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In regular laser, we need:

  1. stimulating photon => this makes the output light coherent
  2. ways to make inversion population
  3. resonant cavity

in the pictures about semiconductor laser, I do not see any incoming photon. I checked it in the book Solid state devices by Banerjee and in the internet. Does semiconductor laser need stimulating photon to produce coherent light?

if it does, what is used to produce that stimulating photon?

if it does not, how is a coherent light output produced?

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    $\begingroup$ You need the first randomly emitted photon into a cavity mode to stimulate the rest. Plenty of other laser types have the same issue. $\endgroup$ – Jon Custer Jul 7 at 2:38
  • $\begingroup$ Could you explain more or direct me to reading source about this? $\endgroup$ – Codelearner777 Jul 7 at 2:41
  • $\begingroup$ The answer below is correct. You always have spontaneous emission to kick start things. $\endgroup$ – Jon Custer Jul 7 at 14:36
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The excited medium in any laser (including a semiconductor laser) produces spontaneous emission of photons in random directions. Only photons that happen to have exactly the right direction (and wavelength) to fit precisely into the lasing cavity will be amplified. But as soon as just one photon "fits", it is amplified and its progeny - the photons whose emission is stimulated by the interaction of a passing photon with an excited atom or molecule in the medium - very quickly constitute the greater part of the photon flux within the cavity.

If there is no cavity but the medium has some long optical paths through it, photons may be amplified while they are on those paths, resulting in what's called "superradiance".

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  • $\begingroup$ thank you for your answer. after convincing myself with your answer, now I am thinking the other way around about why the regular laser need stimulating photon. there should be some spontaneous emission as well in regular laser. it also has defined resonant cavity length. by using the same logic with semiconductor laser like the one in your answer, can regular laser work without stimulating photon? $\endgroup$ – Codelearner777 Jul 7 at 13:58
  • $\begingroup$ At least one photon is required to start lasing. It can be a spontaneously emitted photon, or can be a photon from an outside source, but it needs the right wavelength and direction. $\endgroup$ – S. McGrew Jul 7 at 15:19

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