الفهرس 
AbstractOnly 14 pages are availabe for public view
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Abstract
Optical fiber links are considered an attractive solution for increasing data transmission rates in communication systems. They have been preferred over electrical links for long- or even short-haul transmission distances. Wavelength division multiplexing (WDM) is an alternative optical multiplexing technique that has played an essential role in the development of optical communications, and it allows data transmission at high speed and large capacity. Laser diodes (LDs) are common light sources used to transform electrical signals into optical signals in WDM systems. The direct modulation of LDs is preferred as a data transmission technique to decrease power consumption and reduce the overall system cost. However, direct intensity modulation of LDs causes a time variation in the lasing frequency (i.e., frequency modulation). The interdependence between intensity and frequency modulation is referred to as ”frequency chirp”. In addition, fiber dispersion is a common limiting factor in the optical fiber communication system. Frequency chirp usually causes variations in pulse width, which combine with fiber dispersion and further degrade the performance of the digital system. This degradation imposes limitations on the transmission bit rate and fiber length. Therefore, various attempts have been used for overcoming the dispersion limitation of the standard single-mode fiber (SSMF). Dispersion management methods are widely used in networks for reducing fiber dispersion and hence, improving the fiber link performance. The dispersion management methods include the use of non-zero dispersion-shifted fiber (NZ-DSF), dispersion-compensating fiber (DCF), and fiber Bragg grating (FBG).
The thesis presents a simulation study on the effect of laser parameters on the chirping characteristics of directly modulated LDs and assess their performance for use in 40-Gbps optical fiber links. In addition to studying the effect of dispersion management methods on the performance of 40-Gbps directly-modulated optical fiber links and their application in four-channel WDM fiber systems. The study is based on the numerical integration of the laser rate equations of directly modulated quantum well-distributed feedback LDs and the nonlinear Schrodinger equation of signal propagation down SSMFs.
The first part of the study is to investigate the effects of the linewidth enhancement factor (α) and the gain suppression coefficient (ε) on the laser chirp characteristics and fiber transmission length in a 40-Gbps optical fiber link. The chirp characteristics include the modulated signal waveform, frequency peak-to-peak chirp, and laser output wavelength spectrum. The performance of the 40-Gbps optical fiber link is evaluated in terms of the eye diagram and quality factor of the received laser signal, predicting the maximum fiber length (Lmax) that achieves error-free transmission. The results of this part show that when α increases from 1 to 10, the peak overshoots of the modulated laser relaxation oscillations become strong, resulting in a sharp increase in laser peak-to-peak chirp from 21.8 to 205 GHz, respectively at ε = 0.5×10-17 cm3. Increasing ε to 5×10-17 cm3 dampens out the overshoots, which slightly reduces the peak-to-peak chirp to 19.5 at α = 1 and to 193.3 GHz at α = 10. Although ε improves the laser chirp, it causes a significant wavelength to shift (Δλ) relative to the actual laser wavelength, degrading the efficiency of LD. When ε increase from 0.5×10-17 to 5×10-17 cm3, Δλ increases from 0.02 to 0.177 nm at α = 1, and from 0.212 to 2.28 nm at α = 10, respectively. When the fiber length increases, the influences of α and ε on chirp characteristics become more significant. The predicted Lmax decreases with the increase in α and/or the decrease in ε. When α increases from 1 to 10, Lmax decreases from 5.89 to 0.78 km at ε = 5×10-17 cm3 and from 2.52 to 0.3 km at ε = 0.5×10-17 cm3, respectively.
The second part of the study focuses on the effect of α and dispersion management methods on the performance of 40-Gbps directly-modulated fiber links. The optimal values of the α and the optimal dispersion management method are then identified and applied to design and simulate a four-channel × 40-Gbps WDM fiber system. The dispersion management methods include the use of –NZ-DSF, +NZ-DSF, DCF, and FBG. The obtained results of this part show that regardless of the applied method of dispersion management, the increase in α and/or fiber length reduces the performance of both the 40-Gbps optical link and the WDM system. The use of –NZ-DSF is the least effective method for managing the dispersion effect, followed by +NZ-DSF, then DCF with SSMF, and FBG with SSMF. Regarding the 40-Gbps optical link, when α = 1, the use of SSMF limits the transmission length to 1.6 km, while the length increases to 7.2, 26.5, and 40 km when using –NZ-DSF or +NZ-DSF, DCF with SSMF, and FBG with SSMF, respectively. Increasing α to 3.5, reduces the maximum transmission lengths. Regarding the proposed four-channel × 40-Gbps WDM fiber system, the use of FBG with SSMF is the most effective method for dispersion management at the two key values of α = 1 and 3.5. The maximum transmission length of the WDM system reaches 25 km when α = 1, and reduces to 12 km when α = 3.5.