sentences:  Just prior to firing, the glass is "pumped" to a high-energy state with a series of xenon flash tubes, causing a population inversion of the neodymium (Nd) atoms in the glass.This readies them for amplification via stimulated emission when a small amount of laser light, generated externally in a fibre optic, is fed into the beamlines.The glass is not particularly effective at transferring power into the beam, so to get as much power as possible back out, the beam is reflected through the glass four times in a mirrored cavity, each time gaining more power.