论述了强度调制直接检测(IM-DD)高速密集波分复用(DWDM)系统的二级调制原理,并从马赫曾德尔调制器的传递特性出发推导了四种调制格式[全频率调制归零码(FFMRZ)、半频率调制归零码(HFMRZ)、单边带调制归零码(SSBRZ)、载波抑制归零码(CSRZ)]。根据信号光眼图及归一化频谱描述了各种码型的时频特征。在此基础上分别对各种码型的色散容限、非线性容限进行了数值仿真,通过比较眼图张开度损伤(EOP)发现对于单波长系统载波抑制归零码传输性能最优。最后利用全面的密集波分复用系统模型计算了各种码型的Q因子,发现载波抑制归零码的性能仍然保持最优,这种优势在考虑偏振模色散(PMD)的情况下更为突出。因此载波抑制归零码是强度调制直接检测系统较好的选择。

The fundamental of two stage modulation in intensity modulation direct detection (IM-DD) high-speed dense wavelength-division multiplexing (DWDM) systems is depicted. And based on the transfer characteristic of Mach-Zehnder modulator, the mathematical expressions of four modulation formats are derived including: full frequency modulated return-to-zero (FFMRZ), half frequency modulated return-to-zero (HFMRZ), single sideband modulated return-to-zero (SSBRZ) and carrier suppressed return-to-zero (CSRZ). The characteristics of both time domain and frequency domain are represented according to optical eye diagrams and normalized spectrum. Then the dispersion limit and nonlinearity limit are numerically investigated respectively by comparing the eye opening penalty (EOP), and CSRZ performs better than other formats. At last, the Q-factor, which is relative to the system bit error ratio (BER), is obtained via numerical simulation utilizing comprehensive DWDM system model. The performance of CSRZ still maintains the best and the superiority is more evident when polarization mode dispersion (PMD) is considered. In conclusion, CSRZ is a preferable choice in IM-DD systems.