I was reading about LASER production when I came by the concept of stimulated emission. The book I was reading doesn't elaborate the topic much,so far I have understood what stimulated emission is but not how it happens. When a photon strikes an excited atom the atom falls to it's ground state releasing another coherent photon. To me, what should happen is that atom should gain energy and jump to a higher state(this is what normally happens). Please elaborate this concept. Thank you.