we report on the experimental implementation of single-frequency fiber-laser pumped mid-infrared (mid-ir) transmitter and receiver modules for free-space communications. these modules enable frequency upconversion and downconversion between the 1550-nm telecom wavelength and the mid-ir, thus providing essential free-space transmission links with mid-ir single-frequency lasers in the 3.6 mu m region. specifically, based on difference frequency generation (dfg) in mgo-doped periodically poled linbo3 (mgo: ppln), the mid-ir transmitter produces 9.3-mw power at 3594 nm with 5-w pump power at 1083 nm (< 10 khz linewidth) and 3-w signal power at 1550 nm (< 10 khz linewidth), and the mid-ir receiver reproduces 12-mu w power at 1550 nm with 4.7-w pump power at 1083 nm and 5-mw laser at 3594 nm. the whole modules are integrated into portable and compact devices by incorporating single-frequency fiber lasers, fiber amplifiers, dfg units, and related electronic circuits. in addition, the uses of all polarization-maintaining fiber configuration and wellcontrolled heat dissipation make the mid-ir transmitter and receiver exhibit a long-term stability. (c) 2017 optical society of america