Developments in imaging at millimeter and submillimeter wavelengths

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Aleksi Tamminen Name of the doctoral dissertation Developments in imaging at millimeter and submillimeter wavelengths Publisher School of Electrical Engineering Unit Department of Radio Science and Engineering Series Aalto University publication series DOCTORAL DISSERTATIONS 131/2013 Field of research Radio engineering Manuscript submitted 13 June 2013 Date of the defence 24 September 2013 Permission to publish granted (date) 22 August 2013 Language English Monograph Article dissertation (summary + original articles) Abstract This thesis presents novel experimental results in the fields of milimeterand submilimeterwave imaging, reflectivity studies, as wel as power detector characterization. The overlapping topics share key concepts of beam steering, holography, and antenna measurements. Indirect holographic imaging technique is verified experimentally for the first time. This method allows for coherent detection of a target, using a reference wave and simple direct detection receivers. This is very promising, since receiver complexity is seen as the greatest hindrance to the realization of large imaging arrays. In the holographic method, coherent detection and focusing of the image is a post-detection task. A novel computational technique is introduced, which removes the restrictions on the reference-wave direction. In the experimental work at 310 GHz, the slanted-edge method is applied for estimating the point spread function. Experimental data indicate a decrease in resolution of 20-30 % compared to a diffraction limited case. The noise-equivalent reflectivity difference (NERD) is found to be approximately constant (NERD = 0.002), down to the received signal SNR of 26 dB. Reflectarrays (RAs) are developed and characterized at 120 GHz. The RAs enable beamsteering, and they are designed to be compatible with MEMS microfabrication. Three static RAs are characterized in a near-field measurement range. The measured beamwidths are within 10 % of the simulated ones. The specular reflection from RAs is studied, and the concept of RA efficiency is introduced. The experimentally determined efficiency is found to be 0.11 while simulation results suggest an average efficiency of 0.54. Reflectionand transmission-type phase holograms are used to create a planar wave front, quiet zone (QZ), in a compact test range at both 310 and 650 GHz. The measured QZ variation is ±1.5 dB and ±5° at 310 GHz and ±3 dB and ±25° at 650 GHz. The holograms are suitable for use in radar-cross-section (RCS) measurements. The reflectivity of different radar-absorbing materials (RAMs) is studied in a RCS range. Commercial RAM and low-cost materials are compared for their monostatic reflectivity in an angular range of 45° and 12° at 310 and 650 GHz, respectively. It is found that common materials, such as carpets have reflectivity from −60 to −30 dB, and are suitable for use as RAM. Bolometers used in a submillimeter-wave imager are studied at 321-782 GHz. The antennacoupled microbolometers are characterized in room temperature. They are coupled with an equi-angular spiral antenna and a silicon substrate lens. The beamwidth of the bolometers is found to follow an 8.5°/THz –relation across the band. A low-cost infrared detector is compared with dedicated power meters. It is found to have a sensitivity of 1700 V/W and noiseequivalent power (NEP) of 0.4 uW/rtHz. As such, it can be used as an ad hoc power detector.This thesis presents novel experimental results in the fields of milimeterand submilimeterwave imaging, reflectivity studies, as wel as power detector characterization. The overlapping topics share key concepts of beam steering, holography, and antenna measurements. Indirect holographic imaging technique is verified experimentally for the first time. This method allows for coherent detection of a target, using a reference wave and simple direct detection receivers. This is very promising, since receiver complexity is seen as the greatest hindrance to the realization of large imaging arrays. In the holographic method, coherent detection and focusing of the image is a post-detection task. A novel computational technique is introduced, which removes the restrictions on the reference-wave direction. In the experimental work at 310 GHz, the slanted-edge method is applied for estimating the point spread function. Experimental data indicate a decrease in resolution of 20-30 % compared to a diffraction limited case. The noise-equivalent reflectivity difference (NERD) is found to be approximately constant (NERD = 0.002), down to the received signal SNR of 26 dB. Reflectarrays (RAs) are developed and characterized at 120 GHz. The RAs enable beamsteering, and they are designed to be compatible with MEMS microfabrication. Three static RAs are characterized in a near-field measurement range. The measured beamwidths are within 10 % of the simulated ones. The specular reflection from RAs is studied, and the concept of RA efficiency is introduced. The experimentally determined efficiency is found to be 0.11 while simulation results suggest an average efficiency of 0.54. Reflectionand transmission-type phase holograms are used to create a planar wave front, quiet zone (QZ), in a compact test range at both 310 and 650 GHz. The measured QZ variation is ±1.5 dB and ±5° at 310 GHz and ±3 dB and ±25° at 650 GHz. The holograms are suitable for use in radar-cross-section (RCS) measurements. The reflectivity of different radar-absorbing materials (RAMs) is studied in a RCS range. Commercial RAM and low-cost materials are compared for their monostatic reflectivity in an angular range of 45° and 12° at 310 and 650 GHz, respectively. It is found that common materials, such as carpets have reflectivity from −60 to −30 dB, and are suitable for use as RAM. Bolometers used in a submillimeter-wave imager are studied at 321-782 GHz. The antennacoupled microbolometers are characterized in room temperature. They are coupled with an equi-angular spiral antenna and a silicon substrate lens. The beamwidth of the bolometers is found to follow an 8.5°/THz –relation across the band. A low-cost infrared detector is compared with dedicated power meters. It is found to have a sensitivity of 1700 V/W and noiseequivalent power (NEP) of 0.4 uW/rtHz. As such, it can be used as an ad hoc power detector.