Abstract

Free-space optical (FSO) communications has potential advantages of higher data capacity and lower probability of interception, when compared to radio-frequency communications. However, atmospheric turbulence generally limits performance of FSO links because it induces modal coupling from the fundamental Gaussian mode to many higher-order Laguerre-Gaussian (LG) spatial modes. We review pilot-assisted self-coherent (PASC) approach that can enable turbulence-resilient FSO communications. In PASC, a frequency-offset continuous-wave pilot beam is co-transmitted with the data beam and eventually the two beams are optoelectronically mixed at receiver's photodetector (PD). During square-law mixing in PD, a turbulence conjugate distortion is automatically generated and applied to the distorted data beam. Thus, all the spatial modes can be efficiently mixed between pilot and data beams. As a result, the recovered data quality is not severely affected by turbulence-induced modal coupling effects. We also review the extended applications of PASC approaches, including: (a) increasing spectral efficiency by Kramers-Kronig detection; (b) enhancing system bandwidth by PASC with a PD array; (c) improving PD bandwidth utilization by PASC with self-homodyne detection. Finally, we discuss the enhanced misalignment tolerance by PASC in FSO links.