We experimentally evaluate the performance of Kramers–Kronig (KK) receivers for free-space optical transmission. We transmit a dual-carrier signal, each carrier modulated as a 25 Gbaud 16/32 quadrature-amplitude modulation (QAM) signal. This dual-carrier signal, with overall symbol rate of 50 Gbaud, is detected in a single receiver with about 35 GHz bandwidth. Key parameters such as the carrier-to-signal power ratio, the frequency allocation of the pilot tone, and the frequency separation between both modulated carriers are optimized. In addition, we also study the effect of the receiver bandwidth limitations on the KK technique. We perform long-term measurements, beyond one hour, for both 16 and 32 QAM signals in a 55 m outdoor free-space link during daylight with sunny weather conditions. Our measurements demonstrate that the link operates below the BER threshold assuming 20% forward-error correction (FEC) for the 32QAM signal and 7% FEC overhead for the 16QAM signal. The resulting net data rates are 208 and 184 Gbit/s for the 32 and 16 QAM signals, respectively.

中文翻译：

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使用单光电二极管接收器的 200G 室外自由空间光链路

我们通过实验评估了用于自由空间光传输的 Kramers-Kronig (KK) 接收器的性能。我们传输双载波信号，每个载波调制为 25 Gbaud 16/32 正交幅度调制 (QAM) 信号。这种双载波信号的总符号率为 50 Gbaud，在带宽约为 35 GHz 的单个接收器中被检测到。优化了载波信号功率比、导频频率分配以及两个调制载波之间的频率间隔等关键参数。此外，我们还研究了接收器带宽限制对 KK 技术的影响。我们在阳光明媚的天气条件下，对 55 m 室外自由空间链路中的 16 和 32 QAM 信号进行了超过一小时的长期测量。我们的测量表明，假设 32QAM 信号的前向纠错 (FEC) 为 20%，16QAM 信号的 FEC 开销为 7%，则链路在 BER 阈值以下运行。32 和 16 QAM 信号的净数据速率分别为 208 和 184 Gbit/s。