We studied the influence of GaAs top-layer thickness on the small-signal modulation response and 56 Gb/s four-level pulse-amplitude modulation eye quality of 850 nm vertical-cavity surface-emitting lasers (VCSELs). We considered the proportionality of the gain-saturation coefficient to the photon lifetime. The simulation results that employed the transfer-matrix method and laser rate equations led to the conclusion that the proportionality should be considered for proper explanation of the experimental results. From the obtained optical eyes, we could determine an optimum thickness of the GaAs top layer that rendered the best eye quality of VCSEL. We also compared two results: one result with a fixed gain-saturation coefficient and the other that considered the proportionality. The former result with the constant gain-saturation coefficient demonstrated a better eye quality and a wider optimum range of the GaAs top-layer thickness because the resultant higher damping reduced the relaxation oscillation.

中文翻译：

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用于 56 Gb/s PAM-4 应用的基于 AlGaAs 的 850 nm VCSEL 中 GaAs 顶层的最佳厚度

我们研究了 GaAs 顶层厚度对 850 nm 垂直腔面发射激光器 (VCSEL) 的小信号调制响应和 56 Gb/s 四级脉冲幅度调制眼质量的影响。我们考虑了增益饱和系数与光子寿命的比例。采用传递矩阵法和激光速率方程的模拟结果得出的结论是，应考虑比例性以正确解释实验结果。根据获得的光学眼睛，我们可以确定实现 VCSEL 最佳眼睛质量的 GaAs 顶层的最佳厚度。我们还比较了两个结果：一个结果具有固定的增益饱和系数，另一个考虑了比例性。