We present experimental results on computational submillimeter-wave ghost imaging schemes. The schemes include a dispersive element introducing quasi-incoherent field patterns to the field of view and bucket detection of the back-reflected field across a significantly broad bandwidth. A single bucket detection without discrimination of the field of view into image pixels is used. The imaging experiments at 220-330GHz with dispersive hologram show successful computational ghost imaging of a corner-cube reflector target at 600-mm distance. Two separate image-forming methods are compared: correlation and machine-learning. In the correlation method, the image is formed by integrating the predetermined quasi-incoherent field patterns weighted with the bucket detections. In the machine-learning method, high image quality can be achieved after non-trivial training campaigns. The great benefit of the correlation method is that, while the quasi-incoherent patterns need to be known, no a priori iterative training to the images is required. The experiments with the correlation method demonstrate resolving of the target at 600-mm distance.
We 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.
We present the experimental results of a submillimeter-wave standoff imaging system based on a frequency-diverse hologram and image reconstruction via machine learning at 220-330 GHz. The imaging system operates in a single-pixel, monostatic configuration consisting of a transceiver together with a frequency-diverse phase hologram to interrogate the region of interest with quasi-random field patterns. The spatial reflectivity distribution in the region of interest is embedded in the wide-band frequency spectrum of the back-reflected signal and the images are acquired without mechanical or electrical scanning. Images from a visible-light camera are used as the ground truth of the target elements. The targets are scanned in the region of interest, while the wide-band reflection spectrum for the target is measured. The collected image-signal pair data are used to train a deconvolutional neural network for image reconstruction with the submillimeter-wave reflection spectra as input. In experiments, a corner-cube reflector and a complex test target made of copper foam were imaged in a 28-degree field of view at a distance of 600 mm from the imaging system. The effect of bandwidth on image quality is evaluated using 10-40 GHz bandwidths centered at 275 GHz to image the copper foam target. The resolution in the image predictions was estimated from fitted point-spread functions to be from 12 mm to 30 mm, with the highest resolution at the broadest bandwidth. We have correlated the measured field patterns at the region of interest with the mean squared error (MSE) of the predicted corner-cube images to analyze the effect of field characteristics on imaging accuracy. The results demonstrate increased accuracy in locations with high electric field amplitude and variation over the imaging bandwidth.
The 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° and 90° 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.
We discuss about a fully-staring THz video camera prototype intended for security screening. The camera utilizes so-called kinetic inductance bolometers to detect THz radiation in the bandwidth of 0.3-1 THz. The imaging distance is 2.5 m with the nominal field-of-view of 2 m × 1 m. The camera is equipped with a kilo-pixel detector array, intermediate-scale cryogenics operating at 6 K, and low-noise electronics to read out the whole detector array. Here, we focus on describing the wide field-of-view and close-looking optical system of the imager.
We present a fully-staring THz video camera prototype intended for security screening. The camera utilizes so-called kinetic inductance bolometers to detect THz radiation in the bandwidth of 0.3-1 THz. The imaging distance is 2.5 m with the field-of-view being 2 m × 1 m. The camera is equipped with a kilo-pixel detector array, large field-of-view optics, intermediate-scale cryogenics operating at 6 K, and low-noise electronics to read out the whole detector array. The imaging capabilities of the system are demonstrated through radiometric performance characterization and actual imaging experiments.
We 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.
We propose an alignment strategy for millimeter spectroscopy of cornea that uses imaging to screen for sufficient alignment conditions. The performance of different corneal imaging objectives, in the presence of misalignment, is evaluated. The cornea is illuminated with a TEM00 Gaussian beam at 650 GHz and the beam is swept across the cornea. Images are generated by calculating the coupling between illumination and scattered beams for each illumination beam position and angle. The cornea is displaced at intervals of 500 microns in the transverse and axial directions and with new coupling coefficient maps generated at each misaligned position. Contrast in the misaligned cases are compared to the aligned case via zero normalized spatial cross correlation. The results show a maximum normalized cross correlation of 0.92 for a two-mirror scanning system and 0.74 for a one-mirror scanning objective. The analysis suggests that imaging contrast at 650 GHz can be used to screen for misalignment that would be difficult to detect with MMW.
Kinetic inductance bolometer (KIB) technology is a candidate for scalable submillimeter wave imaging systems, particularly suitable for person security screening applications. We have previously shown that the basic figures of merit are compatible with room-temperature radiometric imaging applications, and demonstrated the functionality of kilo-pixel detector arrays. In this article, we report on our imaging system based on 8208 KIBs organized on a 2D focal plane. We provide an overview on the basic components, including the detectors, optics, and cryogenics, and describe aspects relevant in system integration. Moreover, we demonstrate the capacity in actual concealed object detection by presenting datasets revealing metallic and dielectric objects hidden under the clothes of a test person.
In 2015, Asqella Oy commercialized a passive multi-band submillimeter-wave camera system intended for use in walk-by personnel security screening applications. In this paper we study the imagery acquired with the prototype of the ARGON passive multi-band submm-wave video camera. To challenge the system and test its limits, imagery has been obtained in various environments with varying background surface temperatures, with people of different body types, with different clothing materials and numbers of layers of clothing and with objects of different materials. In addition to the phenomenological study, we discuss the detection statistics of the system, evaluated by running blind trials with human operators. While significant improvements have been made particularly in the software side since the beginning of the testing, the obtained imagery enables a comprehensive evaluation of the capabilities and challenges of the multiband submillimeter-wave imaging system.
We present design and simulations of a reconfigurable reflectarray suitable for use as a sub-reflector within a confocal millimeter-wave imaging system. The performance of various reflectarray designs for use within a confocal imaging system are evaluated, e.g. phase quantization, feeding field. A quantized-particle-swarm optimization algorithm is discussed and applied to the reflectarray phase distribution in order to circumvent common challenges, such as the effects of the periodic phase error across the aperture, in addition to the generation of grating lobes due to aperture discontinuities. The reflectarray elements are based on a microstrip structure integrated with MEMS-based phase shifters in order to yield dynamic reconfigurability of the aperture. Both 1- and 2-bit phase quantization is considered. A reflectarray is characterized in a near-field measurement range; reflections from individual reflectarray elements are imaged at a distance on the order of a wavelength. Both the realized reflection phase and magnitude of the specular reflection are measured, yielding accurate characteristics of reflectarray element performance when compared to standard beam pattern measurements. A near-field measurement system is configured and tested at 120 GHz. The lateral resolution and phase measurement accuracy were measured to be 0:56λ and ±4° respectively. A static reflectarray is tested in the measurement system, and the reflection coefficient of the individual elements measured.
Aleksi Tamminen, Juha Ala-Laurinaho, Sampo Mäkelä, David Gomes-Martins, Janne Häkli, Päivi Koivisto, Pekka Rantakari, Jussi Säily, Reijo Tuovinen, Arttu Luukanen, Markku Sipilä, Antti Räisänen
We present results of experimental characterization of static 650-GHz reflectarrays. The reflectarrays are based on 123-μm circular microstrip patch antennas with tuning stubs as phase shifters. The static reflectarrays are considered as predecessors for active reflectarrays, and therefore the reflectarray elements have two discrete phase-shift values: 0° and -180°. The reflectarrays have 95000 elements, and they have separation of 400 μm. The reflectarrays are fabricated on 150-mm silicon wafers with a ground plane and a 20-μm polyimide substrate atop. The fabricated static reflectarrays are characterized in a near-field measurement range and their beam patterns at the focusing distance of 20 m are calculated with plane-to-plane transform. At this high frequency, fabrication tolerances are difficult to meet and, e.g., over-etching of the antenna and phase-shifting structure may offset the resonance frequency of the reflectarray element by more than its bandwidth.
Aleksi Tamminen, Juha Ala-Laurinaho, David Gomes-Martins, Janne Häkli, Päivi Koivisto, Mikko Kärkkäinen, Sampo Mäkelä, Pekka Pursula, Pekka Rantakari, Markku Sipilä, Jussi Säily, Reijo Tuovinen, Mikko Varonen, Kari Halonen, Arttu Luukanen, Antti Räisänen
Development of a 120-GHz FMCW radar with a reflectarray as focusing element is described. The reflectarray is
realized on a 150-mm silicon wafer and it has 3700 phase-modulating elements on it. The phase shifters have four
discrete values to cover full phase modulation with 90° steps. The reflectarray element is realized with a conductor-backed
coplanar waveguide patch antenna with a phase shifter coupled to it. The required phase modulation for each
reflectarray element is determined with an in-house physical optics simulation combined with genetic-algorithm-based
optimization. The reflectarrays are developed in two stages. First, preliminary reflectarrays with static phase shifters
have been manufactured and tested at 120-GHz antenna measurement range. The static reflectarrays are found to
perform as designed in their capability to steer the beam to a desired direction and to a distance of 3 m. The reflectarrays
have -3-dB beam width from 1.1° to 1.3° depending on the beam tilt. After the preliminary verification with the static
phase shifters, the reflectarrays will be assembled together with actively controlled MEMS-based phase shifters. The
MEMS switches are controlled with dedicated high-voltage CMOS electronics, forming a system-in-a-package (SiP).
First, the MEMS phase shifters are modeled, are being fabricated, and will be measured separately to verify their phase-shifting
capability.
Arttu Luukanen, Juha Ala-Laurinaho, David Gomes Martins, Janne Häkli, Päivi Koivisto, Pekka Pursula, Pekka Rantakari, Jussi Säily, Aleksi Tamminen, Reijo Tuovinen, Markku Sipilä
Recent developments in millimetre to submillimetre-wave imaging radars with excellent ranging resolution provide an
attractive route towards stand-off imaging of concealed explosives at ranges up to several tens of meters. Present systems
typically rely on only one transceiver, coupled with an optomechanical scanning system for image formation. This limits
the image acquisition speed to several seconds/frame. Frame rate can in principle be increased with increasing the
channel count but this adds substantially to the system complexity and cost, while only providing a modest speed
increase. In this paper we present preliminary designs for rapid electronic beam steering system that could provide a way
towards real-time millimetre-wave to submillimetre-wave imaging radars.
We present an active THz-imaging technique, which utilizes holographic process in image retrieval. In this technique,
information of the target is stored in an interference pattern. The pattern is formed with a reference field and a field
reflected from the target. This technique, called indirect holographic imaging, involves only amplitude detection. The
image of the target is formed computationally from the complex field given by the holographic process. An experimental
imaging system operated at 310 GHz is described. Millimeter-wave images of different targets are presented. The imager
performance is described with image signal-to-noise ratio and noise equivalent reflectivity difference, as well as with the
cross-range resolution. The indirect holographic imaging method is assessed with variable system signal-to-noise ratios.
A knife-edge method is utilized to approximate the point spread function of the imaging system. Cross-range resolution
of 0.18° and noise equivalent reflectivity level of 0.002 is achieved with an experimental imager at
310 GHz with 40-cm aperture.
The performance of stand-off imaging systems of concealed weapons in the mm-wave range remains limited by the
relatively poor angular resolution using practical aperture sizes. For this reason, increasing the operating frequency of the
systems is desired, but in practice is hard to realize due to the lack of affordable, low noise amplifiers well beyond 100
GHz. In this paper we present a passive terahertz imaging system which acquires passive terahertz (~200 GHz - ~1 THz)
imagery near video frame rate. The system, one copy of which is built in Finland and the other in the U.S., is based on a
64 pixel linear array of superconducting antenna-coupled microbolometers operated within a commercial cryogen-free
closed cycle cryocooler, and utilizes conical scanning Schmidt optics. Quantitative measurements on the imager
resolution metrics (thermal, spatial and temporal) will be presented. The results from field tests at the Helsinki-Vantaa
airport will be presented.
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