DWDM混合光学系统中帧间和信道内四波混频效应的抑制

本文提出了光码多分址（CDMA）和光密集波分复用（DWDM）的混合系统，全面研究了四波混频（FWM）的影响。在这个系统中，主要存在两个四波混频问题：包括多址干扰（MAI）和码间干扰（ISI）的帧间四波混频和信道内四波混频。结果表明，综合考虑信道间和信道内四波混频的影响，最佳发射功率可选为18 dBm。当发射功率大于18 dBm时，混合系统的误码率（BER）将增加。基于此，本文提出了一种电光相位调制器（EOPM）模块，将其放置在波分复用器之后，通过抑制信道内四波混频的影响，同时调制所有波长信号的相位，从而增加混合系统的非线性容限，这极大地改善了基于OOK传输的光学CDMA-DWDM混合系统的性能。此外，由于多对角线（MD）结构具有零互相关特性，通过使用多对角线识别序列码可以减少多址干扰的影响。结果还表明，CDMA技术与色散相结合有助于降低信道间四波混频的影响。此外，识别序列码间隔在减轻码间干扰中起着至关重要的作用，如结果所示，当识别序列码间隔压缩至比特持续时间的25%时，可以避免码间干扰，此时所提出的混合系统的性能最佳。

Suppression of inter and intra channel four wave mixing effects in optical CDMA over DWDM hybrid system

In this study, a hybrid system of optical Code Division Multiple Access(optical CDMA) and Dense Wavelength Division Multiplexing(DWDM) is proposed with a comprehensive investigation into the effect of four-wave mixing(FWM). In such system, two major FWM problems exist, inter and intra-channel FWM, including multiple access interference(MAI) and inter-symbol interference(ISI). Results show that the optimum transmitted power is 18dBm in order to control the trade-off between inter and intra-channel FWM, where an increase in the BER of the hybrid system at transmitted power above 18 dBm is indicated. Hence, an electro-optic phase modulator(EOPM) module is proposed and placed after the WDM multiplexer to simultaneously modulate the phase of all wavelengths signals to increase the nonlinear tolerance in the hybrid system by suppressing the impact of intra-channel FWM, which is shown to greatly improve the performance of the optical CDMA-DWDM hybrid system based OOK transmission. In addition, the effect of MAI can be reduced by the use of multi-diagonal(MD) identification sequence code, due to the zero cross-correlation property of MD. The results also reveal that the CDMA technology in conjunction with chromatic dispersion helps to reduce the effect of inter-channel FWM. Moreover, the identification sequence code interval plays crucial role in the mitigating of ISI as the results expose that the best performance of the proposed hybrid system can be achieved when the identification sequence code interval squeezed into 25% of bit duration where the avoidance of ISI is guaranteed.