Bariyer Arkası Terahertz Görüntüleme Sistemi

ODTÜ Fizik THz Araştırma Laboratuarı’nda farklı görüntüleme sistemleri kullanılarak, farklı THz ve THz altı görüntüler elde edilmektedir. Bunlardan birincisinde Schottky diyot çarpanlarına dayalı bir THz kaynak Galvo aynaların yardımıyla görüntüleme alanını nokta nokta taramaktadır. İkinci bir sistemde yine Schottky diyot çarpanlarına dayalı başka bir THz kaynak ile Compressive Sensing prensibine uygun olarak görüntüleme yapılmaktadır. Bir başka proje kapsamında ise görüntüleme için GDD alıcı dizgiler geliştirilmektedir. Bu projede THz bariyer arkası görüntüleme sistemlerinin bariyerden gelen gürültüyü yok etmesi için gerekli elektronik bileşenler geliştirilecek ve böylece bahsi geçen üç çalışmanın da kazanç sağlaması sağlanacaktır. Hedeflenen bu amaç için gereken optik dizayn standart üretimi ve satışı yapılan lensler ile sağlanamamaktadır. Bu yüzden gerekli parametrelere sahip lenslerin tasarımı, üretimi ve testi gereklidir. Proje kapsamında uygun bir 3B yazıcı alınıp farklı frekansları akromat şekilde hedefe odaklayan THz merceklerin üretilmesi planlanmıştır

340 GHz imaging system for detection of concealed threat objects at 5 meters stand-off distance (Conference Presentation)

A THz active scanned imaging system is developed for detection of concealed threat objects at a stand-off distance of 5 meters. Single pixel, active imaging system utilizes a continuous wave transceiver unit operating at 340 GHz, based on RF components and Schottky diode rectifiers. The transceiver has a heterodyne detection geometry and has 7 mW total power output which is derived from a 3dB directional coupler (25 dB directivity) and a horn antenna. 2D opto-mechanical scanning is performed using two mirror coupled galvanometer scanners to scan 50x50 cm2 field of view at 5 meters stand-off distance. 2 cm resolution is achieved on the target plane. Based on the opto-mechanical scan speed, frame rate is 2 Hz. 340 GHz provides penetration to common barrier objects such as clothing for detection of concealed threats. Here we present imaging capability of the system and its penetration abilities. Effect of barrier objects to the dynamic range is also investigated. To provide real time screening of the target scene for potential threat objects, its visual image is combined with the acquired THz image of the field of view by using image fusion technique

Compressive sensing imaging through a drywall barrier at sub-THz and THz frequencies in transmission and reflection modes

In this work sub-terahertz imaging using Compressive Sensing (CS) techniques for targets placed behind a visibly opaque barrier is demonstrated both experimentally and theoretically. Using a multiplied Schottky diode based millimeter wave source working at 118 GHz, metal cutout targets were illuminated in both reflection and transmission configurations with and without barriers which were made out of drywall. In both modes the image is spatially discretized using laser machined, 10 x 10 pixel metal apertures to demonstrate the technique of compressive sensing. The images were collected by modulating the source and measuring the transmitted flux through the apertures using a Go lay cell. Experimental results were compared to simulations of the expected transmission through the metal apertures. Image quality decreases as expected when going from the non-obscured transmission case to the obscured transmission case and finally to the obscured reflection case. However, in all instances the image appears below the Nyquist rate which demonstrates that this technique is a viable option for Through the Wall Reflection Imaging (TWRI) applications

Image Reconstruction and Optimization Using a Terahertz Scanned Imaging System

Due to the limited number of array detection architectures in the millimeter wave to terahertz region of the electromagnetic spectrum, imaging schemes with scan architectures are typically employed. In these con fi gurations the interplay between the frequencies used to illuminate the scene and the optics used play an important role in the quality of the formed image. Using a multiplied Schottky-diode based terahertz transceiver operating at 340 GHz, in a stand-o ff detection scheme; the e ff ect of image quality of a metal target was assessed based on the scanning speed of the galvanometer mirrors as well as the optical system that was constructed. Background e ff ects such as leakage on the receiver were minimized by conditioning the signal at the output of the transceiver. Then, the image of the target was simulated based on known parameters of the optical system and the measured images were compared to the simulation. By using an image quality index based on chi(2) algorithm the simulated and measured images were found to be in good agreement with a value of chi(2) = 0.14. The measurements as shown here will aid in the future development of larger stand-o ff imaging systems that work in the terahertz frequency range

Transmission and detection of terahertz radiation in a weakly ionized plasma (Conference Presentation)

Plasma, used as a terahertz (THz) detection medium has promising features. Several studies for mm-wave/THz radiation detection using various kind of methods for plasma creation such as neon indicator lamps [1], gas cells [2] and laser-induced air plasma [3] have been conducted. The interaction between the plasma and various frequency EM waves are still being investigated and in the mm-wave/terahertz range the interaction mechanism is still not well understood. In this study a home-built gas chamber with variable electrode separation is studied using a continuous wave mm-wave/THz measurement systems. A breakdown is induced in gas mixture by applying a bias DC voltage to the electrodes and under sufficient conditions the modulated incident radiation can generate variations in the plasma current which can be measured electronically. The main mechanism of detection is the addition of the electric field of the incident EM radiation to the DC bias field, increasing the total electric field thus excitation collisions. Therefore the EM field is expected to effect the rate of ionization and excitation collisions at most at the regions with maximum total electron energy that is around the cathode dark space [4]. Depending on the orientation of its polarization, the incident EM radiation can also diffuse the signal electrons to the walls of the chamber giving rise to a negative change in bias current, decreasing the signal. Therefore the internal signal gain depends on the electrode geometry and polarization of radiation besides other parameters of the plasma. Several parameters, such as gas pressure, gas species, discharge current, electrode spacing and electrode geometry effect the plasma-THz interaction thus changing the responsivity of the device. The plasma – THz interaction is studied here using a VDI multiplied source (WR2.8AMC). Driven by a frequency tunable Yttrium Iron Garnett (YIG) oscillator the source was modulated electronically providing a frequency tunable output in the 82-125 GHz and 246-375 GHz frequency range by use of a passive tripler. For the gas chamber different gases and gas mixtures are used. Using a Penning mixture, which is a mixture of one type of another gas with miniscule amount of another gas which has a lower ionization voltage than the main gas, a breakdown voltage lower than that of both gases can be obtained. Measurement of changes in the plasma current are carried out for different incident radiation frequencies, different electrode geometries, various gas mixtures and different modulation frequencies

Realization of SV-PWM Motor Control Algorithm using ARM Cortex-M4 Based Microcontroller

Space vector pulse width modulation (SV-PWM), which is the one of the sensorless control techniques of the permanent magnet AC motors, is preferred because it provides voltage and frequency control and produces less harmonics than the sinusoidal pulse width modulation (S-PWM) technique, respectively. In SV-PWM method; the fact that results of the complex equations should be performed in a given period of time by the microcontroller to calculate the required switching time, makes it difficult to implement algorithm in practice. This study includes the implementation of SV-PWM algorithm using ARM Cortex-M4 based microcontroller and experiments conducted on a motor