I know excited atom can emit photon at any direction as long it stays consistent so the total angular momentum and spin states are conserved, this is spontaneous emission. What about stimulated emission? Does it means more photons of same angular momentum and spin are absorbed by the one atom so the only exit is pointing in one specific direction, right? Then the mirror is just to increase the chances for atom to become stimulated, that I know. If I shine laser at any atom do I made it a laser too?
A simple picture for stimulated emission is to imagine that each photon interacts with its surrounding atoms: If a photon flys by an excited atom (= atom, which could emit a photon) the interaction increases the chance that the atom emits a photon "in the same mode" as the "fly-by" photon. This increase of the chance is tiny, if we consider a single photon interacting with a single excited atom. However, if we have many photons in the same mode, these tiny increases add up and the probability that the excited atom emits its photon in the same mode becomes significant.
This is the key idea of stimulated emission. The mirrors are needed to
- reduce the number of allowed modes (see eigenmodes of a vibrating string with two fixed end-points),
- to ensure that the photons going back and forth. Since the increase of the probability (to emit the photon in the same mode) is so tiny, each photon should pass the atom several times. This increasing the effective number of photons in that particular eigenmode.
Just to make clear that the misconception is resolved: It's not the atom, which accumulates many photons "of the same kind", but the cavity (=the space between the two mirrors).
In a laser an external power source (usually electrical) puts many atoms in an excited state. Initially these emit (mostly identical) photons in random directions, but a pair of mirrors reflect a few back and forth. These stimulate emissions in the same direction, and the process builds rapidly until almost all emissions are in phase and are exactly in line with the reflections between the two mirrors. One of the mirrors is partially silvered and allows a fraction of the beam (consistent with the power source) to emerge.