Cross-sections of excited-state absorption at 800 nm in erbium-doped ZBLAN fiber

Excited-state absorption (ESA) fromthe metastable levels 4I13/2 and 4I11/2 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 1.4x10−21 cm2 from the 4I13/2 level and less than 0.1x10−21 cm2 from the 4I11/2 level are in reasonable agreement with former results obtained from a rate-equation simulation of the erbium 3-um laser. The corresponding ESA spectrum under 3-um lasing conditions is derived. At the strongest ground-state absorption around 799 nm, decreasing ESA from the 4I13/2 level is compensated by increasing ESA from the 4I11/2 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-um double-clad fiber laser.