BACKGROUND Three-dimensional (3D) printing, known more formally as additive manufacturing, has become the focus of media and public attention in recent years as the decades-old technology has at last approached the performance necessary for direct production of end-use devices. The most popular forms of standard 3D printing include vat photopolymerization, powder bed fusion, material extrusion, sheet lamination, directed energy deposition, material jetting, and binder jetting, each creating parts layer by layer and offering different options in terms of cost, feature detail, and materials. Whereas traditional manufacturing technologies, such as casting, forging, machining, and injection molding, are well suited for mass production of identical commodity items, 3D printing allows for the creation of complex geometric shapes that can be mass-customized, because no die or mold is required and design concepts are translated into products through direct digital manufacturing. Furthermore, the additively layered approach enables the merging of multiple components into a single piece, which removes the requirement for subsequent assembly operations. Recently, the patents for the original 3D printing processes have begun to expire, which is resulting in a burgeoning number of low-cost desktop systems that provide increased accessibility to society at large. Industry has recognized the manufacturing advantages of these technologies and is investing in production systems to make complex components for jet engines, customized bodies for cars, and even pharmaceuticals. Although standard 3D printing technologies have advanced so that it is now possible to print in a wide range of materials including metals, ceramics, and polymers, the resulting structures are generally limited to a single material, or, at best, a limited number of compatible materials. ADVANCES For the technology to become more widely adopted in mainstream manufacturing, 3D printing must provide end-use products by fabricating more than just simple structures with sufficient mechanical strength to retain shape. Recently, research has resulted in the capability to use new materials with commercial 3D printers, and customized printers have been enhanced with complementary traditional manufacturing processes, an approach known as multiprocess or hybrid 3D printing. Collectively, these advancements are leading to fabrications that are not only geometrically complex, but functionally complex as well. By introducing the robotic placement of components, micromachining for intricate detail, embedding of wires, and dispensing of functional inks, complex structures can be constructed with additional electronic, electromagnetic, optical, thermodynamic, chemical, and electromechanical content. OUTLOOK Multiprocess 3D printing is a nascent area of research in which basic 3D printing is augmented to fabricate structures with multifunctionality. Progress will lead to local manufacturing with customized 3D spatial control of material, geometry, and placement of subcomponents. This next generation of printers will allow for the fabrication of arbitrarily shaped end-use devices, leading to direct and distributed manufacturing of products ranging from human organs to satellites. The ramifications are substantial, given that 3D printing will enable the fabrication of customer-specific products locally and on demand, improving personalization and reducing shipping costs and delays. Examples could include replacement components for grain-milling equipment in a remote village in the developing world, biomedical devices created specifically for a patient in a hospital before surgery, and satellite components printed in orbit, thus avoiding the delays and costs associated with launch operations. The automotive, aerospace, defense, pharmaceutical, biomedical, and consumer industries, among others, will benefit from the new design and manufacturing freedom made possible by multiprocess 3D printing.

http://science.sciencemag.org/content/sci/353/6307/aaf2093.full.pdf

Multiprocess 3D printing for increasing component functionality.

This paper first reviews manufacturing technologies for realizing air-filled metal-pipe rectangular waveguides (MPRWGs) and 3-D printing for microwave and millimeter-wave applications. Then, 3-D printed MPRWGs are investigated in detail. Two very different 3-D printing technologies have been considered: low-cost lower-resolution fused deposition modeling for microwave applications and higher-cost high-resolution stereolithography for millimeter-wave applications. Measurements against traceable standards in MPRWGs were performed by the U.K.’s National Physical Laboratory. It was found that the performance of the 3-D printed MPRWGs were comparable with those of standard waveguides. For example, across X-band (8–12 GHz), the dissipative attenuation ranges between 0.2 and 0.6 dB/m, with a worst case return loss of 32 dB; at W-band (75–110 GHz), the dissipative attenuation was 11 dB/m at the band edges, with a worst case return loss of 19 dB. Finally, a high-performance W-band sixth-order inductive iris bandpass filter, having a center frequency of 107.2 GHz and a 6.8-GHz bandwidth, was demonstrated. The measured insertion loss of the complete structure (filter, feed sections, and flanges) was only 0.95 dB at center frequency, giving an unloaded quality factor of 152—clearly demonstrating the potential of this low-cost manufacturing technology, offering the advantages of lightweight rapid prototyping/manufacturing and relatively very low cost when compared with traditional (micro)machining.

3-D Printed Metal-Pipe Rectangular Waveguides

Additive manufacturing (AM), also known as three-dimensional (3D) printing, has boomed over the last 30 years, and its use has accelerated during the last 5 years AM is a materials-oriented manufacturing technology, and printing resolution versus printing scalability/speed trade-off exists among various types of materials, including polymers, metals, ceramics, glasses, and composite materials Four-dimensional (4D) printing, together with versatile transformation systems, drives researchers to achieve and utilize high dimensional AM Multiple perspectives of the AM of structural materials have been raised and illustrated in this review, including multi-material AM (MMa-AM), multi-modulus AM (MMo-AM), multi-scale AM (MSc-AM), multi-system AM (MSy-AM), multi-dimensional AM (MD-AM), and multi-function AM (MF-AM) The rapid and tremendous development of AM materials and methods offers great potential for structural applications, such as in the aerospace field, the biomedical field, electronic devices, nuclear industry, flexible and wearable devices, soft sensors, actuators, and robotics, jewelry and art decorations, land transportation, underwater devices, and porous structures

Additive manufacturing of structural materials

Three-dimensional-printing technologies have been receiving great attention for a wide variety of applications in recent years for cost effectiveness, eco-friendliness, and process simplicity in complicated structures. This paper describes the challenges and solutions of applying 3-D-printing technologies in fabricating passive millimeter-wave (mmWave) and terahertz (THz) devices. First, we review the state-of-the-art dielectric 3-D-printed passive mmWave and THz devices. Then, we focus on our novel 3-D-printed metallic passive mmWave and THz devices such as horn antennas and waveguides. Next, we analyze the dimensional tolerance and surface roughness of various 3-D-printing technologies in order to provide a guide for choosing an appropriate technology for specific applications. Finally, we summarize the current work and identify the future studies in material powder refinement, surface treatment, optimization of the print process, development of hybrid dielectric and metallic 3-D-printing technology for realizing not only simple mmWave and THz devices but also sophisticated mmWave and THz systems.

Investigation on 3-D-Printing Technologies for Millimeter- Wave and Terahertz Applications

Vertical-external-cavity surface-emitting lasers (VECSELs) are the most versatile laser sources, combining unique features such as wide spectral coverage, ultrashort pulse operation, low noise properties, high output power, high brightness and compact form-factor. This paper reviews the recent technological developments of VECSELs in connection with the new milestones that continue to pave the way towards their use in numerous applications. Significant attention is devoted to the fabrication of VECSEL gain mirrors in challenging wavelength regions, especially at the yellow and red wavelengths. The reviewed fabrication approaches address wafer-bonded VECSEL structures as well as the use of hybrid mirror structures. Moreover, a comprehensive summary of VECSEL characterization methods is presented; the discussion covers different stages of VECSEL development and different operation regimes, pointing out specific characterization techniques for each of them. Finally, several emerging applications are discussed, with emphasis on the unique application objectives that VECSELs render possible, for example in atom and molecular physics, dermatology and spectroscopy.

https://iopscience.iop.org/article/10.1088/1361-6463/aa7bfd/pdf

Optically pumped VECSELs: review of technology and progress

In this work, we designed, built, and tested a low-gain 20 dBi Luneburg Lens antenna using a rapid prototyping machine as a proof of concept demonstrator. The required continuously varying relative permittivity profile was implemented by changing the size of plastic blocks centered on the junctions of a plastic rod space frame. A 12-cm ( 4λ0 at 10 GHz) diameter lens is designed to work at X-band. The effective permittivity of the unit cell is calculated by effective medium theory and simulated by full-wave finite-element simulations. The fabrication is implemented by a polymer jetting rapid prototyping method. In the measurement, the lens antenna is fed by an X-band waveguide. The measured gain of the antenna at X-band is from 17.3 to 20.3 dB. The measured half-power beam width is from 19° to 12.7° while the side lobes are about 25 dB below the main peak. Good agreement between simulation and experimental results is obtained.

A 3-D Luneburg Lens Antenna Fabricated by Polymer Jetting Rapid Prototyping

The design, fabrication, and characterization of a hybrid metal–semiconductor distributed Bragg reflector (DBR) for optically pumped vertical external cavity surface emitting laser are reported. The realization of a pure gold reflector attached to an AlGaAs/AlAs DBR is achieved through the realization of a lithography pattern alternating gold and titanium areas for better surface adhesion. This reduces from 28 to 14, and the number of DBR pairs is needed to achieve a reflectivity above 99.9%. Experimental results are supported by simulations and show an output power beyond 4 W with an optical efficiency of 19% and very low thermal impedance, validating the excellent reflectivity and bonding quality of the device.

Design and Fabrication of Hybrid Metal Semiconductor Mirror for High-Power VECSEL

In this paper, the design, fabrication and characterization of a 3D printed Luneburg lens antenna working at Ka and Q band are proposed. Gradient index control of the lens is based on the mixing ratio of air voids and polymer. The effective dielectric constant of the unit cell was estimated using effective medium theory and extracted from full-wave finite-element simulation results. The diameter of the lens was 7 cm. A 3D polymer jetting rapid prototyping technique was employed to fabricate the lens antenna. In the measurement, the fabricated lens antenna was fed by Ka and Q band waveguides and the measured radiation pattern showed this 3D printed lens works well as a high gain antenna.

Millimeter wave luneburg lens antenna fabricated by polymer jetting rapid prototyping

While Vertical-External-Cavity-Surface-Emitting-Lasers (VECSELs) have been successfully used as ultrafast laser sources with pulse durations in the hundreds of femtosecond regime, the dynamics within the semiconductor gain structure are not yet completely understood. With the high carrier densities inside the semiconductor, nonequilibrium effects such as kinetic-hole burning are expected to play a major role in pulse formation dynamics. Moreover, the nonlinear phase change by the intense light field can induce a complex dispersion, which may potentially limit the achievable pulse durations. To shed light on such nonequilibrium dynamics, we perform in-situ characterization of mode-locked VECSELs. We probe the gain media as well as the intracavity absorber with a femtosecond fiber laser source. For measuring temporal characteristics, we employ an asynchronous optical sampling technique by phase-locking the repetition rate of the VECSEL to a multiple of the probe laser with an adjustable offset frequency. This allows for probing dynamics from femtosecond to nanosecond time scales with scan rates up to hundreds of Hertz without compromise of measurement precision which can be introduced by mechanical delays covering such large temporal windows. With a resolution in the femtosecond range, we characterize gain depletion by the intracavity pulse as well as the gain recovery timescales for different power levels and operation regimes.

Ultrafast in-situ probing of passively mode-locked VECSEL dynamics

This article presents a dual-band amplifier with flexible frequency ratios The amplifier features novel Wilkinson power divider/combiner with dual-band property that allows large frequency ratio (216–49) between its first and second operating bands The measured combining efficiency on a stand-alone combiner is 97% at 225 GHz and 75% at 525 GHz A 225/55 GHz prototype amplifier is tested to verify dual-band functionality Measured linear gains are 1285 dB and 1207 dB at the first and second frequency bands, respectively © 2015 Wiley Periodicals, Inc Microwave Opt Technol Lett 57:2242–2247, 2015

Kokou Gbele

Papers

A 3-D Luneburg Lens Antenna Fabricated by Polymer Jetting Rapid Prototyping

Design and Fabrication of Hybrid Metal Semiconductor Mirror for High-Power VECSEL

Millimeter wave luneburg lens antenna fabricated by polymer jetting rapid prototyping

Ultrafast in-situ probing of passively mode-locked VECSEL dynamics

A dual-band amplifier with flexible frequency ratios