Dissertation/Theses - Department of Electrical and Electronic Engineering
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Post graduate dissertations (Thesis) of Electrical and Electronic Engineering (EEE)2024-03-29T11:16:54ZDesign of an all-optical plasmonic modulator for two micrometer waveband
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Design of an all-optical plasmonic modulator for two micrometer waveband
Choudhury, Dr. Sajid Muhaimin; Shamima Akter Mitu; 0421062320; 623.80414/SHA/2023
In order to meet up the future demand of bandwidth, optical communication wavelength have to be shifted to new wavelength of 2 µm in future. In this study, an all-optical PCM based reflection modulator for upcoming 2 µm communication band have been proposed. The reflectance of the structure can be manipulated by altering the phase of GST by employing optical or electrical stimuli. For designing all optical modulator, optical stimuli has been used to change the phase of GST. Numerical investigation reveals that the nanogear array based structure exhibits a conspicuous changeover from ∼ 99% absorption to very poor interaction with the operating light depending on the switching states of the GST, ending up with 85% modulation depth and only 0.58 dB insertion loss. Due to noticeable differences in optical responses of GST material, a high extinction ratio of 28 dB, a commendable FOM of 49, and 25 MHz switching speed can be achieved which is so far the best modulation performance in this wavelength window. In addition, real-time tracking of the reflectance during phase transition manifests highspeed switching expending low energy per cycle, on the order of sub-nJ. Moreover, we have also proposed another Vanadium di-oxide based modulator which can solve the endurability issue of GST based modulator. This design exhibits a promising result like a large modulation depth of 80%, a high extinction ratio of 15 dB, low switching power of 0.12 pJ/nm2 and 15.8 MHz switching frequency. Hence, given their overall performance, the two devices will be a paradigm shift for the optical modulators for upcoming 2 µm communication technology.
2023-05-03T00:00:00ZDesign of high performance led driver based on SEPIC topology
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Design of high performance led driver based on SEPIC topology
Harun-ur Rashid, Dr. A.B.M.; Fouzia Ferdous; 1014064002 F; 623.13/FOU/2023
LEDs are current driven devices. So it is essential to maintain the stability of LED current. Variation of temperature may cause of instabilities and bifurcations in the LED driver. Driving LEDs from an offline power source faces design challenges like it has to maintain low harmonics in input current, to achieve high power factor, high efficiency and to maintain constant LED current and to ensure long lifetime. This work proposes the technique of harmonics reduction by using parametric optimization of Single Ended Primary Inductor Converter (SEPIC) based LED driver. Without optimization of SEPIC parameters input energy will not be properly transferred to the load and this un-transferred energy will be transmitted to the source. Consequently, the quality of input current will be hampered i.e. harmonics will contaminate the input current. Focussing this, we proposed the design of a non-isolated integrated-stage single-switch constant current LED driver operating in Discontinuous Conduction Mode (DCM) in SEPIC incorporating the design of control circuit with soft start mechanism. This LED driver has achieved a good efficiency (90.6%) and high-power factor (0.98) with reduced harmonics (3.35%). System stability has been determined and simulation studies are performed to confirm the validity of the LED driver circuit. A laboratory prototype is built to verify the functionality and performance of the proposed LED driver.
There are also some design challenges for a multicolor LED driver like precise control of color consistency i.e. to maintain Corelated Color Temperature (CCT) and luminous intensity. A bicolor blended shade white LED system is formed by using warm color LED source of CCT 1000K and cool color LED source of CCT 6500K. The overall CCT is the blended CCT of the two LED sources. This work also approaches the method of nonlinear optimization of LED currents of both sources for reducing deviation of blended CCT from the target in order to achieve desired CCT from the two LED sources of the multicolor LED driver. These optimized LED currents in the two strings are maintained by the control circuit of the SEPIC. The obtained deviation of CCT is 43K, precision is above 99.15%, which is not perceived by human eyes. Another design challenge is to drive LEDs to maintain low harmonics in the input current. This work again proposes the harmonic reduction technique by using parametric optimization of SEPIC based bicolor LED driver. The Non-linear Constrained Parametric Optimization Methodology is deployed to refine the SEPIC parameters to have the least value of Total Harmonic Distortion (THD). The power loss analysis has been performed to minimize losses in order to enhance efficiency. Measured THD (4.37%), P.F. (0.96) and efficiency (92.8%) are satisfactory. Over voltage protection mechanism is incorporated in the experimental driving system for safety count. The system stability has been ensured.
2023-05-27T00:00:00ZModeling and analysis of lead-free perovskite photoconductors for direct conversion x-ray imaging detectors
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Modeling and analysis of lead-free perovskite photoconductors for direct conversion x-ray imaging detectors
Mahmood, Dr. Shaikh Asif; Naznin, Sultana; 1017062247; 623.67/NAZ/2023
In a direct conversion digital X-ray image detector, incident X-rays are converted tochargeandreadoutusingathinfilmtransistor(TFT)arrayforstorageanddisplay.Thedigital X-ray imaging system offers better image quality, reduced X-ray exposure, andthe possibility for real-time imaging compared to the traditional analog system.Thechoiceofthephotoconductoranddesignforoptimumoperationiscrucialforthesupe-rior performance of the detector. In this thesis, a numerical model of X-ray sensitivityassumingaperturbedelectricfieldduetothebulkchargetrappingandionaccumulationnearthephotoconductor/metalinterfacehasbeendeveloped.Thecontinuityequations,trappingrateequations,andPoisson’sequationaresimultaneouslysolvedbynumericalmethod.The continuity equations are solved to calculate charge concentration con-sidering the perturbed electric field.The free carrier concentration and electric fielddistribution are used to determine the photocurrent.The integration of the photocur-rent is the collected charge. The X-ray sensitivity of a photoconductor is the collectedchargeperunitareaperunitexposureofradiation.Thedevelopednumericalmodelhasbeen applied to Cs2AgBiBr6 (CABB) based X-ray detectors. CABB possesses impres-siveoptoelectronicpropertiessuitableforX-raydetectionwithahighlystablestructureand free from toxic Pb. The modeling works in this thesis identify the important fac-torsthatlimitthedetectorperformancesuchasthemobility-lifetimeproductofchargecarriers, ion accumulation, and charge carrier injection from electrodes.The optimaldevicethicknessforanAu/CABB/AuX-raydetectorisestimatedtobe∼350and550µm for 30 keV X-ray energy. It has been found that the optimum dose for maximumsensitivityis∼4.5-7µGyfor30keVX-rayenergy.Inchestradiology,itisdiscoveredthat CABB has higher sensitivity than a-Se by 32% for an applied electric field of 5V/µm. In mammography, CABB has higher sensitivity than a-Se by 5.58% for an ap-pliedelectricfieldof5V/µm.ThisinvestigationcanultimatelyleadtothereductionofpatientexposureindifferentdiagnosticmedicalX-rayimaging.
2023-07-24T00:00:00ZEstimation and enhancement of light extraction efficiency of iii-nitride nanowire based light emitting diodes
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Estimation and enhancement of light extraction efficiency of iii-nitride nanowire based light emitting diodes
Baten, Dr. Md. Zunaid; Tashfiq, Ahmed; 0421062314; 623.81045/TAS/2023
Nanowirearray-basedIII nitridemicroLEDsoperatingfromUVtovisiblerangehaveattracted tremendous attention for many photonics and optoelectronics applications.However, traditionally, these LEDs’ light extraction efficiency (LEE) is numericallyevaluated without considering specific optical polarization characteristics. In this the-sis, we evaluated the LEE of III-nitride UV and visible single nanowire LEDs, a periodicarrayofnanowireemitters,andmicroLEDsconsideringDegreeofPolarization(DOP)using FDTD-based optical simulations. We proposed a weighted average method forcalculating LEE considering DOP from LEE values obtained for TE and TM polariza-tions. The influence of emission polarization on the side and top LEE of single InGaNand AlGaN nanowire LEDs were studied and analyzed using waveguiding nature ofthese nanowire structures. The impact of structural parameters was also studied exten-sively.Considering DOP and material absorption, the maximum calculated top, sideandtotalextractionefficienciesforasingleAlGaNnanowireLEDoperatingat250nmare 11.4%, 75.8% and 87.3%, respectively.Similarly, for a single InGaN nanowire-based LED operating at 400 nm, the maximum top extraction efficiency of 30% isachieved for a nanowire diameter of 125 nm. Here, we have also analyzed the impactof DOP variation on the overall efficiency characteristics. Moreover, a comprehensivestudyhasbeenconductedonthedependenceofLEEonvariousstructuralparame-ters such as nanowire diameter and array periodicity considering DOP. We also pro-posed an optimized geometry for the InGaN and AlGaN periodic nanowire array LEDstructures for maximizing top extraction efficiencies. Considering DOP, we achievedthe maximum top extraction of 67% for InGaN nanowire array-based micro LEDs fornanowire diameter and array periodicity of 50 nm and 350 nm, respectively. Addition-ally, in this work, we have analyzed two methods for LEE enhancement: insertion ofinclined side reflectors and bottom metal reflectors. For InGaN single nanowire LED,wedemonstratedLEEenhancementof28%,withtheinsertionofa50nmthickbottomAl reflector layer. We also studied the influence of inclined side wall reflectors on theenhancement of top extraction efficiencies for nanowire array-based micro LEDs andplanar micro LEDs.Using our optimized geometry and taking DOP into considera-tion, the enhancement of top LEE for InGaN nanowire array-based LED devices withinclined side reflectors having an inclination angle of 30 degrees was calculated to be90.
2023-07-23T00:00:00Z