Developments in imaging at millimeter and submillimeter wavelengths

    Research output: ThesisDoctoral ThesisCollection of Articles

    Abstract

    This thesis presents novel experimental results in the fields of millimeter- and submillimeter-wave imaging, reflectivity studies, as well 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 beam-steering, 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. Reflection- and 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 antenna-coupled 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 noise-equivalent power (NEP) of 0.4 uW/rtHz. As such, it can be used as an ad hoc power detector.
    Translated title of the contributionMillimetri- ja alimillimetriaaltoalueen kuvantamismenetelmien kehittämisestä
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    Supervisors/Advisors
    • Räisänen, Antti, Supervising Professor
    • Ala-Laurinaho, Juha, Thesis Advisor
    Publisher
    Print ISBNs978-952-60-5300-4
    Electronic ISBNs978-952-60-5301-1
    Publication statusPublished - 2013
    MoE publication typeG5 Doctoral dissertation (article)

    Keywords

    • bolometer
    • hologram
    • imaging
    • reflectarray
    • reflectivity

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