Optical Communications, 2nd Edition

9.5 ATMOSPHERIC PULSE SPREADING

Besides the power loss and beam breakup, the atmosphere may also distort the optical waveshape during propagation. This is particularly true for high bandwidth signals such as narrow optical pulses, in which the atmospheric scattering can cause a multipath effect similar to the dispersion in fibers. Scattered pulse fields may be reflected toward the receiver and combine to produce a distorted optical pulse shape. This can be envisioned by the simplified diagram in Figure 9.11, showing a direct plane wave ray line and several scattered paths. The scattered paths are redirected by the particle scattering, but some (paths p₁, p₂, p₃) are eventually scattered back to the collecting receiver. If an optical pulse is transmitted from the source, the pulse signals along the scattered paths arrive with delays relative to the direct path and combine to yield a wider, broadened optical field pulse from that transmitted. This channel dispersion effect is similar to that in fibers.

Figure 9.11. Direct and scattered paths with multiple scattering media.

Figure 9.12. Pulse spreading caused by clouds. α_sc = scattering coefficient. α_e = extinction coemcient.

Experimental studies [10, 11] have measured the pulse spreading after turbulent or cloud transmission. The amount ...