Laser interference

[kkmans]

Laser emits due to the stimulated emission.
As the photons produce in this process are coherent, I would like to ask why they won't undergo interference and produce alternating bright and dark light when they just come out from the laser source.

thanks.

[daniel_i_l]

You answered the question yourself - because of the fact that they're coherent they all have the same phase everywhere - waves only interfere when they have different phases.

[JeffKoch]

In some cases there are oscillations, see http://en.wikipedia.org/wiki/Transverse_mode. The width of the laser medium will set some limit on how collimated the beam can be, which is a diffractive effect. The transverse modes have smoothly-sloped edges, so you don't see the classic diffraction rings you would see if you sent the beam through a hard-edged pinhole.