Self-terminating Laser Transitions
Some solid-state gain media have laser transitions where the lower laser level is metastable and has a long lifetime – even longer than the upper-state lifetime. An example of this situation is the 2.7-μm transition (4I11/2 → 4I13/2) of erbium (Er3+) in fluoride fibers. Here, neither multi-phonon transitions nor the pump radiation can depopulate the lower laser level. In that case, the laser transition is usually self-terminating: lasing is possible only for a short while, until the lower level is so strongly populated that a population inversion is no longer possible. After that, the gain medium requires some time without pumping, until lasing can be restarted.
Sometimes it is possible to cure that problem with some additional dopant which can quench the population of the lower laser level by means of an energy transfer. In the above-mentioned case, praseodymium (Pr3+) codoping has proven to be a good solution [2, 3]. An alternative, but less successful solution is to use a high erbium doping concentration . This introduces various cross-relaxation processes, which can depopulate the lower laser level but also affect the upper-state population. Also, it is sometimes possible to arrange for cooperative lasing from the lower laser level to a still lower energy level.
Self-terminating transitions can also occur with gas lasers, e.g. in nitrogen lasers.
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