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Khodr M.,Hariri Canadian University
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

In order for laser oscillation to occur, the modal gain at the lasing photon energy must equal the total losses. In this work, we analyze and calculate the total losses due to the free carrier absorption, optical waveguide scattering and the laser cavity end losses for PbSe/Pb 0.934Sr 0.066 Se quantum well laser structures. The small confinement factor value causes the free carrier absorption loss to be negligible. The calculated scattering loss values showed a decreasing order for the MQW, MMQW and SCH-SQW structures, for a surface roughness amplitude of 10nm. Increasing the surface roughness amplitude increases these scattering losses even further. However, the calculated cavity loss calculations showed that its values are in an increasing order for the MQW (or MMQW) and SCH-SQW structures. These cavity losses are lowest for uncoated cavity ends. Coating these ends with a quarter wavelength BaF2 layer increases the total cavity loss. In addition, coating the cavity ends with alternating quarter wavelength layers of BaF 2 and CaF 2 also results in an increase in the cavity loss. The increase in cavity loss due to coating is caused by the decrease in the mirrors' reflectivity values. These results show that coating with fluoride layers can best be utilized in applications where high transitivity values are needed. © 2010 SPIE. Source


Khodr M.,Hariri Canadian University
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

Temperature dependence is a key parameter in designing quantum well lasers. In this work, we calculated the effects of temperature on the energy levels and emitted wavelength for PbSe/PbSrSe Single Quantum Well Laser at four different temperatures: 77K, 150K, 200K, 250K, and 300K. This material system is currently being used in Tunable Laser Spectroscopy which plays a key role in detecting biomarker molecules in exhaled breath at wavelengths in the infrared region. We determined the system design parameters to obtain the desired emitted wavelengths associated with certain disease biomarkers as a function of temperature. Our calculated emitted wave lengths are in excellent agreement with experimental data assuming parabolic and nonparabolic energy band structures. Moreover, we calculated the effects of temperature on the confinement factor, gain and current density. The modal gain versus current density curved showed that the nominal current density and the saturation level increases with temperature similar to other material systems. © 2012 SPIE. Source


Khodr M.,Hariri Canadian University
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2011

Threshold current is a key parameter in the design and proper operation of quantum well lasers. In this publication, threshold current analysis and calculations are done on four PbSe/Pb0.934Sr0.066Se quantum well laser structures: SQW, SCH-SQW, MQW, and MMQW. The current work is a continuation to previous publications where energy levels, modal gain, optical confinement, and total losses were published for these four structures assuming the energy bands are nonparabolic. The threshold current as a function of total losses, cavity length, and cavity end mirror reflectivity was obtained for these structures. It is shown that the threshold current decreases with a decrease in the cavity length and then increases at a critical cavity length. The effects of non-parabolicity on the threshold current values are more obvious for short cavities and decreases with an increase in cavity. Whether the SQW or the MQW is the better structure depends on the loss level. At low loss, the SQW laser is always better because of its lower current density where only one QW has to be inverted. At high loss, the MQW is always better because the phenomena of gain saturation can be avoided by increasing the number of QW's although the injected current to achieve this maximum gain also increases. Owing to this gain saturation effect, there exists an optimum number of QW's for minimizing the threshold current for a given total loss. At this typical value, the effects of non-parabolicity on the threshold current values can be neglected without loss of accuracy. However, there is a 20% shift in the output lasing energy that cannot be neglected. © 2011 SPIE. Source


Kasti N.A.,Hariri Canadian University
Advanced Materials Research | Year: 2012

When using structural mechanics to study the deformation of carbon nanotubes (CNTs), one has to pick the structural mechanics properties that are equivalent to the molecular mechanics properties. In a previous publication [1], we have determined the relation between the bending stiffness EI/a used in structural mechanics and the bond bending stiffness C used in molecular mechanics for zigzag carbon nanotubes under simple tension. This paper extends the concept and determines the corresponding relation for simple torsion. We show that the relation obtained is different than that of simple tension; in simple torsion, EI/a is load and chirality dependent. However, for the particular case of a graphene sheet, simple tension and torsion lead to the same value of EI/a, namely C/2. We also include the structural mechanics deformation of the tube that accounts for axial, bending and torsional structural stiffnesses. Unlike simple tension, the deformation in the case of simple torsion has the axial stiffness coupled to the bending and torsional stiffnesses. © (2012) Trans Tech Publications. Source


Ghanem H.,Hariri Canadian University | Zollinger D.,Texas A&M University | Lytton R.,Texas A&M University | Ghanem N.,University of Balamand
Construction and Building Materials | Year: 2012

The main objective of this study is to provide a methodology of testing concrete materials as a combination to assist engineers to effectively mitigate alkali-silica reaction (ASR) in concrete. This involves capturing the combined effects of concrete materials (water cement ratio, porosity, supplementary cementitious materials, etc.) through a method of testing to allow the formulation of mixture combinations resistant to ASR leading to an increase in the life span of concrete structures. Thus, a kinetic model is proposed to determine concrete/aggregate ASR characteristics which are calculated using a mathematical numerical analysis. To relate the effect of material combinations to field performance, the aggregate and concrete characteristics are combined using two mathematical models. The first model predicts the activation energy (E a) of the aggregate at levels of alkalinity similar to field conditions. The second model, generated using the Juarez-Badillo transform, connects the ultimate expansion of the concrete and aggregate, the water cement ratio, and the fly ash content to the E a of the rock. It is expected that the knowledge gained through this work will assist government agencies, contractors, and material engineers, to select the optimum mixture combinations that fits best their needs or type of applications, and predict their effects on the concrete performance in the field. © 2012 Elsevier Ltd. All rights reserved. Source

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