Terahertz (THz) Algılama Uygulamaları için Oda Sıcaklığında Çalışan Deşarj Lambalarının Karekterizasyonu

TÜBİTAK MFAG Proje01.03.2016İlk olarak 1970’lerde araştırılmaya başlayan bir konu olan deşarj lambaları kullanarak THz/milimetre dalga boyu algılama yöntemi günümüzde THz tekniklerine olan ilgi ile tekrar gündeme gelmiştir ve yakın zamanda kendi yaptığımız çalışmalar ile uluslararası alanda yoğun ilgi görmektedir. Yüksek basınçta bir gaz içinde oluşan deşarj sonucu ışıma yapan bu lambalar, oluşan plazma sayesinde ışıma yaparken THz dalga boylarına hassasiyet göstermektedir. Bu çalışmalar ilk olarak Prof. Dr. Natan Kopeika (Ben Gurion Üniversitesi, Nagev, Israil) tarafından gerçekleştirilmiştir. Kendi grubumuzda yaptığımız çalışmalar da bunu desteklemektedir, dahası kurduğumuz zamana dayalı THz ölçüm sistemleri ile yapısı ile ilintili özel THz frekanslarında bu yapıların rezonant etki gösterdiğini tespit eden ilk araştırma grubuyuz. MM dalga boyu/THz algılama için kullanılan bu tip lambaların ucuz olmaları, gerek oda sıcaklığında çalışırken THz dalga boylarını yüksek hassasiyetle tespit edebilmeleri, gerekse de plazmadan geçerken bazı THz frekansların kontrol edilerek filtreleyebilmesi bu tip yapıların farklı sivil ve savunma uygulamalarında kullanabileceklerini göstermektedir. Proje kapsamında yaptığımız çalışmalarda, deşarj lambalarında görülen bu rezonant etkilerin THz algılamadaki rolünü belirleyebilmek için ticari olarak satılan farklı lambaları hem zamana dayalı THz ölçüm sistemlerimiz ile hem de proje kapsamında geliştirdiğimiz sürekli ışıma yapan yüksek frekans THz ölçüm sistemleri ile karekterize ettik. İki türlü ışık kaynağı geliştirildi: 80-125GHz arası ışıma yapan, bu frekans tayfında 1GHz’lik dilimlerde 20mW ortalama güç üreten mm-dalga boyu ışın kaynağı ve 260-380GHz arası ışıma yapan, bu frekans tayfında 1GHz’lik dilimlerde 1mW ortalam güç üreten THz ışın kaynağı. Bu ışın kaynakların temelinde Schottky-diyot temeline dayalı çarpan yapıları süren frekans ayarlanbilir bir YIG osilatör mikrodalga kaynağı kullanılmıştır. MM dalga boyu kaynağı için 9-14GHz arası frekansı ayarlanabilen YIG osilatör kaynağını x9 toplam çarpan Schottky diyot temmelli RF bileşenler kullanıldı. THz frekanslarına ulaşmak için aynı kaynağa x3 çarpan bir Schottky temeline dayalı pasif çarpan bir diyot yapısı kullanıldı. İki sistem için frekansları hava boşluğuna taşımak için ayrı horn antenler kullanıldı. MM dalga boyu ışın kaynağı için konik bir horn anten yapısı kullanılırken, THz ışın kaynağı için piarmit horn anten yapısı kullanıldı. Dağılan ışın kaynağın Gaussiyen ışın dağılımı görünüşünü plano-konveks mercekler kullanarak topladık ve deşarj lamba yapısına odakladık. Deneylerin bir sürümünde deşarj lamba yapısını detektör olarak kullandık, diğer bir sürümünde ise içinden geçen frekansların şiddetlerini bir Golay Cell detetktör yardımıyla ölçtük. 2 THz üretici ve deşarj lamba kullanan alıcı sistemleri geliştirdik, sonra zamana dayalı THz ölçüm sistemlerin deşarj lambalarını daha iyi karekterize edebilmeleri için optimize ettik ve THz dalga boylarının deşarj ortamındaki plazma ile etkileşimini anlamak için benzetim çalışmaları geliştirdik. Bu çalışmalar sonucu rezonant etkilerin THz algılamadaki rolünü belirleyerek bu tip yapıların THz uygulamalarında nasıl ve nerede kullanabileceklerini daha iyi anladık. Deşarj lambaları ışın algılama için kullanıldığında literatürde “Glow Discharge Detector (GDD)” olarak adlandırılmaktadır. GDD lambaları ile bu ölçümlerde şu sonuçları elde ettik: GDD lambaları detektör olarak hem mm dalga boyları hem de THz frekansları için ışının kutuplaşma yönüne hassas (anot-katot arası elektrik alan yönü ışın elektrik alanına paralel ya da anti pararlel olduğu durumlarda algıladığı sinyal maksimum, dikay olduğu durumlarda algıladığın sinyal minimum) GDD lambaları hem optik eksene paralel hem de optik eksene dik konumda mm dalga boyu/THz ışınlarını algılayabiliyor GDD lamba detektör hassasiyeti iki frekans aralığı için neredeyse Golay Cell cihazına eşit (nW/Hz-1/2) GDD lambaları detektör olarak kullanıldığında ışın kaynağına genlik modülasyon uygulamak gerekiyor (“Amplitude Modulation (AM)”); 90kHz modülasyon frekansı için algıladığı sinyal en büyük olarak ölçüldü. Ekipmanların limitasyonları nedeniyle daha yüksek modülasyon frekansları araştırılamadı, bu frekansın MHz civarı olduğunu tahmin ediyoruz Optimize edilen zamana dayalı THz ölçüm sistemleri ve kurulan sürekli THz ışın kaynağı sistemleri ile yaptığımız ölçümlerde anot-katot arası mesafenin belli frekansların yapıdan geçişini etkilediğini gözlemledik. Genel olarak ticari olarak satın alınabilen deşarj lambaların anot-katot arası mesafesinin yaklaşık olarak 1mm olması ile etkileştiği rezonant frekans tayfın 250- 350GHz arası frekanslara denk düşmesi şu sonucu destekliyor: ışın ile plazmanın etkileşiminı arttırmak için anot-katot arası mesafesi dalga boyu ile orantılı olmalı. CST Microwave Studio programını kullanarak yaptığımız ilk benzetim çalışmalarında anot-katot arası mesafeye bağlı olarak plazmanın dielektrik fonsiyonu deşarj sırasında belli frekans aralıkları için minimum geçiş gösterdi. Bu çalışmaların üzerinde yoğunlaşarak anot-katot geometrisini optimize etmeyi hedefliyoruz. Yukarıda alınan sonuçların bir kısmı uluslararası konferansta tam makale bildiri olarak yayınlandı, diğer bir kısmı ise uluslararası (SCI-E) hakemli bir dergide yayınlandı. Projede 3 yapılan çalışmalar bir yüksek lisans öğrencisinin tez çalışmalarını destekledi diğer lisans, doktora düzeydeki öğrencilerin de araştırmalarını destekledi. Dünyada son zamanlarda yoğun ilgi ile araştırılan bariyer arkası görüntüleme sektörü için ucuz, oda sıcaklığında yüksek hassasiyetle çalışan THz detektör teknolojilerin geliştirilmesi eşi benzeri olmayan bir katkı sağlayacağına inanmaktayız. Ülkemizde terör olayları ve benzeri negatif unsurlar karşısında savunma ağırlıklı tespit ve imha teknolojileri kapsamında bu tip detektörlerin önemli bir katkı sağlayacağı ortak görüşündeyiz.First started in the 1970s, which was to use glow discharge lamps to detect THz and millimeter waves has become relevant again in the international arena due to the high interest in developing THz technologies and our recent work in this area. By forming the discharge in a gas kept at high pressure between two leads the lamp generates light, meanwhile the plasma formed drives the device to be sensitive to detection of THz and/or millimeter waves. These investigations were first carried out by Prof. Dr. Natan Kopeika (Ben Gurion University, Nagev, Israel) and his team. Our investigations not only support this, but we were also the first research group to see resonant effects due to the structure of the glow discharge lamp in the THz frequency range. Their low cost coupled with the ability to detect THz waves with high sensitivity at room temperature, or the resonant effects which may point to their potential use as controllable filters shows that these devices can be used in a variety of civilian and defense applications. In this project we investigated using both THz time-domain spectroscopy systems and continuous wave THz systems the role of resonant effects in the detection of THz waves by glow discharge lamps was ınvestigated. Two type of sources were developed: A source emitting in the 80-125GHz spectral region with at least 20mW output power per each 1GHz band and a source emitting in the 260-280GHz spectral range with at least 1mW output power per each 1GHz band in the emission. These sources are based on Schottky-diode based multipliers driven by a frequency tunable YIG oscillator. In the mm-wave source the 9-14GHz tunable YIG oscillator was multiplied (x9) using Schottky diode based RF multipliers. The same source was multiplied by x3 passive multiplier to reach THz frequencies. In both systems horn antennas were used to carry the emission into free space. A conical horn was used for the mm-wave source and a pyramid horn was used for the THz source. The emitted beam had a Gaussian beam profile which was collimated and focused on to the discharge lamp using plano convex lenses. In one embodiment of the experiments the discharge lamp was used as a detector and in one other the transmitted waves through the lamps structure was measured using a Golay Cell. In summary, THz emitter and detector systems based on discharge lamps were constructed and then time-domain THz systems were optimized to characterize these lamps. Furthermore to better understand the THz-wave plasma interaction simulations were performed. These investigations allowed us to better understand how the lamp detects THz waves and also helped us understand where and how to better use such devices. İn the 5 literature discharge lamps when used for light detection are typically called Glow Discharge Detectors (GDD). Using GDD lamps these results were obtained: The GDD lamps when used for detection of THz and mm-waves were found to be sensitive to the direction of the impinging E-Field polarization. (When the anodecathode electric field was parallel or antiparallel the detected signal was maximized, when perpendicular the detected signal was minimized). GDD lamps were able to detect mm-wave/THz radiation for when the structure was parallel and perpendicular to the optical axis. The GDD lamp sensitivity in direct detection was almost equal to a Golay Cell (nW/Hz-1/2) When using GDD lamps as detectors one needs to modulate the source (“Amplitude Modulation (AM)”); the signal response was maximized for a modulation frequency of 90 kHz. Due to the limitations in the detection electronics the response is thought to maximize at higher modulation (MHz) Using optimized THz time-domain spectroscopy systems and CW THz systems we showed that the anode-cathode separation plays a role in the transmission of certain mm-wave/THz frequencies. Since the anode-cathode separation of commercially available discharge lamps are on the order of 1mm, the resonant frequency measured upon transmission fell into the 250-350GHz frequency range which suggests that to increase the interaction of the plasma with the far infrared light one needs to tune the separation on the order of the incoming wavelength Using the commercially available software CST Microwave Studio our first simulation attempts have shown that the transmission of the field decreases for certain frequencies dependent on the anode-cathode separation as well as the dielectric properties of the plasma. We hope to improve these simulations thereby allowing us to better design the anode-cathode geometry to optimize detectivity The results as outlined above have resulted in the publication of one international conference proceeding and one international journal article (SCI-E). The research undertaken in this project directly contributed to the Masters’ thesis of one student and the studies of other doctoral, masters’ and undergraduate students. We believe that cost effective, room temperature THz detectors will have a great impact on the development of behind the barrier (see-through) imaging systems in the world. The 6 development of these technologies will be most beneficial to in Turkey, who has a vested interest in developing detection and threat elimination systems with the ever continuing threat of terror related incidents in our country

A Simple Model for Assessing Millimeter-Wave Attenuation in Brownout Conditions

Flying helicopters in adverse environmental conditions, such as low heights in arid regions, can be dangerous, especially during landing and take-off, since during hovering, the rotors produce a dust cloud of particles. This phenomenon is known as the “brownout” condition. Unlike visible and infrared systems, the radar devices in the microwave or millimeter wave region offer the capability of sufficient transmission through atmospheric obscurants, such as fog, smoke, sand/dust storms, and brownout. In this work, we present a theoretical evaluation of mm-wave (85–100 GHz) attenuation/scattering and power transfer in brownout conditions. The model includes attenuation/scattering prediction and radiant flux, or power collected by the receiver. We are considering the case of sand grain clouds created by helicopter rotor airflow during landing in arid areas. The evaluated scenarios are brownout environments over ranges up to 50 m. The predicted values from the mathematical model are compared with findings in the field and the literature. A simple model for mm-wave power transfer estimation shows satisfactory agreement with the measured values

Improved Performance in the Detection of ACO-OFDM Modulated Signals Using Deep Learning Modules

Free space optical communication (FSO) is widely deployed to transmit high data rates for rapid communication traffic increase. Asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) modulation is a very efficient FSO communication technique in terms of transmitted optical power. However, its performance is limited by atmospheric turbulence. When the channel includes strong turbulence or is non-deterministic, the bit error rate (BER) increases. To reach optimal performance, the ACO-OFDM decoder needs to know accurate channel state information (CSI). We propose novel detection using different deep learning (DL) algorithms. Our DL models are compared with minimum mean square error (MMSE) detection methods in different turbulent channels and improve performance especially for non-stationary and non-deterministic channels. Our models yield performance very close to that of the MMSE estimator when the channel is characterized by weak or strong turbulence and is stationary. However, when the channel is non-stationary and variable, our DL model succeeds in improving the performance of the system and decreasing the signal to noise ratio (SNR) by more than 8 dB compared to that of the MMSE estimator, and it succeeds in recovering the received data without needing to know accurate CSI. Our DL decoders also show notable speed and energy efficiency improvement

Editorial—Special Issue on “Optical and RF Atmospheric Propagation”

This Special Issue presents the latest research and developments in the field of optical and RF propagation sensing, propagation/effects/channel molding, advancements in applications, signal far-field measurements, theoretical/measurement methods for beam handling/processing, military applications, and next-generation network formations, amongst others [...

Infrared image denoising by non-local means filtering

The recently introduced non-local means (NLM) image denoising technique broke the traditional paradigm according to which image pixels are processed by their surroundings. Non-local means technique was demonstrated to outperform state-of-the art denoising techniques when applied to images in the visible. This technique is even more powerful when applied to low contrast images, which makes it tractable for denoising infrared (IR) images. In this work we investigate the performance of NLM applied to infrared images. We also present a new technique designed to speed-up the NLM filtering process. The main drawback of the NLM is the large computational time required by the process of searching similar patches. Several techniques were developed during the last years to reduce the computational burden. Here we present a new techniques designed to reduce computational cost and sustain optimal filtering results of NLM technique. We show that the new technique, which we call Multi-Resolution Search NLM (MRS-NLM), reduces significantly the computational cost of the filtering process and we present a study of its performance on IR images

Fast and Enhanced MMW Imaging System Using a Simple Row Detector Circuit with GDDs as Sensor Elements and an FFT-Based Signal Acquisition System

The relatively high atmospheric propagation of millimeter-waves (MMW) was found to be one of the most critical reasons for the development of reliable sensors for MMW detection. According to previous research works, it has been already shown that incident MMW radiation on a glow discharge detector (GDD) can increase the discharge current. Hence, the electrical mode of detection can be employed to detect the presence of MMW radiation. In this article, a new design of a row detector using GDDs as pixel elements, and the influence of MMW incidence on GDD’s discharge current, were acquired using an elementary data acquisition (DAQ) platform. The DAQ system computes the averaged Fast Fourier Transform (FFT) spectrum of the time signal and returns the FFT results as magnitude based on the level of detection. An FFT-based signal acquisition proved to be a better alternative to the lock-in detection that was commonly used in MMW detection systems. This improved detection circuit provides enhanced noise filtering, thereby resulting in better MMW images within a short time. The overhead expense of the entire system is very low, as it can avoid lock-in amplifier stages that were previously used for signal enhancement. A scanning mechanism using a motorized translation stage (step motor) is involved to place and align the row detector in the image plane. The scanning can be carried out vertically to perform the imaging, by configuring the step motor after selecting the desired step size and position. A simplified version of the MMW detection circuit with a dedicated over-voltage protection facility is presented here. This made the detection system more stable and reliable during its operation. The MMW detection circuit demonstrated in this work was found to be a milestone to develop larger focal plane arrays (FPA) with very inexpensive sensor elements

Design of 4π Directional Radiation Detector based on Compton Scattering Effect

Obtaining directional information is required in many applications such as nuclear homeland security, contamination mapping after a nuclear incident and radiological events, or during the decontamination work. However, many directional radiation detectors are based on directional shielding, made of lead or tungsten collimators, introducing two main drawbacks. The first is the size and weight, making those detectors too heavy and irrelevant for utilization in handheld devices, drone mapping, or space applications. The second drawback is the limited field of view, which requires multiple detectors to cover the whole required field of view or machinery to rotate the narrow field of view detector. We propose a novel 4π directional detector based on a segmented hollow cubic detector, which uses the Compton effect interactions with no heavy collimators. The symmetrical cubical design provides both higher efficiency and 4π detection ability. Instead of traditional two types of detectors (scatterer and absorber) structure, we use the same type of detector, based on GAGG(Ce) scintillator coupled to silicon photomultiplier. Additional advantage of the proposed detector obtained by locating the photon sensors inside the detector, behind the scintillators, which improves the radiation hardness required for space applications. Furthermore, such arrangement flattens the temperature variation across the detector, providing better gain stability. The main advantage of the proposed detector is the ability of 4pi radiation detection for high energy gamma-rays without the use of heavy collimators