How can net output of photons in laser occur?

The way I see it, in both spontaneous and stimulated emission, one atom being de-excited would result in one photon being produced. So if we excite $10^{23}$ atoms, only $10^{23}$ photons would be produced when they de-excite. How can there be a net output of photons then? How is it that lasers can be sustained for a long time at high intensities?

Also, my notes state that when a system is in equilibrium, the rate at which atoms are excited is equal to the rate at which the atoms are de-excited. What is meant by equilibrium in this context?

• Perhaps I'm misreading your first paragraph but just to be clear: In stimulated emission one photon comes in and two photons of precisely the same frequency leave. In the process one atom is de-excited. This is where the photon gain comes from. Jan 12 '14 at 18:59

• But isn't the only benefit to the chain reaction coherence? Without metastable states, the multiple different energy transitions directly caused by the pumping will lead to multiple photons that are not coherent. But each photon will still lead to the excitation and de-excitation of an atom $10^8$ seconds later. A chain reaction might cause more rapid de-excitation but the de-excitation can't be faster than the excitation due to pumping, can it? If it is, the laser won't be sustainable. Am I right? Nov 26 '13 at 11:55