The standard solution for short-haul fiber-optic communications is to deploy noncoherent systems, i.e., to modulate and detect only the light intensity. In such systems, the signal is corrupted with optical noise from amplifiers and with thermal (electrical) noise. The capacity of noncoherent optical links has been studied extensively in the presence of either optical noise or thermal noise. In this paper, for the first time, we characterize the capacity under an average power constraint with both noise sources by establishing upper and lower bounds. In the two extreme cases of zero optical noise or zero thermal noise, we assess our bounds against some well-known results in the literature; improvements in both cases are observed. Next, for amplified fiber-optic systems, we study the trade-off between boosting signal energy (mitigating the effects of thermal noise) and adding optical noise. For a wide spectrum of system parameters and received power levels, we determine the optimal amplification gain. While mostly either no amplification or high-gain amplification is optimal, the best performance is for some parameter intervals achieved at finite gains.

中文翻译：

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何时在非相干传输中使用光放大：一种信息论方法

短距离光纤通信的标准解决方案是部署非相干系统，即仅调制和检测光强度。在此类系统中，信号会被来自放大器的光噪声和热（电）噪声破坏。在存在光噪声或热噪声的情况下，非相干光链路的容量已得到广泛研究。在本文中，我们第一次通过建立上限和下限来表征平均功率约束下两个噪声源的容量。在零光学噪声或零热噪声这两种极端情况下，我们根据文献中的一些众所周知的结果来评估我们的界限；在这两种情况下都观察到了改进。接下来，对于放大光纤系统，我们研究了提高信号能量（减轻热噪声的影响）和增加光噪声之间的权衡。对于广泛的系统参数和接收功率电平，我们确定最佳放大增益。虽然大多数情况下无放大或高增益放大都是最佳的，但最佳性能是在有限增益下实现的某些参数区间。