RF-assisted coherent detection based continuous variable (CV) QKD with high secure key rates over atmospheric turbulence channels
In this invited paper, we experimentally demonstrate high spectral efficiency and large capacity featured free-space optical (FSO) transmission system by using LDPC coded quadrature phase shift keying (QPSK) combined with orbital angular momentum (OAM) multiplexing. The strong atmospheric turbulence channel is emulated by two spatial light modulators on which four randomly generated azimuthal phase patterns from Andrews' spectral density are recorded. The validity of such approach is verified by reproducing the intensity distribution and irradiance correlation function (ICF) from the full-scale simulator. Excellent agreement of experimental, numerical, and analytical results is found. To reduce the azimuthal phase distortion induced by the turbulence emulator, the inexpensive wavefront sensorless adaptive optics (AO) is used. To deal with remaining channel impairments a properly designed large-girth low-density parity-check (LDPC) code is used. To further improve the aggregate data rate, the OAM multiplexing is combined with DWDM and 500 Gb/s optical transmission over the strong atmospheric turbulence channels is demonstrated. By employing both polarization states and frequency-locked lasers, beyond 1 Tb/s serial optical transmission over strong atmospheric turbulence channel can be achieved with the proposed FSO scheme.