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

Excited-state absorption (ESA) from the metastable levels (4)I(13/2) and (4)I(11/2) Of erbium is measured ill 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 x 10(-21) cm(2) from the (4)I(13/2) level and less than 0.1 x 10(-21) cm(2) from the (4)I(11/12) level are in reasonable agreement with former results obtained from a rate-equation simulation of the erbium 3-mu m laser. The corresponding ESA spectrum under 3-mu m 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-mu m double-clad tiber laser.