There are level schemes of laser gain media where a laser transition is difficult to operate because the lower laser level has a high lifetime – possibly higher than that of the upper laser level. In such a situation, ions can accumulate in the lower laser level, thus stopping laser action soon after it started (→ self-terminating laser transitions). This can occur e.g. in some erbium- or thulium-doped heavy metal glasses (e.g. Er:ZBLAN, see also fluoride fibers). The problem can affect amplifiers in the same way as lasers.
Cooperative lasing (also called cascade lasing) constitutes one of several possibilities for solving this problem. Here, the lower laser level is depopulated by laser action on an additional laser transition to a still lower-lying energy level. This can be achieved by making the laser resonator so that it has low optical round-trip losses also at the wavelength of the additional laser transition. Ref.  presents an example, where cooperative lasing has permitted the operation of a fiber amplifier at 1.49 μm. Similarly, cooperative lasing has been applied to holmium  and erbium [2, 4] lasers.
|||M. C. Brierley et al., “Lasing at 2.08μm and 1.38μm in a holmium doped fluoro-zirconate fibre laser”, Electron. Lett. 245 (9), 539 (1988)|
|||M. Pollnau et al., “Three-transition cascade erbium laser at 1.7, 2.7, and 1.6 μm”, Opt. Lett. 22 (9), 612 (1997)|
|||S. Tessarin et al., “Tm3+-doped ZBLAN fibre amplifier at 1.49 μm with co-operative lasing at 1.88 μm”, Electron. Lett. 41 (16), 23 (2005)|
|||S. D. Jackson, “High-power erbium cascade fibre laser”, Electron. Lett. 45 (16), 830 (2009)|
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