William J. Otter

3-D Printed Metal-Pipe Rectangular Waveguides

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.

Hybrid 3-D-Printing Technology for Tunable THz Applications

In recent years, additive manufacturing has experienced rapid growth, due to its inherent capabilities for creating arbitrary 3-D structures, accessibility, and associated low manufacturing costs. This paper first reviews the state of the art in 3-D printing for terahertz (THz) applications and identifies the critical features required for such applications. The future potential for this technology is demonstrated experimentally with the first 3-D-printed, optically controlled THz IQ vector modulator. Here, miniature high-resistivity silicon implants are integrated into metal-pipe rectangular waveguides. The 3-D-printed split-block assembly also houses two packaged infrared laser diodes and a heat sink. The measured performance of a proof-of-principle 4-quaternary amplitude modulation (4-QAM) vector modulator that operates up to 500 GHz is reported. This new hybrid 3-D printing THz technology, which combines semiconductor devices with potentially low-cost, high-performance passive guided-wave structures represents a paradigm shift and may prove to be an ideal solution for implementing affordable transceivers in future ubiquitous THz applications.

Dielectric measurements of nanoliter liquids with a photonic crystal resonator at terahertz frequencies

We present a highly sensitive technique for determining the complex permittivity of nanoliter liquid samples in the terahertz band based on a photonic crystal resonator and microcapillary. Liquids are characterized by using a capillary tube to introduce a ∼4 nl liquid sample into the electromagnetic field of a resonant mode confined by an L3 resonant cavity in a high-resistivity silicon photonic crystal slab. Monitoring the perturbation of the resonant frequency and unloaded Q-factor of the resonant mode at 100 GHz and ∼5800, respectively, allows a samples permittivity to be calculated. An analytical model describing the system response based on perturbation theory and quasi-static analysis of the electric field within the capillary is also presented and found to agree well with FEM simulations and experimental measurements of ethanol-water mixtures of various concentrations for low to moderate loss tangents of the liquid samples. We demonstrate the utility of this approach by measuring the complex permittivity of several bioliquids, including suspensions of red and white blood cells. These results represent a step towards a lab-on-a-chip device for the analysis of extremely small quantities of biological, toxic, explosive, and other liquid types at terahertz frequencies.

3-D Printed Variable Phase Shifter

This letter presents the first fully 3-D printed microwave variable phase shifter. The design methodology for a 3-D printable dielectric flap metal-pipe rectangular waveguide variable phase shifter is described. The ABS building material was independently characterized, revealing a dielectric constant of 2.34 and loss tangent of only 0.0015 at 10 GHz. The predicted and measured performance is given, demonstrating a maximum relative phase shift of 142° at 10 GHz, with a near uniform relative phase shift across the whole of X-band and a low variation in differential-phase group delay.

W-band laser-controlled photonic crystal variable attenuator

A photonic crystal W1 defect waveguide attenuator is demonstrated. Variable attenuation at W-band is achieved by illuminating the W1 defect waveguide with lasers with controllable output power levels. Measurements show that a minimum attenuation level of 45 dB is demonstrated across the whole 92-102 GHz band of operation of the W1 defect waveguide and >60 dB in a narrowband region of the operation.

3-D printing of microwave components for 21st century applications

Additive manufacturing using 3-D printing is an emerging technology for the production of high performance microwave and terahertz components. Traditionally, these components are made by (micro-)machining. However, recent advances in rapid prototyping technology have led to its use in creating high performance and low weight RF components. In this review paper ten state-of-the-art exemplars are described, covering a wide variety of applications (absorbers, waveguides, antennas and lenses) operating over a broad range of frequencies, from 8 to 330 GHz.

Millimeter-wave negative group delay network

This paper describes a negative differential-phase group delay network operating at millimeter-wave frequencies, which can be used to compensate for frequency variations in group delay that may otherwise cause signal distortion. This approach utilizes a defect waveguide with a high Q-factor defect cavity resonator, realized with a high-resistivity silicon-based 2D photonic crystal with engineered waveguide dispersion characteristics. We demonstrate a group delay of -151.8 ns at 99.74 GHz, with a correspondingly high figure of merit value of 3.34.

Technology demonstrators for low-cost terahertz engineering

There is no doubt that terahertz (THz) technology is rapidly growing in interest. Its frequency range lies in a commercially unexploited part of the electromagnetic spectrum. Front-end systems operating in the THz gap, i.e. frequency range between where the performance of conventional electronics falls off and that of photonics increases, have generally existed for expensive scientific applications and have relied on precision free-space (quasi-) optics and often cryogenic cooling. However, in order to move away from less profitable high-end applications, engineering solutions are needed to create a positive spiral of technological growth with more profitable ubiquitous applications. This review paper introduces recent examples of THz technologies that may have the potential for future commercial exploitation.

Microwave-to-terahertz dielectric resonators for liquid sensing in microfluidic systems

The microwave-to-terahertz frequency range offers unique opportunities for the sensing of liquids based on the degree of molecular orientational and electronic polarization, Debye relaxation due to intermolecular forces between (semi-)polar molecules and collective vibrational modes within complex molecules. Methods for the fast dielectric characterization of (sub-)nanolitre volumes of mostly aqueous liquids and biological cell suspensions are discussed, with emphasis on labon- chip approaches aimed towards single-cell detection and label-free flow cytometry at microwave-to-terahertz frequencies. Among the most promising approaches, photonic crystal defect cavities made from high-resistivity silicon are compared with metallic split-ring resonant systems and high quality factor (Q-factor) whispering gallery-type resonances in dielectric resonators. Applications range from accurate haemoglobin measurements on nanolitre samples to label-free detection of circulating tumor cells.

Micro- and millimetre wave measurements of nanolitre biological liquids by dielectric resonators

A variety of dielectric resonator techniques from low GHz frequencies towards 100 GHz has been investigated with respect to their suitability for highly accurate dielectric measurements on liquids within microfluidic systems. Whispering gallery type dielectric resonators have been employed for frequencies from 10 to 40 GHz and 2D photonic crystal slab defect resonators for 100 GHz and beyond. Experiments on organic aqueous solutions and cell suspension were analyzed in terms of their potential for label-free biosensor applications.