Quasi-optical Waveguide Research Articles

Properties of focused combined modes of terahertz laser

Physical features of spatial-energy characteristics of linearly polarized THz laser beams at sharp and moderate focusing in free space are analyzed both theoretically and experimentally. The Rayleigh–Sommerfeld vector theory is used to model propagation of laser beams excited by quasi-optical waveguide resonator modes in free space. Well-known methods of measuring spatial-energy characteristics of laser beams in the THz range are used in the experimental study. It is shown that the intensity of the total electric field of both combined TE0n + EH2n and EН–1n + EH3n modes (n = 1, 2, 3) in the focal region is defined by all three components and has a dip on the axis for both focusing types. The central maxima of the field of these modes become significantly shifted from the geometric foci of the lenses used as the order n of these modes increases. The EН–11 + EH31 mode has the smallest diameter (2.94λ) of the focal spot in the maximum intensity region at sharp focusing. The TE01 + EH21 mode has the smallest diameter (13.65λ) of the focal spot at moderate focusing.

ПОШИРЕННЯ ТЕРАГЕРЦОВИХ ВИХРОВИХ ЛАЗЕРНИХ ПУЧКІВ У ВІЛЬНОМУ ПРОСТОРІ

Subject and Purpose. Currently, numerous ideas and different methods have been in growth for generating vortex beams — areas of the circular motion of the electromagnetic wave energy flow around the so-called phase singularity points caused by a violation of the wave front topological structure. The purpose of this work is to obtain analytical expressions describing the nonparaxial diffraction of wave modes of the waveguide resonator of a terahertz laser during the wave mode interaction with a spiral phase plate. The resulting vortex beams are examined for their physical features in free space propagation. Methods and Methodology. The Rayleigh-Sommerfeld vector theory is adopted to consider the propagation of vortex laser beams generated by wave modes of the quasi-optical waveguide cavity when interacting with a spiral phase plate in different diffraction zones. Results. For the first time, analytical expressions have been obtained to describe the nonparaxial diffraction of wave modes of the waveguide resonator of a terahertz laser, when resonator modes interact with a spiral phase plate at different topological charges, n. The physical features of the resulting vortex beams were studied in their free space propagation. It has been shown that a spiral phase plate modifies the structure of the linearly polarized EH₁₁ mode so that the original (n=0) intensity profile with the maximum energy at the center turns at n=1 and 2 into a ring-like donut shape with an energy hole in the center. The azimuthally polarized TE₀₁ mode has originally (n=0) a ring-shaped intensity. At n=1, this configuration changes to have the maximum intensity in the center. At n=2, it becomes annular again. In the process, the spherical phase front of the beam of the linearly polarized EH₁₁ mode becomes spiral and have one singularity point on the axis, whereas the phase structure of the azimuthally polarized TE₀₁ mode gains a region with two phase singularity points off the axis. Conclusions. The results of the study can effectively facilitate information transfer in high-speed THz communication systems. They can provide a real platform to perform tasks related to tomography, exploring properties of materials, detecting astrophysical sources, which makes them very promising in modern technologies.

Simulation Study on a Planar Quasi-Optical Waveguide Circuit for a W-Band Gyro-TWT With Stability Improvement

In this article, a planar quasi-optical waveguide (PQOW) circuit is proposed for vacuum electron devices (VEDs) for the first time. The circuit is made of a series of metamaterial pillars attached to a pair of parallel plates for phase compensation. It can propagate quasi-optical modes as a pair of concave mirrors does, while the lower order HE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{0}\textit{n}}$</tex-math> </inline-formula> modes experience much heavier circuit loss compared to that of convention confocal waveguide circuits, which can improve the backward oscillation (BWO) stability of gyro-amplifiers. As an example, a W-band gyro-amplifier with PQOW circuits to replace conventional confocal circuits is designed. The 3-D particle-in-cell simulation demonstrated that the circuit is capable of producing stable high-power radiation within 96–108 GHz, with an electron efficiency of about 15% over the entire bandwidth.

The specifics of quasi-optical measurements in the submillimetre waveband

The paper investigates the influence of the electromagnetic field and standing wave structure in a quasi-optical waveguide on the dielectric measurement results and gives recommendations on the layout of measuring circuits.

ПРОБЛЕМЫ МЕТРОЛОГИИ ТЕРАГЕРЦОВОГО ИЗЛУЧЕНИЯ В МЕДИЦИНЕ

The purpose of this work is to consider the issue of safe propagation of terahertz radiation in bio-logical objects. The article gives a brief review of studies of the influence mechanisms of terahertz radiation on biological environments. It considers optical characteristics of blood and its components in the THz frequency range. It is found that the boundaries of the terahertz wavelength range are not precisely defined. It is established that the recently discovered "photon jet" effect allows terahertz ra-diation to penetrate biological objects to considerable depths, due to the effect of a quasi-optical mesoscale waveguide composed of spherical or disk particles. Amplification of small-sized particles that form a photon jet can significantly increase the value of the radiation intensity density at the bio-logical object of study.

Updated Fast Solution for Quasi-Optical Beam Waveguides With Tilted FFT Beam Propagator

An updated fast solution in modeling quasi-optical beam waveguides is reported, where several mirror reflectors are utilized for the beam transformation. The fast Fourier transform Poynting vector ray tracing between virtual apertures (FFT-PVVAs) algorithm has been introduced and applied in modeling the reflected beam, based on the FFT-accelerated beam propagator between parallel apertures, and the Poynting vector ray tracing for the mirror reflection. However, for designing and optimizing a beam path with multireflectors, it is desired that the accumulated precision loss due to the Poynting vector ray tracing can be further reduced. As a modification, angular spectrum transform-based beam propagator between oblique apertures is introduced, so as to reduce the ray tracing path length. The improved method is named as FFT-PVVA-tilted propagation (FFT-PVVA-TP). As validation, beam propagation in a four-mirror beam waveguide is simulated by FFT-PVVA, FFT-PVVA-TP, and physical optics integration. Numerical results suggest that FFT-PVVA-TP provides clearly better accuracy after four times of beam reflection.

Design of second harmonic terahertz gyrotron cavity based on double confocal waveguide

Quasi-optical confocal cylindrical waveguide possesses a lot of good characteristics, such as big power capacity and low mode density, which can suppress the mode competition in beam-wave interaction. So quasi-optical waveguide has a great advantage in designing high harmonic terahertz gyrotrons. For the reason that part of electron beams located in a region of weak field intensity play a limited role in beam-wave interactions, the beam-wave interaction is not efficient in confocal cavity. Motivated by enhancing the beam-wave interaction efficiency of quasi-optical gyrotron, we propose a novel terahertz harmonic gyrotron cavity with double confocal waveguide in this paper. The transverse field distribution and the mode spectrum in double confocal waveguide are analyzed and presented. A 330 GHz second harmonic gyrotron with double confocal cavity is designed, theoretically analyzed and simulated by using a particle-in-cell (PIC) code. The results obtained for double confocal cavity are compared with the results for single confocal cavity, and the physical mechanism of beam-wave interaction enhancement in double confocal cavity is discussed. Theoretical results show that the double confocal cavity is able to increase the coupling strength of beam-wave interaction, thus, to improve the output power and the interaction efficiency of quasi-optical gyrotron. The PIC simulation results suggest that a high-order waveguide mode in double confocal cavity can steadily interact with the high harmonic cyclotron mode of electron beam without mode competition. Driven by a 40 kV, 2 A electron beam with a guiding center radius of 1.65 mm and velocity ratio equal to 1.5, output power of 9.9 kW at 328.93 GHz can be generated in the designed double confocal cavity. The beam-wave interaction efficiency increases from 5.3% in single confocal cavity to 12.4% in dual confocal cavity under the same operation parameters. The double confocal cavity has great potential applications in terahertz band. Moreover, this study indicates that the eigen mode in double confocal waveguide is a kind of hybrid mode superimposed by two independent single confocal waveguide modes. This mode characteristic will be beneficial to designing a multifrequency gyrotron oscillator operated in two modes and two cyclotron harmonics, simultaneously, with a single electron beam used, which provides a new possibility to develop the novel terahertz radiation source.

Multi-Way Quasi-Optical Waveguide Power Divider with 2D Diffraction Approximation and Experimental Verification at Millimeter Wave

In this paper, multi-way quasi-optical parallel-plate waveguide power dividers/combiners are designed and fabricated using the 2D diffraction approximation. Shape optimization technology is applied to shape the cylindrical reflector surface to reconstruct the diffraction field to improve the magnitude and phase balance of the parallel-plate waveguide power dividers. Both a 1-to-6 way quasi-optical waveguide power divider with H-plane horn antenna array and a 1-to-10 way power divider with gap waveguide transition are analyzed and designed, respectively. We fabricated the two designed power devices at millimeter wave for verifying the validity of the design method. The measured average transmission coefficient of the 1-to-6 way power divider is − 10.8 dB from 81 to 110 GHz, corresponding to 50% power combining efficiency, while the measured back-to-back structure of the 1-to-10 way power divider/combiner features an average transmission coefficient to − 2.83 dB corresponding to 72.2% power combining efficiency over the entire W-band. The proposed power dividers/combiners and the efficient optimization method used in their design are believed to be of importance for future power device applications in millimeter wave and terahertz range.

A 0.4-THz Second Harmonic Gyrotron with Quasi-Optical Confocal Cavity

Mode density is very relevant for harmonic gyrotron cavity. Theoretical investigations suggest that quasi-optical confocal waveguide performs low mode density and good mode-selective character. By selecting the appropriate mode and optimizing the cavity parameters, the quasi-optical confocal cavity is suitable for high-harmonic terahertz gyrotron without mode competition. In order to verify the theoretical analysis, a 0.4-THz second harmonic gyrotron has been designed and experimented. Driven by a 40-kV, 4.75-A electron beam and 7.51-T magnetic field, the gyrotron prototype could generate 6.44 kW of output power at 395.35 GHz, which corresponds to an electron efficiency of 3.4%. There is no mode competition between the second harmonic and fundamental observed in the experiments.

PHYSICAL MODELING OF ELECTROMAGNETIC SCATTERING IN THE TERAHERTZ BAND QUASI-OPTICAL DIRECTIONAL STRUCTURES

The paper is devoted to elaboration of the theoretical and experimental basis, methods and recommendations aimed at the development of a new trend in the experimental radio physics -physical quasi-optical waveguide modeling (QWM) of the processes of electromagnetic scattering in the near millimeter (NMM) and submillimeter (SMM) wavelength ranges. The methods of experimental research of the electromagnetic scattering processes in the quasi-optical (QO) directional structures of a «hollow dielectric waveguide» (HDW) class have been developed and generalized. The basis for development of the radio measurement facilities used for physical electromagnetic modeling (EMM) of the object’s scattering characteristics in the NMM and SMM wavelength bands has been elaborated.

Synthesis of quasioptical waveguide mode converters on the basis of the Fox–Li method

We consider a generalization of the Fox-Li method to the problems of analysis and synthesis of weakly irregular quasioptical waveguides. It is shown that the field representation in the form of integral expansions in terms of convergent and divergent cylindrical waves makes it possible to efficiently solve such problems with allowance for the effects of nonparaxiality of wave beams and the vector character of electromagnetic fields. An example of synthesizing a converter of the output radiation of a gyroklystron with an operating frequency of 30 GHz at the TE 5,3 mode to the Gaussian wave beam is considered.

Demonstration of a 140-GHz 1-kW Confocal Gyro-Traveling-Wave Amplifier

The theory, design, and experimental results of a wideband 140-GHz 1-kW pulsed gyro-traveling-wave amplifier (gyro-TWA) are presented. The gyro-TWA operates in the HE(06) mode of an overmoded quasi-optical waveguide using a gyrating electron beam. The electromagnetic theory, interaction theory, design processes, and experimental procedures are described in detail. At 37.7 kV and a 2.7-A beam current, the experiment has produced over 820 W of peak power with a -3-dB bandwidth of 0.8 GHz and a linear gain of 34 dB at 34.7 kV. In addition, the amplifier produced a -3-dB bandwidth of over 1.5 GHz (1.1%) with a peak power of 570 W from a 38.5-kV 2.5-A electron beam. The electron beam is estimated to have a pitch factor of 0.55-0.6, a radius of 1.9 mm, and a calculated perpendicular momentum spread of approximately 9%. The gyro-amplifier was nominally operated at a pulselength of 2 μs but was tested to amplify pulses as short as 4 ns with no noticeable pulse broadening. Internal reflections in the amplifier were identified using these short pulses by time-domain reflectometry. The demonstrated performance of this amplifier shows that it can be applied to dynamic nuclear polarization and electron paramagnetic resonance spectroscopy.

Compression of a microwave pulse using a multi-mirror resonance delay line

We develop the theory of compression of a frequency-modulated pulse in a ring-shaped multi-mirror delay line connected with the input and output quasioptical waveguides by means of a common corrugated mirror. An efficiency of about 60% is obtained in an experimental compressor having a resonant frequency of 34 GHz for the fourfold compression of a pulse with an initial duration of 11 ns.

Beam Combination and Routing at High Power with a Ring-Type Waveguide Millimeter-Wave Resonator

A hybrid quasi-optical waveguide resonating device providing millimeter-wave beam switching and combination at high power is described in this paper. It can be realized, starting from the beam-splitting properties of the rectangular corrugated waveguide with aperture much greater than the wavelength λ, by arranging the waveguides in a resonating ring configuration. This kind of waveguide, cut at an appropriate length, has been proposed for the remote steering (RS) system of the ITER upper electron cyclotron resonance heating (ECRH) launcher, because of its imaging properties. In fact, beam steering can be performed far from the plasma edge since an input beam is transformed into an output beam with the same angle with the waveguide axis as the input one. Multiple imaging properties, derived by the fractional Talbot effect, are applied at waveguide sections cut at fractional lengths and lead to 3-dB beam-splitting properties for a length equivalent to half the length of an RS waveguide. Ring-type resonant devices with two outputs are obtained by setting two or more waveguides in properly arranged loops. The power distribution in the two output channels available can be controlled either mechanically, moving the mirrors used to couple the different sections by fractions of the wavelength λ, or varying the source frequency by a fraction Δε/ε ≪ 1. The exploitation of a second input port allows beams of different gyrotrons with nearly the same frequency to be coupled to the same transmission line. This relatively compact device can be evaluated for application into the ITER ECRH transmission line, with advantages on beam routing control.

Formation of Uniform Field Distributions in a Quasi-Optical Waveguide Resonator Containing a Spherical Convex Reflector

Formation of Uniform Field Distributions in a Quasi-Optical Waveguide Resonator Containing a Spherical Convex Reflector

High-power 140-GHz quasioptical gyrotron traveling-wave amplifier.

We present the design and experimental results of a novel quasioptical gyrotron traveling-wave tube (gyro-TWT) amplifier at 140 GHz. The gyro-TWT produced up to 30 kW of peak power in 2 micros pulsed operation at 6 Hz achieving a peak gain of 29 dB, a peak efficiency of 12%, and a bandwidth of 2.3 GHz. The device was operated in a very higher-order mode of an open quasioptical interaction structure, namely, a confocal waveguide. The diffraction loss from the open sidewalls of the confocal waveguide was used to suppress mode competition in this highly overmoded circuit resulting in a stable single-mode operation. The experiment achieved record high power levels at 140 GHz for a gyro-TWT. These experiments demonstrate the effectiveness of using overmoded quasioptical waveguide interaction structures for generating high power in the millimeter and submillimeter wave bands with a gyro-TWT.

The Quasi-Optical Waveguide Modeling Method for the Measurements of Scattering Matrix Elements in the Near-Millimeter and Submillimeter Wave Bands

The Quasi-Optical Waveguide Modeling Method for the Measurements of Scattering Matrix Elements in the Near-Millimeter and Submillimeter Wave Bands

Quasi-optical waveguide modeling method and microcompact scattering range for the millimeter and submillimeter wave bands

The possibility of using the quasi-optical waveguide modeling (QWM) method in the near millimeter and submillimeter ranges of the electromagnetic spectrum for a study of the backward and forward scattering amplitude and phase characteristics of physical objects or scaled models is discussed in the paper. The method is realized with the aid of a quasi-optical waveguide microcompact range (MCR). The microwave part of MCR is based on the regular section of a hollow dielectric waveguide (HDW) and corresponding quasi-optical components. The results of measurements of backward and forward scattering characteristics for a number of standard objects in the 4-mm wavelength band are presented.

Optimization of the Sectionalized Taper for Quasioptical Waveguides

Optimization of the Sectionalized Taper for Quasioptical Waveguides

Millimeter Wave Solid-State Oscillators with Quasioptical Dielectric Resonators and Waveguides

Millimeter Wave Solid-State Oscillators with Quasioptical Dielectric Resonators and Waveguides