Linked from
The 14 pages that link to Optical pumping, each with the reason it gives.
Fiber laserRelated: Pump light excites the rare-earth ions that supply optical gain.
Ruby laserRelated: A flashlamp pumps chromium ions into excited states before the laser pulse.
Solid-state laserRelated: Flashlamps or diodes excite the active ions in many solid-state lasers.
Nd:YAG laserRelated: Flashlamps or laser diodes optically excite neodymium ions in the crystal.