Optical Communications, 2nd Edition

9.2 EFFECT OF THE ATMOSPHERE ON OPTICAL BEAMS

The discussion in Section 9.1 indicates that radiation propagation at optical wavelengths through the atmosphere are susceptible to additional power losses, beam spreading, and possible beam breakup, depending on the channel conditions. These conditions are also dependent on the beam itself, as shown in Figure 9.5. Figure 9.5a shows the free-space condition in which an optical source produces a transmitter beam of angle θ_b and a power P_a transmitted over a distance Z. As discussed in Section 1.4, the free-space beam generates a beamfront area at Z of A_f = π(θ_bZ)²/4, and a corresponding field intensity inside the beam of P_a/A_f watts/area. A receiver area in the beam collects this field intensity.

Figure 9.5b shows beam propagation in a clear-air channel, exhibiting possible eddies and temperature gradients as field turbulence. As long as the beamfront area is smaller than the turbulence dimension (shown as a flat plane) the field beam is attenuated by the clear-air transmittance but is undistorted, except for possible beam redirection. This causes beam drift and beam defocusing, at the receiver plane, similar to mispointing. The beamfront intensity for uniform beams is now P_aL_a/A_f, where L_a is the wavelength-dependent clear-air transmittance loss and A_f is the wavefront area including spreading. For shaped beams (say Gaussian beams), this beam drift may cause receiver operation at beam edges, producing further power losses even though ...