
COHERENCE 327
Figure 10.11 Speckle size in laser radar. A laser radar system with a diverging Gaussian
beam produces a footprint on a distant target that increases linearly with distance.
as in Section 10.4.3.1, the scattered light behaves like an incoherent source with a diameter
determined by the laser footprint on the target. The equation is the same as Equation 10.66,
but the values are quite different. Let us assume that the beam is Gaussian with a diameter
d
0
= 30 cm, from a carbon dioxide laser with λ = 10.59 μm. Then, at the target, using
Equation 9.23,
d
target
= d
0
1 +
z
2
t
b
2
, (10.68)
where z
t
is the distance from the laser transmitter to the target. ...