Viktor Krozer

THz Active Imaging Systems With Real-Time Capabilities

This paper presents a survey of the status of five active THz imaging modalities which we have developed and investigated during the last few years with the goal to explore their potential for real-time imaging. We start out by introducing a novel waveguide-based all-electronic imaging system which operates at 812 GHz. Its salient feature is a 32-pixel linear detector array heterodyne-operated at the eighth subharmonic. This array in combination with a telescope optics for object distances of 2-6 m reaches a data acquisition speed suited for real-time imaging. The second system described then is again an all-electronic scanner (now for around 300 GHz ), designed for object distances of ≥ 8 m , which combines mechanical scanning in vertical direction, synthetic-aperture image generation in horizontal direction, and frequency-modulated continuous-wave sweeping for the depth information. The third and fourth systems follow an optoelectronic approach by relying on several- to multi-pixel parallel electrooptic detection. One imager is based on a pulsed THz-OPO and homodyne detection with a CCD camera, the other on either continuous-wave electronic or femtosecond optoelectronic THz sources and a photonic-mixing device (PMD) camera. The article concludes with a description of the state of the art of imaging with focal-plane arrays based on CMOS field-effect transistors.

Terahertz Imaging Systems With Aperture Synthesis Techniques

This paper presents the research and development of two terahertz imaging systems based on photonic and electronic principles, respectively. As part of this study, a survey of ongoing research in the field of terahertz imaging is provided focusing on security applications. Existing terahertz imaging systems are reviewed in terms of the employed architecture and data processing strategies. Active multichannel measurement method is found to be promising for real-time applications among the various terahertz imaging techniques and is chosen as a basis for the imaging instruments presented in this paper. An active system operation allows for a wide dynamic range, which is important for image quality. The described instruments employ a multichannel high-sensitivity heterodyne architecture and aperture filling techniques, with close to real-time image acquisition time. In the case of the photonic imaging system, mechanical scanning is completely obsolete. We show 2-D images of simulated 3-D image data for both systems. The reconstruction algorithms are suitable for 3-D real-time operation, only limited by mechanical scanning.

Modeling and design aspects of millimeter-wave and submillimeter-wave Schottky diode varactor frequency multipliers

Design and optimization of Schottky varactor diode frequency multipliers for millimeter and submillimeter wavelengths are generally performed using harmonic balance techniques together with equivalent-circuit models. Using this approach, it is difficult to design and optimize the device and multiplier circuit simultaneously. The work presented in this paper avoids the need of equivalent circuits by integrating a numerical simulator for Schottky diodes into a circuit simulator. The good agreement between the calculated and published experimental data for the output power and conversion efficiency originates from the accurate physical model. The limiting effects of multiplier performance such as breakdown, forward conduction, or saturation velocity are discussed in view of the optimum circuit conditions for multiplier operation including bias point, input power, and loads at different harmonics. It is shown that the onset of forward or reverse current flow is responsible for the limitation in the conversion efficiency.

Coupled Transmission Lines as Impedance Transformer

A theoretical investigation of the use of a coupled line section as an impedance transformer is presented. We show how to properly select the terminations of the coupled line structures for effective matching of real and complex loads in both narrow and wide frequency ranges. The corresponding circuit configurations and the design procedures are proposed. Synthesis relations are derived and provided for efficient matching circuit construction. Design examples are given to demonstrate the flexibility and limitations of the design methods and to show their validity for practical applications. Wideband matching performance with relative bandwidth beyond 100% and return loss RL > 20 dB is demonstrated both theoretically and experimentally. Good agreement is achieved between the measured and predicted performance of the coupled line transformer section.

CMOS detector arrays in a virtual 10-kilopixel camera for coherent terahertz real-time imaging

We demonstrate the principle applicability of antenna-coupled complementary metal oxide semiconductor (CMOS) field-effect transistor arrays as cameras for real-time coherent imaging at 591.4 GHz. By scanning a few detectors across the image plane, we synthesize a focal-plane array of 100×100 pixels with an active area of 20×20 mm2, which is applied to imaging in transmission and reflection geometries. Individual detector pixels exhibit a voltage conversion loss of 24 dB and a noise figure of 41 dB for 16 μW of the local oscillator (LO) drive. For object illumination, we use a radio-frequency (RF) source with 432 μW at 590 GHz. Coherent detection is realized by quasioptical superposition of the image and the LO beam with 247 μW. At an effective frame rate of 17 Hz, we achieve a maximum dynamic range of 30 dB in the center of the image and more than 20 dB within a disk of 18 mm diameter. The system has been used for surface reconstruction resolving a height difference in the μm range.

Design and Realization Aspects of 1-THz Cascade Backward Wave Amplifier Based on Double Corrugated Waveguide

The design and fabrication challenges in the first ever attempt to realize a 1-THz vacuum tube amplifier are described. Implementation of innovative solutions including a slow-wave structure in the form of a double corrugated waveguide, lateral tapered input and output couplers, deep X-ray LIGA fabrication process, and a cascade architecture of the backward wave amplifier are discussed. New knowledge in the field of terahertz vacuum devices brought by intensive simulations and development of advanced fabrication and assembly processes of the micro-structures is highlighted.

Antenna-coupled field-effect transistors for multi-spectral terahertz imaging up to 4.25 THz

We demonstrate for the first time the applicability of antenna-coupled field-effect transistors for the detection of terahertz radiation (TeraFETs) for multi-spectral imaging from 0.76 to 4.25 THz. TeraFETs were fabricated in a commercial 90-nm CMOS process and noise-equivalent powers of 59, 20, 63, 85 and 110 pW/√(Hz) at 0.216, 0.59, 2,52, 3.11 and 4.25 THz, respectively, have been achieved. A set of TeraFETs has been applied in raster-scan transmission and reflection imaging of pellets of sucrose and tartaric acid simulating common plastic explosives. Transmittance values are in good agreement with Fourier-transform infrared spectroscopy data. The spatial distribution of the components in the samples has been determined from the transmission data using principal component analysis.

EM simulation accuracy enhancement for broadband modeling of on-wafer passive components

This paper describes methods for accuracy enhancement in broadband modeling of on-wafer passive components using electromagnetic (EM) simulation. It is shown that standard excitation schemes for integrated component simulation leads to poor correlation with on-wafer measurements beyond the lower GHz frequency range. We show that this is due to parasitic effects and higher-order modes caused by the excitation schemes. We propose a simple equivalent circuit for the parasitic effects in the well-known ground ring excitation scheme. An extended L-2L calibration method is shown to improve significantly the accuracy of the on-wafer component modeling, when applied to parasitic effect removal associated with the excitation schemes.

Towards Three-Dimensional Millimeter-Wave Radar With the Bistatic Fast-Factorized Back-Projection Algorithm—Potential and Limitations

In this paper, we report on a time-domain approach for 3-D synthetic image reconstruction at stand-off distances called the bistatic fast-factorized back-projection (BiFFBP) algorithm. Although the algorithm is suited for multiple purposes, it is applied in this paper to a millimeter-wave radar system that operates in a frequency-modulated continuous-wave mode between 234 and 306 GHz. After initially mapping the bistatic to a quasi-monostatic configuration, the algorithm recursively factorizes both, the aperture positions and the target area. Three-dimensional reconstructions are shown for a simulated point-target in order to evaluate the point-spread-function of the system. In addition, 2-D-imaging is performed on real objects at stand-off distances using a scanner system that consists of 8 transmitters and 16 receivers. Reconstructions with the BiFFBP-algorithm are compared with the global back-projection (GBP) algorithm that serves as a benchmark. The results show that the BiFFBP-approach yields similar results to the GBP with respect to dynamic range in the image and the overall image quality. It is also shown that a resolution of 2 cm can be achieved with relatively few elements, no scanning, and over a large field-of-view.

Oscillator Phase Noise: A Geometrical Approach

We construct a coordinate-independent description of oscillator linear response through a decomposition scheme derived independently of any Floquet theoretic results. Trading matrix algebra for a simpler graphical methodology, the text will present the reader with an opportunity to gain an intuitive understanding of the well-known phase noise macromodel. The topics discussed in this paper include the following: orthogonal decompositions, AM-PM conversion, and nonhyperbolic oscillator noise response.