【正文】 nction ratio at the optimized C value on Q factor for each transmitter module has been observed. It is seen in Fig. 7 that T1 is optimized at C = whereas optimized C for T2 and T3 are ? and ?, respectively. Comparing the three transmitters at an extinction ratio of 20 dB, it is found that T1 offers the best performance while T3 is significantly performing better than T2 and very close to T1. As shown in Fig. 8 for 20Gbps NRZ transmissions the optimal C values for three transmitter modules mentioned previously are recorded to be , ? and ? respectively. The relative performance of the transmitter modules are similar to that observed in the case of 10Gbps shown in Fig. 7. Chirp parameter of , ? and ? optimizes 40Gbps NRZ Duobinary transmission for the three transmitter modules as reported in Fig. 9. The qualitative behavior remains same at 40Gbps also where T1 performs significantly better than T2 and T3. T3, which outperforms T 2, can provide parable performance to T1 at much higher value of extinction ratio. Fig. 6. Comparison of 40 Gbps RZ duobinary transmitter modules 桂林電子科技大 學(xué)畢業(yè) 設(shè)計(jì)（論文）報(bào)告用紙 第 7 頁(yè) 共 29 頁(yè) Fig. 7. Comparison of 10 Gbps NRZ duobinary transmitter modules. Fig. 8. Comparison of 20Gbps NRZ duobinary transmitter modules. Fig. 9. Comparison of 40Gbps NRZ duobinary transmitter modules. V. Conclusion This paper pares different transmitter modules for 10Gbps, 20Gbps and 40Gbps RZ and NRZ duobinary transmission to investigate the value of chirp and extinction ratio of the modulator that provides optimum performance. Analysis reveals that for 10Gbps and 20Gbps RZ duobinary at 20 dB extinction ratio, delayandadd circuit based transmitter performs better than other two. At 40Gbps, single arm MZIM in a delayandadd circuit based transmitter provides parable performance to a dual arm MZIM at relatively higher extinction ratio. Hence, for RZ duobinary, with optimal selection of modulator chirp and extinction ratio, a delayandadd circuit based single arm MZIM can replace a dualarm MZIM. For NRZ duobinary at 10Gbps, 20Gbps and 40Gbps, duobinary filter based 桂林電子科技大 學(xué)畢業(yè) 設(shè)計(jì)（論文）報(bào)告用紙 第 8 頁(yè) 共 29 頁(yè) transmitter performs better than delayandadd circuit based transmitter and hence, a dual arm MZIM can be replaced with negligible penalty in Q factor. VI. Acknowledgement Authors are thankful to the Director, Pilani for extending the facility in the optical munication lab to implement the concept. VII. References [1] S. Walklin, J. Conradi, On the relationship between chromatic dispersion and transmitter filter response in duobinary optical munication systems, Photon. Technol. Lett. 9 (1997) 1005– 1007. [2] W. Kaiser, M. Wichers, T. Wuth, W. Rosenkranz, C. Scheerer, C. Glingener, A. F228。 , as the equation derived in Appendix A. From these two chirpings, two transmitter outputs in terms of electric fields can be expressed by where m is the scaling factor in order to adjust extinction ratio. The transmitter output (in Eq.(3) and (4) by electric field ) will be used as an input field to fiber, which will be discussed in the next section. Fig. 1 shows the calculated and measured eye diagrams at 0 km for the extinction ratio of about 7 dB and 12 dB. The rise and fall time, and 3dB width (cross point) of the pulses were 56 and 100ps, respectively, for the extinction ratio of about 7 dB (Fig. 1(a). and Fig. 1(b), and 42 and 100ps for the extinction ratio of 12 dB (Fig. 1(c) and Fig. 1（ d） , which are the same as the experimental values. We used 128(27) pseudorandom bit pattern with a total 32 768 (215) samples (or 256 samples per bit) for the simulation. Fig. 2 shows transmitter output pulse and two different chirping characteristics generated by Eq.( 3 ) (chirping model 1) and Eq.(4) (chirping model 2). 桂林電子科技大 學(xué)畢業(yè) 設(shè)計(jì)（論文）報(bào)告用紙 第 12 頁(yè) 共 29 頁(yè) (a) and measured (b) optical eye diagrams at 0 km for the extinction ratio ofabout7 dB, and calculated (c) and measured (d) optical eye diagrams for the extinction ratio of12 dB. One divisionrepresents25 ps. . Optical signal transmission over fiber Optical signal transmission through the single mode optical fiber is considered nonlinear, dispersive, and lossy, and therefore the evolution of slowly varying pulse envelope （ A(t )）can be obtained from the non linear Schrodinger equation [ 11 ]: Where β1 is the inverse group velocity, β2 and β3 are the first and secondorder group velocity 桂林電子科技大 學(xué)畢業(yè) 設(shè)計(jì)（論文）報(bào)告用紙 第 13 頁(yè) 共 29 頁(yè) dispersion, a is the absorption coefficient, and γ (=N2ω0/c Aeff ) is the nonlinearity coefficient(N2 is the material nonlinear refractive index and Aeff is the effective core area). The pulse envelope A is assumed to be normalized in order that |A|2 represents the optical power. Nonlinear Schrodinger equation of Eq.(5) is a nonlinear partial differential equation that does not generally lend itself to analytic solution. This equation was solved by the splitstep Fourier method [11] with the parameters :the fiber loss of dB/km, the dispersion of , the dispersion slope of 102ps nm2 km the effective core area of 90 um2 ,and the nonlinear refractive index of 1020 m2 W. . Receiver chara