In normal radiation the atomic transitions from higher to lower states are random and the corresponding energy difference is spherically emitted in all the possible directions. In lasers a larger number of atoms are forced to occupy a lower energy state simultaneously, giving an intense light.
Although transitions are simultaneou but what physical process makes the spherical emissions to a prefered direction?
Due to Stimulated emission and it is purely direction dependent. When the light photon emitted during stimulated emission it will move back and forth inside the cavity whose length is adjusted such that integral multiples of wavelength so the light waves will remain in phase and comes out through partially silvered mirror and hence it will move in same direction.
@ Sreenatha N R
You are absolutely right Sir, this kind of explanation is given in almost all of the books on lasers, but still it doesn't gives the feel how spherical emission converts into linear?
Dear Suresh,
You might be interested in this paper about experiments revealing that ordinary light also remains coherent for a short period right after the moment of emission:
Quote from paper on experiments carried out by J.P. Wesley (page 2):
"It is observed that light from ordinary sources remains coherent for about 10-9 s, so the coherence spatially is about 1 m in length. In lasers, where a greater density of photons can be crowded into the source, the spatially ordered array propagated from such a source can be up to 100 m in length."
See .pdf of this paper attached.
This seems to open the possibility that electromagnetic emission might already be directional at the moment of emission.
It seems to only be an assumption that electromagnetic energy would be emitted as a spherical wavefront from the point of emission when electrons de-excite to settle in orbitals closer to nuclei, instead of being emitted as a permanently localized quantum in a precise direction.
You can find here a Maxwell equations compliant description of how such a permanently localized electromagnetic photon could be internally structured:
https://www.omicsonline.org/open-access/on-de-broglies-doubleparticle-photon-hypothesis-2090-0902-1000153.pdf
Best Regards
André
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