Fiber laser
A laser whose gain medium is an optical fiber doped with rare-earth ions such as ytterbium, erbium or thulium, usually pumped by diode lasers through a surrounding cladding. The dominant high-power laser for industrial cutting and welding.
A fiber laser uses a length of doped optical fiber as its gain medium. Rare-earth ions in the core provide the gain: ytterbium around 1030 to 1080 nm, erbium around 1550 nm, thulium and holmium near 2 µm. The cavity is often formed by fiber Bragg gratings written directly into the fiber at each end, so the light never leaves the glass between the pump and the output. The geometry that makes the fiber laser distinct is the long, thin gain region: meters of gain in a core a few to a few tens of micrometers across.
That geometry solves the two problems that limit bulk solid-state lasers. Heat is spread over the whole length and removed through a large surface, so the thermal lensing that degrades a laser rod at high power is largely absent. The guided mode is set by the fiber, not by the thermal state of the gain medium, so the beam quality stays near diffraction-limited as the power rises. Ytterbium adds a small quantum defect: pumped at 976 nm and lasing at 1070 nm, 8.8% of each absorbed pump photon's energy becomes heat, and at 1030 nm only 5.2%. A 1 kW laser at 1070 nm with an 8.8% defect deposits about 96 W of heat, under 10 W per meter over a 10 m fiber, which forced-air or water cooling on a spool handles easily.
Getting pump power into a small core requires cladding pumping. The fiber is double-clad: the signal is guided in the doped core, while the pump light from multimode diode bars is launched into a much larger inner cladding, typically a few hundred micrometers across, and is absorbed gradually as it crosses the core over the fiber length. Optical pumping through the cladding converts many low-brightness diodes into one high-brightness, near-single-mode beam, and fiber lasers are sometimes described as brightness converters for that reason.
The limit on power is nonlinearity. At 1 kW in a 20 µm core, the intensity is about 320 MW/cm², and over meters of fiber that is enough to drive stimulated Raman scattering and, for narrow-linewidth light, stimulated Brillouin scattering, both of which divert power into unwanted wavelengths or back toward the source. Larger mode areas lower the intensity but invite higher-order modes, and above a threshold power the modes exchange energy thermally (transverse mode instability), which is the main ceiling on single-mode power from one fiber. Continuous-wave single-mode systems reach the 10 kW class; industrial multimode systems combine many such lasers.
Pulsed fiber lasers use the same fiber. Q-switched fiber lasers supply the nanosecond pulses used for marking and engraving; mode-locked fiber oscillators produce femtosecond pulses and are the usual source for optical frequency combs, often followed by fiber amplifiers of the same design. Narrow-linewidth distributed-feedback fiber lasers, with the grating written into a short doped section, serve sensing and coherent applications.
References: D. J. Richardson, J. Nilsson, W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010); M. N. Zervas, C. A. Codemard, IEEE J. Sel. Top. Quantum Electron. 20, 0904123 (2014).