A free-space optical communication system comprising a transmitter and a receiver, the transmitter being configured to transmit an encrypted message to the receiver at the mid-infrared domain, wherein: the transmitter comprises a master mid-infrared optical source configured to generate a mid-infrared signal and a chaos generator configured to generate a chaotic signal by applying external optical feedback to said master mid-infrared optical source, the transmitter being configured to determine said encrypted message from an original message, by applying a message encryption technique to said original message using said chaotic signal, and to send said encrypted message to said receiver through an optical isolator, the receiver comprising a slave mid-infrared optical source, said slave mid-infrared optical source being configured to recover the chaotic signal from the encrypted message by applying chaos synchronization, the receiver further comprising a first detector configured to detect the encrypted message, a second detector configured to detect the chaotic signal, and a message recovery unit configured to recover the original message from the encrypted message detected by said first detector and the chaotic signal detected by said second detector, wherein said external optical feedback consists in re-injecting a part of the emitted light by the master mid-infrared optical source back into the master mid-infrared optical source and wherein the chaos generator comprises a feedback reflector and a mid-infrared polarizer, said external optical feedback being obtained using one or multiple round-trips between the feedback reflector and an emitting facet of the master mid-infrared optical source, said mid-infrared polarizer being adapted to tune the amount of the external optical feedback, the angle of the mid-infrared polarizer defining the feedback strength. the transmitter comprises a master mid-infrared optical source configured to generate a mid-infrared signal and a chaos generator configured to generate a chaotic signal by applying external optical feedback to said master mid-infrared optical source, the transmitter being configured to determine said encrypted message from an original message, by applying a message encryption technique to said original message using said chaotic signal, and to send said encrypted message to said receiver through an optical isolator, the receiver comprising a slave mid-infrared optical source, said slave mid-infrared optical source being configured to recover the chaotic signal from the encrypted message by applying chaos synchronization, the receiver further comprising a first detector configured to detect the encrypted message, a second detector configured to detect the chaotic signal, and a message recovery unit configured to recover the original message from the encrypted message detected by said first detector and the chaotic signal detected by said second detector, wherein said external optical feedback consists in re-injecting a part of the emitted light by the master mid-infrared optical source back into the master mid-infrared optical source and wherein the chaos generator comprises a feedback reflector and a mid-infrared polarizer, said external optical feedback being obtained using one or multiple round-trips between the feedback reflector and an emitting facet of the master mid-infrared optical source, said mid-infrared polarizer being adapted to tune the amount of the external optical feedback, the angle of the mid-infrared polarizer defining the feedback strength.