A new linearization method for optical transmitters based on directly modulated lasers (DMLs), named the Stretched A method, was proposed in Parts I and II of this work. Parts I and II presented the theoretical framework of the method for non-return-to-zero (NRZ) modulation and related detailed simulation and experimental results. Here, we extend the method to pulse amplitude modulation schemes (PAM). Focussing on 4-level PAM (PAM-4), we present the theoretical background and discuss implementation options. A simplified variation of the method for the generation of PAM signals with a significantly lower number of sub-currents is proposed. Simulation studies for PAM-4 transmission at 50 GBaud (100 Gb/s) and an experimental proof-of–principle demonstration at 16 GBaud (32 Gb/s) are reported based on 850 nm vertical-cavity surface-emitting lasers (VCSELs). For PAM-4, products of effective eye diagram areas ( ${\boldsymbol{PS}}$ ) of 0.6 × 10 ⁻⁵ and 43 × 10 ⁻⁵ before and after applying the Stretched A method were measured, demonstrating an improvement ratio of ∼72. The sensitivity and tolerances of the method are analyzed using simulation and experiment.

中文翻译：

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用于光通信的基于 DML 的发射机的线性化方法第三部分：脉冲幅度调制

本工作的第一部分和第二部分提出了一种新的基于直接调制激光器 (DML) 的光发射器线性化方法，称为 Stretched A 方法。第一部分和第二部分介绍了不归零 (NRZ) 调制方法的理论框架以及相关的详细模拟和实验结果。在这里，我们将方法扩展到脉冲幅度调制方案 (PAM)。我们专注于 4 级 PAM (PAM-4)，介绍理论背景并讨论实现选项。提出了一种用于生成具有显着较少子电流数量的 PAM 信号的方法的简化变体。报告了基于 850 nm 垂直腔面发射激光器 (VCSEL) 的 50 GBaud (100 Gb/s) PAM-4 传输模拟研究和 16 GBaud (32 Gb/s) 的实验性原理验证演示. 对于 PAM-4，有效眼图区域的乘积 ( ${\boldsymbol{PS}}$ ) 的 0.6 × 10 ^-5和 43 × 10 ^-5在应用 Stretched A 方法之前和之后进行了测量，表明改进率约为 72。通过仿真和实验分析了该方法的灵敏度和容差。