### Browsing by Author "Tamminen, Aleksi"

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Item 650 GHz imaging as alignment verification for millimeter wave corneal reflectometry(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2022-03-01) Hu, Yong; Baggio, Mariangela; Dabironezare, Shahab; Tamminen, Aleksi; Toy, Brandon; Ala-Laurinaho, Juha; Brown, Elliot; Llombart, Nuria; Deng, Sophie; Wallace, Vincent; Taylor, Zachary; Department of Electronics and Nanoengineering; Zachary Taylor Group; Ville Viikari Group; University of California, Los Angeles; Delft University of Technology; Wright State University; University of Western AustraliaA system concept for online alignment verification of millimeter-wave, corneal reflectometry is presented. The system utilizes beam scanning to generate magnitude-only reflectivity maps of the cornea at 650 GHz and compares these images to a precomputed/measured template map to confirm/reject sufficient alignment. A system utilizing five off-axis parabolic mirrors, a thin film beam splitter, and two-axis galvanometric mirror was designed, simulated, and evaluated with geometric and physical optics. Simulation results informed the construction of a demonstrator system which was tested with a reference reflector. Similarity metrics computed with the aligned template and 26 misaligned positions, distributed on a 0.5 mm x 0.5 mm x 0.5 mm mesh, demonstrated sufficient misalignment detection sensitivity in 23 out of 26 positions. The results show that positional accuracy on the order of 0.5 mm is possible using 0.462 mm wavelength radiation due to the perturbation of coupling efficiency via beam distortion and beam walk-off.Item Beamforming antenna array systems in millimeter-wave sensing applications(2022-05-09) Hiltunen, Paavo; Tamminen, Aleksi; Sähkötekniikan korkeakoulu; Turunen, MarkusItem Calibration Alignment Sensitivity in Corneal Terahertz Imaging(Multidisciplinary Digital Publishing Institute (MDPI), 2022-05-01) Zarrinkhat, Faezeh; Baggio, Mariangela; Lamberg, Joel; Tamminen, Aleksi; Nefedova, Irina; Ala-Laurinaho, Juha; Khaled, Elsayed E.M.; Rius, Juan M.; Romeu, Jordi; Taylor, Zachary; Department of Electronics and Nanoengineering; Zachary Taylor Group; Ville Viikari Group; Department of Electronics and Nanoengineering; Assiut University; BarcelonaTechImproving the longitudinal modes coupling in layered spherical structure contributes significantly to corneal terahertz sensing, which plays a crucial role in the early diagnosis of cornea dystrophies. Using a steel sphere to calibrate reflection from the cornea sample assists in enhancing the resolution of longitudinal modes. The requirement and challenges toward applying the calibration sphere are introduced and addressed. Six corneas with different properties are spotted to study the effect of perturbations in the calibration sphere in a frequency range from 100 GHz to 600 GHz. A particle-swarm optimization algorithm is employed to quantify corneal characteristics considering cases of accurately calibrated and perturbed calibrated scenarios. For the first case, the study is carried out with signal-to-noise values of 40 dB, 50 dB and 60 dB at waveguide bands WR-5.1, WR-3.4, and WR-2.2. As expected, better estimation is achieved in high-SNR cases. Furthermore, the lower waveguide band is revealed as the most proper band for the assessment of corneal features. For perturbed cases, the analysis is continued for the noise level of 60 dB in the three waveguide bands. Consequently, the error in the estimation of corneal properties rises significantly (around 30%).Item Cross polarization and aberrations with Dragonian and equivalent off-axis parabolic mirrors for beam collimation in THz imaging systems(SPIE, 2022) Rezapoor, Pouyan; Tamminen, Aleksi; Ala-Laurinaho, Juha; Dabironezare, Shahab Oddin; Llombart, Nuria; Taylor, Zachary; Department of Electronics and Nanoengineering; Wikner, DA; Robertson, DA; Zachary Taylor Group; Ville Viikari Group; Delft University of TechnologyThe relative benefits of an offset Dragonian reflector compared to equivalent off-axis parabolic (OAP) mirrors for feeding collimated beam to a scanned beam imaging system are investigated. Physical-optics simulation of the Dragonian are performed at 500 GHz. The input is a Gaussian beam with a frequency dependent waist radius fit to the output of a standard Pickett-Potter horn. The collimated output beam properties are characterized, including cross-polarization, beam waist radius, Gaussicity, and M-squared parameter. Next, by sweeping the parameters of an OAP reflector (parent focal length and incidence angle) in the physical-optics simulations, we find the geometry in which the properties of the output beam best match the Dragonian geometry. This reflector is found to be an OAP with 108.22 mm parent focal length and 30. incidence angle. An additional OAP reflector is also considered in these simulations, which is the most often used 90. OAP. The parent focal length is 56.95 mm for this mirror, so that we have a similar beam waist radius in the detector plane. Finally, physical optics simulations reveal that the Dragonian geometry produces much smaller cross-polarization in the detector plane (-23 dB at the beam waist) in comparison with OAP reflector (being -14 dB and -8 dB for 30 degrees and 90 degrees off-axis mirrors, respectively). The 30. OAP reflector is able to produce similar beam quality in terms of phase variation, Gaussicity, and beam waist radius at the detector plane.Item Cross polarization in swept beam THz imaging systems using off-axis parabolic mirrors(2022) Rezapoor, Pouyan; Tamminen, Aleksi; Nefedova, Irina; Ala-Laurinaho, Juha; Llombart, Nuria; Rodilla, Helena; Stake, Jan; Taylor, Zachary; Department of Electronics and Nanoengineering; Zachary Taylor Group; Ville Viikari Group; Delft University of Technology; Chalmers University of TechnologyThe optical behavior of a terahertz imaging system employing a train of four identical off-axis parabolic mirrors with oblique incidence angle illumination is investigated in this work. The aperture filling and aberrations of a single off-axis parabolic mirror when illuminated by a Gaussian terahertz beam at its focus point is measured and simulated. The amplitude of E-field in transverse electric (TE) and transverse magnetic (TM) polarizations at target plane reveals a significant cross polarization, even when there is zero cross polarization at the source beam, amplitude of which is ∼ 33% of TE polarization. The investigation of the E-field on the detector plane reveals that this ratio is ∼ 1.5% at the detector plane, and the cross polarized E-field at the target plane is rotated back to co polarization. Although its amplitude is negligible, the TM distribution at detector plane is bimodal and tilted about the optical axis.Item Curved boundary integral method for electromagnetic fields(Optical Society of America (OSA), 2023-12-18) Lamberg, Joel; Zarrinkhat, Faezeh; Tamminen, Aleksi; Ala-Laurinaho, Juha; Rius, Juan; Romeu, Jordi; Khaled, Elsayed E. M.; Taylor, Zachary; Department of Electronics and Nanoengineering; Zachary Taylor Group; Ville Viikari Group; Department of Electronics and Nanoengineering; Technical University of Catalonia; Assiut UniversityThe angular spectrum method is a rigorous method to synthesize near and far-field electromagnetic beams from planar field distributions. However, this limitation of planar surfaces has restricted its applicability to beams with simple focal planes. We propose a curved boundary integral method (CBIM) to synthesize electromagnetic beams from arbitrary surfaces to address this limitation and expand the method’s scope to synthesize beams from and between shaped objects. This study presents a detailed theoretical framework behind the CBIM and validates its effectiveness and accuracy with a comprehensive set of simulations. Additionally, we present mathematical proof to support our proposal. The proposed method satisfies Maxwell’s equations and significantly benefits optical systems and inverse beam design. It allows for analyzing electromagnetic forward/backward propagation between optical elements using a single method. It is also valuable for optical force beam design and analysis.Item Design and Characterization of Phase Holograms for Standoff Localization at Millimeter and Submillimeter Waves(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2022-01-01) Palli, Samu Ville; Tamminen, Aleksi; Ala-Laurinaho, Juha; Taylor, Zachary D.; Department of Electronics and Nanoengineering; Zachary Taylor Group; Ville Viikari GroupWe present design, simulation, and experimental characterization of dual-band frequency-diverse holograms for distributed beamforming. The holograms operate in the 50-75 GHz (WR-15) and 220-330 GHz (WR-3.4) bands for millimeter-and submillimeter-wave imaging. The holograms are designed to create a dispersive field in the region of interest (RoI) located 600 mm from the aperture. The holograms lie in the front end of an imaging setup and modulate the phase of the incident collimated beam from a parabolic mirror. The distributed beamforming enables interrogation of the RoI so that the measured reflection through the dispersive propagation path conveys the spatial information of the target. Different phase modulation schemes are evaluated, and two prototype holograms are manufactured. The dispersive operation and efficiency of the hologram are characterized with both simulations and measurements. The frequency diversity of the holograms is quantified using singular-value decomposition and spatial-spectral correlation coefficient methods. The results identified a design frequency of 120 GHz, a phase quantization step of π/2 radians, and an added phase of 1.9π radians as a good dispersion-efficiency compromise. A fully connected neural network is trained to localize a corner-cube reflector in the RoI illuminated by the hologram. The localization accuracy follows the diffraction-limited resolution and confirms the best performance for the hologram considered optimal in the design metrics.Item Design of a quasioptical test bench for VNA extenders(2020-11-08) Nefedova, Irina; Moradikouchi, Anis; Baggio, Mariangela; Tamminen, Aleksi; Hu, Yong; Ala-Laurinaho, Juha; Rodilla, Helena; Stake, Jan; Taylor, Zachary; Department of Electronics and Nanoengineering; Zachary Taylor Group; Ville Viikari Group; Chalmers University of Technology; University of California; University of California, Los AngelesThe CAD design, simulated optical performance, and test measurements of a quasioptical test bench for VNA extenders are presented. The system allows straightforward integration of off axis parabolic mirrors and VNA extenders. The mirror orientation reduces aberrations and results suggest optical properties are close to what is expected with geometric ray tracing.Item Design of double-reflector objective for corneal sensing in the 220-330 GHz band(2022) Baggio, Mariangela; Tamminen, Aleksi; Ala-Laurinaho, Juha; Taylor, Zachary; Department of Electronics and Nanoengineering; Zachary Taylor Group; Ville Viikari GroupAn all reflective, Schwarzschild objective design for corneal reflectometry is evaluated in the WR3.4 (220-330 GHz) frequency band. The shadow created by the 60-mm diameter secondary mirror is sufficiently large to mount a standard, OCT system to enable concomitant data acquisition with both modalities while avoiding beam obscuration. The system was fed with a bottle beam comprised of a p= 0, ell = 4 Laguerre-Gaussian mode with a 12.5-mm beam waist radius for increased throughput. The 50-mm focal length system was determined with ray tracing and simulated with an in-house physical optics code using a PEC sphere as a test target. The power coupling coefficient between the feed beam and scattered beam varied from 0.35 at 220 GHz to 0.48 at 330 GHz. While the system demonstrated a clear phase center at less than 1 λ from the geometric focal point, significant edge diffraction prevents the formation of a spherical phasefront, which is needed to achieve phase front matching.Item Developments in imaging at millimeter and submillimeter wavelengths(Aalto University, 2013) Tamminen, Aleksi; Ala-Laurinaho, Juha, Dr., Aalto University, Finland; Radiotieteen ja -tekniikan laitos; Department of Radio Science and Engineering; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Räisänen, Antti, Professor, Aalto University, FinlandThis 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.Item Distributed beamsteering at millimeter waves for imaging and localization(2020-10-19) Pälli, Samu-Ville; Tamminen, Aleksi; Ala-Laurinaho, Juha; Sähkötekniikan korkeakoulu; Taylor, ZacharyThe challenge of current millimeter-wave and submillimeter-wave imaging systems is the scalability of the imaging technology. At millimeter wavelengths, dense detector arrays are either bulky in size or require cryogenic temperatures to operate. Lack of detectors or transceivers is often compensated by use of electronic or mechanical scanning system, increasing cost and complexity. Computational imaging methods with frequency-diverse antennas present a method where a single transceiver can be used to image a scene effectively without moving parts, as the system complexity shifts from the hardware to software. In this master's thesis, a dual-band, frequency-diverse, phase hologram for millimeter-wave imaging system applications is designed and measured. The hologram is designed to operate at frequencies of 60 and 250 GHz with emphasis on enhancing efficiency and frequency diversity. First, a literature review of conventional beamsteering methods in millimeter-wave imaging systems is presented. In addition, computational imaging theory is studied together with previous studies of frequency-diverse antennas. The frequency-diverse phase hologram is designed using MATLAB-based physical-optics simulations. With simulations, the possibility of dual-band operation is studied. Furthermore, a thorough analysis of the effects of different hologram design parameters to efficiency, frequency diversity, and manufacturability is carried out. Based on the simulation results, two holograms are chosen for manufacturing, one with optimal results and one sub-optimal for comparison. To validate the performance of manufactured holograms, a quasi-optical measurement setup is built for millimeter-wave measurements at 50-75 GHz and 220-330 GHz. The optimal version of the manufactured holograms achieved efficiency of 20-40% at 50-75 GHz and 40-50% at 220-330 GHz. Although less than predicted by the simulations, the measured efficiency is sufficient for imaging. The measured frequency diversity of both holograms exceeded what was predicted in simulations. Results indicate that the hologram performs well as a frequency-diverse element in millimeter-wave imaging system at both frequency bands. Future work includes imaging experiments of simple targets using the hologram with trained deep-neural-network backend.Item The Effect of Surface Passivation for Sub-THz Silicon Gradient Refractive Index Lens(2022) Lamminen, Antti; Tamminen, Aleksi; Saarilahti, Jaakko; Ermolov, Vladimir; Pursula, Pekka; Department of Electronics and Nanoengineering; Zachary Taylor Group; VTT Technical Research Centre of FinlandThis paper describes a study of impact of different passivation methods on performance of gradient refractive index (GRIN) lenses. Three different passivation layers on top of lenses are studied: PolySi, PolySi and PECVD silicon oxide (PolySi+SiO2), and PECVD silicon oxide (SiO2). Antenna gains and radiation patterns of fabricated lenses are measured. The study shows that passivation methods have strong impact on silicon GRIN lenses performance. A model with a low-resistivity layer on top of high-resistivity Si can explain observed effects.Item Electromagnetic beam synthesis from spherical surface electric field distributions using modified Fourier optics method(2021-06-16) Lamberg, Joel; Tamminen, Aleksi; Sähkötekniikan korkeakoulu; Taylor, ZacharyConventional methods for evaluating electromagnetic beams created from spherical electric surface distribution may include full-wave simulations, geometric optics (GO), or physical optics (PO). These methods are extensively studied and accurate given the model fidelity and a suitable wavelength range. However, they cannot assess the incident and scattered electric field from multi-layered spherical objects without considerable computational effort. Fourier optics is a powerful method to obtain incident electric field, which can be easily expanded to vector spherical harmonics (VSH) presentation to accommodate spherical geometry. The VHS presentation is mapped with the T-matrix method to evaluate the internal and scattered fields from the multi-layered dielectric sphere. The limitation of this method is that it is nominally compatible with planar interfaces. In this study, the Fourier-optics method is expanded to model beam propagation from arbitrary electric field distributions positioned on spherical surfaces. This method can be applied to synthesize beams from any surface shape within reasonable frequency-dependent surface details. The method’s main idea is that Riemann’s surface integral combines differential surface elements, which can be approximated as locally planar elements. The Fourier-optics method is applied to each of these differential elements, and the total electric field is the sum of the fields created from them by the superposition principle. The method is readily applicable to the field of THz sensing of corneal tissue hydration and geometry due to the cornea’s layered spherical shell. First, a literature review of the general theory of Fourier optics, vector spherical harmonics, and the T-matrix method is presented. The new modified Fourier-optics method is derived, and the result is mathematically proved. The simulation algorithm was implemented in the MATLAB programming language. The simulations with the new method are compared with corresponding wave optics simulations or known analytical solutions. The method differs from the results given by physical optics by -41 dB in the case of a constant field at the calotte, and the difference from the theoretical result is -24 dB in the case of a rectangular aperture. Finally, the minimum size of the quasi-optics focusing element required to achieve sufficient resolution and efficiency for THz imaging is estimated. Reflection simulations at 275 GHz show an efficiency of 80 % when the aperture diameter of the focusing element was 100 mm and was 40 mm from the origin of the spherical mirror. The results suggest that the desired electric field on the spherical surface can be produced with an aperture of this size. In this case, up to 80 % of the reflected power returns to the aperture. In the future, the frequency dependence of the simulation method and field synthesis will be investigated on a spherical surface to analyze the cornea’s reflection spectrum. The work involves iterative optimization, in which case the developed algorithm has to be rewritten without a VSH extension to provide lighter computational capacity.Item Epäsuora holografinen kuvaus millimetriaaltoalueella(Helsinki University of Technology, 2007) Tamminen, Aleksi; Ala-Laurinaho, Juha; TkT, tutkija; Department of Electrical and Communications Engineering; Sähkö- ja tietoliikennetekniikan osasto; Radio Laboratory; Radiolaboratorio; Räisänen, Antti; Prof.Tässä diplomityössä käsitellään radioaaltoihin perustuvaa kuvausta. Erilaisiin kuvausmenetelmiin tutustutaan työn kirjallisuuskatsauksessa. Työ käsittelee erityisesti epäsuoraa holografista kuvausta, johon tutustutaan työn teoreettisessa ja kokeellisessa osuudessa. Työssä on toteutettu epäsuoraan holografiseen kuvaukseen soveltuva laitteisto, jonka tuottamat millimetriaaltokuvat esitellään. Työn ensimmäisessä osassa tehdään jako aktiivisiin ja passiivisiin kuvausmenetelmiin. Eri kuvausmenetelmien ominaisuuksia arvioidaan ja vertaillaan keskenään. Painopiste on sellaisissa kuvausmenetelmissä, jotka soveltuvat turvallisuuteen liittyvään kuvaamiseen, kuten turvatarkastuksiin lentokentillä. Kirjallisuuskatsauksessa käydään läpi eri menetelmiin perustuvaa aiempaa kokeellista työtä, ja saatujen radioaaltokuvien perusteella arvioidaan eri kuvausmenetelmien suorituskykyä. Työn toisessa osassa käsitellään aktiivisen, epäsuoran holografisen kuvauksen teoriaa. Kuvauksen laskennalliseen osuuteen liittyvät tasoaaltospektri ja kompleksisen kentän palautus johdetaan käytäntöön soveltuville diskreeteille suureille. Holografisen kuvauksen perusteoriasta johdetaan kuvauslaitteistolta vaaditut ominaisuudet: apertuurin koko, kentän näytteistysväli apertuurilla ja referenssikentän tulokulma. Kuvauslaitteiston geometriasta saadaan teoreettinen erotuskyky ja näkökenttä. Lisäksi esitellään laajakaistaisen epäsuoran holografisen kuvauksen periaate ja arvioidaan erilaisten referenssikenttien soveltuvuutta epäsuoraan holografiseen kuvaukseen. Työn kokeellisessa osuudessa epäsuora holografinen kuvaus toteutettiin 310 GHz:llä pystypolarisaatiolla. Referenssikenttänä käytettiin sekä tasoaaltoa että Gaussin keilaa. Kuvaus toteutettiin vertailun vuoksi myös suoralla holografisella menetelmällä. Eri kohteista saatuja millimetriaaltokuvia arvioitiin. Paras suorituskyky saatiin 40 x 40 cm2:n apertuurilla käytettäessä Gaussin keilaa referenssikenttänä, jolloin saavutettu erotuskyky 1,5 m:n päässä kohteesta on noin 2 mm. Myös tasoaaltoreferenssi soveltuu epäsuoraan holografiseen kuvaukseen. Käytettäessä 14 x 14 cm2 apertuuria ja tasoaaltoreferenssiä millimetriaaltokuvassa voitiin erottaa alle 1 cm:n kokoisia yksityiskohtia. Lisäksi käsitellään erilaisia keinoja parantaa saatujen kuvien laatua ja helpottaa holografiseen kuvaukseen liittyviä laitteistovaatimuksia. Lopuksi saatuja tuloksia verrataan työn ensimmäisessä osassa käsiteltyjen kuvausmenetelmien tuloksiin.Item Extended legality of curved boundary integral method(Optical Society of America (OSA), 2024-07-29) Lamberg, Joel; Lamberg, Lasse; Tamminen, Aleksi; Ala-Laurinaho, Juha; Taylor, Zachary; Department of Electronics and Nanoengineering; Zachary Taylor Group; Ville Viikari Group; Department of Electronics and Nanoengineering; University of HelsinkiThe angular spectrum method is an efficient approach for synthesizing electromagnetic beams from planar electric field distributions. The electric field definition is restricted to a plane, which can introduce inaccuracy when applying the synthesized beam to curved surface features. The angular spectrum method can also be interpreted as a pure source method defining the field symmetrically with respect to the creation plane. Recently, we generalized that symmetric field method to arbitrary source distributions, which are valid at any point on compact, regular surface Ω in R3. We call this approach the Curved Boundary Integral method. The electromagnetic fields synthesized with this method satisfy the Helmholtz equation and are adjusted via amplitude and phase at the desired surface. The fields are obtained as a relatively simple integral. However, restrictions on where in space the synthesized field is valid were included in the mathematical proof length to avoid obscuring the main points. These restrictions can be significant depending on the shape and degree of curvature of surface Ω. In this article, we remove these restrictions so that the integral representation of the electromagnetic beam becomes valid at all points r ∈ R3 \ Ω, with a minor restriction. Its modification can work even on Ω. We demonstrate the importance of this extended legality with a source field parametrized into the torus surface. The electromagnetic radiation of this structure would not be valid at any point in space without this extension. Finally, we show that by changing the order of integration, the field singularity at each source point is eliminated.Item Extraction of thickness and water content gradients in hydrogel-based, water-backed corneal phantoms via submillimeter wave reflectometry(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2021-11-01) Tamminen, Aleksi; Baggio, Mariangela; Nefedova, Irina; Sun, Qiushuo; Presnyakov, Semyon; Ala-laurinaho, Juha; Brown, Elliot; Wallace, Vincent; Macpherson, Emma; Maloney, Thaddeus; Kravchenko, Natalia; Salkola, Mika; Deng, Sophie; Taylor, Zachary; Department of Electronics and Nanoengineering; Department of Bioproducts and Biosystems; Zachary Taylor Group; Ville Viikari Group; Bio-based Materials; University of Birmingham; Higher School of Economics; Wright State University; University of Western Australia; University of Warwick; Revenio Oy; University of California, Los AngelesAbsolute thickness and free-water-content gradients in gelatin-based corneal phantoms with physiologically accurate radii of curvature and aqueous backing were extracted via coherent submillimeter-wave reflectometry at 220-330 GHz. Fourier-domain-based calibration methods, utilizing temporal and spatial gating, were developed and yielded peak-to-peak amplitude and phase clutter of 10-3 and 0.1°, respectively, for signal-to-noise ratios (SNRs) between 40 and 50 dB. Total 12 phantoms were fabricated. Calibration methods enabled quantification of target sphericity that strongly correlated with optical-coherence tomography-based sphericity metrics via image segmentation. The extracted free-water volume fraction varied less than 5% in the five phantoms whose fabrication yielded the most spherical geometry. Submillimeter-wave-based thickness accuracy was better than 111 μm (∼λ/9) with an average of 65 μm (∼λ/17) and standard deviation of 44 μm (∼λ/25) for phantoms with physiologically relevant geometry. Monte-Carlo simulations of measurement noise and uncertainty limits agree with the experimental data analysis and indicate a lower thickness accuracy limit of 33 μm, and water-content sensitivities of 0.5% and 11.8% for the anterior and posterior segments, respectively. Numerical analysis suggests that the measurement fidelity was SNR limited and identified optical path length ambiguities within the cornea where a continuum of thickness/water gradient pairs produces statistically insignificant differences in complex reflection coefficient for finite SNR. This is the first known submillimeter-wave measurement technique, which is able to extract both the thickness and water-content gradients from a soft-tissue phantom, with a water backing, without the need for ancillary measurements.Item A fully-staring THz video camera with wide field-of-view and close-looking optics(2020-01-01) Luomahaara, Juho; Mäyrä, Aki; Aikio, Mika; Sipola, Hannu; Grönberg, Leif; Timofeev, Andrey; Tappura, Kirsi; Rautiainen, Anssi; Ala-Laurinaho, Juha; Tamminen, Aleksi; Vesterinen, Visa; Leivo, Mikko; Gao, Feng; Vasama, Hannu; Luukanen, Arttu; Hassel, Juha; Department of Electronics and Nanoengineering; Salmon, Neil A.; Gumbmann, Frank; Centre of Excellence in Quantum Technology, QTF; Ville Viikari Group; VTT Technical Research Centre of Finland; Asqella OyWe present a passive, fully-staring THz imaging system for security screening purposes. The camera is equipped with a multi-kilo-pixel detector array of kinetic inductance bolometers, cryogenics operating above 5 K, and a multiplexed low-noise electronics to readout the whole detector array. After briefly describing the system, we concentrate on the optical system of the instrument. We discuss the trade-offs related to the optics and justify the adopted lens material and design. Furthermore, we show measurement results characterizing the related optical performance.Item Heijastusryhmäantenni 120 GHz:n tutkasovellukseen - antennielementtien karakterisointi lähikenttämittauksin(2014-06-16) Mäkelä, Sampo; Tamminen, Aleksi; Sähkötekniikan korkeakoulu; Räisänen, AnttiTämän diplomityön käsittelemä heijastusryhmäantenni, jota tutkitaan lähikenttämittauksin, on suunnitteilla 120 GHz:n lähialueen millimetriaaltotutkasovellukseen. Kehitteillä oleva sähköisesti keilaava heijastusryhmäantenni sisältää noin 3700 antennielementtiä, joita ohjataan mikrosähkömekaanisilla (MEMS) vaiheensiirtimillä. Ennen sähköisesti keilaavaa prototyyppiä antennista on valmistettu mittauksia varten staattiset versiot, joissa MEMS-vaiheensiirtimet on korvattu siirtojohdoilla, ja niiden pituutta muuttamalla aikaansaadaan sopiva vaiheensiirto. Sen toimintaperiaate, suunnitteluperusteet, analyysi ja valmistettujen antennien ominaisuudet on selitetty työn kirjallisuuskatsauksessa. Antennin säteilykuviot on mitattu kolmen metrin etäisyydeltä, ja niitä on verrattu laskennallisiin vastaaviin. Tasoaaltospektri ja diffraktiohyötysuhde on määritetty mittaamalla antennin apertuurikenttää lähietäisyydeltä. Yksittäisten antennielementtien karakterisointiin kehitetty heijastusmittausmenetelmä on mallinnettu kalibroinnin ja mitattujen parametrien erottelun varmistamiseksi. Mitatut säteilykuviot vastaavat hyvin laskennallisia keilanleveyden ja -suunnan osalta, mutta mitatussa tasoaaltospektrissä havaittiin voimakas peiliheijastuskomponentti. Valmistetuissa antenneissa ilmeni ylietsaantumista, mikä kasvattaa elementtien resonanssitaajuutta. Sen havaittiin kasvaneen noin 132.7 GHz:iin, missä pääkeilan amplitudi oli 7.4-8.7 dB:ä suurempi kuin 120 GHz:llä mitatuissa. Tutkittavalla heijastusmittausmenetelmällä onnistuttiin erottelemaan sekä amplitudi että suhteellinen vaiheensiirto erilailla viritetyille elementeille. Heijastuskertoimen vaiheensiirto sijoittuu 20 asteen ja 190 asteen välille ja suunniteltu 90 asteen vaiheensiirto saavutettiin vain avoimella piirillä päätetyllä 195 µm:n ja oikosuljetulla 200 µm:n siirtojohdolla. Mitattu diffraktiohyötysuhde vaihtelee 0.02:n ja 0.2:n välillä, mikä on linjassa tasoaaltospektristä holografisella projektiomenetelmällä määritetyn kanssa.Item Holograms with neural-network backend for submillimeter-wave beamforming applications(SPIE - The International Society for Optical Engineering, 2020-01-01) Tamminen, Aleksi; Pälli, Samu-Ville; Ala-Laurinaho, Juha; Aspelin, Arthur; Oinaanoja, Akseli; Taylor, Zachary; Department of Electronics and Nanoengineering; Zachary Taylor Group; Ville Viikari Group; Department of Electronics and Nanoengineering; Zachary Taylor GroupWe present a new method to carry out localization based on distributed beamforming and neural networks. A highly dispersive hologram, is used together with a terahertz spectrometer to localize a corner-cube reflector placed in the region of interest. The transmission-type dielectric hologram transforms input pulse from the spectrometer into a complex pattern. The hologram causes complicated propagation paths which introduce delay so that different parts of the region of interest are interrogated in a unique way. We have simulated the emitted pulses propagating through the hologram. The hybrid simulation combines the finite-difference and physical optics methods in time domain and allows for evaluating the dispersion and directive properties of the hologram. The dispersive structure is manufactured of Rexolite and it has details resulting in varying delay from 1 to 19 wavelengths across the considered bandwidth. The spectrometer is configured in reflection mode with wavelets passing in to the region of interest through the hologram. A data-collecting campaign with a corner-cube reflector is carried out. The effective bandwidth for the localization is from 0.1 THz to 2.1 THz, and the measured loss is 57 dB at minimum. The collected data is used to train a fully-connected deep neural network with the known corner-cube positions as labels. Our first experimental results show that it is possible to predict the position of a reflective target in the region of interest. The accuracy of the prediction is 0.5-0.8 mm at a distance of 0.17 m.Item Imaging experiments with a 340-GHz FMCW radar and frequency-diverse holograms(2023) Pälli, Samu Ville; Tamminen, Aleksi; Hiltunen, Paavo; Rexhepi, Sazan; Bonmann, Marlene; Bryllert, Tomas; Robertson, Duncan A.; Ala-Laurinaho, Juha; Stake, Jan; Taylor, Zachary; Department of Electronics and Nanoengineering; Hedden, Abigail S.; Mazzaro, Gregory J.; Raynal, Ann Marie; Zachary Taylor Group; Ville Viikari Group; Department of Electronics and Nanoengineering; Chalmers University of Technology; University of St AndrewsWe present recent developments of a standoff imaging system based on a frequency-diverse phase hologram and deep neural networks. The single-pixel imaging system operates in a monostatic configuration consisting of a 340-GHz FMCW radar and a frequency-diverse phase hologram to interrogate the radar down range direction with spatially varying, frequency-dependent field patterns. The measured back-reflected signal contains spatial reflectivity information from the target, and the fast chirp rate of the radar enables real-time imaging performance. Together with simultaneously acquired visible-light images, a deep neural network integrated into the submillimeter-wave data readout electronics can map the received signal onto a 2D image without mechanical or active electrical beam scanning. In experiments, we have collected submillimeter-wave and visible-light data of a moving target in the region of interest with a 60-Hz frame rate. The results suggest that the system can image the moving target with a resolution comparable to the theoretical diffraction limit. The minimal hardware complexity and good imaging performance of the demonstrated computational submillimeter-wave imaging system support its potential as a cost-effective and easily deployable solution for various imaging applications.