Abstract
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 phaseshift 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 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , 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.
Highlights
Free-space optical (FSO) communications take many advantages over traditional microwave systems for their high modulation bandwidth, free license spectrum and enhanced security [1], [2]
The results show that the bite error rate (BER) can be effectively reduced and the optical phase conjugation (OPC) correction is tightly affected by the OPC receiving aperture in strong turbulence regimes
The dual-hop free-space optical (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
Summary
Free-space optical (FSO) communications take many advantages over traditional microwave systems for their high modulation bandwidth, free license spectrum and enhanced security [1], [2]. Due to the all optical phase conjugation (OPC) techniques, the compensations based on OPC for atmospheric turbulences have much shorter response time (10−9–10−7 sec) and less limitations of the affordable spatial frequencies [14]. If node B fails, it can switch the traffic to the protection path and the traffic will back to node A while the phase conjugation technique can remove the distortions and ensure the operation of the protection path In such a system, dual-hop FSO links play an important role to ensure the system available.
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