Demonstration of Turbulence Resiliency in a Mode-, Polarization-, and Wavelength-Multiplexed Free-Space Optical Link Using Pilot-Assisted Optoelectronic Beam Mixing

We experimentally demonstrate turbulence-resilient free-space optical (FSO) coherent communications with multiple multiplexed data channels in different orthogonal domains, including mode, polarization, and wavelength. The turbulence resiliency is enabled by utilizing pilot-assisted optoelectronic mixing of the received beams, wherein one pilot is being mixed with its corresponding beam. By transmitting additional continuous-wave (CW) pilot tones, whose frequencies are offset from the data-carrying beams, and mixing all pilot tones and beams in a single free-space-coupled photodetector (PD) at the receiver, the turbulence-induced modal coupling could be efficiently suppressed. The paper first discusses an experimental demonstration of a turbulence-resilient mode-division-multiplexed (MDM) FSO communication link using two orbital angular momentum (OAM) beams. Each OAM beam carries an independent 2-Gbit/s quadrature-phase-shift-keying (QPSK) data channel. Experimental results indicate that (i) the channel crosstalk under both the weaker and stronger turbulence distortions are measured to be lower than ∼−25.7 dB, (ii) the turbulence-induced OAM modal coupling could be effectively suppressed for the emulated 15 random turbulence realizations, and (iii) the error-vector magnitudes (EVM) of the multiplexed QPSK data channels are measured to be lower than by 17.4% and 19.8% under the relatively weaker and stronger turbulence effects, respectively. The paper subsequently describes an experimental demonstration of a turbulence-resilient FSO link with eight 0.5-Gbit/s QPSK data channels multiplexing two OAM beams (
l
= +1 &
l
= −2), two polarizations, and two wavelengths. The measured bit-error rates (BERs) of all data channels under turbulence effects are below the 7% forward-error-correction (FEC) limit, with power penalties less than ∼3.7 dB.