Resonator Axis Research Articles

Comparison of Inductive and Capacitive End Couplings in the Design of a Combline Microwave Cavity Filter for the E1 Galileo Band

Abstract. Inductive and capacitive end couplings in the design of combline microwave filters are compared. For instance, a combline microwave cavity filter for the E1 Galileo band (1559–1591 MHz, 2 % fractional bandwidth) is considered. The inductive end coupling is composed of an L-shaped pin in galvanic contact with the end resonator, while the capacitive end coupling is realized by a straight pin parallel to the end resonator's axis. Although both end couplings can realize the desired external quality factor, the capacitive end coupling is easier to manufacture, while the inductive end coupling is less sensitive to the design parameter. An excellent agreement between synthesized (5th-order Chebyshev mask with 0.05 dB ripple) and measured responses is observed for the realized prototype of the filter. The insertion losses at the center frequency are 1.72 and 1.53 dB for the inductive and capacitive end couplings, respectively. The spurious-free ranges are up to 8.23 and 8.92 GHz for inductive and capacitive end couplings, respectively.

Higher-order fractal transverse modes observed in microlasers

Two classes of higher-order, fractal spatial eigenmodes have been predicted computationally and observed experimentally in microlasers. The equatorial plane of a close-packed array of microspheres, lying on one mirror within a Fabry-Pérot resonator and immersed in the laser gain medium, acts as a refractive slit array in a plane transverse to the optical axis. Edge diffraction from the slit array generates the high spatial frequencies (>104 cm−1) required for the formation of high-order laser fractal modes, and fractal transverse modes are generated, amplified, and evolve within the active medium. With a quasi-rectangular (4-microsphere) aperture, the fundamental mode and several higher-order eigenmodes (m = 2,4,5) are observed in experiments, whereas only the m = 1,2 modes are observed experimentally for the higher-loss resonators defined by triangular (3-microsphere) apertures. The fundamental and 2nd-order modes (m = 1,2) for the 4-sphere aperture are calculated to have qualitatively similar intensity profiles and nearly degenerate resonant frequencies that differ by less than <0.1% of the free-spectral range (375 GHz) but exhibit even and odd parity, respectively. For all of the observed fractal modes, the fractal dimension (D) rises rapidly beyond the intracavity aperture array as a result of the high spatial frequencies introduced into the mode profile. Elsewhere, D varies gradually along the resonator axis and 2.2 < D < 2.5. Generating fractal laser modes in an equivalent optical waveguide is expected to allow the realization of new optical devices and imaging protocols based on the spatial frequencies and variable D values available.

Insert Misalignment Studies of a Coaxial-Cavity Gyrotron—Full-Wave Approach

Insert misalignment studies are carried out for a coaxial-cavity gyrotron with triangular corrugations on the insert wall using the full-wave approach, space harmonics method (SHM). By applying electromagnetic boundary conditions, dispersion relation is derived for a coaxial-cavity with misaligned insert. Misalignment of the insert caused by axial displacement as well as tilting of the insert axis from the outer resonator axis are taken into consideration in the study. As the electromagnetic fields in the interaction space vary with the insert misalignment, due to this the mathematical formulations of the beam-coupling coefficient, wall losses, RF interaction efficiency, and output power are modified. For a 2-MW, 220-GHz gyrotron, the insert misalignment studies are carried out using the proposed full-wave model with the help of our in-house code Gyrotron Design Suite. For validation, the results obtained using the SHM approach are compared with that of the surface impedance model (SIM) approach. In addition, comparison studies between SIM and SHM approaches are performed for the practically developed 170-GHz, 2-MW coaxial-cavity gyrotron with both triangular and rectangular corrugations in the insert.

A multimodal axial array resonator and its application in radiofrequency (RF) volume coil designs for low-field open magnetic resonance imaging (MRI)

A multimodal axial array resonator and its application in radiofrequency (RF) volume coil designs for low-field open magnetic resonance imaging (MRI)

Dynamic wavefront distortion in resonant scanners.

Dynamic mirror deformation can substantially degrade the performance of optical instruments using resonant scanners. Here, we evaluate two scanners with resonant frequencies >12kHz with low dynamic distortion. First, we tested an existing galvanometric motor with a novel, to the best of our knowledge, mirror substrate material, silicon carbide, which resonates at 13.8 kHz. This material is stiffer than conventional optical glasses and has lower manufacturing toxicity than beryllium, the stiffest material currently used for this application. Then, we tested a biaxial microelectromechanical (MEMS) scanner with the resonant axis operating at 29.4 kHz. Dynamic deformation measurements show that wavefront aberrations in the galvanometric scanner are dominated by linear oblique astigmatism (90%), while wavefront aberrations in the MEMS scanner are dominated by horizontal coma (30%) and oblique trefoil (27%). In both scanners, distortion amplitude increases linearly with deflection angle, yielding diffraction-limited performance over half of the maximum possible deflection for wavelengths longer than 450 nm and over the full deflection range for wavelengths above 850 nm. Diffraction-limited performance for shorter wavelengths or over larger fractions of the deflection range can be achieved by reducing the beam diameter at the mirror surface. The small dynamic distortion of the MEMS scanner offers a promising alternative to galvanometric resonant scanners with desirable but currently unattainably high resonant frequencies.

Folded microstrip antenna with isotropic radiation pattern

AbstractIn contrast to existing antennas with polarization and directional characteristics, we propose a novel isotropic antenna that provides equivalent vertical and horizontal polarization in all directions. The final design was guided by simulations, with the performance characteristics of the fabricated antenna verified experimentally. In the proposed antenna, two λ/4 folded microstrip antennas were integrated into one antenna in a “+” shape to generate the isotropic radiation pattern, with the input impedance matched to the feeding position. The resonant frequency, horizontal width, radiation pattern, and axis ratio were adjusted by changing the length of the microstrip line. Both simulations and experiments confirmed that the radiation patterns of vertical and horizontal polarization on each plane show the characteristics of an isotropic antenna with nondirectional radiation pattern.

Development of an Electrostatic Comb-Driven MEMS Scanning Mirror for Two-Dimensional Raster Scanning.

Microelectromechanical System (MEMS)-based scanning mirrors are important optical devices that have been employed in many fields as a low-cost and miniaturized solution. In recent years, the rapid development of Light Detection and Ranging (LiDAR) has led to opportunities and challenges for MEMS scanners. In this work, we propose a 2D electrostatically actuated micro raster scanner with relatively large aperture. The 2D scanner combines a resonant scanning axis driven by an in-plane comb and a quasistatic scanning axis driven by a vertical comb, which is achieved by raising the moving comb finger above the fixed comb finger through the residual stress gradient. The analytic formula for the resonant axis frequency, based on the mechanical coupling of two oscillation modes, is derived and compared with finite element simulation. A prototype is designed, fabricated, and tested, and an overall optical Field-of-View (FoV) of about 60° × 4° is achieved. Finally, some possibilities for further improvement or optimization are discussed.

Experimental characterization of aero-acoustic performance of micro-perforated materials in a wind tunnel

Mitigating the propagation of low frequency noise in ducted flows represents a challenging task since wall treatments have often limited dimensions. Micro-perforated panels have been successfully used backed by a cavity constituting a bulk-reacting resonator or in combination with honeycomb for a locally reacting system. In this work, a cost-efficient methodology for the study and characterization of the aero-acoustic properties of flush-mounted micro-perforated resonators has been developed. Although Laser Doppler Velocimetry is a non-intrusive technique, it requires delicate instrumentation to obtain an estimation of the acoustic velocity and pressure fluctuations at several points over a wall liner. Here, the attenuation has been estimated from sound pressure level measurements performed with two nosecone microphones positioned along a vertical line centered on the resonator axis. Experiments have been performed in a wind tunnel in presence of a low-speed turbulent boundary layer of air fully developed over different samples made up of micro-perforated sheets or porous materials with surface roughness. The micro-perforates have been flush-mounted over a cylindrical cavity of depth 30 mm situated on the floor of the test section. The ability of bulk-reacting resonators for reducing the acoustic or flow-induced noise has been assessed in comparison with locally reacting treatments.

Self-ordering and cavity cooling using a train of ultrashort pulses

A dilute atomic gas in an optical resonator exhibits a phase transition from a homogeneous density to crystalline order when laser illuminated orthogonal to the resonator axis. We study this well-known self-organization phenomenon for a generalized pumping scheme using a femtosecond pulse train with a frequency spectrum spanning a large bandwidth covering many cavity modes. We show that due to simultaneous scattering into adjacent longitudinal cavity modes the induced light forces and the atomic dynamics becomes nearly translation-invariant along the cavity axis. In addition the laser bandwidth introduces a new correlation length scale within which clustering of the atoms is energetically favorable. Numerical simulations allow us to determine the self-consistent ordering threshold power as function of bandwidth and atomic cloud size. We find strong evidence for a change from a second order to a first order self-ordering phase transition with growing laser bandwidth when the size of the atomic cloud gets bigger than the clustering length. An analysis of the cavity output reveals a corresponding transition from a single to a double pulse traveling within the cavity. This doubles the output pulse repetition rate and creates extra substructures in close analogy to a time crystal formation in the cavity output. Simulations also show that multi-mode operation significantly improves cavity cooling generating lower kinetic temperatures at a much faster cooling rate.

Continuously tunable fiber laser based on Fano resonance filter of thin-fiber-taper-coupled conical microresonator

Through Fano resonance, the resonant modes exhibiting peak-like shapes on the bandpass spectrum of the thin-fiber-taper-coupled conical microresonator are generated. For the first time, this thin-fiber-taper-coupled conical microresonator functions as a Fano filter, and is applied as a tunable internal-cavity bandpass filter for an Erbium-doped fiber ring laser. If the size of the conical microresonator in the coupling junction is adjusted by displacing the fiber taper along the resonator axis, the wavelength of the output laser is able to be tuned linearly and continuously with a coefficient of 0.225 nm/mm.

INCREASE OF EFFICIENCY OF ACCELERATION OF THE TEST ELECTRON BUNCH BY SEQUENCE OF ELECTRON BUNCHES IN THE RECTANGULAR DIELECTRIC RESONATOR AT PLASMA FILLING

A result of numerical simulation of acceleration of the test electron bunch following a sequence of relativistic electron bunches in the rectangular dielectric resonator when filling the drift channel with plasma of different density is provided. We have found out that at change of plasma density the linear growth of energy gain of test bunch electrons is observed. It is connected with structure ordering of longitudinal accelerating electric field in the resonator and also with increase in its amplitude. It is due to that at periodic injection of drive bunches on resonator axis the areas with the increased plasma electrons density are formed.

Double patch sensor for identification of stress level and direction

In this article microstrip sensor for evaluation of stress direction and value was proposed. Developed sensor consists of two patches (resonators) of various sizes. Both patches have different operating frequencies and different angles between resonator axis and the direction of mechanical excitation—resonators are arranged along orthogonal axes. Proposed transducer enables determination of the direction and value of stress in the range from 0 to 90°. Patches are fed by T-junction power divider, so they use one excitation port. The resonators were developed for operating frequencies of 2.65 and 2.75 GHz, so a small difference (100 MHz) compared to the sensor with one resonator was obtained between the two resonant frequencies. It enables better use of measuring hardware capabilities, which leads to faster and more accurate measurements. The proposed sensors were evaluated using numerical analysis and measurements. Moreover, the algorithm for identification of direction and stress level was proposed. It uses artificial neural network approximator. The algorithm was positively verified.

Effect of Insert Misalignment on a Triangular Corrugated Coaxial Cavity Gyrotron

In this paper, the field analysis of a triangular corrugated coaxial cavity with misaligned insert is carried out for a megawatt-class sub-terahertz wave gyrotron. In misalignment analysis, both parallel displacement of the insert axis and tilting of the insert axis with the outer resonator axis are considered. Graff's addition theorem is used for deriving the eigenvalue equation of the coaxial cavity with such a misaligned insert. Mathematical formulations are carried out to include the effect of insert misalignment on the beam coupling coefficient and ohmic wall loading of the outer cavity and insert of the coaxial cavity gyrotron. The effect of structural misalignment of the insert on the operation of 2-MW, 220-GHz coaxial cavity gyrotron is analyzed. It is shown that insert misalignment reduces RF output power as well as increases ohmic wall loading of the insert beyond the prescribed cooling limit. Compared to tilting of the insert, parallel displacement of the insert causes more deterioration in the gyrotron operation. Comparative studies are also performed for a coaxial cavity with rectangular slots on the insert.

SYNTHESIS OF NANOSCALED MAGNETIC MATERIALS ON THE BASIS OF OXIDE SYSTEMS AND MANUFACTURING OF NON-RECIPROCAL COMPOSITE ELEMENTS BASED ON THEM

Nanoscaled particles of nickel-zinc ferrites solid solutions were synthesized by the method of precipitation from aqueous solutions, as initial reagents were used nitrates solutions, namely Fe(NO3)3, Ni(NO3)2 and Zn(NO3)3. Cylindrical composite resonant elements in a form of systems “high-Q non-magnetic dielectric – magnetic film” were manufactured using synthesized particles. As a non-magnetic dielectric was used barium tetratitanate BaTi4O9 with a doping of ZnO. For this the film-forming suspension consisted of synthesized ferrite particles and photopolymer was prepared. The suspension was applied on the end face of the dielectric resonator and polymerized by the UV radiation. Magnetic field of electromagnet was applied to the composite resonators during the magnetic film polymerization process (parallel or normal to the resonators axis).
 Absorption spectra of composite resonators in the waveguide measuring cell under influence of an external magnetic field were obtained. Spectra confirm the opportunity to control the resonance frequency of the composite structure and energy absorption in it using the external permanent magnetic field. The tuning of composite resonators parameters was more efficient for the resonators with the film polymerized in the parallel to the resonator axis field. For the frequency shift and electromagnetic isolation were obtained the values of 71 MHz and 53 dB respectively but with a high “straight” energy losses in the area of ferromagnetic resonance. So, the measuring cell with the composite resonator located in it was simulated using an Ansys HFSS software. Respective absorption spectra were obtained. It is shown that it is possible to manufacture composite resonators with a needed for the practical applications characteristics level (reverse losses were 34 dB, straight losses were 0.8 dB) and a tuning ability for the development of superheterodyne transmitters, dynamic filters for microwave technics, microwave valves, etc.

A Novel Indirect Quasi-Static Actuation Concept for 2D MEMS Micromirrors

We present a 2D MEMS raster scanning micromirror based on a novel actuation concept for the (quasi-)static axis. This is based on the distortion of the angular momentum generated by the primary resonant axis using a secondary oscillation. After introducing a model of the device dynamics using a rigid body approximation, several simulation studies are carried out to investigate the nonlinear dynamics of such a device. In the next step, a fully operational MEMS design is shown, that is actuated by piezoelectric as well as electromagnetic drive schemes. After manufacturing, we performed a series of experiments to obtain the characteristics of the resonant as well as the quasistatic axis, proving the viability of the proposed actuation concept. [2018-0250]

Determination of RF resonator axis inclination to beam axis in electron-positron storage ring

We proposed and tested a method of experimental determination of the upper limit of RF resonator axis inclination to the beam axis. The electric field transverse component by such a perturbation deflects the particle trajectory. In the horizontal plane, a distortion of the closed orbit leads to a difference in the energies and spin precession frequencies of electrons and positrons in a storage collider with a single ring.

Effect of inhomogeneous temperature fields on acoustic streaming structures in resonators.

Acoustic streaming in 2D rectangular resonant channels filled with a fluid with a spatial temperature distribution is studied within this work. An inertial force is assumed for driving the acoustic field; the temperature inhomogeneity is introduced by resonator walls with prescribed temperature distribution. The method of successive approximations is employed to derive linear equations for calculation of primary acoustic and time-averaged secondary fields including the streaming velocity. The model equations have a standard form which allows their numerical integration using a universal solver; in this case, COMSOL Multiphysics was employed. The numerical results show that fluid temperature variations in the direction perpendicular to the resonator axis influence strongly the streaming field if the ratio of the channel width and the viscous boundary layer thickness is big enough; the streaming in the Rayleigh vortices can be supported as well as opposed, which can ultimately lead to the appearance of additional vortices.

Electrical tuning of the polarization state of light using graphene-integrated anisotropic metasurfaces.

Plasmonic metasurfaces have been employed for moulding the flow of transmitted and reflected light, thereby enabling numerous applications that benefit from their ultra-thin sub-wavelength format. Their appeal is further enhanced by the incorporation of active electro-optic elements, paving the way for dynamic control of light's properties. In this paper, we realize a dynamic polarization state generator using a graphene-integrated anisotropic metasurface (GIAM) that converts the linear polarization of the incident light into an elliptical one. This is accomplished by using an anisotropic metasurface with two principal polarization axes, one of which possesses a Fano-type resonance. A gate-controlled single-layer graphene integrated with the metasurface was employed as an electro-optic element controlling the phase and intensity of light polarized along the resonant axis of the GIAM. When the incident light is polarized at an angle to the resonant axis of the metasurface, the ellipticity of the reflected light can be dynamically controlled by the application of a gate voltage. Thus accomplished dynamic polarization control is experimentally demonstrated and characterized by measuring the Stokes polarization parameters. Large changes of the ellipticity and the tilt angle of the polarization ellipse are observed. Our measurements show that the tilt angle can be changed from positive values through zero to negative values while keeping the ellipticity constant, potentially paving the way to rapid ellipsometry and other characterization techniques requiring fast polarization shifting.This article is part of the themed issue 'New horizons for nanophotonics'.

Divergence and polarisation of radiation of a wide-aperture chemical DF laser with an unstable resonator

Influence of a plane-parallel plate, placed inside an unstable resonator and entirely overlapping the beam cross section, on the polarisation and divergence of wide-aperture DF-laser radiation is studied at various angles of plate inclination to the resonator axis. It is shown that the plate inside a resonator has no effect on the radiation divergence and only requires a corresponding shift of a convex mirror in order to keep the resonator confocal. A noticeable change in the radiation polarisation has been observed only at angles of plate inclination greater than ; at angles of above the radiation becomes actually linearly polarised.

Reformulation of the Fourier–Bessel steady state mode solver

The Fourier–Bessel resonator state mode solver is reformulated using Maxwell's field coupled curl equations. The matrix generating expressions are greatly simplified as well as a reduction in the number of pre-computed tables making the technique simpler to implement on a desktop computer. The reformulation maintains the theoretical equivalence of the permittivity and permeability and as such structures containing both electric and magnetic properties can be examined. Computation examples are presented for a surface nanoscale axial photonic resonator and hybrid {ε,μ} quasi-crystal resonator.