Ytterbium-doped fiber
Silica fiber doped with Yb3+ ions, the gain medium behind most high-power fiber lasers and amplifiers around 1 micron. A two-manifold level scheme with a quantum defect as small as five percent makes it the most power-efficient widely used laser gain medium.
Ytterbium in silica is a spectroscopist's dream by way of being boring: the Yb ion has exactly two level manifolds within reach, the ground manifold and the excited manifold. There is no higher level to absorb from the excited state, no pathway for the concentration quenching that plagues erbium at high doping, and nothing for a second pump photon to do. The ion absorbs near 915 or 976 nm and emits between roughly 1010 and 1120 nm, and that is the whole story.
Why it wins on power. The quantum defect, the fraction of pump energy lost as heat, is : pumping at 976 nm and lasing at 1030 nm wastes only 5.2 percent. Combined with the clean level scheme, this allows slope efficiencies above 80 percent and, just as importantly, very little heat per watt of output, which is what ultimately limits any high-power laser. Erbium, by comparison, throws away about a third of its pump energy; see the EDFA entry for why telecom accepts that.
Pumping. The 976 nm absorption peak is strong but only a few nanometers wide, so it wants wavelength-stabilized diode pumps; the 915 nm band is several times broader and forgiving, at the cost of lower absorption. High-power systems use double-clad fiber: the doped single-mode core sits inside a large multimode inner cladding that carries the pump, so cheap, low-brightness diode light is converted, watt by watt as it crosses the core, into diffraction-limited signal. A fiber laser is, in this sense, a brightness converter.
How far it scales. Near-single-mode continuous-wave outputs beyond 10 kW have been demonstrated from a single fiber, and combined industrial systems reach the 100 kW class; ytterbium fiber lasers now dominate industrial metal cutting and welding. The practical ceilings are stimulated Raman scattering, photodarkening (a slow, Yb-specific loss increase linked to charge-transfer states), and above all transverse mode instability, a thermally written grating that suddenly transfers power from the fundamental mode to higher-order modes at multi-kilowatt levels.
Ultrafast side. The same broad gain supports pulses of about 100 fs, and ytterbium fiber chirped-pulse amplifiers have become the default engine of industrial femtosecond machining and a common basis for mode-locked frequency combs at 1 micron.
References: Richardson, Nilsson & Clarkson, J. Opt. Soc. Am. B 27, B63 (2010); Jauregui, Limpert & Tünnermann, Nat. Photonics 7, 861 (2013).