Mikko Leivo

Passive terahertz camera for standoff security screening

We describe the construction and performance of a passive, real-time terahertz camera based on a modular, 64-element linear array of cryogenic hotspot microbolometers. A reflective conical scanner sweeps out a 2 m x 4 m (vertical x horizontal) field of view (FOV) at a standoff range of 8 m. The focal plane array is cooled to 4 K in a closed cycle refrigerator, and the signals are detected on free-standing bridges of superconducting Nb or NbN at the feeds of broadband planar spiral antennas. The NETD of the focal-plane array, referred to the target plane and to a frame rate of 5 s(-1), is 1.25 K near the center of the array and 2 K overall.

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.

Stand-off passive THz imaging at 8-meter stand-off distance: results from a 64-channel real-time imager

At present, the imaging of concealed weapons and contraband is primarily carried out at a relatively short stand-off range of a few meters mainly because of spatial resolution considerations. In order to maintain a reasonable aperture size, there is a desire to extend the operating frequency towards 1 THz. In this paper we report the progress on a video-rate THz camera demonstrator which utilizes broadband antenna-coupled microbolometers as detectors, operated within a turnkey commercial closed-cycle cryocooler. A full system has been integrated consisting of 64 parallel sensors and readout electronics, and reflective Schmidt camera optics incorporating a conical scanner for real time imaging.

Passive THz imaging system for stand-off identification of concealed objects: results from a turn-key 16 pixel imager

Passive imaging of concealed objects at stand-off distances in excess of a few meters requires both excellent spatial, thermal and temporal resolution from the terahertz imaging system. The combination of these requirements while keeping the overall system cost at a reasonable level has been the motivation for this joint work. The THz imaging system under development is capable of sub-Kelvin NETD at video frame rates. In this paper we report the first imaging results from a 16-pixel array of superconducting antenna-coupled NbN vacuum-bridge microbolometers, operated within a cryogen-free, turn-key refrigerator. In addition to the system overview, we shall also address the uniformity of the detectors and present passive indoors raster-scanned imagery.

Measured performance of a high-resolution passive video-rate submillimeter-wave imaging system demonstrator for stand-off imaging

In the paper we present the performance of our new 128 -channel passive submillimeter-wave camera, capable of cm-scale resolution at 5 m standoff and frame rates up to 10 fps. The measured resolution metrics for the system will be presented.

Passive real-time submillimetre-wave imaging system utilizing antenna-coupled microbolometers for stand-off security screening applications

At present, the imaging of concealed weapons and contraband is primarily carried out at a relatively short stand-off range of a few meters mainly because of spatial resolution considerations. In order to maintain a reasonable aperture size, there is a desire to extend the operating frequency towards 1 THz. In this paper we report the progress on a video-rate THz camera demonstrator which utilizes broadband antenna-coupled microbolometers as detectors, operated within a turn-key commercial closed-cycle cryocooler. A full system has been integrated consisting of 64 parallel sensors and readout electronics, and reflective Schmidt camera optics incorporating a conical scanner for real time imaging. At present, the system provides near real time submillimetre-wave video imagery at 6 frames per second.

A passive, real-time, terahertz camera for security screening, using superconducting microbolometers

We describe a broadband terahertz camera based on a modular 64-element linear array of hot-spot microbolometers Unlike many superconducting sensor arrays, the readout for this array is performed by entirely uncooled electronics; no SQUIDs or cryogenic HEMTs are employed. The operating principles for the microbolometer and the readout scheme are described and compared with those of similar superconducting sensors.

Passive three-colour submillimetre-wave video camera

Stand-off detection for concealed weapons is one of the applications for passive submillimetre-wave imaging. The operating frequency (neglecting technology limitations) is often a compromise between the diffraction-limited angular resolution for a fixed maximum aperture diameter, and the extinction of the signal in obscurant layers: At high frequencies towards the 1 THz mark, excellent angular resolution is readily achievable with modest aperture diameters, while scattering and attenuation by clothing is high which creates potentially more clutter rather than improving detection capability. At lower frequencies towards 100 GHz, attenuation and scattering by clothing is much less pronounced, albeit at significantly reduced spatial definition thanks to increased diffraction. In order to avoid the above-mentioned compromise, we have constructed a three-band passive imaging system operating at effective centre frequencies of 250 GHz, 450 GHz and 720 GHz. Aspects of the system will be presented.

Imaging with Modular Linear Arrays of Cryogenic Nb Microbolometers

We present ultrawideband imagery obtained with modular, 8-element, superconducting Nb microbolometer arrays. Conically scanned images are presented and compared with raster-scanned images obtained on the same arrays and from similar NbN arrays at VTT. Statistical data on detector non-uniformity, and methods for mitigating and compensating it are described. Low-noise readout is accomplished with room-temperature electronics using the transimpedance scheme of Pentilla et al. Characterization of spatial resolution, noise-equivalent temperature difference, and spectral response is done using metrology tools - standard targets, mm-wave blackbodies, and variable filters - that have been developed at NIST for this purpose.

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.