【正文】 refractive index of the electrooptic waveguide changes accordingly, thus slowing down the light wave and hence inducing a delay on the optical signal. The induced delay corresponds to the phase change, thus an EOPM is able to manipulate the phase of the light wave carrier [9]. where Vπ is the driving voltage required to create a phase shift, V(t) is a timevarying driving signal voltage and Vbias is dc bias voltage. Optical field Eo at the output of the EOPM is given as: Eo(t) = Ei (t)ej?(t) (2) Optical intensity modulator uses two EOPMs in a parallel structure to form a Mach–Zehnder interferometer monly known as the Mach–Zehnder intensity modulator (MZIM) [9]. Input optical signal splits equally in the two arms of the MZIM which are actually EOPMs for modulating the phase of the optical carrier. At the output, the two arms are coupled either constructively or destructively to provide intensity modulated optical pulses. Fig. 1. Duo binary transmitter module with dualarm MZIM (T1). Fig. 2. Duo binary transmitter with singlearm MZIM delayandadd circuit (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 3 頁 共 29 頁 Fig. 3. Duobinary transmitter with singlearm transmitter and filter circuit (T3). MZIM can be of two types: singlearm MZIM and dual arm MZIM. In singlearm MZIM only one single driving voltage is applied to the either arm of MZIM [9] and the output transmitted optical field Eo(t) is given as: Existence of the phase term in Eq. (3) shows that the chirping effect is present, thus we can say that the singlearm MZIM generated signals are not chirpfree. Particular structure of the MZIM can only minimize the chirping (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]. So, we can bandlimit the signal at [f0 177。 , 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 共 29 頁 (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 共 29 頁 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