Does the power of a laser beam follow the inverse square law?
https://www.quora.com/Is-the-light-from-lasers-reduced-by-th...
ADDED. Yes at distances from the source greater than 3.3 meters
> (...) if you measure the intensity of a beam with a photodiode (say) 1 inch from the laser and then 10 inches from the laser, you will not find 1/100th of the light with your fixed area detector. If you do this same experiment with an incandescent light bulb and make your closest measurement far enough away (much, much greater than the size of the filament), you will observe this square law decrease.
[1] https://groups.google.com/forum/#!topic/sci.optics/P-8VmDlY4...
The real answer is, as usual, "it's complicated". But not very so.
The beam width is roughly constant with distance close to the focal point, so there the inverse square law does not apply. Far from the focal point, it's the 3D angle that's roughly constant, which means that the law does apply there.
A laser beam has a direction, as such doesn't spread its influence equally in all directions.