High-order Mode Operation Research Articles

High-Power Emission via Large-Area VCSELs With Single High-Order Mode Operation

High-Power Emission via Large-Area VCSELs With Single High-Order Mode Operation

A THz High-Order Mode Operation BWO Based on a Composite Structure

A THz High-Order Mode Operation BWO Based on a Composite Structure

Wideband and Aperture-Efficient Traveling-Wave Endfire Antenna Designs Using the First Higher-Order Transmission Line Mode

In this article, we propose a traveling-wave endfire antenna based on the first higher-order transmission line (TL) mode, and it features wide bandwidth, high gain, and large aperture efficiency. We propose two essential conditions for the realization of the proposed wideband endfire antenna. First, the dispersion curve of the first higher-order mode should be made close to that of the air line across a wide frequency band. Second, the fundamental mode should be suppressed to allow single higher-order mode operation across a wide frequency band. Following these principles, we provide a design for an endfire antenna based on a periodically slotted parallel stripline, where the periodic meandered slots suppress the fundamental mode and hence allow single higher-order mode radiation. To verify the proposed approach, a prototype has been designed and fabricated. The antenna can achieve endfire radiation within a 1:2 frequency band (8.5–17.5 GHz), with an average gain of 10.7 dBi, and large aperture efficiency up to 6.38 dBi/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda ^{2}_{0}$ </tex-math></inline-formula> .

Cold Characteristics of Overmoded Ridge- and Groove-Loaded Folded Waveguides for High-Power Traveling Wave Tubes

The TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> high-order mode operation characteristics of the ridge-loaded (RL) and groove-loaded (GL) folded waveguide slow wave structures (FWG SWSs) with two electron beam channels are studied for high-power traveling wave tubes (TWTs) at the terahertz (THz) frequency regime. Operating in a higher order mode means that the SWSs can have larger structure dimensions to reduce the fabrication difficulty at the THz frequency band and use higher current to increase the output power. For FWG SWSs operating in the high-order mode, it is difficult to suppress backward wave oscillation (BWO) of fundamental mode. In this article, we proposed the RL and GL FWG SWSs to suppress BWO and studied the dispersion, coupling impedance, and transmission loss of them. The results show that the RL structure can substantially increase the start oscillation length of the interaction circuit and avoid using many circuit sections for spurious mode depression which ease the assembly process of the tube. The GL structure can broaden the bandwidth with high-order mode operation but with the ease of BWO of fundamental mode. The straight RL and GL waveguides are used to explain the above results, and the effect of electron beam channel position on coupling impedance is also studied.

High‐gain magnetic‐dipole quasi‐Yagi antenna using higher‐order‐mode dielectric resonator with near‐zero‐index metamaterial

For the first time, the rectangular dielectric resonator (DR) operating in higher-order TE3δ1 mode is investigated and used as a magnetic-dipole driver to design quasi-Yagi antenna with high gain. For further enhancing the antenna gain, a near-zero-index (NZI) metamaterial (NZIM) is proposed instead of the traditional directors and put in the front of DR driver. It is a simple structure and composed of a set of the parallel metallic lines printed on a substrate along the end-fire direction. Benefiting from the higher-order mode operation of the DR and NZIM, the realized gain of the proposed antenna can reach 10.3 dBi, including the gain improvement of 2 dBi resulting from the employed NZIM. To verify the design concept, the prototype of quasi-Yagi DR antenna with NZIM is fabricated and characterized. The measured results agree very well with the simulated results.

Generating High-Order Transverse Patterns in Optically Pumped Semiconductor Lasers

High-order pattern formation in an optically pumped semiconductor laser (OPSL) under a selective pumping with varying spatial overlap between the pump beam and transverse modes is explored. In contrast to transverse pattern generation by off-axis pumped solid-state lasers where the mode order can be flexibly increased, experimental results reveal that the selective pumping fails to realize high-order mode operation in OPSLs when the pump-to-mode size ratio and pump power are insufficiently large. On the other hand, several high-order patterns belonging to the Hermite-Laguerre-Gaussian (HLG) modes are observed when scanning a large-ratio pump beam to specific positions on the gain chip. These HLG modes are experimentally confirmed to mainly originate from the transverse non-uniformity of present OPSL chip, while their structural features cannot be simply correlated with the pump scanning positions. Nevertheless, it is believed that the first-time observation of pure HLG modes under the large-ratio pumping can offer useful insights into the high-order pattern manipulation in OPSLs.

Single Flip-Chip Packaged Dielectric Resonator Antenna for CMOS Terahertz Antenna Array Gain Enhancement

A single dielectric resonator antenna (DRA) capable of enhancing the antenna gain of each element of a $2\times 2$ terahertz (THz) antenna array realized in a 0.18- $\mu \text{m}$ CMOS technology is proposed in this paper. The DRA implemented in a low-cost integrated-passive-device technology is flip-chip packaged onto the CMOS antenna array chip through low-loss gold bumps. By designing the DRA to work at the higher order mode of TE $_{3,\delta,9}$ , only a single DRA, instead of conventionally needing four DRAs, is required to simultaneously improve the antenna gain of each element of the $2\times 2$ antenna array. This not only simplifies the assembly process, but it can also reduce the assembly cost. Moreover, the DRA can provide great antenna gain enhancement because of being made of high-resistivity silicon material and higher order mode operation. The simulated antenna gain of each on-chip patch antenna of the $2\times 2$ CMOS antenna array can be increased from 0.1 to 8.6 dBi at 339 GHz as the DRA is added. To characterize the proposed DRA, four identical power detectors (PDs) are designed and integrated with each element of the $2\times 2$ THz antenna array. By measuring the voltage responsivity of each PD output, the characteristics of each antenna of the antenna array with the proposed DRA, including the gain enhancement level and radiation pattern, can be acquired. The measurement results match well with the simulated ones, verifying the proposed DRA operation principle. The four PDs with the proposed DRA are also successfully employed to demonstrate a THz imaging system at 340 GHz. To the best of our knowledge, the proposed DRA is the one with the highest order operation mode at THz frequencies reported thus far.

Simulation study of a high-order mode terahertz radiation source based on an orthogonal grating waveguide and multiple sheet electron beams.

Generally, it is difficult for the common backward wave oscillator (BWO) to produce powerful THz radiation when the operating frequency increases to a high level such as over 1 THz due to the very small structural dimensions. The concept of generating powerful THz radiation from the interaction between high-order mode THz wave and multiple sheet electron beams is a promising solution to address the issue. For the high-order mode operation, a novel orthogonal grating waveguide is proposed, which is relatively ease of fabrication compared with the overmoded structure based on the double staggered grating waveguide. A high-order mode BWO based on the orthogonal grating waveguide and multiple sheet electron beams is studied by simulations. Particle-in-cell simulations show that the BWO can provide over 1.08 W power in the frequency range of 1.18-1.30 THz. Such a methodology opens up a new way to extend the BWO's operating frequency to a higher level and provides a potential solution for developing compact powerful THz radiation sources with wide tunable bandwidth.

340-GHz Low-Cost and High-Gain On-Chip Higher Order Mode Dielectric Resonator Antenna for THz Applications

A low-cost and high-gain on-chip terahertz (THz) dielectric resonator antenna (DRA) is proposed in this work. The DRA consists of a low-loss dielectric resonator (DR) made of high-resistivity silicon material and an on-chip feeding patch realized in a 0.18-μm CMOS technology for exciting the desired electromagnetic (EM) mode. The DR can be easily fabricated to the required dimension by wafer dicing of a 2-in silicon wafer. With a 500-μm thick DR, a higher order mode of TE δ, 1 , 7 can be excited, which greatly enhances the antenna gain. Such higher order mode operation also provides a reliable design. If a fundamental mode is selected, the DR thickness is around 100 μm at THz frequencies, which not only requires an additional wafer thinning process, but the wafer is also easily broken during the fabrication process. The feeding patch is used to excite the TE δ, 1 , 7 mode. Moreover, its ground plane also prevents the EM field from leaking into the lossy CMOS silicon substrate, which improves the antenna efficiency. The simulated antenna gain can be 7.9 dBi while providing radiation efficiency of 74% at 341 GHz with 7.3% bandwidth. To characterize the DRA performance, an identical CMOS imager is designed to be integrated with the proposed DRA and an on-chip patch antenna. By comparing the measured responsivity of these two imagers, the gain improvement of the DRA over the on-chip patch antenna can be obtained. Three samples are measured to evaluate the robustness of the proposed antenna over process variation. The measured results show that the maximum gain improvement of 6.7 dB can be acquired at 327 GHz. The proposed DRA with the integrated CMOS imager is also employed to successfully demonstrate a THz transmissive imaging system at 327 GHz. To the best of authors’ knowledge, this is first higher order mode DRA working at THz frequencies.

Terahertz electron cyclotron maser interactions with an axis-encircling electron beam

To generate terahertz radiation via the electron cyclotron maser instability, harmonic interactions are essential in order to reduce the required magnetic field to a practical value. Also, high-order mode operation is required to avoid excessive Ohmic losses. The weaker harmonic interaction and mode competition associated with an over-moded structure present challenging problems to overcome. The axis-encircling electron beam is a well-known recipe for both problems. It strengthens the harmonic interaction, as well as minimizing the competing modes. Here, we examine these advantages through a broad data base obtained for a low-power, step-tunable, gyrotron oscillator. Linear results indicate far more higher-harmonic modes can be excited with an axis-encircling electron beam than with an off-axis electron beam. However, multi-mode, time-dependent simulations reveal an intrinsic tendency for a higher-harmonic mode to switch over to a lower-harmonic mode at a high beam current or upon a rapid current rise. Methods are presented to identify the narrow windows in the parameter space for stable harmonic interactions.

A Novel Wideband Differentially-Fed Higher-Order Mode Millimeter-Wave Patch Antenna

This paper presents a novel differentially-fed wideband patch antenna working at millimeter-wave band. The antenna is fabricated using conventional low-cost printed-circuit-board and plated-through-hole technologies and the patch size of the element has been enlarged by exploiting higher-order mode operation for good fabrication tolerance. The antenna element is a 0.86 guided wavelength ( $\lambda_{\rm g}$ at 60 GHz) long patch shorted at one end. A slot of 0.09 $\lambda_{\rm g}$ in width is introduced on the shorted patch and the distance between the slot center to the other end of the patch is 0.5 $\lambda_{\rm g}$ . The shorted patch resonates at ${\rm TM}_{30}$ mode and the 0.46 $\lambda_{\rm g}$ patch, created by presence of the slot, operates at ${\rm TM}_{10}$ mode. The combination of these two modes broadens the impedance bandwidth of the element to 18%. Asymmetric radiation patterns in the E-plane and large cross-polarization levels in the H-plane generated by a single patch are eradicated by feeding two mirrored elements differentially. The designed differentially-fed prototype has more than 18% measured impedance bandwidth, stable gain at 9–10 dBi and symmetric and stable radiation patterns across the operating band from 56 to 67 GHz.

Dispersion properties of plasma-filled metallic photonic crystal slow-wave structure

Plasma-filled slow-wave devices provide a new way to develop high efficiency and high power vacuum-electron microwave sources, but their theoretical analysis and simulation is difficult. This paper introduces the wheel spoke antenna to excite signals for analyzing the dispersion characteristics of resonant cavity with plasma-filled metallic photonic crystal slow-wave structure (SWS). Influences of parameters of the SWS and plasma density on dispersion characteristics of the SWS are studied. Results show that there is little difference in dispersion characteristics obtained by wheel spoke antenna excitation of signals and other methods without plasma filling. When plasma fills in the SWS, the frequency of zero mode is consistent with the previous results obtained by other methods. Hence, both the results with and without plasma filling demonstrate that the wheel spoke antenna signal-excitation method is effective. Moreover, decreasing the thickness of wheel spoke antenna properly and the distance between the antenna and reflection surface of the metal plate can reduce the wheel spoke antenna influence on the cavity resonance frequency. Furthermore, thicker antenna can excite the slow wave field easily, while thinner antenna can excite the resonant mode easily. Besides, the outer radius and thickness of the SWS plate have little influence on the dispersion characteristics, while the period length and the inner radius of the SWS plate have greater influence on the dispersion characteristics. In addition, the dispersion curves of frequency and phase velocity will move to higher frequency regions with the increase of plasma density. Further, the influence of plasma filling on low-order modes is greater than that on higher order modes. It is also found that the higher-order mode operation can reduce the size of cavity and the velocity of the electron beam.

Periodically Allocated Reentrant Cavity Klystron

A novel klystron topology is proposed to overcome the frequency limits of conventional klystrons. In contrast to the conventional klystron approach, where the distance between the cavities placed along the drift tube is a function of the plasma frequency wavelength, the new configuration exploits the interaction property of a sequence of numerous cavities allocated at fixed short distance along the drift tube. The remarkable advantage is to obtain good performance by the use of cavities with low R/Q, as in the case of high-frequency and high-order mode operation. A frequency operation up to 136 GHz with 20-dB gain and an output power up to a few tens of watts are demonstrated. The promising performance of the periodically allocated reentrant cavity klystron makes it attractive for millimeter-wave applications.

A method of suppressing mode competition in a coaxial localized-defect Bragg resonator operating in a higher-order mode

A coaxial localized-defect Bragg resonator has potential applications in high-power CARM oscillators. When it operates at sub-terahertz and terahertz frequencies, a higher-order mode is always required so as to get enough large geometry size. Analysis shows that higher-order mode operation may cause undesired mode competition due to the localized defect coupling the operating mode with its neighboring modes. A simple but efficient method is presented to solve the mode competition problem, where Hamming windowing-function distribution is separately applied to both sides of the localized defect.

Operation of a Ka-band Harmonic-Multiplying Gyrotron Traveling-Wave Tube

Amplification studies of a Ka-band harmonic-multiplying gyrotron traveling-wave tube (gyro-TWT) are reported. A coupled cavity with inner- and outer-intersecting couplings and precise control of the focusing magnetic field are used to improve the coupling between the fundamental and the harmonic and to adapt the sensitivity of the effect of focusing magnetic field on the beam-wave interaction in the gyro-TWT to high-order mode operation. Experimental results show that 75 kW of peak output power with a bandwidth of 1.06% and a gain of more than 25 dB can be reached at a pulse voltage of 67.2 kV and a pulse current of 15.8 A.

Comparison between Single-mode Treatment and Multi-mode Treatment of a Coaxial Bragg Resonator for a Terahertz Cyclotron Autoresonance Maser Oscillator

As a coaxial Bragg structure has more complex geometry configuration than a cylindrical or a planar Bragg structure, for simplicity of calculation a single-mode treatment is often adopted. However, heterogeneity of a coaxial Bragg structure always gives rise to the multi-mode conversion, which can change the performance of the resonator. Based on the scattering matrix method, detailed comparison between the single-mode treatment and the multi-mode treatment is presented to a coaxial Bragg resonator for a cyclotron autoresonance maser (CARM) oscillator operating in the higher-order mode TE61 at frequency of 0.35 THz. Results indicate that there is a little difference between the results obtained by these two treatments to the upstream reflector due to its shallow ripple amplitude, whereas to the downstream reflector, due to its relatively deep ripple amplitude, an apparent difference occurs at the operating frequency and consequently it gives notable difference to the frequency response and Q-factor of the whole resonator. Optimization shows possible application of a coaxial Bragg resonator with a single higher-order mode operation in constructing a terahertz CARM oscillator.

900-mW high brightness buried ridge lasers by selective area epitaxy

A study has been made of high power single lateral mode buried ridge lasers fabricated by selective area epitaxy. Several ridge thicknesses have been evaluated simultaneously in a single fabrication run. These lasers operate purely in the fundamental mode to output powers in excess of 450 mW, after which they are subject to beam steering or higher order mode operation. For weakly guided lasers, the output remains in a narrow lobe [full-width at half-maximum (FWHM)] = 4/spl deg/-6/spl deg/, stable at a given current value, up to output powers of 900 mW in CW tests.

Short submillimeter operation of the Planar Orotron

Recent experimental results demonstrating Planar Orotron operation in the short submillimeter wavelength regime of 1 mm to 400 μm are presented. This device belongs to the class of Smith-Purcell free electron lasers that utilize periodic metal grating structures to support electromagnetic waves with phase velocities lower than the speed of light. The J· E interaction between this slow wave and a mildly relativistic electron beam (25–150 keV) produces coherent radiation. Supporting theory involving dispersive and tuning characteristics of specific grating geometries is discussed. Single-particle gain calculations for fundamental and higher order mode operation are presented.

An X-band voltage tunable magnetron using a &amp;#920;-mode interdigital structure

Circuit coupling considerations make conventional pi mode VTM operation above C band an increasingly difficult task. This paper reviews a program, sponsored under U. S. Army Electronics Command Contract No. DA- 36-039-AMC-02199E, in which many of the restraints on higher frequency VTM operation have been relaxed through the use of the N/2±1 modes of an interdigital structure. Among the principal advantages of this higher order mode operation are: (a) circuit sizes in band comparable to conventional S band devices; (b) the opportunity for heavy coupling, and (c) a reduction in cathode radiation. The major complicating factor is that mode separation is not equivalent to the pi mode ease and that the two null field patterns of the N/2±1 modes are spatially degenerate. These problems, though, have been overcome by careful choice of vane number and by the use of mode positioners.

The effect of nonuniform population distributions in masers

The inverted population in a cavity maser has spatial dependence, produced by standing waves in the cavity, if the maser material is of the order of the cavity dimensions. It is shown that higher order mode operation of a maser, in which many nodes exist in the signal and pump fields, does not appreciably degrade the small-signal performance and, in fact, improves the saturation behavior of the maser amplifier. However, higher pump powers are required. A dielectric cavity maser (DCM), composed of a ruby rod separating two metal end plates, is described which exploits this phenomenon. Both the strength of the maser action, as measured by the paramagnetic filling factor and the saturation behavior of the device have been calculated and measured. When operated as an X -band amplifier, pumped at 35 Gc/s, gain bandwidth products up to 141 Mc/s have been observed. Power outputs greater than 0.1 mW have been measured using the DCM as an amplifier or an oscillator. Improvement in the saturation behavior of ∼20 dB compared to other masers is reported.