Chen Junda, Hu Zihe, Yang Zheng, Chen Yizhao, Zhang Mingming, Zeng Yifan, Li Weihao, Cheng Chen, Zhou Ziwen, Wang Xuefemg, Liu Deming, Tang Ming
Opt Express. 2023 Sep 25;31(20):32044-32057. doi: 10.1364/OE.486833.
Nonlinear frequency division multiplexing (NFDM) systems, especially the eigenvalue communications have the potential to overcome the nonlinear Shannon capacity limit. However, the baud rate of eigenvalue communications is typically restricted to a few GBaud, making it challenging to mitigate laser frequency impairments such as the phase noise and frequency offset (FO) using digital signal processing (DSP) algorithms in intradyne detections (IDs). Therefore, we introduce the polarization division multiplexing-self-homodyne detection (PDM-SHD) into the NFDM link, which could overcome the impact of phase noise and FO by transmitting a pilot carrier originating from the transmitter laser to the receiver through the orthogonal polarization state of signal. To separate the signal from the carrier at the receiver, a carrier to signal power ratio (CSPR) unrestricted adaptive polarization controlling strategy is proposed and implemented by exploiting the optical intensity fluctuation of the light in a particular polarization rather than its direct optical power as the feedback. Optical injection locking (OIL) is used subsequently to amplify optical power of pilot carrier and mitigate the impact of signal-signal beat interference (SSBI). Additionally, the effects of cross-polarization modulation (XPolM) and modulation instability (MI) in long haul transmission are explored and inhibited. The results show that the tolerable FO range is about 3.5 GHz, which is 17 times larger than the ID one. When 16-amplitude phase shift keying (APSK) or 64-APSK constellations are used, identical Q-factor performance can be obtained by using distributed feedback (DFB, ∼10 MHz) laser, external cavity laser (ECL, ∼100kHz), or fiber laser (FL, ∼100 Hz), respectively, which demonstrates that our proposed PDM-SHD eigenvalue communication structure is insensitive to the laser linewidth. Under the impact of cycle slip, the Q-factor difference of 16-APSK signal between the ECL-ID system and ECL-SHD system can be up to 8.73 dB after 1500 km transmission.
非线性频分复用（NFDM）系统，尤其是特征值通信，有潜力突破非线性香农容量限制。然而，特征值通信的波特率通常限制在几吉波特，这使得在零差检测（ID）中使用数字信号处理（DSP）算法来减轻诸如相位噪声和频率偏移（FO）等激光频率损伤具有挑战性。因此，我们将偏振分复用 - 自零差检测（PDM - SHD）引入到NFDM链路中，通过将源自发射机激光器的导频载波通过信号的正交偏振态传输到接收机，该方法可以克服相位噪声和FO的影响。为了在接收机处将信号与载波分离，提出并实现了一种载波与信号功率比（CSPR）不受限制的自适应偏振控制策略，该策略利用特定偏振态下光的光强波动而非其直接光功率作为反馈。随后使用光注入锁定（OIL）来放大导频载波的光功率并减轻信号 - 信号拍频干扰（SSBI）的影响。此外，还研究并抑制了长距离传输中交叉偏振调制（XPolM）和调制不稳定性（MI）的影响。结果表明，可容忍的FO范围约为3.5 GHz，比ID系统大17倍。当使用16幅度相移键控（APSK）或64 - APSK星座时，分别使用分布反馈（DFB，10 MHz）激光器、外腔激光器（ECL，100kHz）或光纤激光器（FL，~100 Hz）可获得相同的Q因子性能，这表明我们提出的PDM - SHD特征值通信结构对激光线宽不敏感。在周跳的影响下，经过1500 km传输后，ECL - ID系统和ECL - SHD系统之间16 - APSK信号的Q因子差异可达8.73 dB。
