This work addresses the possibility of a high average power coherent light source in the 4.8 micrometers atmospheric window. The approach is to frequency-double the 9.55 micrometers CO2 laser line, yielding 4.775 micrometers . In order to achieve efficient conversion, the intensity of the laser is increased by intracavity resonant modulation, or 'mode-locking'. Efficient conversion in available lengths of doubling material requires an intensity of at least 20 MWcm-2. However, the most efficient CO2 lasers operate with a pulse duration of at least 1 microsecond(s) ec, the characteristic time for energy transfer from N2 to CO2 at 1 atmosphere. Without modulation a fluence of at least 20 Jcm-2 would therefore be needed for high overall system efficiency. With modulation at a 1:10 or better mark-to-space ratio, this fluence is reduced to the 2 Jcm-2 range that typifies the surface damage threshold of available materials. The doubling material chosen for this work was AgGaSe2, silver gallium selenide. A relatively long crystal (35 mm) was used in Type 1 phase-matching. A mode-locked pulse train was generated using a TEA CO2 laser pumped for a duration of 3 microsecond(s) ec, containing 1 nsec pulses spaced by 40 nsec. The 9.55 micrometers line was selected either by injection or by the use of an intracavity grating.