Ashim Kumar Saha received BScEngg (Electrical & Electronic Eng) degree from Khulna University of Engineering & Technology (KUET) Khulna Bangladesh in 2004 and stood 3rd in order of merit in his class. He started his teaching career as a Lecturer of Computer Science & Engineering Department of University of Development Alternative (UODA) Dhanmondi Dhaka Bangladesh on September 2004. On May 2005, he joined Electrical & Electronic Engineering Department of Engineering Faculty, American International University - Bangladesh (AIUB), Banani Dhaka as a Lecturer. Since then he has been teaching in the undergraduate classes at AIUB. In April 2008 he became an Assistant Professor of Electrical & Electronic Engineering Department of Engineering Faculty, American International University - Bangladesh (AIUB), Banani Dhaka. Ashim Kumar Saha has been involved in research on Vertical Cavity Surface Emitting Laser (VCSEL) since May 2006. Currently, he is working on Quantum Well and Quantum Dots.
Abstract: In this paper, reflectivity of a Distributed Bragg Reflector (DBR) has been computed by considering the effects of changes of wavelength on the changes of the refractive index of the materials of DBR layers. The intrinsic losses of the materials have been included in the computation of the reflectivity of the DBR. It has been found that the effect of change of the wavelength on the refractive index of the DBR materials reduces the Full Width Half Maxima (FWHM) of the stop band significantly which is expected to improve the laser characteristics. If the FWHM is reduced, the thickness of the active layer of a VCSEL can also be reduced which will further reduce the threshold current of the device. It has been found that the intrinsic losses of the materials have a significant effect on the reflectivity of a DBR. It has also been found that peak reflectivity of a 20 pair AlAs/GaAs DBR reduces by 0.2% after including the intrinsic losses (with a value of the intrinsic losses α = 10 cmâ1).
Abstract: The performance of Erbium Doped Fiber Amplifier (EDFA), Single Mode Fiber (SMF) and Dispersion Compensating Fiber (DCF) in a dynamic and reconfigurable Wavelength Division Multiplexing (WDM) system has been demonstrated in this simulation study. Eight WDM channels, each channel running at 40 Gbps, are transmitted through SMF, DCF and EDFAs. The effect of long distance transmission of optical signals to the value of OSNR, Q factor and bit error rate (BER) performance in the WDM optical network, keeping the Bit Error Rate (BER) or eye pattern in acceptable range has been observed. The WDM system trial using EDFAâs, SMF and DCF shows acceptable but deteriorated eye patterns and bit error penalties upto 420 km of transmission distance. Output power of the system was kept almost constant (-12 dBm) for the whole simulation process. Our simulation model can inhibit dispersion to a minimum possible limit due to the application of DCF (dispersion value is -85 ps/nm/km) along with the single mode fiber (dispersion value is 17 ps/nm/km). Gain flatness is also maintained by keeping the value of input power and output power equal while performance of the network is analyzed.
Abstract: A simplified analytical model for the calculation of the Signal-to-Noise Ratio (SNR) in Multi Wavelength Erbium-Doped Fiber Amplifier (EDFA) cascades is used to observe the effect of different wave lengthâs light sources on SNR. In this work, the wavelengths chosen for the analysis is 1330 nm and 1550 nm. It has been found that the SNR for 1550 nm is higher than 1330 nm wavelength. The effect of changing the amplifier spacing on SNR has been observed for fixed transmission length with different Gain-Loss differences (ï(i)) and for different transmission lengths. For a 3000 km long transmission line with gain-loss difference ï(i) = -0.15 dB, maximum SNR of 20.45 dB for 1550 nm wavelength and 19.8 dB for 1330 nm wavelength light sources has been found. These maximum SNRs occur for the amplifier spacing of 44.8 km. Maximum amplifier spacing to obtain a fixed SNR (15 dB) for different ï(i) and variable transmission lengths has also been observed for 1330 nm 1550 nm.
Abstract: In this paper, the reflecting mirrors of a Vertical Cavity Surface Emitting Laser (VSCEL) have been formed by arranging the Distributed Bragg Reflector (DBR) layers in four different ways to explore new ways of forming such lasers. Using such DBR based VCSEL the effect of external feedback is shown. Using Sampled Transfer Matrix Method (STMM), the amount of optical feedback inside a VCSEL from the external reflector has been computed. Four different arrangements of DBR mirror including traditional quarter wavelength thick HIGH and LOW refractive index DBR were used to compute the performance parameters of the VCSEL. The variations of threshold current, differential quantum efficiency and output power due to different positions of the external reflector have been computed for four of the above mentioned different VCSELs. For each case the computed results were compared with the theoretical and experimental results obtained by other researchers. Good agreements have been observed.
Abstract: In this work, novel ideas of computing the threshold current and turn on delay of a Vertical Cavity Surface Emitting Laser (VCSEL) using a different set of equations have been presented. Threshold current of a VCSEL has been computed by solving the rate equations for different values of the injection current. In this work, threshold current of a VCSEL has been found to be 0.18 mA which is very much close to the value of 0.179 mA obtained after computing by using the formula of other researchers. Turn on delay of a VCSEL has also been computed by solving the rate equations. Results obtained by using the proposed method have been found to be in close agreement with those of the other researchers.
Abstract: In this paper, a new analytical model has been presented to calculate the internal electric field distribution of a vertical cavity surface emitting laser (VSCEL). Using Sampled Transfer Matrix Method (STMM), the field equations have been deduced for this model. Using this model one can determine the amount of optical feedback inside a VCSEL from the external reflector. The variations of threshold current, differential quantum efficiency and output power due to external reflector positions obtained by this model have been computed. The obtained results were compared with the theoretical and experimental results obtained by other researchers and good agreement has been observed.
