Laser Interference: Why No Alternating Bright and Dark Light?

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SUMMARY

Laser light is produced through stimulated emission, resulting in coherent photons that maintain the same phase. This coherence prevents the formation of alternating bright and dark light patterns, as interference requires waves with differing phases. The width of the laser medium influences beam collimation, introducing diffractive effects that affect the transverse modes of the beam. Consequently, the smooth edges of these modes eliminate the classic diffraction rings typically observed with hard-edged apertures.

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  • Understanding of stimulated emission in lasers
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  • Basic principles of diffraction and transverse modes
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Physicists, optical engineers, and students studying laser technology and wave optics will benefit from this discussion.

kkmans
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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.
 
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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.
 
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.
 

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