Quasi-optical Components Research Articles

3-D Printed Plug and Play Prototyping for Low-Cost Sub-THz Subsystems

Polymer-based additive manufacturing using 3-D printing for upper-millimeter-wave ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ca.</i> 100 to 300 GHz) frequency applications is now emerging. Building on our previous work, with metal-pipe rectangular waveguides and free-space quasi-optical components, this paper brings the two media together at G-band (140 to 220 GHz), by demonstrating a compact multi-channel front-end subsystem. Here, the proof-of-concept demonstrator integrates eight different types of 3-D printed components (30 individual components in total). In addition, the housing for two test platforms and the subsystem are all 3-D printed as single pieces, to support plug and play development; offering effortless component assembly and alignment. We introduce bespoke free-space TRM calibration and measurement schemes with our quasi-optical test platforms. Equal power splitting plays a critical role in our multi-channel application. Here, we introduce a broadband 3-D printed quasi-optical beamsplitter for upper-millimeter-wave applications. Our quantitative and/or qualitative performance evaluations for individual components and the complete integrated subsystem, demonstrate the potential for using consumer-level desktop 3-D printing technologies at such high frequencies. This work opens-up new opportunities for low-cost, rapid prototyping and small-batch production of complete millimeter-wave front-end subsystems.

A Fourier Optics Tool to Derive the Plane Wave Spectrum of Quasi-Optical Systems [EM Programmer's Notebook

Quasi-optical (QO) antennas employ feeds in combination with lenses or reflectors. Design optimization of such antennas can be expensive due to the high computational cost associated with the analysis of electrically large and multiscale structures. This issue's “EM Programmer's Notebook” column describes a freely available MATLAB-based tool for the fast analysis of QO antennas. It facilitates the design of QO antenna feeds through the introduction of a field-matching methodology. It could also be useful in the codesign of feeds and QO components. The tool provides valuable complimentary capabilities to commercial packages.

Polymer-Based 3-D Printed 140-220 GHz Low-Cost Quasi-Optical Components and Integrated Subsystem Assembly

Few examples of individual polymer-based 3-D printed quasi-optical component types have been previously demonstrated above ca. 100 GHz. This paper presents the characterization of polymer-based 3-D printed components and complete subsystems for quasi-optical applications operating at G-band (140 to 220 GHz). Two low-cost consumer-level 3-D printing technologies (vat polymerization and fused deposition modeling) are employed, normally associated with microwave frequencies and longer wavelength applications. Here, five different quasi-optical component types are investigated; rectangular horn antennas, 90° off-axis parabolic mirrors, radiation absorbent material (RAM), grid polarizers and dielectric lenses. As an alternative to conventional electroplating, gold-leaf gilding is used for the polarizer. A detailed investigation is undertaken to compare the performance of our 3-D printed antennas, mirrors and RAM with their commercial equivalents. In addition, a fully 3-D printed, RAM-lined housing with central two-axis rotational platform is constructed for performing two-port measurements of a quasi-optical horn-mirror-polarizer-mirror-horn subsystem. Measured results generally show excellent performances, although the grid polarizer is limited by the minimum strip width, separation distance and metallization thickness. The ultra-low cost, ‘plug and play’ housing is designed to give a fast measurement setup, while minimizing misaligning losses. Its RAM lining is designed to suppress reflections due to diffraction from components under test that may cause adverse multi-path interference. Our work investigates each component type at G-band and integrates them within subsystem assemblies; operating at frequencies well above those normally associated with low-cost consumer-level 3-D printing technologies. This opens-up new opportunities for rapid prototyping of complete low-cost front-end quasi-optical upper-millimeter-wave subsystems.

Beam Profiling of a Commercial Lens-Assisted Terahertz Time Domain Spectrometer

To undertake THz spectroscopy and imaging, and accurately design and predict the performance of quasi-optical components, knowledge of the parameters of the beam (ideally Gaussian) emitted from a THz source is paramount. Despite its proliferation, relatively little work has been done on this in the frame of broadband THz photoconductive antennas. Using primarily pinhole scanning methods, along with stepwise angular spectrum simulations, we investigate the profile and polarization characteristics of the beam emitted by a commercial silicon-lensintegrated THz photoconductive antenna and collimated by a TPX (polymethylpentene) lens. Our study flags the limitations of the different beam profiling methods and their impact on the beam Gaussianity estimation. A non-Gaussian asymmetric beam is observed, with main lobe beam waists along x and y varying from 8.4 ± 0.7 mm and 7.7±0.7 mm at 0.25THz,to1.4±0.7 mm and 1.4 ± 0.7 mm at 1 THz, respectively. Additionally, we report a maximum cross-polar component relative to the ON-axis co-polar component of -11.6 dB and -21.2 dB, at 0.25 THz and 1 THz, respectively.

Coherent Fourier Optics Model for the Synthesis of Large Format Lens-Based Focal Plane Arrays

Future sub millimeter imagers are being developed with large focal plane arrays (FPAs) of lenses to increase the field of view (FoV) and the imaging speed. A full-wave electromagnetic analysis of such arrays is numerically cumbersome and time-consuming. This article presents a spectral technique based on Fourier optics combined with geometrical optics for analyzing, in reception, lens-based FPAs with wide FoVs. The technique provides a numerically efficient methodology to derive the plane wave spectrum (PWS) of a secondary quasi-optical component. This PWS is used to calculate the power received by an antenna or absorber placed at the focal region of a lens. The method is applied to maximize the scanning performance of imagers with monolithically integrated lens feeds without employing an optimization algorithm. The derived PWS can be directly used to define the lens and feed properties. The synthesized FPA achieved scan losses much lower than the ones predicted by standard formulas for horn-based FPAs. In particular, an FPA with scan loss below 1 dB while scanning up to ±17.5° (~±44 beam-widths) is presented with directivity of 52 dBi complying with the needs for future sub millimeter imagers. The technique is validated via a physical optics code with excellent agreement.

Symmetry and Finite-Size Effects in Quasi-Optical Extraordinarily THz Transmitting Arrays of Tilted Slots

Extraordinarily transmitting arrays are promising candidates for quasi-optical (QO) components due to their high frequency selectivity and beam scanning capabilities owing to the leaky-wave mechanism involved. We show here how by breaking certain unit cell and lattice symmetries, one can achieve a rich family of transmission resonances associated with the leaky-wave dispersion along the surface of the array. By combining 2-D and 1-D periodic method of moments (MoM) calculations with QO terahertz (THz) time-domain measurements, we provide physical insights, numerical, and experimental demonstration of the different mechanisms involved in the resonances associated with the extraordinary transmission peaks and how these evolve with the number of slots. Thanks to the THz instrument used, we are also able to explore the time-dependent emission of the different frequency components involved.

Three‐dimensional printing technologies for terahertz applications: A review

Three-dimensional (3D) printing technologies enables fast prototyping of complex 3D objects with ever improving printing qualities. To date, 3D printing has been found useful in areas such as manufacturing, industrial design, aerospace, dental and medical industries, and many others. In this article, we review recent advances of 3D printing technologies for terahertz (THz) applications. Different 3D printing technologies and printable materials are first discussed and compared. 3D-printed THz components and devices, which are categorized as waveguides/fibers, antennas, and quasi-optical components, are further demonstrated. It is found that the performances and functionalities of 3D-printed THz devices have been greatly enhanced, while the operating frequencies have been increased from the lower end of THz range to over 1 THz region. With further development of novel materials and printing techniques, it is believed that 3D printing technologies will play an important role in the realization of THz components for efficient control and manipulation of THz waves.

Design and implementation of quasi-optical components for the upgrade of the TCV EC-system

The EC-system of the TCV tokamak is progressively being upgraded with the addition of two MW-class dual-frequency gyrotrons (84 and 126 GHz/2s/1MW). In order to connect the two gyrotrons to the existing low field side and top launchers, new waveguide routing from gyrotron hall to TCV tokamak was designed and dedicated Matching Optics Units (MOU) have been developed. The internal optics of the system have been determined aiming at optimal coupling to the HE11 waveguides. The laws of quasi-optics were used to find quadratic surfaces to shape an incoming Gaussian beam representative of the gyrotron output into a beam matching the proper field distribution at the waveguide entrance and with HE11 content compatible with the system requirements. A solution with one flat (movable) mirror and two shaping mirrors was found and characterized with the physical optics code GRASP. The resulting field distribution is then truncated and projected onto the HE11 component to evaluate the design solution (coupled power at the waveguide entrance >98.4% and HE11 content >96.8% for both frequencies). The model and the results of this analysis will be presented and compared to a model based on the Rayleigh–Sommerfeld scalar diffraction integral. GRASP was also used to evaluate preliminary misalignment effects in terms of coupled power to the waveguide and the MOU design moved to the manufacturing phase. In parallel to the gyrotron integration and to extend the level of flexibility of the TCV EC-system, a modular closed divertor chamber is developed, requiring the X3 top-launcher to be redesigned. Preliminary antenna conceptual design studies including new curvature to cope with the requirements of modularity and flexibility will be presented.

A Quasi-Optical Testbed for Wideband THz On-Wafer Measurements

We propose a novel quasi-optical topology to enable ultra-wideband on-wafer characterization in the THz regime. Using quasi-optical components, we implement a high-performance directional coupler and a noncontact probing scheme, to interrogate on-wafer devices and circuits. The use of quasi-optics allows superior signal isolation and wideband operation due to the frequency independent nature of focusing components. Additionally, the simple topology is amenable to integration with photonics-based THz emitters and detectors to enable a true THz testbed for vector measurement capabilities. A proof-of-concept testbed is designed and evaluated in the 300–340 GHz band using free-space and on-wafer measurements. The on-wafer characterization of passive devices shows good agreement with simulated results and repeatability of <2% for reflection coefficient magnitude and <4° for phase deviation.

Millimeter-Wave Faraday Rotation From Ferromagnetic Nanowires

Nickel nanowires embedded in track-etched polycarbonate membrane filters have been used to impart Faraday rotation on millimeter wave signals with frequencies of 61.25 GHz. A customized apparatus for measuring Faraday rotation was designed and built using quasi-optical components. Initial experiments were performed to ensure that the non-reciprocal polarization rotation, attributed to the Faraday effect, had direct proportionality to the strength of a static magnetic field applied in the direction of the millimeter wave propagation. Experiments were also performed to rule out the possibility of measuring reciprocal polarization rotation due to birefringence and to calculate the Faraday rotation angle in these artificial ferrites. A Verdet constant was extracted from experimental data and found to have a relatively high value of approximately $\text{25.5}\times \text{10}^{3}$ rad $\cdot$ T $^{-1}\cdot$ m $^{-1}$ .

Aerosol Jet Printed Quasi-Optical Terahertz Components

This paper presents the design, fabrication, and characterization of aerosol jet printed quasi-optical components for operation in the terahertz (THz) spectral range. Performance of each of these components is compared to their counterpart fabricated by a lithographic process. Two band-stop filters, one designed for operation at 250 GHz and another at 550 GHz, as well as a bandpass filter for operation at 250 GHz, are designed and characterized. Additionally, polarizing screens are presented, demonstrating high-resolution capabilities of the aerosol printing technique. Enhancement of the printed structures by means of an electroless metal plating process is demonstrated using an aerosol jet printed seed layer.

Reception Power Pattern of Distributed Absorbers in Focal Plane Arrays: A Fourier Optics Analysis

Passive imaging cameras at millimeter and sub-millimeter wavelengths are currently entering a new era with the development of large format arrays of direct detectors. Several of these arrays are being developed with bare absorbing meshes without any antenna coupling (lens or horn) structures. The design of such arrays is typically done resorting to geometrical considerations or basic broadside plane wave incidence analysis. This paper presents a spectral technique for the analysis of such focal plane arrays in reception using Fourier Optics, which is valid also for moderately skewed incident angles. The analysis constitutes a step improvement with respect to previously used methods by providing an accurate and efficient way to estimate the point-source angular response and the throughput from a distributed incoherent source of an absorbing mesh in the focal plane of a quasi-optical component (e.g., a parabolic reflector or lens). The proposed technique is validated with full-wave simulations. After presenting the analysis, this paper compares the performance of arrays of bare absorber in the focal plane of a quasi-optical component to those of corresponding antenna based arrays. It is found that absorbers lead to a comparable tradeoff, in terms of spill-over and focusing efficiency, only for very tight samplings. For larger samplings, the focusing efficiency of absorbers is significantly lower than the one for antennas.

Quasi-Optical Orthomode Splitters for Input–Output of a Powerful &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${W}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Band Gyro-TWT

Three different designs of an external microwave system for feeding and extracting power to and from a gyrotron traveling-wave tube (gyro-TWT) through one oversized port are presented and discussed. As it shown, such a microwave system should operate as a splitter which splits two wavebeams with mutually orthogonal polarizations incoming through one port into two spatially separated ports or transmission lines transporting each wavebeam individually. The orthomode splitters presented in this paper were designed to be used with a gyro-TWT producing 350- and 50-kW maximum pulsed and average powers, respectively, with a 3-dB bandwidth of 10 GHz at the center frequency of 95 GHz. Such a high-power level and wide bandwidth at short millimeter waves make quasi-optical components to be the most appropriate solutions in contrast to low-mode standard waveguide circuits.

3-D Printed Millimeter-Wave Lens Systems at 39 GHz

This letter explores the potential of 3-D printing technologies for the development of free-space quasi-optical components operating at 39 GHz. Within this study, the design process, simulation, fabrication, and testing of the lens systems will be detailed and explored. The two highest gain lens systems produced from this study are the subjects of primary interest. The first lens system produced is unique in that the lenses are prealigned using a printed support structure, and the entire system is printed as a single unit. Meanwhile, the second system to be printed is unique in that it uses a separate cavity and individually printed lenses to allow for in-test experimentation and improved resolution of the printed lenses. The single print biconvex lens system achieved a gain of 12.9 dB, while the second lens system achieved a gain of 18.6 dB, with both results matching the simulation results to within 2 dB. This corroboration between results and simulation stands as an excellent justification for future progression to higher frequency systems.

Multibeam Antenna Technologies for 5G Wireless Communications

With the demanding system requirements for the fifth-generation (5G) wireless communications and the severe spectrum shortage at conventional cellular frequencies, multibeam antenna systems operating in the millimeter-wave frequency bands have attracted a lot of research interest and have been actively investigated. They represent the key antenna technology for supporting a high data transmission rate, an improved signal-to-interference-plus-noise ratio, an increased spectral and energy efficiency, and versatile beam shaping, thereby holding a great promise in serving as the critical infrastructure for enabling beamforming and massive multiple-input multiple-output (MIMO) that boost the 5G. This paper provides an overview of the existing multibeam antenna technologies which include the passive multibeam antennas (MBAs) based on quasi-optical components and beamforming circuits, multibeam phased-array antennas enabled by various phase-shifting methods, and digital MBAs with different system architectures. Specifically, their principles of operation, design, and implementation, as well as a number of illustrative application examples are reviewed. Finally, the suitability of these MBAs for the future 5G massive MIMO wireless systems as well as the associated challenges is discussed.

Design and characterization of a W-band system for modulated DNP experiments

Magnetic-field and microwave-frequency modulated DNP experiments have been shown to yield improved enhancements over conventional DNP techniques, and even to shorten polarization build-up times. The resulting increase in signal-to-noise ratios can lead to significantly shorter acquisition times in signal-limited multi-dimensional NMR experiments and pave the way to the study of even smaller sample volumes. In this paper we describe the design and performance of a broadband system for microwave frequency- and amplitude-modulated DNP that has been engineered to minimize both microwave and thermal losses during operation at liquid helium temperatures. The system incorporates a flexible source that can generate arbitrary waveforms at 94GHz with a bandwidth greater than 1GHz, as well as a probe that efficiently transmits the millimeter waves from room temperature outside the magnet to a cryogenic environment inside the magnet. Using a thin-walled brass tube as an overmoded waveguide to transmit a hybrid HE11 mode, it is possible to limit the losses to 1dB across a 2GHz bandwidth. The loss is dominated by the presence of a quartz window used to isolate the waveguide pipe. This performance is comparable to systems with corrugated waveguide or quasi-optical components. The overall excitation bandwidth of the probe is seen to be primarily determined by the final antenna or resonator used to excite the sample and its coupling to the NMR RF coil. Understanding the instrumental limitations imposed on any modulation scheme is key to understanding the observed DNP results and potentially identifying the underlying mechanisms. We demonstrate the utility of our design with a set of triangular frequency-modulated DNP experiments.

Millimetre wavelength variable focusing antenna for power beaming and active denial systems

A design of a millimetre wavelength antenna with adjustable focal length is demonstrated in this study. This antenna is designed to operate in the near field regime, and has the ability to change the focal plane position. The structure of the antenna is based on a Cassegrain configuration in order to deliver maximum power density to a pre-defined focal plane. Possible applications are radioactive power beaming and active denial systems. The analysis and development of this antenna were carried-out using the Gaussian beam model and the ABCD matrix method in order to find the focus position and spot size as a function of the axial distance. The development of the quasi-optical components of the Cassegrain antenna, simulation results and experimental results are described in this work.

ϵ-near-zero (ENZ) graded index quasi-optical devices: steering and splitting millimeter waves

Graded index ϵ-near-zero (GRIN-ENZ) quasi-optical components such as beam steerers and power splitters are designed, simulated and analyzed. The GRIN-ENZ medium is realized using stacked narrow hollow waveguides whose infinite array shows the first transmission peak at 1.0002 THz. Several GRIN-ENZ beam steerers to channel a normal incident plane wave to different output angles (15, 45, 65 and 80°) with good impedance matching (low reflection) are analytically and numerically demonstrated using planar structures with a thickness of 5λ0 = 1500 μm along the z-axis. Moreover, symmetrical and asymmetrical power splitters are designed with output angles (±45°) and (−80, +35°), respectively.

Quasi 3D ECE imaging system for study of MHD instabilities in KSTAR

A second electron cyclotron emission imaging (ECEI) system has been installed on the KSTAR tokamak, toroidally separated by 1/16th of the torus from the first ECEI system. For the first time, the dynamical evolutions of MHD instabilities from the plasma core to the edge have been visualized in quasi-3D for a wide range of the KSTAR operation (B0 = 1.7∼3.5 T). This flexible diagnostic capability has been realized by substantial improvements in large-aperture quasi-optical microwave components including the development of broad-band polarization rotators for imaging of the fundamental ordinary ECE as well as the usual 2nd harmonic extraordinary ECE.

Study of a millimeter-wave squint indirect holographic algorithm suitable for imaging with large field-of-view

In this paper a millimeter-wave (MMW) squint indirect holographic method is presented, which is suitable for imaging with a large field-of-view. The proposed system employs the squint operation mode to remove the background and twin-image interferences, which achieves a similar effect to off-axis holography but leaves out the large-aperture quasi-optical component. The translational scanning manner enables a large field of view and ensures the image uniformity, which is difficult to realize in off-axis holography. In addition, a corresponding imaging algorithm for the presented scheme is developed to reconstruct the image from the recorded hologram. Some imaging results on typical objects, obtained with electromagnetic simulation, demonstrate good performance of the imaging scheme and validate the effectiveness of the image reconstruction algorithm.