“…Their CW or average power outputs also range from the highest (>9 kW from C 0 2 [58]) to the "lowest" (that is, the lowest of practical interest, the stabilized He-Ne laser used for metrological applications [59]). They utilize an amazing variety of pumping methods, e.g., electrical excitation in the form of glow discharges, both R F and dc excited (He-Ne and C 0 2 ) ; arc discharges, RF, dc, and pulsed (ion lasers) ; special discharges (the ultraviolet Nz laser [60] and recent types of C 0 2 lasers [61]); optical excitation (He lamp pumping of Ce vapor [62] and CO flame pumping of C 0 2 [63]); chemical excitation (the H F laser [64] and the D F pumped .C02 laser [65]); thermal excitation (expansioninverted C 0 2 [66]) ; and various combinations (photodissociated CH,I [67], electrically formed HBr [68], etc.). Many of these gas lasers could be used for communication, but because of the wide selection of operational characteristics available to the user, only the most useful few have received the serious attention and development necessary to realize performances approaching their optimum as practical optical communication sources.…”