Rare Earth Doped Fiber

Most popular solid-state gain media are the rare-earth-doped laser crystals and glasses. These media are doped with rare earth ions – most commonly trivalent and rarely divalent ions (in special laser devices). The rare earth ions replace other ions of similar size and same valence in the host medium. For example, an Nd3+ ion in Nd:YAG (yttrium aluminum garnet) substitutes an yttrium (Y3+) ion.

Table: Common laser-active rare earth ions and host media and important emission wavelengths.

ION	COMMON HOST MEDIA	IMPORTANT EMISSION WAVELENGTHS
neodymium (Nd3+)	YAG, YVO4, YLF, silica	1.03–1.1 μm, 0.9–0.95 μm, 1.32–1.35 μm
ytterbium (Yb3+)	YAG, tungstates, silica	1.0–1.1 μm
erbium (Er3+)	YAG, silica	1.5–1.6 μm, 2.7 μm, 0.55 μm
thulium (Tm3+)	YAG, silica, fluoride glasses	1.7–2.1 μm, 1.45–1.53 μm, 0.48 μm, 0.8 μm
holmium (Ho3+)	YAG, YLF, silica	2.1 μm, 2.8–2.9 μm
praseodymium (Pr3+)	silica, fluoride glasses	1.3 μm, 0.635 μm, 0.6 μm, 0.52 μm, 0.49 μm
cerium (Ce3+)	YLF, LiCAF, LiLuF, LiSAF, and similar fluorides	0.28–0.33 μm

Active fibers not only guide light, but also provide laser amplification. In these fibers, some amount of rare earth ions is incorporated into the fiber. The steps involved in active fiber amplification are:

	1	The pump light, typically at a shorter wavelength than the signal to be transmitted, is injected into the fiber.
	2	The laser-active ions absorb the pump light and get excited into some metastable states.
	3	The excited ions can now amplify signal light via stimulated emission.

Figure: Energy level structure of the trivalent erbium ion, and some common optical transitions.

Wide range of methods exist for the active optical fiber fabrication. The fabrication methods broadly classified as:

	1	Preform-based methods which is the most important method for the production of glass fibers
	2	Direct fiber production methods which is the oldest fabrication technique. It is less suitable for producing ultra-pure fibers with very low losses. This is because it is difficult to avoid any contamination with material from the crucible. This method is commonly used for fabrication of plastic optical fibers. An example is fabrication based on extrusion