【正文】 ng (xcut MZIM). It has been found that a small amount of chirp is useful for transmission [9]. Dualarm MZIM has push– pull arrangement where the dual drive voltages V1(t) and V2(t) are inverse to each other and thus, able to pletely eliminate the chirping effect in the modulation. The transmitted op tical field can be written as: Fig. 4. Comparison of 10 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 4 頁 共 29 頁 Fig. 5. Comparison of 20 Gbps RZ duobinary transmitter modules. The use of optical duobinary transmitter with the dualarm MZIM is the usual choice in transmitter design at high data rate, however, dual arm configuration demands more stringent requirement of symmetry to be met [10]. Singlearm MZIM with duobinary filter can also be used where duobinary filter can be approximated by a low pass filter with halfpower cutoff at approximately one fourth the data rate. At this cutoff frequency, the spectral occupancy of the modulated optical field is restricted to [f0 177。 (Bit rate)/2], where f0 is a nominal continuous wave (CW) frequency [11]. The frequency spectrum of an ideal duobinary signal exhibits the first nulls at [f0 177。 (Bit rate)/2] while preserving at the same time, sufficient information for later reconstruction of the signal [11]. The duobinary filter used for simulation is a fifth order lowpass Bessel filter. Duobinary signal generation can also be achieved by directly applying the RZ/NRZ signal to the delayandadd circuit followed by band limiting filter of bandwidth (Bit rate)/2. III. Performance measure IV. System description and results A performance measure criterion provides us the platform for investigation and analysis. In this work we have considered the semianalytic BER evaluation technique for the estimation of the Bit Error Rate (BER) and BER equivalent Qfactor is considered as performance measure criterion. In the present analysis, we are considering three types of duobinary transmitter modules viz. (1) duobinary transmitter based on pushpull configuration of dualarm MZIM (hereafter denoted by T1), (2) singlearm MZIM with delayandadd circuit (here after denoted by T2) 桂林電子科技大 學(xué)畢業(yè) 設(shè)計(jì)（論文）報(bào)告用紙 第 5 頁 共 29 頁 and (3) the singlearm MZIM followed by a duobinary filter (hereafter denoted by T3). Simulative analysis has been performed using mercial package OptSimTM. We carried out our analysis at three different bit rates of 10, 20 and 40G bps. The simulation setup typically prises of three sections: duobinary generation, . transmitter section, optical filter or channel and the receiver section. Receiver section which is mon in the link design for all three abovementioned transmitter modules consists of a PIN photo detector having the responsivity of A/W followed by a fourth order electrical Bessel low pass filter of 30 GHz bandwidth centered at 1550 nm which provides filtered data to the BER estimator for performance measurement. We optimize the modulator chirp C for all three transmitter modules at an Extinction ratio of 20 dB. To find the value of C that optimizes system performance, we vary the C value from 4 to +2. Fig. 1 shows the simulation setup of NRZ duobinary transmission using T1 module. Optical link consisting of Single Mode Fiber (SMF) of 80 km, 20 km and 4 km lengths have been considered for duobinary transmission at 10, 20 and 40Gbps respectively. SMF has the following specifications: attenuation loss of dB/km, dispersion and dispersion slope of 16 ps/kmnm and ps/nm2km at 1550 nm, respectively, effective area of 80 m2 and refractive index of 1020 m2/ is followed by an EDFA that provides a fixed power of 3 dBm and has a noise figure of dB. In this simulative work, an optical laser having 10 MHz line width has been considered. To carry out simulation, first RZ duobinary transmission has been considered and then the RZ pulse generator in Figs. 1–3 has been replaced with NRZ pulse generator to obtain NRZ duobinary transmission. For a particular data rate optimal value of C has been figured out for each type of transmitter module. Qfactor of T1 at optimized C for extinction ratio of 20 dB has been considered as reference to pare with T2 and T3. Extinction ratio at which T2 and T3 offers parable performance to T1 is investigated. Fig. 4 reports that at 10Gbps RZ duobinary transmissions, T1 is optimized at C = while T2 and T3 optimize at ? and ? C values, respectively. It is observed that at 20 dB extinction ratio T2 performs better than T1 and T3. Considering T1 with 20 dB extinction ratio as reference, it can be interpreted that T2 achieves the performance of T1 at a relatively lower value of extinction ratio. Fig. 5 records optimum C value of , ? and ? for T1, T2, T3 respectively used in 20Gbps RZ duobinary data transmission. Comparison of different transmitter modules at 20 dB extinction ratio shows similar qualitative behavior as seen in Fig. 4. Fig. 6 shows that C value of , ? and ? ensures highest Q factor for the three transmitter modules respectively. At 40Gbps RZ duobinary transmitted from T1 provides 桂林電子科技大 學(xué)畢業(yè) 設(shè)計(jì)（論文）報(bào)告用紙 第 6 頁 共 29 頁 better performance than T2 and T3. The performance of T1 at 20 dB extinction ratio can be achieved at a higher value of extinction ratio using T2. Figs. 7–9 provides a platform to pare NRZ Duobinary transmission at different data rate of 10, 20 and 40Gbps for the three transmitter modules. Effect of exti