Abstract
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.
Highlights
Compared with radio, free-space optical (FSO) communications have gained substantial interest due to their higher data capacity and lower probability of interception[1,2,3]
Owing to a random spatial and temporal refractive index distribution, the turbulence effects can induce a transverse, spatially dependent wavefront distortion to the Gaussian beam[27]. Since such distortion induces modal power coupling, the electrical field of the data beam (Edata) at the receiver aperture can be expressed as a superposition of LG modes[12,28]:
A turbulent Intensity modulation/direct detection (IM/DD) FSO link (that is, S(t,f) is amplitude-only encoded) may suffer from turbulence-induced modal-coupling loss if an single-mode fibre (SMF)-coupled PD is used because higher-order modes are not efficiently captured by the SMF13
Summary
Free-space optical (FSO) communications have gained substantial interest due to their higher data capacity and lower probability of interception[1,2,3]. FSO systems can benefit from simultaneously recovering the data beam’s amplitude and phase to enable complex modulation formats[5,6] such as quadrature amplitude modulation (QAM)[7]. FSO systems can recover both amplitude and phase by using coherent detection, which mixes the data beam with a receiver Gaussian local oscillator (LO) beam[5,9,11]. Atmospheric turbulence generally limits coherent detection because it induces power coupling of the data beam from the Gaussian mode to other Laguerre–Gaussian (LG) spatial modes[12,13,14].
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