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|PERFORMANCE ENHANCEMENT OF LONG HAUL ULTRA DENSE WDM SYSTEMS
|Kaler, R. S.
Sharma, Enakshi Khular
|The burgeoning demand for broadband services such as database queries, home shopping, video-on-demand, remote education, telemedicine and video conferencing has pushed the existing networks to their limits. This demand was mainly fueled by the brisk proliferation of Personal Computers (PC) together with the exceptional increases in their storage capacity and processing capabilities and the widespread availability of the internet. Hence, the necessity, to develop high-speed optical technologies in order to construct large capacity networks arises. Two of the most popular multiplexing techniques available in the optical domain that are used in the building of such high capacity networks, are Wavelength Division Multiplexing (WDM) and Optical Time Division Multiplexing (OTDM). Dense Wavelength Division Multiplexing (DWDM) has drastically increased the capacity of optical transmission systems. Its inherent advantages have made it the current favorite multiplexing technology for optical networks. OTDM has made it possible to increase the bit rate transmissions on single wavelengths. Therefore, merging these two techniques to get very high-speed hybrid WDM/OTDM networks brings about the merits of both multiplexing technologies. These optical multiplexing technologies would solve the "fiber exhaust” problem and is expected to be the core technology in the all-optical networks of the future. An optical fiber enables the transmission of many signals over long distances because of its huge transmission bandwidth (in THz) and low losses window of operation. Long Haul links are prone to losses and attenuation which forms the major limiting factor imposed by optical components or by fiber which degrades the system performance and limits the appreciable reach of the signals. In order to compensate for these effects, opto-electronic regenerators were used to perform the 3R of Reshaping, Retiming and Retransmitting the signal. Unfortunately it has also resulted in increased cost of the system. Therefore, Optical Amplifiers (OA) was introduced to boost the signals without going through the optical to electrical signal and vice versa conversions. Erbium Doped Fiber Amplifier (EDFA) was first modeled in 1987 by David Payne at University of Southampton, UK. It is a short length of erbium fiber which acts an amplifying media in the entire C band (1530nm-1570nm). In the scenario of DWDM channels, EDFA causes signal to noise ratio being differentially spread among the whole number of channels due to the various non-linear effects and phase noises iii present in the optical amplifiers. Therefore, Hybrid Optical Amplifiers (HOA) which is formed by combinations of various Conventional Optical Amplifiers (COA) comes as possible solution to provide improved performance in terms of gain flatness and larger gain bandwidth for DWDM systems. Various advanced modulation formats can also aid in enhanced capacity systems. In this thesis, a step is made towards building, up scaled capacity WDM systems in a cost effective way. Such a system would require tailored and optimized real time parameters of the various components used throughout the complete system design. Initially, the investigations are carried out using 64, 32 and 16 laser sources operating at wavelength from 1549.5nm with spacing of 0.4nm, 0.35nm, 0.25nm and 0.1nm and with different bit rates. It is found that the output power and BER is greatly affected by longer fiber links, higher bit rates and increased number of channels as inter channel crosstalk and other non-linearities comes into play. Therefore it is demonstrated for various cases the minimal allowed spacing which gives acceptable BER of 10-9 to 10-12 and Quality factor >15dB. With these guidelines more channels and larger distances coverage could be scaled. Further, a simplex OTDM system at (4 x 40Gbps) = 160Gbps is also characterized in terms of pulse width of the short pulses transmitted and the control power margin for the OTDM demultiplexer. The system is also optimized for the input power/channel. FWHM pulse width of 5ps serves optimum with control margin after 22dBm which degrades the performance. A 4 x 40Gbps RZ-DPSK high speed DWDM system was investigated for 110km reach. Further, the number of channels is augmented and the system tested with various combinations of optical amplifiers (HOA) proposed for a DWDM system. It is reported that the Raman-EDFA HOA gives improved performance results as compared to Raman-SOA and EDFA-SOA amplifiers. A 160 x 2.488 Gbps DWDM system at 0.2nm channel spacing is evaluated with different HOA and COA. It is reported that Raman-EDFA provides Q factor >21dB with acceptable high output power of 10dBm. EDFA is reported to perform better than the other COA with Quality factor >18dB. The implemented DWDM system performs well with NRZ modulation format. At higher bit rate and higher channel systems, the RZ reportedly suffers from more dispersion and hence it gives reduced performance. A 96 channel Ultra Dense DWDM system at 0.1nm is designed further and its performance is checked with Raman-EDFA for long haul applications. In the prospective of long haul communication system, it is recommended that 60 km is the optimum span distance of iv standard single mode fiber (SSMF) at which the proposed HOA achieves a 2070 km transmission distance with acceptable performance of Q factor >15dB. The EDFA obtained a maximum of 1794km with the same optimized span. The Dispersion Compensated Fiber length to correctly negate the dispersion effects is 9km. The HOA are also implemented on the combined capacity enhanced DWDM-OTDM systems to achieve the long distance performance with the established performance metric of BER <10-9 . A 320Gbps and 640Gbps system is implemented with HOA with different kinds of Raman fibers. The HOA is implemented using different fibers like the Dispersion Shifted Fiber (DSF) and Dispersion Compensated Fiber (DCF). It is reported that Raman-EDFA HOA gives a higher system BER <10-8 with DCF in the context of 640Gbps system. A high system output power > 6dBm is noted with 0dBm input power. The long distance transmission for both 320Gbps and 640 Gbps system topologies are notified. A distance of 345.044km for 8 x 4 x 10Gbps, 180.176km for 8 x 8 x 10Gbps and 184.02km for 16 x 4 x 10Gbps is reported. An orthogonal launch of the 640 Gbps system reports the repeater less single span distance of 207.02km. Further, a high speed 16 x 10 Gbps DWDM is modulated with phase modulation schemes of DPSK (Differential Phase-Shift Keying) and DQPSK (Differential Quadrature Phase Shift Keying) modulated system at narrow channel spacing with different hybrid optical amplifiers. It is reported that for any of the combinations of HOA used in the scenario of lesser input power (<0dBm), the linear modulation schemes of NRZ and RZ is preferable. If the input channel power is >= 0dBm then DPSK and DQPSK could be used. Q factors > 30dB for NRZ > 29dB for RZ, 21dB for DQPSK and 18dB for DPSK is obtained. These phase modulated schemes are not prone to system nonlinearities that results due to the increased input power as the information is carried in phase and not amplitude. The bandwidth efficient high capacity system with up to 640Gbps is investigated with different modulation formats and different kinds of fiber using the Raman-EDFA. It is reported that 8 x 8x 10Gbps DWDM-OTDM systems provide a highest ever reported gain >19dBm with HOA. The Noise Figure (NF) is minimum being around 5dB. The OSNR achieved is higher >20dB. A comparison with a different topology using 16 channels i.e. 16 x 4 x10Gbps system reportedly shows a HOA gain > 12dB and Q factor > 19.27dB. The HOA is observed to perform well even in the scenario of 640 Gbps with NRZ modulation. An extinction ratio of 18dB is noted for this case. A 320Gbps system with 8 x 4 x 10Gbps v system reports a gain as high as 25dB with HOA is reported with Q factor >20dB. The high speed orthogonally launched 16 x 40Gbps DWDM system was also modulated with Duo binary and compared with simplex binary for single span reach distance. Thus this thesis facilitates different proposed system implementations with newer configurations of Optical Amplifiers by tracking several system level challenges while acknowledging the limitations of existing system. Therefore, this study establishes the design and optimization of capacity enhanced Dense Wavelength Division Multiplexing with Hybrid Optical Amplifiers in the fiber optical communication system resulting in the aid of revolutionary growth of internet traffic for terrestrial fiber backbone networks.
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