Juha Ala-Laurinaho

Real-time passive terahertz imaging system for standoff concealed weapons imaging

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.

Beam Switching Conformal Antenna Array for mm-Wave Communications

In this letter, the design and measurement results of a millimeter-wave beam switching antenna are presented. Several antenna prototypes have been fabricated and measured in planar and conformal installations. The developed prototype consists of three antenna arrays of 16 patch elements in each array and it is operating at 61 GHz with linear polarization. A single-pole three-throw semiconductor switch is attached to the manufactured prototype using copper pillar attachment. The main beam direction of the convex antenna can be changed by switching between the antenna arrays in the prototype. Half sphere 3-D radiation pattern measurements have been performed. For the convex prototype, bent on the cylinder with radius 25 mm, beam switching with -32° / + 34° degrees is achieved.

Rapid beamsteering reflectarrays for mm-wave and submm-wave imaging radars

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.

Reflectarray for 120-GHz beam steering application: design, simulations, and measurements

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.

A systematic design method for CRLH periodic structures in the microwave to millimeter-wave range

Bloch analysis in conjunction with a full-wave simulation is used in order to design a composite right/left-handed (CRLH) transmission line (TL). We analyze the parasitic components introduced by loading elements and inter-cell coupling, and present details to the compensation required in order to maintain the balanced state of the periodic TL. The method is shown to yield a useful tool for the design of CRLH periodic TLs up to millimeter-wave frequencies for structures with arbitrary metallization. The performance of the method is shown numerically at 26 and 77 GHz. Experimental verification is done at 26 GHz, comparing radiative properties, dispersion, and attenuation constant of a through-broadside scanning leaky-wave antenna.

Reflectarray Design for 120-GHz Radar Application: Measurement Results

In this paper, we present design and experimental results on reflectarrays at 120 GHz. The offset-fed reflectarrays consist of conductor-backed coplanar patch antennas with phase-shifting stubs. Three 138-mm reflectarrays are lithographically fabricated and evaluated in a near-field measurement range. Their measured beam patterns are compared to the theoretical ones. The theoretical -3-dB beam width is 60-64 mm at 3-m distance from the reflectarray. Measured beam widths of the different reflectarrays deviate less than 10% from the theoretical values. The beam pointing is found to be close to theoretical, whereas the sidelobe level is up to 5 dB higher. The efficiency, alignment accuracy, and surface shape of the reflectarray are studied with near-field imaging of the reflectarray aperture field. The measured average efficiency is 0.11 whereas the predicted average efficiency is 0.54. The low efficiency is most likely due to over-etching of the structures of the reflectarray element, and could be improved in future fabrication processing rounds. Beam pattern measurement close to the main beam is well suited for evaluating the beam width and pointing accuracy, but it gives little information on the element performance. We propose near-field imaging of the reflectarray to evaluate both element efficiency and phase shift.

Indirect holographic imaging: evaluation of image quality at 310 GHz

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.

Phaseless Characterization of Broadband Antennas

A new efficient method for broadband antenna characterization from phaseless acquisitions in the frequency-domain (FD) is presented. The phase-retrieval technique is based on an extrapolation of the off-axis indirect holography. In common with the conventional approach, the power of the interferometric field of the antenna under test (AUT) and a reference antenna, whose field is known in advance, as well as the power of the AUT alone, is measured at the desired frequencies. Nevertheless, the phase retrieval is accomplished independently at each spatial point by filtering in the time-domain (TD) rather than in the k-space. Thus, the dependency of the phase retrieval on the position accuracy is reduced and it can be accomplished simultaneously at all frequencies without resorting to iterative schemes. Moreover, it yields a less dense sampling and a phase-retrieval algorithm not dependent on the geometry of the acquisition. The method is illustrated with a numerical example in the W-band as well as with two near-field (NF) measurement examples in the Ka- and W-bands.

Towards video rate imaging at submillimetre-waves — Finnish developments of passive multi-band imaging and holographic submm-wave beam steering at VTT

Imaging at submillimetre-wave (SMMW) frequencies is of considerable interest for security applications due to potentially superior performance at longer stand-off ranges in comparison to mm-wave imaging thanks to reduced diffraction. We have demonstrated passive broad-band video rate imaging system operating at a centre frequency of about 600 GHz, capable of 10 frames/second imagery of a 2 m × 1 m field-of-view at a stand-off distance of 5 meters. In addition, a multi-band system centred around 250, 450 and 720 GHz will be discussed. Multi-band imagery is interesting given its potential for rudimentary materials differentiation, a capability that would substantially benefit e.g. security imaging applications. The passive imaging activities are complemented by activities towards developing rapid electronic beam steering capability for imaging submm-wave radars. Results from two projects aiming at constructing such beam steering systems at 120 GHz and at 650 GHz are presented.

Experimental Determination of DRW Antenna Phase Center at mm-Wavelengths Using a Planar Scanner: Comparison of Different Methods

A study of the phase center position of dielectric rod waveguide (DRW) antennas (silicon and sapphire) for different millimeter-wave (mm-wave) frequencies is presented in this document. Phase center position is determined using data obtained by planar scanning and analyzed by means of different methods: least squares fit method with and without weighting coefficients and plane wave spectrum (PWS) analysis method. A study of the radiation pattern and phase center position for different mm-wave frequencies is provided and the results of the different methods are presented and compared.