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Emission properties of Er 3þ in Ga 2 S 3 -GeS 2 -Sb 2 S 3 glasses at the mid-infrared region were investigated from the viewpoint of their dependence on the concentration of the active ion and the glass composition. In the Judd-Ofelt analysis, no variation in omega parameters were observed when GeS 2 was replaced by Ga 2 S 3 , while O 2 increased as Sb 2 S 3 was replaced by Ga 2 S 3 . This is due to the structural similarity and difference between the glass network units, GaS 4 and GeS 4 tetrahedra, and SbS 3 pyramid. Clear mid-infrared emissions were observed at 2750 and 4300 nm assigned to the 4 I 11/2 ! 4 I 13/2 and 4 I 9/2 ! 4 I 11/2 transitions, respectively. The lifetime of the initial level of the 4.3 mm emission, 4 I 9/2 , rapidly decreased with the Er 3þ concentration because of the cross relaxation of this level, which can take place even at considerably low Er 3þ concentration. The cross-relaxation processes were suppressed by the increase in the content of Ga 2 S 3 because the solubility of Er 3þ ions in the glasses increases with the Ga 2 S 3 content.

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