of an oversized annular electron beam. In this paper, the simulations of field evolution using the three-dimensional code MAGIC are carried out and results are presented. An investigation of 2D Bragg structures obtained by corrugating the inner surface of the outer conductor of a coaxial waveguide or by lining the surface of a smooth waveguide with a dielectric material, which has a bi-periodic permittivity, has been conducted. Experimental studies of 2D Bragg structures were also undertaken and the good agreement between experimental measurements and theoretical predictions is demonstrated. Measurements of a 7 cm diameter annular electron beam produced by a high-current accelerator to be used to drive the FEM are presented and the experimental set-up discussed.

Theory of free-electron maser with two-dimensional distributed feedback driven by an annular electron beam

The use of two-dimensional (2D) distributed feedback is considered as a method of providing spatially coherent radiation from an oversized annular electron beam. To realize the feedback mechanism, 2D Bragg structures formed from doubly-corrugated waveguide sections of coaxial geometry are suggested. The properties of two types of coaxial cavities formed using such structures are compared: a single-section 2D Bragg cavity and a two-mirror cavity. The eigenmodes of both cavities are found and their high selectivity over both azimuthal and longitudinal indices was demonstrated. Time-domain analyses of the excitation of the cavities by an annular electron beam were carried out. The influence of the cavity parameters on the oscillation regime is analyzed and discussed. It was shown that for a specific set of 2D Bragg cavity parameters it is possible to obtain a regime of steady-state oscillations when the transverse size of the beam exceeds the wavelength by a few orders of magnitude, while outside this parameter space multimode oscillation takes place. The design of a 2D Bragg free-electron maser oscillator based on a high-current accelerator at the University of Strathclyde is discussed.

Experimental studies of two-dimensional coaxial Bragg structures for a high-power free-electron maser

The experimental studies of two-dimensional (2D) coaxial Bragg structures are presented. These structures, which realize 2D distributed feedback, have been recently proposed as a method of producing gigawatt power level spatially coherent radiation from a free-electron maser driven by a large-size relativistic electron beam of annular geometry. The experimentally obtained frequency dependence of transmission coefficients for the 2D Bragg structures are in good agreement with theoretical predictions that demonstrates the operation of the two-dimensional Bragg scattering mechanism.

Experimental and theoretical study of 2D Bragg structures for a coaxial free electron maser

Experimental results are presented of the first observation of two-dimensional distributed feedback in a coaxial 2D-Bragg structure. The use of two-dimensional distributed feedback has been proposed as a method of producing spatially coherent radiation from extremely powerful large sized relativistic electron beams. To obtain coherent high-power (GW) microwave and millimetre wave radiation, 2D Bragg resonators are needed to overcome the problems of mode selection and synchronisation of radiation from different parts of an oversized beam. The design and cold microwave measurements of a 2D Bragg coaxial structure for use in a Free Electron Maser (FEM) driven by an annular electron beam of circumference 25 times larger than the radiation wavelength is presented. The cavity for the 2D Bragg FEM consists of two 2D Bragg coaxial reflectors separated by a regular coaxial waveguide. The formation of pure two-dimensional distributed feedback without a mixture of one-dimensional parasitic feedback was demonstrated for the first time in this series of experiments. The first experimental comparison of 2D and 1D Bragg structures was also conducted and good agreement between experimental results and theoretical predictions was observed. The eigenmodes of the two-mirror cavity were calculated and it was shown that a single-mode steady-state operation regime could be obtained in a FEM based on such a novel cavity.

Progress of the Strathclyde Free Electron Maser experiment using a 2D Bragg structure

The use of two-dimensional (2D) Bragg resonators of coaxial geometry, realising two-dimensional distributed feedback, is considered as a method of providing additional mode selection and producing spatially coherent radiation from oversized annular electron beams. The main goals of the project are to prove operability of a FEM using 2D Bragg structures and compare the performance of the FEM using the novel 2D Bragg resonators with conventional 1D Bragg resonators.