![]() ![]() ![]() Since data are encoded as distinct vectors, QAM I/Q constellation points can be arranged in a two-dimensional array 4. Alternatively, 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. Often, an amplitude-only-modulated Gaussian data beam (for example, as in pulse-amplitude modulation (PAM)) is transmitted and recovered 2 since data are encoded as distinct amplitude levels, the data constellation points of PAM lie on a one-dimensional line in the two-dimensional in-phase (I) and quadrature (Q) constellation 4. We demonstrate a 12 Gbit s −1 16-QAM polarization-multiplexed free-space optical link that is resistant to turbulence.Ĭompared 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. 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. Subsequently, a photodetector mixes all corresponding pairs of the beams’ modes. 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. Here, we use a pilot-assisted self-coherent detection approach to overcome this problem. 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. 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.
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