Excited State Absorption and Cross Sections for Er-Doped Glasses
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EXCITED STATE ABSORPTION AND CROSS SECTIONS FOR Er-DOPED GLASSES S. ZEMON, G. LAMBERT, W. J. MINISCALCO, L. J. ANDREWS, and B. T. HALL, GTE Laboratories Incorporated, 40 Sylvan Road, Waltham, MA 02254 ABSTRACT Pump-excited-state-absorption (ESA) measurements on Er 3รท-doped phosphates, fluorophosphates, and silicate bulk glasses indicate that ESA cross sections are approximately equal to ground state absorption (GSA) cross sections in the 800-nm band. The oscillator strengths of the ESA and GSA bands are also approximately equal, in qualitative agreement with Judd-Ofelt calculations. Fluorozirconate samples were found to have substantial populations in the upper excited states for the measurement conditions used and ESA transitions originating from four excited states were identified. Fluorozirconate fiber amplifiers and lasers at 1.55 gim, therefore, would have decreased efficiency for 800-nm pumping. INTRODUCTION Erbium has been shown to be an important dopant for fiber lasers and amplifiers operating at 1550 nm [1]. However, excited state absorption (ESA) of pump photons occurs in several of the principal pump bands and is a major impediment to device implementation [2, 3]. In particular, for Er3+-doped silica fibers pumped at = 800 nm, the absorption band appropriate for AlGaAs diode lasers, strong ESA seriously degrades performance [2, 3]. We present the results of ESA measurements on four glass compositions potentially useful for active fiber devices, i.e., phosphates, fluorophosphates, fluorozirconates, and silicates. To our knowledge, Er 3 +ESA has only been reported for silica [2,3] and fluoride [3] fibers. Data were taken on bulk material because of the difficulty of synthesizing fibers for a large number of different glasses. Saturation effects were used to estimate the population inversion and thus convert absorption data into cross sections. Data on ESA cross sections (Ge) and ground state absorption (GSA) cross sections (ag) are presented for the region between 780 and 900 nm, which includes ESA bands at =800 and =850 nm. Over much of the 800-nm band 0 e = ag for the phosphates, fluorophosphates, and silicates. Our results are in qualitative agreement with an analysis based upon the Judd-Ofelt approximation [4,5] which involved calculating ESA and GSA oscillator strengths. The fluorozirconates were found to have substantial populations in the first four excited states for our measurement conditions and ESA transitions originating from these states are identified. EXPERIMENTAL DETAILS The choice of glass types was guided by a Judd-Ofelt analysis [5]. The samples which were picked to be studied had a ratio of ESA to GSA oscillator strengths of =1, a factor of two improvement over the ratio found for silica fibers. The glass compositions are listed in Table 1 for the low-fluorine fluorophosphate (LG 11), high-fluorine fluorophosphate (LG14), silicate (LG22), phosphate (LG28), and fluorozirconate (FG 114) samples. The erbium concentrations (N0 ) are also included. The fluorophosphate, phosphate, and silica
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