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MIT International Journal of Electronics and Communication Engineering, Vol. 2, No. 1, Jan. 2012 pp. (1-4) 2ISSN 2230-7672 (c) MIT Publicationstechniques for compensation of dispersion by differential delaymethod including the impact of higher order dispersion termsKaler et al. [5] discussed the limitations due to Group VelocityDispersion (GVD) on transmission distance, bit rate and laserlinewidth including the higher order dispersion effects. Thepower penalty analysis for different realistic weight functionsfor combating the pulse broadening effects of group-velocitydispersion in a fiber-optic communication link usingdifferential time delay method with higher-order dispersionterms [6] was discussed. Further, the propagation of signaland noise in the transmission medium to observe the validityof higher order dispersion terms [7] was described.The comparison of pre-compensation, post-compensationand symmetrical-dispersion compensation schemes for10 Gb/s NRZ links using standard and dispersion compensatedfibers was also investigated[8].Sukhwinder Kaur et al. [9] hasdiscussed the symmetrical compensation scheme by varyingthe lengths and dispersion values for the Standard Single ModeFiber (SSMF).The performance of the SSMF is tested for 20to 60 km fiber length. It is shown that the DCF qualitativelyimproves the performance of the optical fiber links.Section III discusses the proposed work for the SymmetricalCompensation. Results for the simulation are discussed andvalidated by comparisons among the exsisiting and proposedmethod in Section VI, followed by the concluding remarksfor various configurations under symmetrical compensationin the Section VII.DISPERSION COMPENSATING FIBERThe Dispersion Compensating Fiber (DCF) is the predominanttechnology for dispersion compensation. It consists of anoptical fiber that has a special design such as providing a largenegative dispersion coefficient while the dispersion of thetransport fiber is positive. A proper length of DCF allows thecompensation of the chromatic dispersion accumulated over agiven length of the transport fiber, although standard moduleswith predetermined dispersion values (with a typicalgranularity corresponding to the dispersion of 20 km of SSMF)are commercially available. The main advantage of thistechnology is the fact that it provides a broadband operationwith a smooth dispersion property and good opticalcharacteristics.DISPERSION COMPENSATING MODULEThe Dispersion Compensating Module (DCM) is proposedby using the Symmetrical optimization for the optical fibersusing Pre & Post compensation both. Dispersion managementcan be achieved with various combinations of fiber layout.Symmetrical compensation which uses both the techniques inone link as shown in Figure 1.The DCM module comprises of equal number of sectionsof SMF & DCF.Figure 1: Dispersion management usingsymmetric compensationSYSTEM MODELLINGTo achieve optimum dispersion compensation, a dispersionmanagement scheme is modelled using a single link forsymmetrical configuration consisting of SMF = 120 km andthe lengths of DCF sections are varied for 24 km,30 km and35 km for Pre-compensation and Post compensation both. Nopolarization effects and non-linear effect for the fiber areconsidered. The fiber parameters for SMF and DCF are listedin Table 1 and the simulation model is shown in Figure 2.In this technique, a partial compensation of second-orderdispersion by employing DCF electrically filtered by usingBessel filter (low pass), bandwidth 8 dB. As a measure ofsystem performance Q factor and BER are evaluated from eyediagrams obtained from the simulations in the systemconfiguration 10Gb/s.Figure 2: Simulated model for the symmetrical dispersion compensation
MIT International Journal of Electronics and Communication Engineering, Vol. 2, No. 1, Jan. 2012 pp. (1-4) 3ISSN 2230-7672 (c) MIT PublicationsTable 1: Simulation parameters used for thesymmetrical compensationSimulation SMF DCF pre- DCF post-Parameters compensation compensationLength (km) 120 24 24Dispersion 16 -80 -80[ps/km/nm]Loss (dB/km) 0.2 0.6 0.6Nonlinear 1.26677 1.8 1.8Coefficient(Wkm) -1Transmitter & NRZ — —ReceiverParametrsBit rate [Gb/s] 10 — —Figure 4: Pre-compensation fiber length = 30 km, SMF =120 km, Post-Compensation fiber length = 24 km. Q Value(dB) = 20.53, BER = 1.8e-0.26RESULTS AND DISCUSSIONBy taking into account the dispersion management schemewhich is employed using a single link consisting of equalnumbers of 120 km SMF and DCF sections of 24 km, 30 kmand 35 km for pre-compensation and post-compensation. Theobservations made are summarized in Table 2.Table 2: Simulation results for the symmetricalcompensation for various lengths of pre & post compensationPre Post Q Value BERcompensating compensating (dB)Fiber length Fiber length(Km)(Km)24 24 30.76 1e-04030 24 20.53 1.8e-0.2630 30 15.814 3.47e-01035 35 7.168 0.01117Figure 5: Pre-compensation fiber length = 30 km, SMF =120 km, Post-Compensation fiber length = 30 km,Q Value(dB) = 15.814, BER = 3.47e-01Figure 6: Pre-compensation fiber length = 35 km, SMF =120 km, Post-Compensation fiber length = 35 km,Q Value(dB) = 7.1682, BER = .01117Figure 3: Pre-compensation fiber length = 24 km, SMF =120 km, Post-Compensation fiber length = 24 km,Q Value(dB) = 30.76, BER = 1e-0404For the proposed method, a comparison made betweenthe various configurations taken, shows that if the DCF lengthfor the Pre-compensation and Post-compensation fiber =24 km, SMF = 120 km; the estimated values of the Q Factor =30.7602 dB, BER = 1e-040 and Jitter = 0.024919ns Figure 3.
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