A Tm-Ho co-doped optical fiber laser exhibits significant fluorescence between 1.6 and 2.1 micrometers . Such a fiber can from the basis of a gas detection system, since many gases of interest have overtone absorption bands in this wavelength region: in particular, carbon dioxide, methane, arsine, replacement gases for refrigerants, and nitrous oxide. Using this fiber we have demonstrated a simple shceme for the detection of overtone absorption bands of carbon dioxide in the 2 micrometers region. The detection sensitivity for carbon dioxide with this present method is of the order of 1%.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.