If your beam is perfectly coherent, then its cross section has the same area regardless of the distance, so you should be getting full power. However, no beam i

If your beam is perfectly coherent, then its cross section has the same area regardless of the distance, so you should be getting full power. However, no beam is perfectly coherent, so it’s still O(1/r^2), and the better coherence, the smaller constant you have hidden in the big-O notation.