In free-space optical communications that use both amplitude and phase data modulation (for example, in quadrature amplitude modulation (QAM)), the data are typically recovered by mixing a Gaussian local oscillator with a received Gaussian data beam. However, atmospheric turbulence can induce power coupling from the transmitted Gaussian mode to higher-order modes, resulting in a significantly degraded mixing efficiency and system performance. Here, we use a pilot-assisted self-coherent detection approach to overcome this problem. Specifically, we transmit both a Gaussian data beam and a frequency-offset Gaussian pilot tone beam such that both beams experience similar turbulence and modal coupling. Subsequently, a photodetector mixes all corresponding pairs of the beams’ modes. During mixing, a conjugate of the turbulence-induced modal coupling is generated and compensates the modal coupling experienced by the data, and thus the corresponding modes of the pilot and data mix efficiently. We demonstrate a 12 Gbit s⁻¹ 16-QAM polarization-multiplexed free-space optical link that is resistant to turbulence.

在同时使用幅度和相位数据调制的自由空间光通信中（例如，在正交幅度调制 (QAM) 中），通常通过将高斯本地振荡器与接收到的高斯数据光束混合来恢复数据。然而，大气湍流会引起从透射高斯模式到高阶模式的功率耦合，导致混合效率和系统性能显着下降。在这里，我们使用导频辅助自相干检测方法来克服这个问题。具体来说，我们同时传输一个高斯数据光束和一个频偏高斯导频光束，使得两个光束都经历相似的湍流和模态耦合。随后，光电探测器混合所有相应的光束模式对。在混合过程中，生成湍流引起的模态耦合的共轭并补偿数据所经历的模态耦合，从而有效地混合导频和数据的相应模式。我们演示了一个 12 Gbit s⁻¹抗湍流的 16-QAM 偏振复用自由空间光链路。