Data-format-independent all-optical transmitter and receiver modules for free-space optical communications in the $3.8\mu \mathrm{m}$ region have been developed, essentially consisting of Ti-indiffused, periodically poled $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ waveguides. Using these modules, conversion of C-band radiation to/from the mid-infrared (MIR) via difference-frequency generation was demonstrated. More than $10\mathrm{mW}$ of MIR power could be generated with the transmitter unit; the internal low-power conversion efficiency was $69\%∕\mathrm{W}$ . A free-space MIR link was set up with a $-41\mathrm{dB}$ fiber-to-fiber loss; only $-15\mathrm{dB}$ are due to the parametric processes.