Shahram Mohammadnejad

Iran University of Science and Technology, Teheran, Tehrān, Iran

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Publications (23)16.36 Total impact

  • Nima Jouyandeh, ali bahrami, shahram mohammadnejad
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    ABSTRACT: In this paper, the sub-layers of AlGaAs/GaAs double junction (DJ) solar cell have been redesigned in order to achieve an optimum cell structure. It has been deduced with cooperation of detailed balance limit theory and structural behaviour of AlGaAs, that the Al0.45Ga0.55As is the best choice for top cell’s material in AlGaAs/GaAs DJ solar cell. Also, there is a trade-off between peak tunnelling current and transparency in tunnel junction which makes Al0.07Ga0.93As as the optimum tunnel junction of AlGaAs/GaAs cell. Finally, a smoothed reflectance senary-layer structure based on modified-DBR has been proposed to be used as anti-reflection coating of proposed structure. Also, the thickness and doping concentration level of different layers have been optimized.
    Journal of Modern Optics 03/2014; · 1.16 Impact Factor
  • Nima Jouyandeh, ali bahrami, shahram mohammadnejad
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    ABSTRACT: In this paper, the multilayer antireflection coatings have been designed with modifying the thickness of two and three paired layer DBR structure. Our proposed DBR-based tructures show antireflection behavior in spite of the reflection treatment in traditional DBR structures. Firstly, the proposed structures are designed to be equivalent with the theoretical ideal triple layer antireflection coating. Therefore, the problem of finding a suitable material for the middle layer of triple structure has been solved. Simulation results show the significant equivalency for the reflectance of proposed structures with the ideal triple layer ARC in the same wavelength and incident angles. Also, the design of structures has been changed in order to present the constant reflectance coefficient over a wide range of the wavelengths. This structure enhances the Omni-directionality of the multilayer antireflection coatings.
    Chinese Physics B 01/2014; 23:28803-28803. · 1.15 Impact Factor
  • Masoomeh Taherkhani, Shahram Mohammadnejad
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    ABSTRACT: Integrated photonic devices are expected to play a promising role in the field of quantum information science. In this paper we propose two schemes for generating polarization-mode entangled photon pairs based on titanium-indiffused waveguide on periodically polled lithium niobate by using simultaneous quasi-phase-matching of Type-I and higher order Type-0 spontaneous parametric down conversion processes in one of them and Type-II in another. The photon pairs are emitted at the wavelength of 812 nm suitable for quantum computation applications within a bandwidth of 14 and 0.2 nm, and the generation rate of the degenerate sources is 44,360 and 91 pairs/(s GHz mW) respectively, in a 1-cm long waveguide. These degenerate sources can provide maximally entangled photon pairs as the Tangle of the sources is as high as 0.9999 and 1, accordingly.
    Optical and Quantum Electronics 11/2013; · 1.08 Impact Factor
  • S. Mohammadnejad, Nima Jouyandeh Abkenar, A. Bahrami
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    ABSTRACT: In this paper, we have used detailed balance limit model in order to achieve optimum single junction solar cell for different outdoor conditions. Also, maximum achievable efficiency for GaAs-based double junction solar cells has been calculated and effects of the concentration ratio on optimum point have been investigated. Although 1.9/1.0 eV is achieved as the optimum combination of band gaps for double junction solar cells, but simulation results show some changes in value of top cell’s optimum band gap when the GaAs is used as bottom cell. Finally, effect of changing temperature on behavior of GaAs-based double junction solar cell has been studied under 1sun black body, AM0 and AM1.5 spectrums.
    Indian Journal of Physics 10/2013; · 1.79 Impact Factor
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    Shahram Mohammadnejad, Iman Beiranvand, Ali Bahrami
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    ABSTRACT: Efficiency on the order of 25% is now achievable on silicon crystalline solar cells. Quantum dot solar cells increase the cell's efficiency greatly. Bose–Einstein equations in efficiency of solar cells are appeared. These equations can't be solved in the form of conventional but should be solved by numerical methods. In this study, a combination of descriptive and empirical approaches to both library and archival data collection, research and field tests have been. First, the document library and Internet references keywords used throughout this study were repeated search Journals and articles related to this research has been some of the key factors and principles derived from these sources and to analyze them this research have been conduced using Matlab. The results show that roots of the denominator of the integrand should not be included in calculating the Bose–Einstein integral because in this case the function will diverge. In quantum computing numbers are within in FEMTO and the acceptable error would be much smaller.
    5 th Iranian Conference on Applied Mathematics; 09/2013
  • Shahram Mohammadnejad, Nima Jouyandeh Abkenar, Ali Bahrami
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    ABSTRACT: In this paper, the effect of doping concentration and layer thickness on the performance of tunnel junctions is studied. We investigate the behavior of single, double and triple layer structures of tunnel junctions. Triple layer structure shows better performance in comparison with the other structures and can reach the higher tunneling current besides lower voltage drop. Also, the behavior of the triple layer tunnel junction with different doping concentration profiles is studied. We propose a new normal distribution profile for doping concentration in multilayer tunnel junctions which shows better performance in comparison with the linear and graded doping concentration profiles. The higher α parameters in normal distribution enhance the device performance with increasing the smoothness of doping variations in the center and edge of the tunnel junction. Finally, we examine different thicknesses of triple layer tunnel junction in order to achieve the optimum structure.
    Optical and Quantum Electronics 04/2013; · 1.08 Impact Factor
  • Ali Bahrami*‡, Shahram Mohammadnejad, Nima Jouyandeh Abkenar
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    ABSTRACT: In this paper, the effects of antireflection coatings on the performance of GaAs solar cells are theoretically investigated. Also, the conversion efficiency, short circuit current and open circuit voltage of the solar cell are calculated in various thicknesses and refractive indices of coating materials. Single and double layer coatings are utilized in order to achieve the highest performance. Simulation results show an optimum point for the thickness and refractive indices of the materials used as single and double layer antireflection coatings. Finally, 16.97% conversion efficiency, 27.91mA short circuit current and about 0.944V open circuit voltage are achieved for GaAs single cell with a low 5nm TiO2 thickness in Al2O3/TiO2 double layer coating. But the lower thickness of TiO2 increases the incident angle dependency of the reflectance.
    INTERNATIONAL JOURNAL of RENEWABLE ENERGY RESEARCH. 01/2013; 3(1):80-83.
  • Nima Jouyandeh, Ali bahrami, shahram mohammadnejad
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    ABSTRACT: Theoretically optimized triple layer antireflection coating can be replaced with four layer LHLH structure around the designed wavelength. In this paper, triple layer and its four layer equivalent structure have been utilized as antireflection coating of GaAs solar cell and the reflectance coincidence of these structures is proved. The wavelength of 650nm has been assumed for design of multilayer structure. Also, the six layer antireflection coating is made with inserting one more HL pair in the structure. We design multilayer structures by introducing X parameter whic describes 4- and 6-member set of thicknesses and finding the optimum combination of thicknesses. Simulation results confirm that the optimum case of six layer structure presents better performance in comparison with the best case of four layer coating.
    Journal of Optoelectronics and Advanced Materials 01/2013; 15:960-965. · 0.52 Impact Factor
  • S. Mohammadnejad, A.B. Madineh, M.N. Sarvi
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    ABSTRACT: There are many devices used today on modern spacecraft to accurately determine their position. Star trackers (space cameras) are the most accurate device in use for determining a spacecraft's position. The star tracker operates automatically, getting images of star patterns within its Field of View (FOV). The stars observed by the camera can then be identified and the orientation of the spacecraft can be calculated. A star tracker is composed of a baffle for removing stray lights, optical system, image sensor, microprocessor and memory. In this paper, basic principles for designing a baffle system, such as determination of baffle dimensions, arrangement of vanes inside the baffle, and coating of surface have been reviewed. We first determined dimensions of a two-stage conical baffle. Then, we calculated positions and height of vane to prohibit diffusely reflected light inside the baffle from entering into the photometer. In order to evaluate performance of the designed baffle system, we have used a ray tracing program using a Monte Carlo method and calculated attenuation factor.
    Communication Systems, Networks & Digital Signal Processing (CSNDSP), 2012 8th International Symposium on; 01/2012
  • M. Taherkhani, S. Mohammadnejad
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    ABSTRACT: We propose two basic transformations which can be used for manipulating modal qubits in Ti:LiNbO3 photonic quantum circuits. These include the mode analyzer, a device that separates the even and odd components of a state into two separate spatial paths and a modal phase-flip gate which introduces a phase shift of π to the odd component of a two-mode photon state.
    Communication Systems, Networks & Digital Signal Processing (CSNDSP), 2012 8th International Symposium on; 01/2012
  • S. Mohammadnejad, A. Rostami, M.N. Sarvi
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    ABSTRACT: Nowadays, star trackers are widely used in satellites and spacecraft for three-axis attitude estimation. Star trackers are the most accurate instrument among the others. A Star tracker is made up of hardware and software (firmware). The software (firmware) is a set of algorithm that runs in a microprocessor. These algorithms include centroiding, star identification and attitude determination. Star identification algorithm needs a database to identify stars in the image. In this paper, we describe a novel method of constructing database by considering practical issues. We discuss about main catalog, base catalog, database and ordering it to improve the speed of star identification algorithm. This database was used in our star identification algorithm.
    Communication Systems, Networks & Digital Signal Processing (CSNDSP), 2012 8th International Symposium on; 01/2012
  • Ali Bahrami, Shahram Mohammadnejad, Ali Rostami
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    ABSTRACT: This article presents an all-optical multi-mode interference switch in which the non-linear directional coupler is utilized to realize a passive phase shifter. The proposed structure can be used either as a 1 × 2 or a 2 × 2 switch, with two inputs applied simultaneously in the latter case. The operation of the device is mainly based on the phase difference of the inputs of the multi-mode interference section. The beam propagation method is used for the design and simulation of the structure. The simulation results approve the low sensitivity on wavelength and fabrication tolerances. The crosstalk of the structure is equal to −31.6 dB.
    Fiber and Integrated Optics 05/2011; 30(3):139-150. · 0.28 Impact Factor
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    Shahram Mohammadnejad, Sobhan Roshani, Mehdi Nasiri Sarvi
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    ABSTRACT: The fixed pattern noise (FPN) is an important parameter in imaging devices such as CCD or CMOS sensors that becomes more important when we consider about precision. There is no simple technique to remove FPN without any inconvenience. An algorithm is proposed which can reduce both, gain and offset FPN noise. At first the gain FPN of all pixels will be unified. Then, the same procedure applies to the offset FPN. For a particular application in a star tracker, one can exploit stars image and its dark background or satellite motion to calibrate the camera during mission. The efficiency of proposed algorithm has been proved by simulation results. This technique also can be used in any camera or infrared devices that use focal arrays system to observe the scenes.
    01/2011;
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    Shamsolah Salemian, Shahram Mohammadnejad
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    ABSTRACT: In optical quantum communication, quantum state measurement is necessary. This paper proposes a new technique for realization of polarization filter based on planar lightwave circuit (PLC). This filter is used for quantum state discriminator in quantum communication and also as a Bell-state analyzer in quantum repeater. Electro-optics interferometer has been used in design and implementation of polarization filter. We use lithium niobate as a wafer material and Ti:LiNbO3 for waveguide. Two directional couplers have been used in this device. The length and spacing of these directional couplers have been designed so that each polarization is routed in specific output. The proposed device has one input and two outputs. If polarization of the input photon is vertical, then this photon will appear in output 1, otherwise if the input photon has horizontal polarization, it appears in output 2. For vertical polarization input, the power overlaps integral (POI) shows that isolation between two outputs is 14.96 dB. As to horizontal polarization input, the isolation between two outputs is 13.8 dB. The designed polarization filter has length of 33 mm and width of 60 μm. This device is very suitable for use in integrated optics.
    Optik - International Journal for Light and Electron Optics 01/2011; · 0.77 Impact Factor
  • S. Salemian, S. Mohammadnejad
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    ABSTRACT: The implementation of quantum communication protocols in long optical fibers is limited by several decoherence mechanisms. In this contribution we review this mechanisms and analysis their effect on the quantum states. Because of asymmetry in the real fibers, the two orthogonal polarization modes are propagated at different phase and group velocities. The difference in group velocities results in Polarization Mode Dispersion (PMD). It is created by random fluctuations of the residual birefringence in optical fibers, such that the State of Polarization (SOP) of an optical signal will turn randomly over time, in an unpredictable way. In optical quantum communication, quantum state measurement is necessary. The bottleneck for communication between far apart nodes is the increasing of the error probability with the length of the channel connecting the nodes. For an optical fiber, the value of absorption and depolarization of a photon (i.e., the qubit) increases exponentially with the length of the fiber.
    Telecommunications (ConTEL), Proceedings of the 2011 11th International Conference on; 01/2011
  • parastoo.khademi, shahram mohammadnejad
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    ABSTRACT: In this paper we apply an external electric field to two different molecules based on benzene dithiol molecule and analysis the effects of external field in the parameters of these molecules, like energy, dipole moment and the HOMO surfaces. The method used to calculate the parameters is Density function Theory (DFT) with Gaussian 03 package. We show that the benzene dithiol molecule with two donor acceptor groups has strong gate effects than the single benzene dithiol molecule and is a better choice for use as molecular transistors.
    010 3rd International Conference on Computer and Electrical Engineering (ICCEE 2010); 10/2010
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    shahram mohammadnejad, nasrin ehteshami
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    ABSTRACT: n this paper we present a novel dispersion compensating photonic crystal fiber with defected core. The small central defect of air hole can flexibly control the chromatic dispersion properties of this kind of photonic crystal fiber. The 2-D finite difference frequency domain method (FDFD) with perfectly matched layers (PML) is used to investigate dispersion properties. By varying the size of defected core, it is possible to obtain high negative dispersion coefficient. The proposed photonic crystal fiber is suitable for broadband dispersion compensation in wavelength division multiplexing (DWDM) optical communication systems.
    Mechanical and Electronics Engineering (ICMEE), 2010 2nd International Conference; 08/2010
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    Shamsolah Salemian, Shahram Mohammadnejad
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    ABSTRACT: Quantum entanglement distribution is an essential part of quantum communication and computation protocols. Here, linear optic elements are employed for the distribution of quantum entanglement over a long distance. Polarization beam splitters and wave plates are used to realize an error-free protocol for broadcasting quantum entanglement in optical quantum communication. This protocol can determine the maximum distance of quantum communication without decoherence. Error detection and error correction are performed in the proposed scheme. In other words, if there is a bit flip along the quantum channel, the end stations (Alice and Bob) can detect this state change and obtain the correct state (entangled photon) at another port. Existing general error detection protocols are based on the quantum controlled-NOT (CNOT) or similar quantum logic operations, which are very difficult to implement experimentally. Here we present a feasible scheme for the implementation of entanglement distribution based on a linear optics element that does not need a quantum CNOT gate. Keywordsquantum entanglement–quantum communication–quantum error detection–decoherence–error correction
    Chinese Science Bulletin 01/2010; 56(7):618-625. · 1.37 Impact Factor
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    S. Salemian, S. Mohammadnejad
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    ABSTRACT: An entangled photon source is an essential tool for quantum information experiments, such as tests of Bell inequalities, quantum teleportation and entanglement swapping. We report the generation of polarization-entangled photon pairs using grating structures of nonlinear materials such as Ti-diffused lithium niobate. This approach is useful for quantum integrated optics because the size of this element is less than 325 micron. Polarization-entangled photon pairs are created using the nonlinear optical process of type-II spontaneous parametric down-conversion. The pump laser at 800 nm was doubled to a wavelength of 400 nm, which was used to produce SPDC in the grating structure of Lithium Niobate. The photon pairs were generated at wavelengths 692 nm and 950 nm.
    Applied Electronics, 2009. AE 2009; 10/2009
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    Kiazand Fasihi, Shahram Mohammadnejad
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    ABSTRACT: We have proposed a three-port high efficient channel-drop filter (CDF) with a coupled cavity-based wavelength-selective reflector, which can be used in wavelength division multiplexing (WDM) optical communication systems. The coupling mode theory (CMT) is employed to drive the necessary conditions for achieving 100% drop efficiency. The finite-difference time-domain (FDTD) simulation results of proposed CDF which is implemented in two dimensional photonic crystals (2D-PC), show that the analysis is valid. In the designed CDF, the drop efficiency larger than 0.95% and the spectral line-width 0.78 nm at the center wavelength 1550 nm have been achieved.
    Optics Express 06/2009; 17(11):8983-97. · 3.55 Impact Factor