A very low energy-absorption coating and related methods for use with high power CO2 lasers includes at least a thin layer of ThF4 (16) used as a water/moisture barrier, in combination with BaF2 (14), for various optical elements. When used in connection with a ZnSe substrate (12) or any other suitable material (such as for a focusing lens), the coating (10) extends the useful life by helping prevent moisture adsorption that otherwise may occur within 2-3 days of contact with air. The coating (10) may comprise (1) a BaF2 layer (14) of approximately optical quarter wave thickness, (2) a thin 200-300 Angstrom layer of ThF4 (16) used as a water barrier (3) a thin 1000-2000 Angstrom layer of ZnSe (18), and (4) an optional ZnSe layer (20) of optical half wave thickness. Among other applications, the coating (10) provides very low energy absorption for a 10.6 um CO2 laser at a value