Two-dimensional Bragg Structures Research Articles

Bandgap Control of Two-Dimensional Coaxial Bragg Structures With Helically Corrugated Conductors

The untunable bandgap is a difficult problem for Bragg structures constructed from corrugated metallic waveguides, and becomes a major barrier for applications. Based on the coupled-mode theory, this paper presents the principle as well as a corresponding theoretical model of bandgap control of two-dimensional coaxial Bragg structures under the condition of a fixed ripple shape. It is shown that such control can be achieved by varying the angular deviation between the outer and inner corrugations, which can be done by rotating one of the conductors relative to the other. The effects of the proposed method on transmission bandgap control of two example structures were investigated by theoretical analysis and electromagnetic simulation. The results confirmed the validity of the proposal, and showed that such a control method can be used to manipulate the number and location of the working bands under multiple bandgap operation. These peculiarities provide potential applications of two-dimensional coaxial Bragg structures in constructing tunable passive and active high-power microwave devices.

Theoretical and experimental studies of highly selective planar two-dimensional Bragg structures based on dielectric waveguides in the terahertz frequency range

Based on theoretical approach and three-dimensional modeling using the CST Microwave Studio code, planar dielectric two-dimensional Bragg structures in terahertz frequency range were developed and manufactured. Proof-of-principle electrodynamic experiments on the “cold” testing of these structures were carried out. It is shown that the experimental results are in good agreement with the theoretical predicts, including the existence of the highest Q mode inside the Bragg reflection band in the absence of periodicity defects.

Coupled-mode analysis for two-dimensional coaxial Bragg structures with helical ripples

Making use of field expansion and the equivalent boundary, an analytical model is set up to describe the multi-mode intercoupling in two-dimensional coaxial Bragg structures with either one or both of the conductors corrugated with helical ripples. The coupled-mode equations and the explicit formulae of the coupling coefficients for all possible mode combinations are derived, and the general mode coupling rules are discussed. Based on the analytical model, the dependence of the coupling coefficients on structural parameters of the two-dimensional coaxial Bragg structures with overmoded operation is investigated by numerical calculation. The results show that the additional conductor provides more flexibility for adjustment of coupling coefficients compared with other types of two-dimensional Bragg structures with a single conductor. Taking advantage of this feature, a two-dimensional coaxial Bragg structure can provide strong reflection with high mode purity under overmoded operation by suppressing the parasitic coupling using an optimized combination of corrugated outer and inner conductors. In this way, the structure has potential applications in the design of high-Q improved Bragg resonators for high-power microwave oscillators operating in the millimeter wave or terahertz range.

Theoretical and experimental studies of dielectric two-dimensional Bragg structures for development of spatially-extended heterolasers

Theoretical and experimental studies of dielectric two-dimensional Bragg structures for development of spatially-extended heterolasers

Generation of high-power Cherenkov superradiance pulses using 2D periodic slow-wave structures based on oversized cylindrical waveguides

We propose to stabilize the azimuthal structure of superradiance pulses using two-dimensional (2D) periodic Bragg structures that simultaneously play the role of a slow-wave system. Analysis performed in the framework of a quasi-optical approach is confirmed by the results of 3D particle-in-cell modeling that demonstrates the possibility of using the proposed system for generating multi-GW nanosecond radiation pulses in the millimeter waveband.

Synchronization of radiation in an oversized coaxial Ka-band backward wave oscillator using two-dimensional Bragg structure

A coaxial Ka-band backward wave oscillator with a two-dimensional Bragg structure located at the output of the interaction space has been studied. This structure has a double-period corrugation and provides azimuthal electromagnetic energy fluxes, which act on the synchronized radiation of an oversized tubular electron beam. Proof-of-principle experiments were conducted based on the Saturn thermionic accelerator ($300\text{ }\text{ }\mathrm{keV}/200\text{ }\text{ }\mathrm{A}/2\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$). In accordance with simulations, narrow-band generation was obtained at a frequency of 30 GHz and a power level of 1.5--2 MW. As a result, the possibility of using a two-dimensional distributed feedback mechanism in oscillators of the Cherenkov type has been demonstrated.

A spatially developed coaxial 30-GHz backward wave oscillator with radiation synchronization by a two-dimensional Bragg structure

A coaxial backward wave oscillator with a two-dimensional Bragg structure mounted at the exit from the interaction space is studied theoretically and experimentally. Azimuthal wave fluxes arising in this structure permit one to synchronize radiation of different fractions of a tubular relativistic electron beam with a large diameter. A narrowband radiation of the 8-mm range with a megawatt level of power has been obtained in the experimental prototype of the backward wave oscillator. The prototype was implemented on the basis of the Saturn accelerator (300 keV/200 A/1 μs), as the perimeter of the system constituted more than 15 wavelengths.

Two-dimensional distributed feedback lasers based on static and dynamic Bragg structures**Reported at the Conference on ‘Laser Optics’, St. Petersburg, Russia, 2011.

In order to increase the output power of DFB lasers, we consider the possibility of using two-dimensional distributed feedback. Within the framework of this scheme, the feedback circuitincludes four partial wave fluxes propagating in mutually orthogonal directions, which makes it possible to provide coherent radiation from a spatially extended planar active medium characterised by large values of the Fresnel parameter. By analogy with the onedimensional distributed feedback, the wave coupling can be ensured by using both the structures with a periodically varying effectiverefractive index (static two-dimensional Bragg structures) and the gain modulation (photo-induced two-dimensional Bragg structures). Within the semiclassical approximation, the initial conditions andnonlinear dynamics of lasers with the above-described two-dimensional Bragg structures are analysed. Self-similarity conditions are found, allowing one to scale the laser parameters with increasing active region size, which is accompanied by an increase in the integratedoutput power.

High-intensity super-radiance pulses generated by an active medium placed in a two-dimensional Bragg structure

The possibility is shown of using two-dimensional Bragg structures for generating short super-radiance (SR) pulses of high intensity. The structures provide spatial synchronisation of the radiation in two orthogonal directions in the active medium with large Fresnel parameters. In contrast to the SR effects without Bragg structure, an efficient generation of SR arises in an active medium with the dimensions not exceeding the inverse increment with the intensity by several orders of magnitude greater than the SR intensity of the same medium in a free space.

Two-dimensional distributed feedback lasers with excitation of TE waves in the active medium

Two-dimensional Bragg resonators with the coupling of TE- and TM-polarised waves are proposed. The selective properties of such resonators are analysed. Within the semiclassical approach, the nonlinear dynamics of laser radiation with a two-dimensional (in the xy plane) distributed feedback is studied, at which TE-polarised waves, propagating in the ±z directions, are amplified in the active medium (in particular, based on quantum wells). The latter, on a two-dimensional Bragg structure, are scattered into TM-polarised waves propagating in the ±x directions. These partial wave flows do not interact with the active medium but provide the spatial radiation synchronisation. The conditions of the solution self-similarity are obtained with increasing the dimensions of the active medium and the corresponding increase in the integral output power. It is shown that when an additional end mirror is mounted, almost unidirectional radiation coupling can be realised.

Using two-dimensional Bragg structures for the synchronization of radiation in planar backward wave oscillators

We propose to use two-dimensional Bragg resonance structures for the spatial synchronization of emission in planar backward wave oscillators (BWOs) pumped by wide sheet electron beams. It is demonstrated that the synchronization regime is stable with respect to changes in the detuning between the free-running BWO generation frequency and the Bragg frequency.

Generation of spatially coherent radiation in free-electron masers with two-dimensional distributed feedback

The results of theoretical and experimental studies on the generation of spatially coherent electromagnetic radiation in a planar free-electron maser with two-dimensional distributed feedback are reported. A two-dimensional Bragg structure is used at the initial part of the interaction space to ensure the transverse synchronization of the radiation. The possibility of the narrowband generation in the 75-GHz frequency band is demonstrated experimentally for a sheet kiloampere electron beam whose width is 20 times larger than the wavelength.

Observation of the high-Q modes inside the resonance zone of two-dimensional Bragg structures

Electrodynamical properties of two-dimensional Bragg resonators of planar geometry are studied in the frame of the geometrical-optical approach and in three-dimensional simulations. Results of the theoretical analysis are corroborated by data obtained in “cold” microwave measurements. Formation of the high-Q eigenmodes in the vicinity of the Bragg resonance frequency has been demonstrated.

Experimental observation of high-Q modes at the center of a resonance band of two-dimensional Bragg structures

The electrodynamic characteristics of two-dimensional Bragg resonance structures of planar geometry have been studied. The measured frequency dependences of the coefficients of transmission and transverse scattering agree well with theoretical analysis within the framework of the geometric optics approximation and with the results of full-scale three-dimensional numerical simulations. Experimental data confirm the existence of high-Q modes in the vicinity of the frequency of the exact Bragg resonance.

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 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.

A two-dimensional distributed feedback used for synchronization of a multibeam planar free-electron maser system

We suggest to use a two-dimensional distributed feedback for synchronizing the radiation of a multibeam generator representing a system of planar free-electron maser (FEM) units, each FEM being power supplied with a ribbon-shaped relativistic electron beam. It is demonstrated that various FEM units can be coupled by transverse electromagnetic energy fluxes arising in two-dimensional Bragg structures.

Possible use of two-dimensional Bragg structures in an FEL amplifier powered by a sheet electron beam

It is shown that two-dimensional doubly-periodic Bragg structures can be used effectively in the amplification schemes of free electron lasers powered by sheet relativistic electron beams. The presence of such a structure ensures spatial coherence of radiation from different parts of the electron beam when the input signal propagates across the electron flux. Theoretical analysis shows that the gain in the regenerative scheme can reach 50 dB.