Browsing by Author "Wongpaibool, Virach"

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    Effect of Dispersion on SS-WDM Systems
    Wongpaibool, Virach (Virginia Tech, 1998-09-18)
    The purpose of this thesis is to investigate the effect of dispersion on a spectrum-sliced WDM (SS-WDM) system, specifically a system employing a single-mode optical fiber. The system performance is expressed in term of the receiver sensitivity defined as the average number of photon per bit Np required for a given probability of bit error Pe. The receiver sensitivity is expressed in terms of two normalized parameters: the ratio of the optical bandwidth per channel and the bit rate m=B0/Rb=B0T, and the transmission distance normalized by the dispersion distance z/LD. The former represents the effect of the excess beat noise caused by the signal fluctuation. The latter represents the effect of dispersion. The excess beat noise can be reduced by increasing the value of m (increasing the optical bandwidth B0 for a given bit rate Rb). However, a large m implies that the degradation due to the dispersion is severe in a system employing a single-mode fiber. Therefore, there should be an optimum m resulting from the two effects. The theoretical results obtained from our analysis have confirmed this prediction. It is also shown that the optimum m (mopt) decreases with an increase in the normalized distance. This suggests that the dispersion strongly affects the system performance. The increase in the excess beat noise is traded for the decrease in the dispersion effect. Additionally, the maximum transmission distance is relatively short, compared to that in a laser-based system. This suggests that the SS-WDM systems with single-mode fibers are suitable for short-haul systems, such as high-speed local-access network where the operating bit rate is high but the transmission distance is relatively short.
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    Improvement of Fiber Optic System Performance by Synchronous Phase Modulation and Filtering at the Transmitter
    Wongpaibool, Virach (Virginia Tech, 2003-01-31)
    In this dissertation the performance of a novel variant of a return-to-zero (RZ) modulation format, based on square-wave phase modulation and filtering of a continuous-wave (CW) signal, is investigated and compared with various modulation formats considered in the literature. We call this modulation format continuous-wave square-wave (CWSW). With CWSW an RZ pulse train is generated by phase modulating the CW signal by a periodic square-wave phase function having an amplitude of and frequency of half the bit rate, and then filtering the signal. The filter performs phase-to-amplitude conversion, resulting in an alternate-sign RZ pulse train, which is shown to be resistant to fiber dispersion. The alternate-sign RZ pulse train is then amplitude modulated with the data before the transmission. Alternate signs between adjacent pulses makes this signal format robust to impairments caused by the optical fiber, similar to a conventional alternate-sign RZ signal format. However, the unique property of the CWSW signal format is that individual pulses can induce peak intensity enhancement (PIE), a phenomenon by which the peak of a pulse increases during the initial propagation in the presence of dispersion. The PIE in effect delays the decrease in the pulse peak, which represents the signal level for bit 1. Thus, the eye opening at the receiver is improved. An analytically tractable model is developed to explain the occurrence of the PIE, which cannot be achieved with a conventional pulse shape. The sources of performance degradations for different modulation formats in single-channel 40 Gb/s systems are also discussed in this dissertation. Various transmission system configurations of practical interest are considered and the performance of CWSW is compared with alternative modulation formats. It is found that the CWSW signal format performs significantly better than the other considered modulation formats in systems not employing dispersion compensation and is comparable to the others in dispersion-managed systems. Furthermore, the transmitter configuration of the CWSW signal format is simpler than the other approaches.