Hakan Altan - Profile on Academia.edu (original) (raw)

Papers by Hakan Altan

NATO Science for Peace and Security Series B: Physics and Biophysics, 2017

The use of general descriptive names, registered names, trademarks, service marks, etc. in this p... more The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.

Elektriksel Ölçüm ve Terahertz Teknolojileri

Response of a Glow Discharge Detector to Far Infrared Radiation

IEEE Transactions on Plasma Science, Dec 1, 2022

Performance of an Active THz Imaging System for Recognition of Concealed Faces

Journal of Infrared, Millimeter, and Terahertz Waves

The metal to insulator transition (MIT) in vanadium dioxide (VO2) has played a key role in develo... more The metal to insulator transition (MIT) in vanadium dioxide (VO2) has played a key role in development of devices in the terahertz frequency range. In the present study, the use of VO2 patterned as a metamaterial surface or coupled as a homogeneous layer with a metallic metamaterial surface on top is investigated in terms of performance. High quality VO2 thin films were deposited on c-cut sapphire substrates by using the dc magnetron sputtering technique. A change in resistivity by a factor of 104 MIT in VO2 was observed allowing to investigate its use as a controllable metamaterial. The layer was patterned using a unique geometry (four-cross shaped) that operates in the THz frequency range. To understand its performance as a tunable THz filter, the four-cross structure fabricated from VO2 is compared to one fabricated from Au on VO2 bare film using UV lithography and ion beam etching techniques. The spectral performances of metamaterials was assesed using THz-Time Domain Spectrosco...

TRay: active THz imaging autonomous biometric secured access control and tracking gate system

Millimetre Wave and Terahertz Sensors and Technology XI

Recent security concerns worldwide have made it necessary to put a large number of people through... more Recent security concerns worldwide have made it necessary to put a large number of people through detailed security inspections in critical infrastructure and border entrances. It is desired to implement inspection processes, which will enhance security while providing improvements in several aspects, such as answering the high throughput at crowded places, standardizing control processes, minimizing operator errors and increasing the feel of security, without making individuals uncomfortable as possible. To answer these requirements, Active Terahertz Imaging Autonomous Biometric Security Access Control and Tracking Gate System, or TRay, integrates various security technologies. TRay offers epassport control, biometric identity check (fingerprint, face etc.) and THz imaging for concealed object detection. During the inspection process, the person using the gate is guided through e-passport control and biometric identity check steps by following a set of instructions, without requiring the assistance of an operator. The person will also be scanned for concealed objects by a THz imaging system. Locations of possible threat objects and contraband, made of metal and, in addition, ceramic, plastic, liquids etc. are identified using object detection algorithms. The system is able to generate a warning in case of a detected object, for further inspection of the location of the body part by security personnel. THz imaging system operates at a standoff distance, which allows scanning to be performed at any point during the inspection process, eliminating the need to use confined spaces for body scanning. Less human intervention and misunderstandings, decreased waiting time at passport/identification checkpoints, improved public satisfaction, standardized control duration, optimized use of space, easy tracking of passenger transitions, generating alarms in case of undesired transitions, 24/7 operation, forged document usage prevention and decreased workload on the security personnel are further advantages of the system, which is unique in terms of the features it combines.

Journal of Sensors

Angle random walk (ARW) is a critical noise component of a typical gyroscope alongside with bias ... more Angle random walk (ARW) is a critical noise component of a typical gyroscope alongside with bias instability noise. ARW has dominant effects especially in short-term accuracy. The measurement uncertainty degrades with the deterioration in ARW resulting in the lowered overall gyroscope accuracy. Many inertial navigation applications such as satellite control and gyro-compassing require low ARW. For an interferometric fiber optic gyroscope, lowest detectable rotation is proportional to scale factor of sensor and inversely proportional to optical bandwidth of light source used in the gyroscope. In this study, using a novel pump laser control closed-loop method, gyroscope performance is able to significantly enhance including signal-to-noise ratio (SNR). Fiber optic gyroscope includes an Yb-doped amplified spontaneous emission (ASE) source with broad emission spectra of 15 nm bandwidth used as light source in order to improve gyroscope sensitivity. The final ARW performance is about 0.0...

Comparison of terahertz and ultrasound imaging in the assessment of oral soft tissue lesions

Oral Surgery, Oral Medicine, Oral Pathology and Oral Radiology

THz Imaging Based on THz Spatial Light Modulation

Yoğun Plazmaların Terahertz Dalgaboyları İle İncelenmesi

AMAÇ VE GEREKÇE Maddenin 4 halinden biri olan ve evrendeki varolan maddenin %99 unun bu halde old... more AMAÇ VE GEREKÇE Maddenin 4 halinden biri olan ve evrendeki varolan maddenin %99 unun bu halde olduğu düşünülen plazma, günümüz bilim alanlarında yoğun ilgi ile araştırılmaktadır. Özellikle sanayi ve enerji ile ilgili alanlarda plazmaların uygulamaları üzerine çalışmalar sürdürülmektedir. Oluşturulan plazmaların incelenmeleri için elektronik yöntemler geliştirilmiştir. Elektronik “probe” olarak literatürde geçen bu tekniklerin ortak özelliği plazmanın içine sokulan bir cismin plazma içindeki yoğunluk ve sıcaklık ölçümlerine dayalıdır. Fizik bölümümüzde geliştirdiğimiz ve Türkiye’de ilk ve tek özelliğini taşıyan atımlı terahertz spektroskop sistemleri ile plazmanın bu özelliklerini hiç plazmaya değmeden veya bir cisim sokmadan ölçmeyi hedefliyoruz. KAPSAM Önerilen projede mevcut atımlı Terahertz spektroskop sisteminden elde edilen terahertz tayf spektrumu ile hem RF hem de DC yöntemler kullanılarak elde edilen plazmaların elektrik ve optik özelliklerinin incelenmesi önerilen projenin ...

Tunable Metamaterial Filters Fabricated from High Temperature Superconductors

Compressive Sensing Imaging and Image Fusion at sub THzfrequency in Transmission Mode

2019 IEEE MTT-S International Microwave and RF Conference (IMARC), 2019

Over the past few years, the Terahertz (THz) band has been seen as quite promising in fields of s... more Over the past few years, the Terahertz (THz) band has been seen as quite promising in fields of spectroscopy and imaging. Most of the current manufacturing methods for THz emission and detection devices are limited due to the associated cost and complexity, therefore alternative ways have been sought driven by the availability of new technologies. From recent research in the field of imaging, commercially available Glow Discharge Detectors (GDDs) have been found to be an interesting solution because of their low cost and efficiency in detecting electromagnetic (EM) radiation from microwave to higher frequencies of the electromagnetic spectrum. Several studies have been carried out in the scope of millimeter/sub-millimeter waves to understand how its detection mechanism changes for higher frequencies and how the mechanisms of plasma generation influence the device's performance. In this paper, we aim to study the interaction between THz waves and the structure of a GDD, namely, its reflection, transmission and losses. Numerical modelling of electromagnetics, by solving the Maxwell's equations, has been undertaken for this type of device using different gas pressures of the lamp. Our results show that these devices are not optimized for the THz domain.

Compressive Sensing Imaging at Sub-THz Frequency in Transmission Mode

NATO Science for Peace and Security Series B: Physics and Biophysics, 2017

Due to lack of widespread array imaging techniques in the THz range, point detector applications ... more Due to lack of widespread array imaging techniques in the THz range, point detector applications coupled with spatial modulation schemes are being investigated using compressive sensing (CS) techniques. CS algorithms coupled with innovative spatial modulation schemes which allow the control of pixels on the image plane from which the light is focused onto single pixel THz detector has been shown to rapidly generate images of objects. Using a CS algorithm, the image of an object can be reconstructed rapidly. Using a multiplied Schottky diode based multiplied millimeter wave source working at 113 GHz, a metal cutout letter F, which served as the target was illuminated in transmission. The image is spatially discretized by laser machined, 10 × 10 pixel metal apertures to demonstrate the technique of spatial modulation coupled with compressive sensing. The image was reconstructed by modulating the source and measuring the transmitted flux through the metal apertures using a Golay cell. Experimental results were compared to reference image to assess reconstruction performance using χ2 index. It is shown that a satisfactory image is reconstructed below the Nyquist rate which demonstrates that after taking into account the light intensity distribution at the image plane, compressive sensing is an advantageous method to be employed for remote sensing with point detectors.

The evaluation of various designs for ytterbium-doped fiber-based superfluorescent source at 1μm wavelength

The development of powerful sources with wide spectral coverage is important for many sensing app... more The development of powerful sources with wide spectral coverage is important for many sensing applications. In this paper, various superfluorescent fiber sources (SFS) at 1μm Wavelength using Ytterbium doped (Yb-doped) fiber is studied. Different configurations such as single pass forward (SPF), single pass backward (SPB), double pass forward (DPF), double pass backward (DPB), are compered in terms of spectral bandwidth, power, central wavelength stability and power stability. Double-pass bi-directional sources are employed using loop mirrors. For all cases, maximum power is compared at various laser diode power. Mean wavelength stability is measured between temperature of -40/+60°C to assess potential applications.

Millimeterwave imaging with compressed sensing: Effects of modulation frequency and data type

2018 26th Signal Processing and Communications Applications Conference (SIU), 2018

Millimeter wave imaging is a promising field that can be used in many areas, primarily in defense... more Millimeter wave imaging is a promising field that can be used in many areas, primarily in defense and security. The lack of high-resolution two-dimensional imaging detectors that work in this band, as well as the cost of measurements in this frequency band being high, make single-pixel imaging configuration attractive. Data acquisition with compressed sensing is preferred to reduce the number of measurements. In this study, an experimental study of millimeter-wave imaging at 60 GHz center frequency is presented. The masks used in compressed sensing are designed to have a new mask in each row/column motion to approach real time imaging. The effect of the acquired data type and modulation frequency on the reconstructed image is investigated.

Radiation Pattern Characterization of Terahertz Photoconductive Antennas Using Time-Domain Spectroscopy System

2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, 2018

This paper presents the radiation pattern characterization of terahertz photoconductive antennas ... more This paper presents the radiation pattern characterization of terahertz photoconductive antennas using a new technique. The proposed technique is implemented using a modified terahertz time domain spectroscopy by applying a full mapping of the THz beam. The proposed method was also compared with analytical methods and numerical solvers. The comparison of the measured results with that of the simulations shows a half-power beam width error of 1.5°and an optical path difference error of 8%. To the best of our knowledge, this is the first time terahertz time domain spectroscopy system is used to characterize the radiation pattern of terahertz photoconductive antennas.

Terahertz Propagation in Tissues and Its Thickness Limitation

The terahertz region offers a multitude of possibilities for new diagnostic techniques in biology... more The terahertz region offers a multitude of possibilities for new diagnostic techniques in biology. Recent advancements in terahertz sources and detectors have allowed such systems to be employed in investigations of biological tissues. The main interaction with such media is evident in the water content, which can be detrimental in certain instances such as deep tissue diagnostics and also be beneficial in others such as the detection of certain types of cancers. The field has developed to deal with such challenges, and the improvement in overall signal to noise of especially pulsed diagnostic methods has led to the achievement of penetration depths on the order of millimeters, which has opened up a variety of new investigations. These investigations are described with special emphasis on diagnostics on hard tissues such as teeth

Investigation of doped and undoped VOx films for terahertz microbolometer applications

Terahertz Emitters, Receivers, and Applications XII, 2021

The microbolometer technology has proved its potential in the Infrared (IR) region due to its low... more The microbolometer technology has proved its potential in the Infrared (IR) region due to its low fabrication costs, and room temperature operation, making this technology desirable to be used in various applications, and this interest has recently expanded into the Terahertz (THz) region as well. The detection in microbolometers is achieved through the absorption of THz radiation which subsequently heats up and is sensed by the temperature sensitive material at the core of the device. This temperature sensitive material is typically based on VOx, which exhibits a sufficient change in resistance with temperature. While this temperature sensitive material is useful in the IR, the low energy of the THz wave compared to the background radiation makes it a challenge to operate the device at room temperature and show a large change in resistance with respect to the slight change in temperature. Metal doped VOx films can show a better performance however these effects are not well understood in the THz region. In this study, Tungsten (W) doped and undoped VOx films are fabricated and then analyzed using Time Domain THz Spectroscopy. The DC electrical properties of the films as well as their optical behaviors in the region of 0.2-2.0 THz are analyzed as a function of temperature. The metal doping is seen to affect the overall electrical and optical response of the film. Understanding this dependence is key to achieving a better film for applications in the THz region.

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

Millimetre Wave and Terahertz Sensors and Technology IX, 2017

Plasma, used as a terahertz (THz) detection medium has promising features. Several studies for mm... more 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.