a lowest-order Gaussian-beam wave
We assume the transmitted wave is a unit-amplitude Gaussian beam that, in the plane of the emitting aperture of the transmitter at z =0, can be expressed as
where F0 and W0 denote the phase front radius of curvature and beam radius, respectively, and k is the optical wave number.
The free-space optical wave in the receiver plane at z = L is another Gaussian beam described by
注：高斯准直光束对应的F0 =inf，即θ0=1；而高斯聚焦光束对应的F0 =L，即θ0=,0，θ=0
the higher-order TEMmn Hermite-Gaussian beams
Laguerre-Gaussian beam 
