Mature solid state pulsed mid-IR laser technology will be adapted to diode-pumped and CW operation to support GHz bandwidth free space optical communications (FSOC). A primary challenge is how to continuously depopulate the lower layer level while the system is producing laser output. Such depopulation is needed in order to achieve CW operation. We will develop approaches that preserve the highest possible overall efficiency, and that are compatible with scaling the laser output power to 5 watts during Phase II. Existing wideband mid-IR modulation techniques will be adapted to the FSOC application. Since currently available modulation techniques in this wavelength band have greater insertion loss than the more mature modulation techniques used in shorter wavelength FSOC systems, a master oscillator power amplifier approach will be considered for the laser transmitter to maximize overall system efficiency. Possibilities for spatial modulation, such as use of multiple orbital angular momentum states in the transmitted beam, will be considered in optimization of covertness and system power efficiency.
