200 G Outdoor Free-Space-Optics Link Using a Single-Photodiode Receiver

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 CHz 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.