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In solid state lasers a light source pumps the medium, which is a crystal. A crystal is excited and emits infrared light, until then everything is clear.

What I want to know: after excitement, what controls the flow of those photons emitted to oscillate in the cavity?

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  • $\begingroup$ I think you could undelete your deleted question, it is imho salvageable, but requires some discussion, how to make it more clear. Here is the list of your recently deleted questions. (Only you can see this list, I only can show its link.) $\endgroup$ – peterh Dec 7 '20 at 22:41
  • $\begingroup$ I guess the question is clear and crystal it concerning the size of the medium gain I did edited but still closed. $\endgroup$ – autodidact Dec 8 '20 at 10:26
  • $\begingroup$ Yes, but it is hard to understand your English. But your posts are comprehensible, but it requires a little thinking, what you want to say. This causes a downwind for you. You need to live with it together a little bit, until your English do not develop enough on the SE. Until then, do everything what you can to make your text the best possible. (Not I was the close voter.) $\endgroup$ – peterh Dec 8 '20 at 10:28
  • $\begingroup$ If your question is closed, it is not so bad: 1. you still can get useful information in the comments, 2. you have some chance to reopen it (only if you did not delete it). $\endgroup$ – peterh Dec 8 '20 at 10:31
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If a photon hits an already excited atom, it can make it to de-excite and also eject another photon. That is induced emission. This new photon will have the same properties (phase, direction) than the original one.

If more than 50% of the atoms are excited, this causes an exponential increase of the photons in the system, all in the same phase and direction.

The cavity length must be an integer multiple of the photon wavelength, with mirrors on both ends. (One of the mirrors can be a little bit transparent, to enable the beam to get out.) To get the laser beam, the photons must be reflected many times on the mirrors.

Photons whose direction was not exactly perpendicular to the mirrors, will soon leave the system (among with the other photons they made with the induced emission). This is one of the losses which make laser beams highly ineffective.

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  • $\begingroup$ what control the phase and direction its what I want to know, for example, in ruby laser the light pumped to the medium gain is 90ْ to the phase and direction of the photons resonating between the two mirrors (cavity) $\endgroup$ – autodidact Dec 8 '20 at 10:22
  • $\begingroup$ Direction is controlled by that the photons not exactly perpendicular to the mirrors, leave the laser soon. Phase is controlled by two effects: 1. induced emission creates photons in the same phase as the original one, 2. the distance between the mirrors is an integer multiple of the photon wavelength, resulting that the photons reflected from the mirrors will have the same phase. $\endgroup$ – peterh Dec 8 '20 at 10:24

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