Condition for keeping polarization invariant on propagation in space-to-ground optical communication downlink

We consider a Gaussian Schell-model (GSM) laser beam applied in space-to-ground optical communication downlink. Based on the derived analytic expression of cross-spectral density matrix of GSM laser beam, the sufficient conditions of the polarization properties remaining invariant through the space-to-ground optical communication downlink are obtained. The conditions reveal when the source light is linear polarization beam or the three coherence lengths ( δ_xx, δ_xy and δ_yy) are equal to each other, the polarization properties unchanged upon propagation. With full consideration of the coherence lengths and the zenith angle, the conditions of keeping polarization properties invariance is verified by numerical calculation.