How does a laser produce coherent light?

1 Answer
Dec 20, 2014

A laser produces coherent light through a process known as STIMULATED EMISSION.

You may know that light is the result of spontaneous emission of a photon by an electron undergoing a transition in an atom.

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As you can see in the picture the electron (that previously absorbed energy and jumped to the higher level) returns to the lower level almost immediately emitting a photon of light of frequency proportional to the gap between the 2 levels (can be a red, green, yellow....photon);
In white light, like from the Sun, you have electrons doing a lot of different spontaneous rapid transitions at different times so you have all the colors (frequencies) and phases (incoherent light).

In a laser you have only a color (only one frequency) and all the photons are basically...twins!!!! They all have the same laser light is highly coherent.
To achieve this you must use a substance with kind of "parking" levels where the electrons can stay for a while (they are called metastable).
When a lot of electrons end up parked on these parking levels they can then fall all together emitting all the "same" kind of photons.

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In the picture you can see how the atoms of the lasing material ended up having their electrons parked in the metastable level #m_a#. These electrons stay there for a while and can build up a good sized population (population inversion). It is sufficient a little kick (a thermal stimulation or an incoming photon) to make all the electrons fall down to metastable level #m_b# emitting photons all of the same frequency and phase.
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