Electromagnetic Propagation If we were able to generate a perfectly collimated laser in vacuum, would it have any propagation loss due to distance?
 A: There is no solution to Maxwell’s equations that represents a perfectly collimated laser beam. Even in the optimal case there will be diffraction from the aperture. 
The actual solution to Maxwell’s equations that is closest to what you describe would be a simple plane wave. You can think of it as a laser with perfect coherence and an infinitely large aperture so there is no diffraction. A plane wave in vacuum would indeed have no propagation loss in free space. 
A: Look at Fraunhofer diffraction.
"Fraunhofer diffraction deals with the limiting cases where the light appoaching the diffracting object is parallel and monochromatic, and where the image plane is at a distance large compared to the size of the diffracting object."
It pretty much fits your conditions.
$y =$ width of the center peak.
$D =$ distance from slit to measured plane.
$y^2$ is proportional to $D^2$
 plus a small constant.
Consider the end of the laser to be the slit.
It has to spread out. The wider the slit the slower it spreads. 
The shorter the wavelength the slower it spreads.
But it spreads.
The math assumes a plane wave and the laser probably isn't. But we can approximate it as a sum of plane waves with different diameters at the slit, and each of them spreads. 
