Andrej Švigelj

Spectroscopic Terahertz Imaging at Room Temperature Employing Microbolometer Terahertz Sensors and Its Application to the Study of Carcinoma Tissues

A terahertz (THz) imaging system based on narrow band microbolometer sensors (NBMS) and a novel diffractive lens was developed for spectroscopic microscopy applications. The frequency response characteristics of the THz antenna-coupled NBMS were determined employing Fourier transform spectroscopy. The NBMS was found to be a very sensitive frequency selective sensor which was used to develop a compact all-electronic system for multispectral THz measurements. This system was successfully applied for principal components analysis of optically opaque packed samples. A thin diffractive lens with a numerical aperture of 0.62 was proposed for the reduction of system dimensions. The THz imaging system enhanced with novel optics was used to image for the first time non-neoplastic and neoplastic human colon tissues with close to wavelength-limited spatial resolution at 584 GHz frequency. The results demonstrated the new potential of compact RT THz imaging systems in the fields of spectroscopic analysis of materials and medical diagnostics.

Comparison of terahertz technologies for detection and identification of explosives

We present results on the comparison of different THz technologies for the detection and identification of a variety of explosives from our laboratory tests that were carried out in the framework of NATO SET-193 “THz technology for stand-off detection of explosives: from laboratory spectroscopy to detection in the field” under the same controlled conditions. Several laser-pumped pulsed broadband THz time-domain spectroscopy (TDS) systems as well as one electronic frequency-modulated continuous wave (FMCW) device recorded THz spectra in transmission and/or reflection.

A high performance room temperature THz sensor

Resonant THz antenna-coupled micro-bolometers are considered as a potential candidates for room temperature THz imaging, as well as spectroscopic applications. Micromachining technology is found to be well-suitable to fabricate a micro-meter bolometer sensor suitable for MEMS implementation. The sensitivity of the sensor is determined to be up to 1000V/W and the noise equivalent power (NEP) – is down to 5pW /√Hz. The sensor parameters are designed to be easily implemented with a low cost standard preamplifier array which increases the pixel sensitivity to 106V/W without compromising the noise equivalent power.

THz-vision system with extended functionality

A near real-time THz-vision system is presented in the paper. The most important part of it is the THz sensors focal plane array operating at the room temperature, featuring low NEP (5pW/√Hz) and high sensitivity (1e6 V/W). Its architecture allows direct digital processing of the output signal. The system performance is upgraded with large parallel processing of up to 64 channels. The second important building block is the FM THz source used for illumination. A wide FM range, of up to ±10% of the central frequency allows using the system for various applications. The THz source is a solid-state source using a GHz range frequency synthesizer followed by frequency multipliers and microwave amplifiers. Such a compact THz source can cover the lower region of the THz spectrum, i.e. below 1THz using different frequency bands. The band selection depends on the application. Three different areas of applications are discussed in the paper: 3D imaging of hidden objects as one of the most attractive features of the presented system, an accurate range finder with the resolution within a fraction of the wave length and a narrow band CW spectrometer operating in the FM range of the source.

A compact THz imaging system

The objective of this paper is the development of a compact low cost imaging THz system, usable for observation of the objects near to the system and also for stand-off detection. The performance of the system remains at the high standard of more expensive and bulkiest system on the market. It is easy to operate as it is not dependent on any fine mechanical adjustments. As it is compact and it consumes low power, also a portable system was developed for stand-off detection of concealed objects under textile or inside packages. These requirements rule out all optical systems like Time Domain Spectroscopy systems which need fine optical component positioning and requires a large amount of time to perform a scan and the image capture pixel-by-pixel. They are also almost not suitable for stand-off detection due to low output power. In the paper the antenna - bolometer sensor microstructure is presented and the THz system described. Analysis and design guidelines for the bolometer itself are discussed. The measurement results for both near and stand-off THz imaging are also presented.

Advanced THz sensor array for precise position and material properties recognition

The precise position of objects in the industrial process, assembly lines, conveyers, or processing bins is essential for fast and high quality production. In many robotized setups the material type and its properties are crucial. When several types of materials or parts are used, material recognition is required. Advanced robotics systems depend on various sensors to recognize material properties, and high resolution cameras with expensive laser measuring systems are used to determine the precise object position. The purpose of this paper is to present how the THz sensor and THz waves can be applicable for such precise object position sensing and its material properties in real time. One of the additional features of such a THz sensor array is also the ability to see behind barriers that are transparent for THz waves. This allows the system to obtain precise dimensions, position, and material properties of the object, which are invisible for visible light or anyhow obscured to other vision systems. Furthermore, a 3D THz image of the object can also be obtained and, in cases when a visual picture is available, its fusion with a THz image is possible. In the paper a THz sensor array, operating at a 300GHz central frequency and at room conditions is presented, together with the proposed vision system description. The target is illuminated with a frequency modulated, solid state THz source, and provides output power around 1mW. By mixing of the illuminating and reflected signals, the resulting difference frequency signal is obtained. Its amplitude and phase carry all relevant information of the target. Some measurement results are also shown and discussed.

3D THz range finder of concealed objects

Numerous applications require fast and accurate range measurement of concealed objects. As THz waves penetrate almost all materials except metals, they are a candidate to perform this task. In this paper a system consisting of an illumination THz source and a THz detector array is presented. A solid state 300GHz THz source signal is frequency modulated and guided to a detector array using a beam splitter. The detector array consists of ultra-sensitive bolometers with 1000V/W sensitivity and NEP of 5pW/√Hz. Their square law sensitivity characteristic allows mixing of the reflected wave with the illumination wave, resulting in a mixed product with a difference frequency of few kHz proportional to the target distance, the frequency modulation span, and its rate. Parallel signal processing of all 16 signals results in fast range detection of the target. All 16 detectors provide four readings per second for all X and Y positions of the image. A compact and portable system with 40GHz frequency modulated source provides Δf/Δt = 160GHz/sec. This corresponds to a 1kHz difference frequency per one meter distance of the target. A resolution accuracy in the micrometer range has been achieved using advanced signal processing. In the paper the processing algorithms and the obtained results are presented and discussed. The complete hardware structure of the system is described together with the required signal processing procedures. The advantage of the presented system is that it operates at room temperature and is therefore cost effective and very robust.