WHAT Ra? My laser's power decreases with the square in the distance travelled!!?
Yes.
As the laser beam diverges, the beam diameter is increasing while the power density decreases proportionally.
The inverse square law applies.
No, I am confusing the inverse square law with the beam divergence. qumefox is right. What I meant is that the power density decreases as the beam diverges. The beam divergence angle determines the ratio at which the W/m^2 is reduced.So a laser behaves like an ordinary antenna but with an enormous amount of gain. Ya?
If you use values at very short distances, it's not as close, but that is because the "point source" is a virtual point that's actually behind the laser. That is one reason we calculate divergence at the furthest distance possible - to make minimize the effect of the initial diameter.