ABSTRACT
Rare earth ions have a long history in optical and magnetic applications. Among these, luminescent devices using single crystals, powders, and glasses have been particularly important. Rare earths have important characteristics that distinguish them from other optically active ions: they emit and absorb over narrow wavelength ranges, the wavelengths of the emission and absorption transitions are relatively insensitive to host material, the intensities of these transitions are weak, the lifetimes of metastable states are long, and the quantum efficiencies tend to be high, except in aqueous solutions. These properties all result from the nature of the states involved in these processes and lead to excellent performance of rare earth ions in many optical applications. Devices that provide gain, such as lasers and amplifiers, must have low scattering losses, and one is restricted to using single-crystal or glass hosts. Whereas in many applications crystalline materials are preferred for reasons that include higher peak cross sections or better thermal conductivities, the versatility of glasses and the broader emission and absorption spectra they provide have led to the use of rare earth doped glasses in many applications. Nowhere is this more true than in the area of optical fiber devices. Table 1 lists the wavelengths, transitions, and features of fiber lasers and amplifiers that had been reported as of 1991. Oxide and fluoride hosts are distinguished in the table, because the latter provide more metastable states and greater transparency at wavelengths beyond 2000 nm. The high intensities and long interaction lengths made possible by fiber waveguides make these devices vastly superior to their bulk glass counterparts
Type of host
rable in performance with laser-pumped, bulk, crystalline lasers, and as continuous-wave (cw) amplifiers they are even better. The major exception is in applications requiring highenergy pulses, for which the small volume of the optical fiber cannot provide as much energy storage as bulk media.
