A rare earth doped fiber, in which trivalent ion of rare earth (RE) is doped into the core as an active material, has transition lines in the wavelength range below 3 μm as shown in Fig. 4.3 .

A doped amount of typical RE ion, Er3+, Nd3+, and Yb3+, is 100 ppm (part per million) or more (10,000 ppm (1%) or more). RE ion is incorporated into RE2O3 in the silica glass. For the high doping concentration, an RE distribution can be clustered (clustering). The clustering can be avoided to some extent by doping Al2O3, P2O5, or so on together, which makes RE ion easily melt. Doping of Al2O3 also smooths and expands the fluorescence spectrum. In order to increase the refractive index, GeO2 is added into the core.

Figure 4.3: Emission wavelength range for rare-earth-doped fiber

In this section, we describe the characteristics of rare earth doped fiber. Rare earth doped fiber is employed for Yb fiber laser and Er fiber laser, which is used in general. In particular, Yb doped fiber is explained in detail. Fig. shows Yb3+, Er3+, and Tm3+ energy structures, which principally contribute to the laser oscillation.

Figure 4.4: Energy level diagram of rare earth ions. (a) Yb3+, (b) Er3+, and (c) Tm3+

(1) Ionic interaction

When an optical fiber contains of a different or the same kinds of rare earth ions with high concentration, energy transfer with or without luminescence can occur among rare earth ions. In the energy transfer process, an ion, which emits the energy, is called a donor, while an ion, which receive the energy, is called an acceptor. The energy transfer can yield concentration quenching and intensified sensitivity.

(2) Tm doped fiber

By exciting Tm doped fiber with ca. 793 nm light, Tm fiber laser oscillating at 2 μμm range is obtained. Tm fiber laser is known as an eye-safe laser, because light in 1.4 2.6 μμm range is hard to reach at the retina. In the 2 μμm range, a biological cell or tissue has a local maximum of absorption by water, while there is an atmospheric window, in which the transmission property for the air is good). Therefore, Tm fiber laser is employed for medical laser mes, laser-based distance measurement system, light detection and ranging (LIDAR, lader system using light instead of radio wave), and range finder. In a recent Tm fiber laser, 53 % slope efficiency and 1 kW or higher average power at 2045 nm is realized.