I'm curious about the properties of ruby. It does seem to glow when light hits it. Why do ruby lasers only seem to work with intense flash lamps? Can you not create a weak laser with a constant running light source?
Shorter answer: Population inversion (more electrons excited than not) is required for sustained stimulated emission. If the excitation time of the medium is short (which is governed by rather complex physics) and/or the energy source is insufficient to maintain this inversion, the resultant photon generation is incoherent (typically, fluorescent), not coherent.