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. Therefore, such lasers are often suitable only for pulsed operation, unless additional measures are taken to depopulate the lower laser level (see below). That limitation of such lasers has also led to the term transient laser.
Another examples for self-terminating lasers are the copper vapor laser, as described in the article on metal vapor lasers, and nitrogen lasers.
Achieving Continuous Laser Operation
For achieving continuous-wave operation of a laser with a self-terminating transition, one needs to implement a sufficiently effective process for depopulating the lower laser level. Several techniques with that purpose have been implemented:
- One may introduce an additional dopant which can quench the population of the lower laser level by means of an energy transfer. In the above-mentioned case of the 2.7-μm erbium laser, 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.
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|||E. Poppe et al., “980 nm diode-pumped continuous wave mid-IR (2.7 μm) fiber laser”, Electron. Lett. 34, 2331 (1998), doi:10.1049/el:19981582|
|||B. Srinivasan et al., “High-power 'watt-level' CW operation of diode-pumped 2.7 μm fiber lasers using efficient cross-relaxation and energy transfer mechanisms”, Opt. Express 4 (12), 490 (1999), doi:10.1364/OE.4.000490|
|||S. D. Jackson et al., “Diode-pumped 1.7-W erbium 3-μm fiber laser”, Opt. Lett. 24 (16), 1133 (1999), doi:10.1364/OL.24.001133|