Performance Optimization of Hybrid Optical Amplifiers for Dense Wavelength Division Multiplexed System

Abstract

The optical fiber is the enabling and promising technology used in almost all the trunk lines of existing networks. It is also capable of allowing the transmission of many signals over long distances because of its huge transmission bandwidth (in THz) and low losses. For multichannel transmission and to efficiently utilize the fiber bandwidth the dense wavelength division multiplexing (DWDM) is a promising technique, in which a single fiber can independently carry several optical channels with different wavelengths. In optical fiber communication, the attenuation is the major limiting factor imposed by optical components or by fiber itself which degrades the system performance and limits the reach of the signals. To compensate these effects opto-electronic regenerators are used to reshape, retime or retransmit the signal, but simultaneously it increase the cost of the system. On the other hand, optical amplifiers boosts the signals without going through the costly conversions from optical to electrical signal and vice versa. But due to the various non-linear effects and phase noise present in the optical amplifier, its use is restricted to limited applications in optical communications. So, there is demand of optical amplifiers which provide better performance (in term of transient performance, power crosstalk, gain flatness, larger gain bandwidth etc.) for DWDM systems. In order to achieve these objectives it is utmost important to design, characterize and investigate an appropriate optical amplifier for high capacity DWDM systems. As such, new ways to extend the bandwidth, improve the noise and dynamic properties of present-day amplifiers are constantly pursued. This thesis facilitates this process by tracking several system level challenges while acknowledging the limitations of existing devices. The hybrid optical amplifier is an enabling and promising technique for future DWDM multi-terabit systems to minimize the impairments due to fiber nonlinearities and to enhance the gain bandwidth and/or gain flatness. This thesis mainly design, characterize and optimize the new hybrid optical amplifiers for DWDM system and further various important aspects has been addressed, such as gain flatness; gain bandwidth product; transient phenomena; crosstalk etc. Initially, the various combinations of optical amplifiers are proposed for a DWDM system and the impact of reduced channel spacing has been investigated in terms of output power, eye closure, bit error rate and quality factor. It is reported that the Raman-EDFA HOA provides better results with a maximum covered single span distance (220 km). Then we have further characterized this Raman-EDFA scheme in denser or scaled up version and addressed methods to achieve better flat gain characteristics without using any other costly components like a gain equalizer, multiple pumping scheme etc. It is observed that, with an input signal power of 3 mW, a flat gain of >10 dB is obtained across the frequency range from 187 to 190.975 THz with a gain variation of <4.5 dB without using any gain-flattening technique. Further, a novel net gain analytical model of Raman-EDFA HOA is derived by taking the actual conditions and the various parameters are optimized using genetic algorithm. This optimized Raman-EDFA HOA shows the improved results in the term of gain which is >17.5 dB with a gain variation of less than 1.48 dB. The obtained noise figure is also lowest (< 4 dB/ channel) ever reported for a Raman-EDFA HOA at reduced channel spacing. In order to check the performance of proposed Raman-EDFA HOA in various applications, the different high speed DWDM system and up-scaled LAN/ MAN have been considered. Initially, the hybrid Raman and EDFA is investigated for a high speed 16×40 Gbps differential phase-shift keying modulated system at different channel spacings. Then as a means of efficient utilization of bandwidth, a new optical data format based on simultaneous non-return-to-zero and polarization-shift-keying modulations has been proposed and investigated for Raman-EDFA for high speed DWDM system. In addition to further increase the speed with large spectral efficiency, the hybrid OTDM-DWDM technique is also investigated on the proposed HOA. The improved performance in the terms of gain ripple, cost, crosstalk, speed, etc. has been reported after comparing it with current state-of-the-art schemes. In respect to long haul communication system, it is found that, 70 km is the optimum span distance at which the proposed HOA achieves a 2450 km transmission distance with acceptable performance. Further, we introduce optimal and heuristic solutions for the scaled-up and denser version of sample LAN/MAN. The solution is based on placing hybrid optical amplifiers for high capacity DWDM links and by placing conventional optical amplifiers over local links with the goal of minimizing the total number of optical amplifiers needed in the network, hence reducing its cost. As far as the cost and size of the components are concerned, the integrated optics technology can achieved it by the placing the several optical component into a single substrate. Our one next step to further improve this technology is to develop optical amplifiers that can be integrated with these devices. Such amplifiers can serve to compensate for the losses in splitters or other components, and can also serve as preamplifiers for active devices such as detectors. In this work, we proposed the various configurations of these low cost waveguide amplifiers. In first configuration, a novel flat-gain optical amplifier is proposed using a hybrid configuration with an Er-Yb co-doped waveguide amplifier and a semiconductor optical amplifier for 100×10 Gbps DWDM system. On the other hand, in second configuration a new split-band hybrid waveguide amplifier (HWA) is proposed using parallel configuration of Er-doped waveguide amplifier and Er-Yb co-doped waveguide amplifier for C+L-band DWDM system. This HWA plays a role to boost the DWDM signals and giving a larger gain while keeping the little power/gain variations over effective gain bandwidth product. Inter-channel crosstalk in optical amplifiers and the resulting output transients is a serious issue, particularly in high-speed and reconfigurable networks. Output power transients can lead to burst errors during detection and in the worst case cause damage to vital system components. A new all-optical hybrid control method for mitigating transients on the channel level is presented. Also, the proposed HOAs and HWAs are recommended for high capacity ultra-DWDM after checking the acceptable level of crosstalk at reduced wavelength spacing. The impact of word length of a pseudo-random bit sequence and the input laser power on nonlinear crosstalk induced by the different HOAs has also been examined. This study establishes the design, characterization and optimization of hybrid optical amplifiers in the fiber optic communication system.