In a fiber laser, the laser gain medium is a length of optical fiber. This gain fiber, with its core doped with rare earth ions, typically consists of an outer glass or polymer coating; an inner cladding or pump core, which acts as a waveguide for pump light; and the active doped core, which carries the signal light and which absorbs pump light carried in the inner cladding to amplify the laser signal.
This fiber geometry allows the lasers to be easily coupled to high-power, commercially available multimode diode lasers (with relatively low beam quality) as pump sources.
Commercial fiber lasers commonly use fiber Bragg gratings (FBGs) to form resonators, which avoids the use of free-space mirrors and eliminates the need for realignment during the fiber laser’s lifetime. The result is a simple, compact, and robust setup. Fiber lasers can operate in continuous wave (CW) or pulsed modes. Pulsing can be accomplished by passive or active mode-locking or Q-switching (using, for example, saturable absorbers such as SESAMs), or gain switching, in which the pump source itself is pulsed. In the 2 μm wavelength region, mode-locked Tm/Ho-based fiber lasers have achieved pulse widths approaching 100 fs, and Q-switched fiber lasers have achieved pulse widths in the tens of nanoseconds.
AdValue Photonics’ 2 μm fiber lasers use Tm-doped fiber to offer high output power with multiple pulse widths and spectral widths. The high output power is the result of the very high quantum efficiency of Tm-doped laser systems, and the mature technology of high-power GaAlAs laser diode pumps at 0.8 μm. In particular, Tm-doped fiber lasers can benefit from the phenomenon of cross-relaxation: because the energy of the pump photon is more than twice that of the laser transition, a single pump photon, at 0.8 μm, can lead to the excitation of two ions and generate two signal photons at 2 μm. With a sufficiently high doping concentration of Tm3+ ions (greater than 2 weight percent) in the gain medium, Tm-doped fiber lasers have shown quantum efficiency exceeding 100 percent in some experiments.
Summarized from: “Fiber Laser-The 2 µm Market Heats Up”, OPTICS & PHOTONICS NEWS JULY/AUGUST 2014, Page 34-41. Authors: Jihong Geng and Shibin Jiang, AdValue Photonics.