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This question already has an answer here:

As far as the working of lasers is concerned, I was convinced till the part that the light flashes on a ruby stone and the electrons are excited and then again go back to their ground state and then they emit the photons(stimulated emission). This emitted photons in turn are absorbed by electrons of other atoms and this continues...

The photons emitted are emitted in all the directions. But they are reflected again and again such that they all get in a straight line. But, each reflection would have taken up some energy from photon hence, wavelength would change. And it is not certain that all the photons will undergo same number of reflections and it isn't certain either that all the excited electrons will undergo same transitions. Then how do we get resultant light that is of one wavelength.

Nor, am I convinced that all the emitted photons will be in phase with each other. That is coherence, right?

P.S.: I am not getting this concept well :(

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marked as duplicate by CuriousOne, Carl Witthoft, user36790, ACuriousMind, Gert Feb 17 '16 at 3:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ There is some overlap, but this question is more basic $\endgroup$ – Peter Diehr Feb 16 '16 at 11:11
  • $\begingroup$ You have made some invalid assumptions and that isn't helping you. Photons are emitted in many directions; the ones not emitted along the cavity axis are "lost" ; the ones which are produced via stimulated emission follow the phase and direction of the stimulating photon. And most certainly reflection does not change the photon energy $\endgroup$ – Carl Witthoft Feb 16 '16 at 13:32
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It's the cavity mirrors which control the repetitions of the lasing process. You need a material with an appropriate energy structure: susually a multi-level system which is easy to excite, but slower to decay back to the ground state, otherwise the statistics are against you.

Then you need a way to pump in energy, otherwise nothing happens.

Then you need the cavity mirrors: at one end place the best mirror you can find, close to 100% refective; at the other end use a less perfect mirror, so that some of light can escape.

Now the pump sets the stage: excites the system, and puts energy into the meta stble state. These states decay randomly - random times and random directions. This is called spontaneous emission. Many of these emissions are lost due to going in bad directions, absorption by defects, etc. The pump keeps filling the empty states.

But some spontaneous emissions hit the cavity mirrors at angles which send them back and forth trough the cavity ... and of these we are interested in the ones that pass close to other excited atoms, for then they can stimulate emission of identical photons.

It is the stimulated emission which generates the coherent output, for they have identical momentum and phase. And these, in turn, stimulate more photon emissions. It is a cascade.

So crystal or gas, you need a material to work with; you need a pump to supply energy; and you need cavity mirrors. This the simplest form for a laser.

Added: the cavity length determines the supported, stable modes of the lasing process. Wavelengths that "fit" are enhanced, much like tuning a musical instrument. Cavity design is an important element of laser performance.

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  • $\begingroup$ Yet, it didn't answer why the wavelength is the same... We get a particular distinct color laser .. Therefore, it is sure that the wavelength is the almost* same. But why? $\endgroup$ – brainst Feb 16 '16 at 12:30
  • $\begingroup$ I added a note on wavelength selection through cavity tuning. There are a lot of things that can be done, including the use of materials that absorb certain undesired wavelengths, to use of mirror coatings to enhance reflection of the preferred wavelength - but cavity length is the most basic. $\endgroup$ – Peter Diehr Feb 16 '16 at 13:36

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