Abstract-THz Imaging Using Uncooled Wideband Direct Detection Focal Plane Arrays

Sven van Berkel,  Ozan Yurduseven ;  Angelo Freni,  Andrea Neto,  Nuria Llombart,

http://ieeexplore.ieee.org/document/8016629/


In millimeter and submillimeter-wave radiometric imaging systems, a persistent goal is the increase in the speed of acquisition of the image while maintaining a high sensitivity. Typically, the highest sensitivity is achieved by cryogenically cooling the detectors, specifically in astronomical applications. However, for the purpose of low-cost imaging applications, it is desirable to operate at room temperature. Without cryogenically cooling, the electronic noise introduced by the detectors becomes dominant, making the detectors less sensitive. Resorting to detection architectures containing amplification circuitry might be impractical for implementation in large focal plane arrays (FPAs) fabricated in integrated technologies. This contribution derives the focal plane architecture that maximizes the imaging speed of radiometers operating at room temperature without using any amplification circuitry. It is shown that in such scenario a practical image acquisition speed can still be achieved when a very broad portion of the THz-band is exploited. Ultimately, the imaging speed is maximized when the FPA is undersampled, implying a tradeoff in the size of the optics. The analysis is substantiated by a case study with recently developed wideband leaky lens antenna feeds operating from 200 to 600 GHz.

Abstract-Surface wave control for large arrays of microwave kinetic inductance detectors

Stephen J. C. Yates, Andrey M. Baryshev, Ozan Yurduseven, Juan Bueno, Kristina K. Davis, Lorenza Ferrari, Willem Jellema, Nuria Llombart, Vignesh Murugesan, David J. Thoen, Jochem J. A. Baselmans

(Submitted on 7 Jul 2017)

https://arxiv.org/abs/1707.02142

Large ultra-sensitive detector arrays are needed for present and future observatories for far infra-red, submillimeter wave (THz), and millimeter wave astronomy. With increasing array size, it is increasingly important to control stray radiation inside the detector chips themselves, the surface wave. We demonstrate this effect with focal plane arrays of 880 lens-antenna coupled Microwave Kinetic Inductance Detectors (MKIDs). Presented here are near field measurements of the MKID optical response versus the position on the array of a reimaged optical source. We demonstrate that the optical response of a detector in these arrays saturates off-pixel at the ∼−30 dB level compared to the peak pixel response. The result is that the power detected from a point source at the pixel position is almost identical to the stray response integrated over the chip area. With such a contribution, it would be impossible to measure extended sources, while the point source sensitivity is degraded due to an increase of the stray loading. However, we show that by incorporating an on-chip stray light absorber, the surface wave contribution is reduced by a factor >10. With the on-chip stray light absorber the point source response is close to simulations down to the ∼−35 dB level, the simulation based on an ideal Gaussian illumination of the optics. In addition, as a crosscheck we show that the extended source response of a single pixel in the array with the absorbing grid is in agreement with the integral of the point source measurements.

Abstract-Optomechanical System Design for Dual-Mode Stand-Off Submillimeter Wavelength Imagers

Erio Gandini,  Jan Svedin,   Tomas Bryllert,  Nuria Llombart

http://ieeexplore.ieee.org/document/7929367/

In this paper, the practical tradeoffs for designing submillimeter wavelength imagers based on optomechanical systems combined with focal plane arrays (FPAs) are presented. The architecture of these systems differs for operation at short and long ranges. General formulas to derive the effective field of view of diffraction limited quasi-optical systems in these two scenarios are shown. These formulas can be used to evaluate the performance of a specific optical system implementation. As an application example, we present the design of an optomechanical system that can operate at both ranges in a modular approach. The presented implementation achieves an effective field of view, which is 70% of the canonical one. The proposed solution consists of a linear FPA of eight active transceivers combined with a raster scan technique. The system for short-range scenario is a side-fed dual-reflector Dragonian architecture because of its good scanning performance when illuminated by an FPA. Thanks to the system’s small aperture, the scanner is arranged after the primary mirror, without causing additional scan loss. The Dragonian system is then used to illuminate a confocal dual-reflector architecture to magnify its aperture, and can be used in the long-range scenario. The scanner in this case is before the main aperture and it has to be considered in the performance optimization of the optical system since it adds phase aberration loss.

Terahertz Radar “Maps” Targets In Long-Distance Pat-Down

http://www.mwrf.com/Article/ArticleID/23809/23809.html
Due to concerns over privacy, efficiency, and accuracy, security screening at airports and other public places is constantly being improved. At NASA’s Jet Propulsion Laboratory, a 675-GHz imaging radar with the potential to perform such screenings from a distance has been proposed by Ken B. Cooper, Robert J. Dengler, Nuria Llombart, Bertrand Thomas, Goutam Chattopadhyay, and Peter H. Siegel. This radar can conduct rapid “frisk” or “pat-down” types of searches of persons as far away as 25 m via a focused, low-energy terahertz beam.
The team picked the 675-GHz band because it benefits from low atmospheric attenuation. At the same time, it provides sufficiently high spatial resolution for a favorable tradeoff between antenna size and standoff range. To achieve sub-centimeter-range resolution, the radar relies on the frequency-modulated-continuous-wave (FMCW) radar technique in combination with a bandwidth of nearly 30 GHz. To optimize the radar’s range resolution, a software-calibration procedure compensates for signal distortion from radar waveform nonlinearities.
The radar achieves low-noise, high-dynamic-range detection with a combination of a heterodyne RF architecture, low-noise chirp source, and 675-GHz transceiver. With its quasi-optical design, it allows low-distortion, fast beam scanning for single-pixel imaging. The portable laboratory prototype operates in FMCW mode over a 28.8-GHz bandwidth, currently centered at 676.7 GHz. With peak output power below 1 mW, it is well within health safety limits. See “THz Imaging Radar for Standoff Personnel Screening,” IEEE Transactions On Terahertz Science And Technology, Sept. 2011, p. 169.