Lasers phonon-terminated

The most common laser transition,has operated both pulsed and cw in crystals, however low temperatures are usually required. Phonon-terminated laser action has also been reported for Ho + in BaY2Fg (76). 

To obtain stimulated emission between two energy levels, a population inversion is necessary. This is usually achieved by excitation into a third level (or levels) which rapidly and efficiently transfers its energy to a metastable upper laser level. A generalized energy level scheme for laser action is shown in fig. 35.3. If the terminal laser level is the ground state, then more than one-half of the ions must be excited to obtain an inverted population. If, instead, the terminal level 2 is above the ground state, then only an excited-state population sufficient to overcome the Boltzmann thermal equilibrium population in the terminal level is needed. This reduces the pumping requirements. In phonon-terminated lasers, level 2 is a vibrational-electronic state. The four-level laser scheme depicted in fig. 35.3 is representative of that employed for most rare earth lasers. 

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