If lasers are collimated, what causes them to decollimate? Their production system seems to suggest a completely linear, collimated light source, but they do spread out over large distances. The same holds for synchrotrons. Why does this happen?
[Physics] Lasers and Collimation
laser
Best Answer
There is a fundamental limit to the collimation of a laser due to diffraction. Assuming the laser beam profile is a uniform disk it will be diffracted to an Airy disk at large distances, and the angular spread is approximately given by:
$$ \theta \approx 1.22 \frac{\lambda}{d} $$
where $d$ is the beam diameter. Assuming a diameter of 1 mm, which seems a reasonable estimate for most lasers I've seen, you get an angular divergence of about 0.6 milliradians for 500nm light.
I know next to nothing about the design of lasers, but Wikipedia reports the divergence as commonly less than 1 milliradian, which fits with the above estimate.