Automatic mitigation of dynamic atmospheric turbulence using optical phase conjugation for coherent free-space optical communications

Coherent detection can provide enhanced receiver sensitivity and spectral efficiency in free-space optical (FSO) communications. However, turbulence can cause modal power coupling effects on a Gaussian data beam and significantly degrade the mixing efficiency between the data beam and a Gaussian local oscillator (LO) in the coherent detector. Specifically, for widely used single-mode-fiber (SMF)-coupled coherent detectors, such degradation is mainly caused by the significantly reduced efficiency when coupling the multi-mode data beam into the SMF. Optical phase conjugation (OPC) in a photorefractive crystal can “automatically” mitigate turbulence by (a) recording a back-propagated turbulence-distorted probe beam, and (b) creating a phase-conjugate beam that has the inverse phase distortion of the medium as the transmitted data beam. However, previously reported crystal-based OPC approaches for FSO links have demonstrated either: (1) a relatively fast response time of 35 ms but at a relatively low data rate (e.g., <1 Mbit/s), or (2) a relatively high data rate of 2-Gbit/s but at a slow response time (e.g., >60 s). Here, we report an OPC approach for the automatic mitigation of dynamic turbulence that enables both a high data rate (8 Gbit/s) data beam and a rapid (<5 ms) response time. For a similar data rate, this represents a 10,000-fold faster response time than previous reports, thereby enabling mitigation for dynamic effects. In our approach, the transmitted pre-distorted phase-conjugate data beam is generated by four-wave mixing in a GaAs crystal of three input beams: a turbulence-distorted probe beam, a Gaussian reference beam regenerated from the probe beam, and a Gaussian data beam carrying a high-speed data channel. We experimentally demonstrate our approach in an 8-Gbit/s quadrature-phase-shift-keying coherent FSO link through emulated dynamic turbulence. Our results show an up to ∼10-dB improvement in the free-space-to-SMF coupling efficiency for the data beam under dynamic turbulence with a bandwidth of up to ∼260 Hz (Greenwood frequency). Our approach has the potential to significantly increase the resilience of high-performance coherent FSO links to turbulence.