【正文】 modulator based transmitters. The two different α parameters [12,13] are given by and Where ?(t)is phase change and S(t) . is instantaneous output power change as a function of 桂林電子科技大 學(xué)畢業(yè) 設(shè)計(jì)（論文）報(bào)告用紙 第 11 頁 共 29 頁 time, and VA and VB are peak voltages applied to two electrodes. From Eq.(1)., chirping (?(t) / d t ) can be calculated from S(t). and the measured α parameters [15]. The other chirping can be calculated from 227。 (Bit rate)/2]. So, we can bandlimit the signal at [f0 177。 編號： 畢業(yè)設(shè)計(jì) (論文 )外文翻譯 （原文） 學(xué) 院： 信息與通信學(xué)院 專 業(yè)： 通信工程 學(xué)生姓名： 李忠斌 學(xué) 號： 1000210219 指導(dǎo)教師單位： 信息與通信系 姓 名： 樊孝明 職 稱： 講師 20xx 年 6 月 8 日 第 1 頁 共 29 頁 Investigation of modulator chirp and extinction ratio in different RZ and NRZ duobinary transmitter modules for performance optimization Abstract In this work we present a parative investigation of modulator chirp and extinction ratio in different transmitter modules for 10Gbps, 20Gbps and 40Gbps RZ and NRZ duobinary transmission. For parative analysis three types of transmission modules have been considered viz. push–pull configuration based on dual arm MZIM, delayandadd circuit based single arm MZIM and a duobinary filter followed by single arm MZIM. For each case, the modulator chirp has been optimized with an extinction ratio of 20 dB. Investigation has been carried out to find extinction ratio of single arm MZIM used for RZ and NRZ duobinary transmission that offers system performance parable to dual arm MZIM at 10, 20 and 40Gbps. The results help choosing the best suitable transmitter module with optimized modulator chirp and extinction ratio. I. INTRODUCTION In high speed optical munication systems duobinary modulation is an effective solution that provides a better spectral efficiency and reduces the performance degradation owing to dispersion and nonlinear effects [1–3]. In the 1980s and the early 1990s, direct modulation of semiconductor laser was the mostly used technique. However, direct modulation has several limiting factors like it induces unwanted chirps which results in spectra broadening of the signal, hence causing severe dispersion penalties [4]. Directlymodulated optical signals experience fluctuations in intensity due to Relative Intensity Noise (RIN) of the semiconductor laser. Nonzero line width of laser sources introduces laser phase noise thus for high speed transmission direct modulation is usually not preferred. Hence, external modulation has been an essential choice for the high speed longhaul munication [5,6]. External modulators remove the large amount of wavelength chirping which will otherwise be included if laser diode is directly modulated. Mainly two types of semiconductor external modulators are available viz. ElectroOptic Modulator (EOM) and ElectroAbsorption Modulators (EAM) [6]. Of these two modulators, EOM that changes the band gap energy with applied electric field is mostly preferred because of various advantages like linear response characteristic, high extinction ratio, ability to control phase, frequency or amplitude of the light wave carrier owing to the properties of electro optic material. Even for very low value of driving voltage EOM is able to achieve high speed 桂林電子科技大 學(xué)畢業(yè) 設(shè)計(jì)（論文）報(bào)告用紙 第 2 頁 共 29 頁 modulation [7,8]. II. Theory In optical domain, data modulation is achieved using two types of modulators: optical phase modulator and optical intensity modulator. An ElectroOptic Phase Modulator (EOPM) uses only one electrode. When a driving voltage is applied to the electrode, the 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 chirpi