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We report on an erbium cascade laser in a fluorozirconate fiber. Lasing on the transition 4I11/2→4I13/2 at 2.71 μm is supported by colasing on the transition 4S3/2→4I9/2 at 1.72 μm. This recycles the excitation that is lost via excited-state absorption and avoids the saturation of the output power. Threshold at 2.71 μm is 33 mW launched pump power at 791 nm. The measured slope efficiency of 22.6% is relatively close to the 29.1% stokes-efficiency limit. An output power of 158 mW is obtained, limited only by the 1.43 W power available from the Ti: sapphire pump laser. Output power is 15 and slope efficiency 2.5 times higher than reported in previous publications.

Excited-state absorption (ESA) from the metastable levels 4 I 13/2 and 4 I 11/2 of erbium is measured in a fluorozirconate fiber in the wavelength range 780-840 nm. Using a pump-and probe-beam technique and choosing the pump wavelength such that the perturbation by pump ESA is minimized in the measurement, it is possible to determine the effective ESA cross sections, despite the fact that the excitation is distributed among two metastable levels. The derived ESA cross sections at 793 nm of 1.4 × 10 −21 cm 2 from the 4 I 13/2 level and less than 0.1 × 10 −21 cm 2 from the 4 I 11/2 level are in reasonable agreement with former results obtained from a rate-equation simulation of the erbium 3-µm laser. The corresponding ESA spectrum under 3-µm lasing conditions is derived. At the strongest ground-state absorption around 799 nm, decreasing ESA from the 4 I 13/2 level is compensated by increasing ESA from the 4 I 11/2 level, i.e., ESA losses cannot be avoided when pumping around 800 nm. This result is of relevance for possible high-power diode pumping of an erbium 3-µm double-clad fiber laser.

We report on an upconversion cascade laser in an erbium-doped ZBLAN fiber emitting simultaneously on the three transitions 4 S 3/2 ! 4 I 9/2 at 1.7 mm, 4 I 11/2 ! 4 I 13/2 at 2.7 mm, and 4 I 13/2 ! 4 I 15/2 at 1.6 mm. At moderate pump powers, the laser transition at 1.6 mm supports 2.7-mm lasing and permits a slope efficiency at 2.7 mm of 15% versus launched pump power. Above the threshold of upconversion lasing at 1.7 mm, the slope efficiency at 2.7 mm increases to 25.4%. Taking pump excited-state absorption into account, this value represents more than 90% of the theoretical slope efficiency. A transversely single-mode output power of 99 mW is achieved at 2.7 mm.

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