This paper proposes multilevel linear polarization shift keying (MLPolSK) modulation, based on a high polarization-dependent gain optical amplifier (PDG-OA), for spectral efficient free-space optical (FSO) communication. At the transmitter end, multiple linear states of polarization (SOPs) are adopted, and different SOPs are mapped for MLPolSK modulation according to the PDG-OA. First, in the receiver, scintillation is effectively mitigated by a polarization-independent semiconductor optical amplifier (SOA) with gain saturation. Next, various SOPs are transformed into different signal intensity levels by an OA with high PDG characteristics. Finally, transformed signal is directly detected by a single photodiode (PD) with multilevel-thresholds decision. The feasibility of the proposed technique is evaluated experimentally using a reflective semiconductor optical amplifier (RSOA), with a high PDG-up to 20 dB. A Mach–Zehnder modulator (MZM)-based fading simulator is used to accommodate lognormal distribution simulated turbulence effects into experiments. Experimental results illustrate that multilevel linear SOPs can be distinguished effectively by the proposed technique. Consequently, the MLPolSK detection is simplified, and the spectral efficiency (SE) is improved by up to 2 bit/s/Hz with effective scintillation mitigation in FSO.

中文翻译：

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用于频谱高效自由空间光通信的基于高 PDG-OA 的 MLPolSK 调制

本文提出了基于高偏振相关增益光放大器 (PDG-OA) 的多级线性偏振移键控 (MLPolSK) 调制，用于光谱高效的自由空间光 (FSO) 通信。在发射端，采用多种线性偏振态（SOP），根据PDG-OA映射不同的SOP进行MLPolSK调制。首先，在接收器中，具有增益饱和的偏振无关半导体光放大器 (SOA) 有效地减轻了闪烁。接下来，通过具有高 PDG 特性的 OA，将各种 SOP 转换为不同的信号强度级别。最后，转换后的信号由具有多级阈值决策的单个光电二极管 (PD) 直接检测。所提出的技术的可行性使用反射半导体光放大器 (RSOA) 进行了实验评估，该放大器具有高达 20 dB 的高 PDG。基于 Mach-Zehnder 调制器 (MZM) 的衰落模拟器用于将对数正态分布模拟湍流效应纳入实验。实验结果表明，所提出的技术可以有效地区分多级线性 SOP。因此，MLPolSK 检测得到简化，频谱效率 (SE) 提高了高达 2 比特/秒/赫兹，并在 FSO 中有效地抑制了闪烁。实验结果表明，所提出的技术可以有效地区分多级线性 SOP。因此，MLPolSK 检测得到简化，频谱效率 (SE) 提高了高达 2 比特/秒/赫兹，并在 FSO 中有效地抑制了闪烁。实验结果表明，所提出的技术可以有效地区分多级线性 SOP。因此，MLPolSK 检测得到简化，频谱效率 (SE) 提高了高达 2 比特/秒/赫兹，并在 FSO 中有效地抑制了闪烁。