Abstract In this paper, we experimentally explore using both mode and space (i.e., aperture) diversity to improve system reliability and performance of a free-space optical (FSO) link under atmospheric turbulence. When a system has a limited number of transmitter and receiver apertures, introducing mode diversity to the existing space diversity scheme might further enhance the system performance. We utilize orbital angular momentum (OAM) modes for mode diversity as an example. A 100-Gbit/s quadrature phase-shift keying FSO link is demonstrated through emulated turbulence using two transmitter and receiver aperture pairs. For each aperture pair, a Gaussian beam and an OAM beam are transmitted and received, and all four beams carry the same data stream. Our experimental results indicate that introducing mode diversity to space diversity systems could help to further reduce the variance of received signal power, thereby improving system tolerance to turbulence under a given detection threshold. Bit-error-rates (BERs) mostly below 3.8 × 10−3 are achieved under emulated turbulence with a Fried parameter of 0.4 mm. The dependence of system performance on different diversity schemes, OAM topological charge differential, and aperture spacing are also investigated.

中文翻译：

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在使用模式和空间分集的 100-Gbit/s QPSK 自由空间光链路中提高系统对湍流的容忍度

摘要 在本文中，我们通过实验探索使用模式和空间（即孔径）分集来提高大气湍流下自由空间光 (FSO) 链路的系统可靠性和性能。当系统的发射机和接收机孔径数量有限时，将模式分集引入现有的空间分集方案可能会进一步提高系统性能。我们将轨道角动量 (OAM) 模式用于模式多样性作为示例。100-Gbit/s 正交相移键控 FSO 链路通过模拟湍流使用两个发射器和接收器孔径对进行演示。对于每个孔径对，发送和接收一个高斯波束和一个 OAM 波束，并且所有四个波束都携带相同的数据流。我们的实验结果表明，在空间分集系统中引入模式分集有助于进一步降低接收信号功率的方差，从而在给定的检测阈值下提高系统对湍流的容忍度。大多数低于 3.8 × 10−3 的误码率 (BER) 是在模拟湍流下实现的，弗里德参数为 0.4 mm。还研究了系统性能对不同分集方案、OAM 拓扑电荷微分和孔径间距的依赖性。