A new scheme is proposed to mitigate the atmospheric turbulence effect in coherent free-space optical (FSO) communications by employing optical phase conjugation (OPC) to compensate the signal distortion. The compensation performance of the dual-hop FSO transmission link is simulated using the phase screen method for quadrature phase-shift keying (QPSK) signals. The results show that the bite error rate (BER) can be effectively reduced and the OPC correction is tightly affected by the OPC receiving aperture in strong turbulence regimes. The dual-hop FSO link with the OPC correction is experimentally demonstrated in a turbulence-tunable atmospheric cell, where the OPC is achieved by using degenerate four-wave mixing (DFWM) in a 110-m-long highly nonlinear fiber. At the BER of × − , the power penalty for dual-hop link with OPC correction decreases about 1.2 dB and 2 dB compared with the single-trip link and the non-compensation dual-hop link. The result demonstrates that the OPC unit can improve the performances of the coherent FSO communication, and it has the potential to be extended to higher-order modulation formats and higher bit rate processing.

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A comprehensive multiparameter model is proposed for underwater wireless optical communication (UWOC) channels to integrate the effects of absorption, scattering, and dynamic turbulence. The simulation accuracy is further improved by the combined use of the subharmonic method and the strict sampling constraint method by comparing the phase structure function with the theoretical value. The average light intensity and scintillation index are analyzed using the channel parameters of absorption, scattering, turbulence, flow velocity, and transmission distance. Under weak or medium turbulence, the bit error rate (BER) performance can be effectively improved by increasing the transmitting light power. The power penalty of a 50 m UWOC channel is 5.8 dBm from pure seawater to ocean water and 1.0 dBm from weak turbulence to medium turbulence, with the BER threshold of 10−6.