A cladding-pumped doped optical fiber glowing on an optical table. Rare-earth-doped optical fibers are used for light amplification in telecommunications and fiber lasers. Under appropriate optical pumping conditions, population inversion can be achieved leading to the optical gain for the signal to be amplified. Cladding pumping technique was developed in the late 1990s and found its primary use in industrial high-power fiber lasers by the early 2000s. The blue light seen is spurious, very low-power fluorescence, whereas most of the optical power (invisible infrared) is confined within the fiber.