Department of Physics
http://41.89.164.27:8080/xmlui/handle/123456789/211
Thu, 10 Oct 2024 21:10:17 GMT2024-10-10T21:10:17ZCoherent High-Speed Signal Transmission in Passive Optical Networks
http://41.89.164.27:8080/xmlui/handle/123456789/2100
Coherent High-Speed Signal Transmission in Passive Optical Networks
Cherutoi, Henry; Waswa, David; Choge, Dismas
A comparative study of digital M-ary Quadrature Amplitude Modulation (16-QAM) and QPSK modulationformats for high-speed transmission is presented. Based on VPI optical simulation software. This paper builds a high-bit rate dual polarization (DP) QPSK and16-QAM modulation formats coherent optical transmission system for Passive Optical Networks (PON). Higher-order modulation formats could be used to provide huge data capacity, extended coverage, and long-reach connections. Channel impairments are mitigated by using digital signal processing (DSP) for dispersion compensation, carrier frequency recovery, and carrier phase estimation. The performance of our system is analysed on Bit Error Rate (BER=1e-9), the threshold for a communication system. For a 32-Splitter PON configuration operating with 16-QAM modulation format, a data rate of 80 Gb/s per ONU is achieved after 24km using standard single mode fibre (SSMF). The results indicate that the proposed schemes are a promising solution for coherent high-speed transmission in PONs.
Wed, 01 Nov 2023 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/21002023-11-01T00:00:00ZMechanical and Elastic Properties of Selected 211 MAX Phases: A Density Functional Theory Study
http://41.89.164.27:8080/xmlui/handle/123456789/2086
Mechanical and Elastic Properties of Selected 211 MAX Phases: A Density Functional Theory Study
ARUSEI, Geoffrey; et al.
MAX Phases are a class of ternary materials that have continued to play a greater role in the field
of materials science due to their unique properties that bridge the gap between metals and ceramics
which have uses in a wide range of applications. However, in spite of these progress, there are still
a lot of open questions and properties that needs to be understood. The Mechanical and elastic
properties of (Nine) MAX phase materials, namely, Ti2AlC, Ti2AlN, Ti2GaC, Ti2GaN, Ti2PbC,
Ti2CdC, Ti2SiC, Ti2GeC and Ti2SnC have been investigated using the density functional theory
within the generalized gradient approximations as expressed in Quantum Espresso and VASP
codes. The values of elastic anisotropy, Young’s modulus, Poisson ratio and shear modulus
revealed that the compounds were indeed stable and ductile. Out of all the nine MAX Phase
materials, Ti2PbC and Ti2CdC are more stable than the other considered compounds. The findings
of this study suggest that the nine MAX phases are potential candidates in various industrial
applications requiring hard materials.
Sun, 01 Oct 2023 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/20862023-10-01T00:00:00ZIN SITU TECHNO-ENVIROECONOMIC ANALYSIS OF A ROOFTOP PHOTOVOLTAIC (PV) BACKUP SYSTEM IN THE TROPICAL CLIMATE
http://41.89.164.27:8080/xmlui/handle/123456789/1743
IN SITU TECHNO-ENVIROECONOMIC ANALYSIS OF A ROOFTOP PHOTOVOLTAIC (PV) BACKUP SYSTEM IN THE TROPICAL CLIMATE
CHERUIYOT, WILKINS KOSGEI
Solar PV generators are increasingly being deployed in the built environment as stand-alone or backup power systems to supply electricity either solely or during power outages respectively. Diesel generators (DGs) are applied routinely as standby power systems by many enterprises and institutions, especially in developing countries where power outages are real and frequent due to unstable national grids. The current pursuit of low-carbon and sustainable source of energy places photovoltaic (PV) power system in advantageous position as a substitute for a DG backup system. An existing off-grid 780 Wp PV system installed as a backup power supply in a learning institution in western part of Kenya was studied both experimentally and theoretically. Technical, economic and environmental analyses were carried out to determine its performance under the local outdoor conditions at the site for a period of one year in 2020. Irradiance estimation models were also validated by experimental data to determine appropriate model(s) for the site. Plane-of-array (POA) solar radiation was measured with solar cell sensor installed at the surface of PV modules and a charge controller/inverter unit with the capability to measure and log real time output current (I) and voltage (V) was used to generate I-V characteristic data. PVsyst software was used to simulate the PV system and generate optimized theoretical results for the site which were compared with experimental results. Available energy was determined as 3202.80 kWh/year, average array efficiency of 11.71%, FF of 0.66, array yield of 4.89 kWh/kW, reference yield of 5.51 kWh/kW, capacity factor as 19.8%, annual average performance ratio (PR) as 76.0%, and average array losses as 0.54 kWh/kW. Economic results for the PV system show that the payback time (PBT) is∼ 6.38 years, LCC of $3057.93, levelized cost of energy (LCOE) of0.045 $/kWh and operation and maintenance (O&M) is17.32 $/year. For the diesel generator (DG),PBT was 4.25 years and O&M was 262.80 $/year for a lifespan of 5 years and assuming that it operates 2 hours per day of blackouts, LCC of $7792.75 and LCOE of 0.324 $/kWh. Environmental results show that the total annual amount of CO2 emissions avoided when PV is used instead of DG power backup system was 5.84 tCO2/year giving an average cost parameter (penalty for CO2 generation) of $9.62. Simulation results gave the available energy as 3746.40 kWh/year, reference yield of 5.55 kWh/kW, array yield of 4.18 kWh/kW, array losses of 0.61 kWh/kW, capacity factor as 21.23%, FF as 0.68, PR as 73.6% and PV array efficiency of 13.19%. The average amount of CO2 emission avoided was 7.95 tCO2/year with annual environmental cost of $116.18. Angstrom-Prescott and Iqbal models were found to be the most accurate for the site having the lowest values of mean absolute percentage error (MAPE) of 8.5% and 8.9% and root mean square error (RMSE) of 0.252 and 0.302 respectively. In conclusion, technically, the low FF (<<1) indicate that the system is not operating at its optimum, which can be attributed to how the PV system was installed. The LCOE results show that PV power is cheaper by a factor of seven than that of the diesel generator, and the amount of CO2 avoided is at least 0.44 tCO2/month. The PV power presents net benefits over diesel power in all performance indices evaluated, and hence can be used as a reliable and affordable replacement for DG backup systems in tandem to the global quest to transition to clean and sustainable energy sources and attainment of the SDGs 7 and 13.
Fri, 01 Jul 2022 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/17432022-07-01T00:00:00ZAb initio insights into Graphene-Zirconium disulfide/diselenide heterostructure as electrode material for alkali-ion batteries
http://41.89.164.27:8080/xmlui/handle/123456789/1605
Ab initio insights into Graphene-Zirconium disulfide/diselenide heterostructure as electrode material for alkali-ion batteries
King’ori, Gladys W.; et. al...
In the search for an energy storage medium with higher electronic conductivity, rate performance and moderate volume expansion, van der Waals heterostructures
are a promising alternative. Herein, the potential of graphene (Gr) and Zirconium dichalcogenide (ZrX2, X = S, Se) van der Waals heterostructures for applications as
battery electrodes is explored using density functional theory (DFT) calculations. Through intercalating alkali ions (Li and K) between the Gr and ZrX2, we obtain low
energy activation barriers, indicating that Gr-ZrX2 van der Waals heterostructures can be candidates for high rate performance electrode application.
DFT calculations also indicate that the Gr-ZrX2 heterostructure formation is energetically favoured with better volume expansion as compared to bilayer Gr and
ZrX2. The calculated open circuit voltage (OCV) for K intercalation was 0.66 V (Gr-ZrSe2) and 0.77 V (Gr-ZrS2), hence suitable for anodic application in Potassium-ion
batteries (KIB). The calculated OCV for Li interaction was 2.83 V (Gr-ZrSe2) and 2.95 V (Gr-ZrS2) hence suitable for cathodic applications
Fri, 01 Jan 2021 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/16052021-01-01T00:00:00ZINTEGRATION OF VCSEL WITH MODULATED RAMAN PUMP FOR DATA AND CLOCK TRANSMISSION IN OPTICAL COMMUNICATION
http://41.89.164.27:8080/xmlui/handle/123456789/1590
INTEGRATION OF VCSEL WITH MODULATED RAMAN PUMP FOR DATA AND CLOCK TRANSMISSION IN OPTICAL COMMUNICATION
OSIEMO, DOUGLAS MOMANYI
The evolution of ultrafast optical technologies has led to the realization of low cost,
low latency and high speed optical networks. Existing telecommunication
infrastructure does not provide sufficient capacity to satisfy future demands in terms
of extend of coverage, bandwidth and number of served customers. Fibre-optic
technology for data transmission is subject to dynamic development fueled by the
increasing demand for large bandwidth and reach extension. High capacity VCSELs
are the most desired transmitters for data transmission but, they are limited to short
distances. Since high powered Raman lasers are compatible with VCSELs, integration
of the two unlocks the puzzle of enhanced data and RF clock signal distribution
system. First, a 1550 nm VCSEL was directly modulated with 8.5 Gbps data to
counter-propagate with modulated backward Raman pump carrying 8.5 Gbps data
over a 50.7 km True Wave Reach fibre to realize enhanced full-duplex bidirectional
data transmission system. We report error free data transmission with receiver
sensitivity of -14.42 dBm, -14.38 dBm and -13.00 dBm for signal analysis with unmodulated
backward Raman, modulated backward Raman pump analysis and signal
analysis with modulated backward Raman respectively at BER threshold of 10-9.
Secondly, unidirectional data transmission system was realized by employing
modulated forward Raman pump instead of modulated backward Raman. Modulated
Raman pump simultaneously amplifies the VCSEL channel and transmits data over
the transmission link, therefore maximizing the network efficiency. Minimum
receiver sensitivity of −14.38 dBm was attained for a transmission configuration with
un-modulated forward Raman pump. Thirdly, performance of DFB laser on 2 GHz, 4
GHz and 6 GHz RF clock signal distribution was investigated. At 10 kHz offset
frequency lowest phase noise of -119.93 dBc/Hz was recorded for 2 GHz signal.
Fourthly, we realized a technique of exploiting forward and backward Raman
pumping to both disseminate clock tones and amplify data signal simultaneously. 10
Gbps data from directly modulated VCSEL was coupled into a 50.7 km fibre to
counter propagate with 4 GHz modulated Raman pump and repeated the same for
forward pumping where the data co-propagated with 4 GHz clock. Amplified errorfree
data transmission with minimum receiver sensitivity of -14.8 dBm for modulated
forward Raman and distributed stable clock signal over 50.7 km True Wave Reach
fibre was achieved. A maximum phase noise of -100.6 dBc/Hz at 10 kHz offset
frequency was experimentally measured for 50.7 km fibre transmission with
backward Raman pumping. Finally, long term stability measurement on RF clock
signal was analyzed using Allan deviation for DFB laser. Allan deviation of 1.49e-12 /s
and 1.63 e-12 /s was obtained for B2B and 24.69 km fibre respectively at 100 s
averaging time for 2 GHz clock signal. The results show the suitability of VCSEL
light source and Raman amplification for application in integrated optical
transmission links. This work offers an integrated VCSEL-based transmission system
which satisfy the requirements for the next generation optical fibre networks
regarding long reach, enhancement, duplex data/clock dissemination with a single
fibre upstream and downstream transmissions.
Sat, 01 Jan 2022 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/15902022-01-01T00:00:00ZFIELD DEPENDENCE OF THE PROPERTIES OF HIGH-TEMPERATURE SUPERCONDUCTORS
http://41.89.164.27:8080/xmlui/handle/123456789/1584
FIELD DEPENDENCE OF THE PROPERTIES OF HIGH-TEMPERATURE SUPERCONDUCTORS
MASINDE, FRED WEKESA
High temperature superconductivity mechanism has remained an elusive concept for a long time. Field dependence of the properties of high temperature superconductors could be on the brink of solving this puzzle. This Study shows that there is a strong field dependence on the theory of high temperature superconductors (HTSCs). High temperature superconductors of transition temperature, 𝑇𝑐> 77 K (boiling point of liquid nitrogen) were studied. This research established that there are effects of applied magnetic field on a two dimensional square lattice, effective magnetic flux per plaquette and electric field on oscillating Cooper pairs parallel to copper oxide planes, and their effects on the thermodynamic properties of HTSCs. Second quantization formalism was used in determining the thermodynamic properties of HTSCs such as the energy gap and transition temperature. The effect of electric field E on an oscillating Cooper pair along the 𝐶𝑢𝑂2plane was found to give the value of Δ = 62.5 meV. The ratio (𝛼) of flux through the lattice cell to one flux quantum indicates that there is no Meissner expulsion of flux as the applied magnetic field H increases through the lattice cell. The value of 𝑇𝑐 at which superconductivity disappears was found to be 177K for YBCO/BSCCO systems. The lattice spacing a at 300Kwas 2.3Å. The value of 𝛼 was2.28 x 10−7 when 𝐻𝑐2 = 3.0 x 106G and a = 3Å.Studies on the effect of effective magnetic flux per plaquette Φ established that increase in values of Φ leads to increase in values of𝑇𝑐 and forΦ = 5.84𝐺/𝑚2, 𝑇𝑐 = 300K.
Fri, 01 Jan 2016 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/15842016-01-01T00:00:00ZNUCLEATION OF SUPERCONDUCTING VORTICES BY EXCITING ACOUSTIC STANDING WAVES IN A TYPE - II SUPERCONDUCTOR
http://41.89.164.27:8080/xmlui/handle/123456789/1583
NUCLEATION OF SUPERCONDUCTING VORTICES BY EXCITING ACOUSTIC STANDING WAVES IN A TYPE - II SUPERCONDUCTOR
OGANDO, FELIX OCHIEN’G
Superconductivity nucleation phenomenon in superconductors under an applied magnetic field close to the upper critical field HC3 has been studied by many authors. Physicists Saint-James and De Gennes were the first to study the surface nucleation phenomenon for a semi-infinite super-conductor occupying the half space and placed in an applied magnetic field which is parallel to the surface of the superconductor. When the applied magnetic field is spatially homogeneous and close to Hc3, a superconducting layer or sheath nucleates on a portion of the surface at which the applied field is tangential to the surface. In the case of non-homogeneous applied magnetic fields, interior nucleation may occur first when the applied field is decreased below the upper critical field. The research is interested in the rotational velocity of a magnitude that would generate a fictitious magnetic field that exceeds Hc1 (the upper limit of the magnetic field for type – I superconductors). The magnitude of Hcl is around 0.2 T. This requires that the angular velocity, ω be around 109 s-1 and it is not possible to experimentally attain such a high value for a mechanical rotation. However, such local rotations could be generated in a superconductor by high frequency ultrasound and we could study the possibility of the nucleation of a vortex by sound. A superconducting cylinder rotated at an angular velocity of ω about its symmetry axis develops a magnetic moment 𝐌. It is this possibility of the nucleation of a vortex by sound that has been theoretically studied in this thesis. For a vortex to enter a superconductor, the Gibb’s free energy of the system must be lowered. Calculations that showed how sound enters the problem were done and equations connecting the generated magnetic fields Bs, rotating cylinder thickness d, rotational angular velocity Ω, ultrasound angular velocity ω, amplitude Uo and wavelength λ, were derived. Calculations and analysis revealed the following major results; the rotating cylinder thickness d and the ultrasound wavelength λ are connected by the equation, d = 0.5 λ; to achieve high rotational angular velocities, Ω desired for the superconducting cylinder, its thickness d must be kept very small; the generated magnetic field and rotational angular velocity of the superconducting cylinders are connected by the equation, Bs = 6.82 x 10-3 Ω. These results imply that it is possible to generate values of fictitious magnetic fields Bs, of the order 103 T that exceed Hc1
Wed, 01 Jan 2014 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/15832014-01-01T00:00:00ZTHERMODYNAMIC PROPERTIES OF HIGH TEMPERATURE SUPERCONDUCTORS BASED ON THE t-J-d MODEL
http://41.89.164.27:8080/xmlui/handle/123456789/1568
THERMODYNAMIC PROPERTIES OF HIGH TEMPERATURE SUPERCONDUCTORS BASED ON THE t-J-d MODEL
RAPANDO, WAKHU BERNARD
Many theories that attempt to describe the superconducting state have suffered significant
failures, leaving the t-J model as the only model which effectively captures the physics of
the strong correlations inherent in this problem. However, many aspects of this model
are quite complex and the thermodynamic properties of the superconducting state need to
be analyzed carefully. In this thesis, second quantization techniques involving Bogliubov Valatin transformation have been used to diagonalize the t-J model Hamiltonian so that
the thermodynamic properties of high temperature superconductors can be studied.
Formulae for ground state energy, , specific heat, , and entropy, , of high
temperature superconductors have been derived in the framework of the t-J model.
Additionally, the superconducting dipole Hamiltonian has been diagonalized and the sum
of the t-J and dipole Hamiltonians obtained resulting in what is now referred to as the „t- J-d model‟. Transition temperature for LSCO in the t-J formalism is obtained as
and in the t-J-d system. Transition temperature for YBCO in the
t-J formalism is obtained as and in the t-J-d model. Both
models predict Tc that is higher than the experimental value of 90K but remarkably close
to the known values. Highest heat capacity of the superconducting state of the t-J model
is found to be while the highest entropy value is 3.15×10-3
eV/K for
high-Tc superconductors. The total energy of the system increases exponentially with the
temperature.
Thu, 01 Jan 2015 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/15682015-01-01T00:00:00ZNUCLEON-NUCLEON INTERACTION IN RHENIUM- 186Re.
http://41.89.164.27:8080/xmlui/handle/123456789/1562
NUCLEON-NUCLEON INTERACTION IN RHENIUM- 186Re.
SIRMA, KENNETH KIPCHUMBA
Properties of nuclear matter and large finite nuclei have been studied using different types
of nucleon-nucleon (NN) interaction potential. In this study, focus is on the thermodynamic
properties of a large nucleus of Rhenium ( Re 186
75 ) using NN-interaction potential. Rhenium
has been chosen because of its applications in technology, it’s used in engines of airplanes,
missiles and high temperature thermo-couplers thus it may equally have an application at
lower temperatures. The NN-interaction is used as a perturbation and general methods of
quantum mechanics as well as many-body techniques have been used to calculate the
energy, the heat capacity and the entropy. It is found that the heat capacity exhibits a phase
transition at a critical temperature of 0.144K. The heat capacity increases linearly with
increase in excitation energy at constant temperature and this is due to changes in the
internal energy of nuclei. The entropy of the nuclei increases linearly with temperature and
becomes zero at absolute 0K. These results suggests the possibility of Rhenium, a fermi system exhibiting superfluid properties below the critical temperature Tc~0.144K.
Wed, 01 Jan 2014 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/15622014-01-01T00:00:00ZROLE OF LONG – RANGE ELECTRON - PHONON AND COULOMB INTERACTIONS IN HIGH - TC CUPRATE SUPERCONDUCTORS
http://41.89.164.27:8080/xmlui/handle/123456789/1557
ROLE OF LONG – RANGE ELECTRON - PHONON AND COULOMB INTERACTIONS IN HIGH - TC CUPRATE SUPERCONDUCTORS
KIPROTICH, TANUI PETER
Superconductivity is the disappearance of the electrical resistance of certain materials at
certain critical temperatures called transition temperatures. This phenomenon was
discovered in 1911 and is one of the most interesting and sophisticated discoveries in
condensed matter physics. Superconducting materials have long been classified into two
categories, viz low temperature (conventional) and high temperature ( unconventional)
supercunductors and the current work deals with the later type. High temperature
superconductors have transition temperatures above 30 K (-243.150C) and are further
grouped into pnictides and cuprates. Cuprates are copper oxide superconductors. This
study investigated the role of long – range electron phonon and Coulomb interactions in
high - Tc cuprate superconductors. In the study, the electron – phonon and Coulomb
Hamiltonian was derived using frozen phonon method. The expectation value of the Hepc
was calculated using second quantization and many body techniques. The equation for
the energy of the system at ground state was obtained from the product of the expectation
value of Hepc and the thermal activation factor, exp (-E1/kT). The equation relating
specific heat and absolute temperature was obtained from the first derivative of the
energy of the system at ground state with respect to absolute temperature. The equation
relating entropy and absolute temperature was obtained from the specific heat equation,
using integral calculus. From the equations relating specific heat and entropy with
absolute temperature, values of specific heat and entropy against absolute temperature
were calculated. In these calculations, the onsite energy of copper (Ed) was fixed at 2.0 x
10-6
eV. The onsite energy of oxygen (Ep), hybridization energy of oxygen and copper
bands (tpd), the electron – phonon interaction energy, (gep) and energy due to repulsion of
copper holes occupying the same orbital (ud), were varied. From the results, it was found
out that increase in the parameters Ed, tpd, gep and ud leads to increase in the transition
temperature from 30 K to 90 K. It was further found that entropy and specific heat
decrease with increase in the parameters. It can therefore be concluded that long range
electron – phonon and Coulomb interactions increase the transition temperature of
superconducting cuprates.
Fri, 01 Jan 2016 00:00:00 GMThttp://41.89.164.27:8080/xmlui/handle/123456789/15572016-01-01T00:00:00Z