Automatic mitigation of dynamic atmospheric turbulence using optical phase conjugation for coherent free-space optical communications
Huibin Zhou, Xinzhou Su, Yuxiang Duan, Yue Zuo, Zile Jiang, Muralekrishnan Ramakrishnan, Jan Tepper, Volker Ziegler, Robert W. Boyd, Moshe Tur, Alan E. Willner
Abstract
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., <1Mbit/s), or (2) a relatively high data rate of 2-Gbit/s but at a slow response time (e.g., >60s). 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 (<5ms) 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 ∼260Hz (Greenwood frequency). Our approach has the potential to significantly increase the resilience of high-performance coherent FSO links to turbulence.