Higher-order TE Modes Research Articles

Расчет фильтра для выделения излучения третьей гармоники циклотронной частоты сильноточного гиротрона Ka-диапазона

The calculation of a mode filter for a circular waveguide designed to isolate a high-order TE mode consistent with its field has been carried out. A theoretical justification and results of electrodynamic modeling are given using the example of identifying the third harmonic TE12,4 mode of the gyrotron against the background of predominant radiation at the fundamental cyclotron harmonic in the TE4,2 mode.

Scalable silicon-based mode-order converters assisted by tapered metal strip layer

On-chip multimode applications require efficient mode sources with various different mode-orders, so the scalable mode-order converters would be preferred but they are still not available at present. Here, we propose a scalable silicon-based mode-order conversion scheme that can achieve the mode-order conversion from input fundamental TE mode to arbitrary output higher-order TE mode in a conversion length of only 2.5 μm. Four higher-order modes (TE1 ∼ TE4) are analyzed as examples with mode conversion efficiencies >90%, crosstalks <−12 dB and insertion losses <2.8 dB, respectively. Further, the working bandwidths are relatively broad and fabrication tolerances are calculated in detail. With these features, such device scheme would help generate a serious of higher-order modes in compact sizes which would address the requirements of on-chip multimode applications, e.g., improving the mode channel numbers of on-chip mode-division multiplexing.

Integrated TM-through/TE-converted polarization beam splitter based on z-cut lithium niobate-on-insulator platform

Lithium niobate-on-insulator (LNOI) platform has been regarded as the new driving force to push the developments of integrated photonics, where efficient polarization management and mode conversion are the fundamental issues to be solved. In this work, we propose a triple waveguide-based TM-through/TE-converted polarization beam splitter (PBS) integrated on the z-cut LNOI platform, where the device works on the phase matching of optical mode. To improve the device performance and shorten the device size, we have added two cutting angles on both ends of the intermediate waveguide and optimized the length overlap between two evanescent coupling regions. From results, polarization extinction ratio 26.15 dB, insertion loss 0.47 dB for input TM0 mode, and mode conversion efficiency 99.6%, crosstalk − 22.82 dB, insertion loss 0.12 dB for input TE0 to output TE1 mode have been obtained, respectively. So, efficient polarization separation and mode conversion have been achieved simultaneously on-chip. Further, the proposed device scheme can be extended to output other higher-order TE modes via adding the width of the cross output waveguide. We hope such device scheme can make for the compact integration and capacity improvement of on-chip optical interconnects through multiplexing transmissions on the LNOI platform.

Generation of High-Order Waveguide Modes with Reduced Symmetric Protection

Single-mode operation in a rectangular waveguide is very important, since the existence of multimodes may arouse distortions of carried signals due to the modal dispersion. High-order mode means multiple periods of field in the transverse cross section, with distinct electromagnetic properties from the dominant mode. Although numerous methods are proposed to achieve mode selection within the waveguide, they only permit specific high-order modes to survive in a frequency range. However, it still remains a great challenge to control the bandwidth freely for single-mode operation. Here, we propose an approach for high-order mode generation, in which the desired mode operates in the band gap of other modes, and the working bandwidth can be engineered independently by reducing the symmetry of the loaded periodic metallic ridges in the waveguide. Two examples are demonstrated, where only ${\mathrm{TE}}_{20}$ and ${\mathrm{TE}}_{30}$ modes are supported. The proposed method can also be applied to generate other high-order TE modes in the future.

RF Behavior of a Coaxial Interaction Structure for 0.24-THz, 2-MW Gyrotron

The coaxial interaction structure for a 0.24-THz, 2-MW gyrotron is designed, and its RF behavior is analyzed for the beam current of 60 A and the beam voltage of 90 kV. The searching of a suitable, very high-order TE mode, which satisfies all the technical conditions required to generate 2-MW RF power, is an important and critical step in the development of a coaxial-cavity (COCA) gyrotron. The detailed analysis of mode selection is described in this article. The geometry of the axially corrugated insert is optimized using an in-house code COCA. Based on the optimized geometry of the coaxial insert and the cylindrical cavity, the coupling coefficient and the start oscillation current are computed to analyze the mode competition for the operating mode TE42,28. The particle-in-cell (PIC) algorithm-based simulation tool is used for the beam–wave interaction computation. Very fine meshing is implemented in the simulations, considering the high frequency and fine geometry of the coaxial insert. The electron beam parameters and the cavity magnetic field are optimized first aiming at the RF power ≥2 MW and frequency 240 GHz. The final results of the cavity design confirm 2.2-MW RF power and 239.975-GHz frequency at 9.74-T magnetic field.

A novel dual resonance long period waveguide grating based highly sensitive refractive index sensor with reduced temperature sensitivity

In this paper, we are proposing a novel structure to reduce the temperature cross sensitivity of a refractive index (RI) sensor based on dual resonance long period waveguide grating in a metal clad ridge waveguide, operating near the dispersion turning point. The core of the waveguide is considered to be made of two parts having opposite thermo-optic coefficients with grating written in one part only. Considering the coupling between the fundamental TE mode and a higher order TE mode, we have shown that by adjusting the thickness of the two parts of the core, it is possible to achieve blueshifts for both the resonance wavelengths with the increase in temperature unlike the case of a conventional dual resonance spectrum where two resonance wavelengths show opposite shifts. As a result, an extremely low temperature cross sensitivity ∼0.006 nm/0C has been achieved, whereas the dual resonance spectrum holds its conventional behavior with the change in ambient RI, showing an extremely high RI sensitivity ∼57,260 nm/RIU near water RI 1.33. Further, it has been observed that as we approach the dispersion turning point, temperature sensitivity decreases gradually whereas RI sensitivity increases, leading to a more accurate ambient RI measurement

Polarization-selective waveguide holography in the visible spectrum.

We propose and experimentally demonstrate a polarization-selective waveguide hologram at optical wavelengths, based on an all-dielectric metamaterial multilayer system. We show that two spatially separated or overlapped holographic images can be produced with two orthogonally polarized beams, incorporated into a binary computer generated hologram (CGH). These two images can be combined into a single 3D stereoscopic image observable using linearly or circularly polarized glasses. The two polarized beams can also be utilized to construct radially and azimuthally polarized "vortex" beams. The fundamental and first higher-order TM and TE modes of an optical waveguide are used to guide the two polarization states with distinct propagation constants. The two guided waves act as spatially distinct reference waves such that the integrated, on-chip hologram can distinguish the two and provide two independent images corresponding to the two polarizations. Polarization selective waveguide holograms can be used in a diverse set of applications, from chip-scale displays and augmented reality (AR) to optical trapping.

Enhancement of the quadrupole interaction of an atom with the guided light of an ultrathin optical fiber

We investigate the electric quadrupole interaction of an alkali-metal atom with guided light in the fundamental and higher-order modes of a vacuum-clad ultrathin optical fiber. We calculate the quadrupole Rabi frequency, the quadrupole oscillator strength, and their enhancement factors. In the example of a rubidium-87 atom, we study the dependencies of the quadrupole Rabi frequency on the quantum numbers of the transition, the mode type, the phase circulation direction, the propagation direction, the orientation of the quantization axis, the position of the atom, and the fiber radius. We find that the root-mean-square (rms) quadrupole Rabi frequency reduces quickly but the quadrupole oscillator strength varies slowly with increasing radial distance. We show that the enhancement factors of the rms Rabi frequency and the oscillator strength do not depend on any characteristics of the internal atomic states except for the atomic transition frequency. The enhancement factor of the oscillator strength can be significant even when the atom is far away from the fiber. We show that, in the case where the atom is positioned on the fiber surface, the oscillator strength for the quasicircularly polarized fundamental mode HE$_{11}$ has a local minimum at the fiber radius $a\simeq 107$ nm, and is larger than that for quasicircularly polarized higher-order hybrid modes, TE modes, and TM modes in the region $a<498.2$ nm.

RF Behavior of Cylindrical Cavity Based 240 GHz, 1 MW Gyrotron for Future Tokamak System

In this paper, we present the RF behavior of conventional cylindrical interaction cavity for 240 GHz, 1 MW gyrotron for futuristic plasma fusion reactors. Very high-order TE mode is searched for this gyrotron to minimize the Ohmic wall loading at the interaction cavity. The mode selection process is carried out rigorously to analyze the mode competition and design feasibility. The cold cavity analysis and beam-wave interaction computation are carried out to finalize the cavity design. The detail parametric analyses for interaction cavity are performed in terms of mode stability, interaction efficiency and frequency. In addition, the design of triode type magnetron injection gun is also discussed. The electron beam parameters such as velocity ratio and velocity spread are optimized as per the requirement at interaction cavity. The design studies presented here confirm the realization of CW, 1 MW power at 240 GHz frequency at TE46,17 mode.

Microwave dielectric stepped-index flat lens antenna

Dielectric stepped-index flat lens antennas for operation at 12 GHz are presented. A brick-shaped dielectric with a permittivity profile optimized for focusing is sandwiched between the metallic plates of an open-ended parallel-plate waveguide. A tapered slot antenna is placed at the focal point of the dielectric lens, thereby creating antennas with high directivity of 16.8 and 15.8 dBi, respectively. In the two versions of the antenna, the parallel-plate waveguide operates in TEM-mode and in the first higher-order TE-mode, respectively. The dielectric profile is realized by appropriate mixtures of alumina ceramic powder and microscopic hollow glass spheres, realizing permittivity ranging from εrel= 1.31 to εrel= 3.24. The design of the complete antennas is based on geometrical optics followed by optimizations with a full-wave electromagnetic solver. Measurements show good agreement with simulations.

Mode hybridization and conversion in silicon-on-insulator nanowires with angled sidewalls.

The mode property and light propagation in a tapered silicon-on-insulator (SOI) nanowire with angled sidewalls is analyzed. Mode hybridization is observed and mode conversion between the TM fundamental mode and higher-order TE modes happens when light propagates in a waveguide taper which is used very often in the design of photonic integrated devices. This mode conversion ratio is possible to be very high (even close to 100%) when the taper is long enough to be adiabatic, which might be useful for some applications of multimode photonics. When the mode conversion is undesired to avoid any excess loss as well as crosstalk for photonic integrated circuits, one can depress the mode conversion by compensating the vertical asymmetry in the way of reducing the sidewall angle or introducing an optimal refractive index for the upper-cladding. It is also possible to eliminate the undesired mode conversion almost and improve the desired mode conversion greatly by introducing an abrupt junction connecting two sections with different widths to jump over the mode hybridization region.

Mode conversion in tapered submicron silicon ridge optical waveguides

The mode conversion in tapered submicron silicon ridge optical waveguides is investigated theoretically and experimentally. Two types of optical waveguide tapers are considered in this paper. One is a regular lateral taper for which the waveguide width varies while the etching depth is kept the same. The other is a so-called "bi-level" taper, which includes two layers of lateral tapers. Mode conversion between the TM fundamental mode and higher-order TE modes is observed in tapered submicron silicon-on-insulator ridge optical waveguides due to the mode hybridization resulting from the asymmetry of the cross section. Such a mode conversion could have a very high efficiency (close to 100%) when the taper is designed appropriately. This enables some applications e.g. polarizer, polarization splitting/rotation, etc. It is also shown that this kind of mode conversion could be depressed by carefully choosing the taper parameters (like the taper width, the etching depth, etc), which is important for the applications when low-loss propagation for the TM fundamental mode is needed.

Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires

A novel concept for an ultracompact polarization splitter-rotator is proposed by utilizing a structure combining an adiabatic taper and an asymmetrical directional coupler. The adiabatic taper structure is singlemode at the input end while it becomes multimode at the other end. When light propagates along the adiabatic taper structure, the TM fundamental mode launched at the narrow end is efficiently (close to 100%) converted to the first higher-order TE mode at the wide end because of the mode coupling between them. By using an asymmetrical directional coupler that has two adjacent waveguides with different core widths, the first higher-order TE mode is then coupled to the TE fundamental mode of the adjacent narrow waveguide. On the other hand, the input TE polarization does not change when it goes through the adiabatic taper structure. In the region of the asymmetrical directional coupler, the TE fundamental mode in the wide waveguide is not coupled to the adjacent narrow waveguide because of phase mismatch. In this way, TE- and TM- polarized light are separated while the TM fundamental mode is also converted into the TE fundamental mode. A design example of the proposed polarization splitter-rotator is given by using silicon-on-insulator nanowires and the total length of the device is less than 100μm. Furthermore, only a one-mask process is needed for the fabrication process, which is compatible with the standard fabrication for the regular photonic integrated circuits based on SOI nanowires.

Parallel Plate Patch Antenna With Internal Rectangular Coupling Patches and TE$_{\rm N0}$ Mode Excitation

A parallel plate waveguide antenna is presented with microstrip patches as radiating elements. Rectangular patches are also placed inside the waveguide as coupling field structures from the waveguide to the radiating elements. The connections from the coupling to the radiating patches are made by means of vias. The field propagated along the parallel plate waveguide corresponds to a high order TE mode, creating virtual smaller rectangular waveguides in the parallel plate structure. An antenna prototype has been manufactured and measured. A 60% efficiency and 27 dBi antenna gain were obtained; with a 6.5% reflection bandwidth. Nevertheless, the resonant behavior becomes evident in the radiation pattern measurements.

The mode characteristics of a symmetrical slab waveguide with single-negative-material cladding

The propagating characteristics of a symmetrical three-layer slab waveguide with mu-negative (MNG) media are studied in detail. It is found the MNG waveguide can propagate not only quick waves but also slow waves of TE0 and TE1 modes. The properties of this guided mode are different from those of the left-handed waveguide and the conventional waveguide. In MNG slab waveguide, the region of effective refractive index of the guided mode is wider than those of the other two. In addition, the TE0 fundamental guided mode does not exist at all, while the higher-order TE guided modes and all TM guided modes do exist. However the surface wave of TE0 and TE1 modes can emerge under a certain condition. The propagation constant of higher-order TE mode is greater than that of the TM mode of the same order. Two TE modes of the same order can coexist in the MNG, which is closely related to the structural parameters. The energy fluxes of two TE modes are oppositely directed inside and outside the inner layer, therefore the total energy flux can be negative.

Field Patterns in Double Circular-Groove Guide by Finite Element Method

The electromagnetic power coupling in symmetrical and asymmetrical double circular-groove guides have been analyzed by finite element method (FEM) in this paper. The electrical field patterns of the dominant mode and the first higher-order TE mode have been presented. The electromagnetic field of the dominant mode distributes with a concentration in the two grooves and the region between grooves as well for symmetrical double circular-groove guide, while it distributes with a concentration in the groove with larger radius for the asymmetrical double circular-groove guide. The electrical field patterns of the first higher-order TE mode in symmetrical and asymmetrical double circular-groove guide are also presented. The figures in this paper have important values in design of circular-groove guide power coupler for millimeter wave applications.

Design of trapezoidal-ridge waveguide by finite-element method

Transmission characteristics of single and double trapezoidal-ridge waveguides, are analysed by the finite-element method. Design data of cutoff wavelength and single-mode bandwidth for different ridge dimensions are presented in tabulated forms. Field patterns of dominant mode and the first higher-order TE mode in both waveguides are also illustrated. Numerical results provide an extension to the existing design data of rectangular-ridge waveguides and are considered to be helpful in practical applications.

Field Patterns in Asymmetrical Double Groove Guide by Finite Element Method

The electromagnetic power coupling in asymmetrical double-groove guide is analyzed by finite element method (FEM) in this paper. The electrical field patterns of the dominant mode and the first higher-order TE mode have been presented. The electromagnetic field of the dominant mode is distributed with a concentration of energy in the wide groove region, while the electromagnetic field of the first higher-order TE mode is distributed with a concentration of energy in the narrow groove region. The results in this paper have important values in design of groove guide power coupler for millimeter wave applications.

Finite Element Analysis of Field Pattern in Multiple Groove Guide

Rectangular groove guide with multiple grooves has been analyzed by finite element method (FEM) in this paper. The electrical field patterns of the dominant mode and the first higher-order TE mode have been presented for groove guide with single-, double-, triple-, and quadruple-rectangular grooves. The electromagnetic field of the dominant mode is distributed with a concentration in the groove regions, while the electromagnetic field of the first higher-order TE mode is mainly distributed in regions between grooves. The low loss characteristic has been found not only in single-groove guide, but also in multiple groove guide.

Finite element analysis of the TE modes in truncated-circular waveguide

Truncated-circular waveguide are used in some microwave and millimeter-wave components. The field patterns in truncated-circular waveguide are desired but usually difficult to be computed by conventional methods. In this paper, they are efficiently determined by using finite element method. The electrical field lines of the dominant TE mode and the first sixteen higher-order TE modes are presented. The calculated results in this paper have important values for us to understand the transmission characteristics of truncated-circular waveguide, and will be of practical significance in designing waveguide components in microwave and millimeter wave engineering.