A method, a system and a device for encrypting an optical signal to be transmitted via an optical fiber communication link by causing controlled chromatic dispersion of said signal. The controllable decryption device, as well as the controllable decryption device, can be implemented in the form of a variable dispersion compensation module controlled by a decryption key. In the system, the encryption device and the decryption device are controlled in synchronism by an encryption key and a decryption key respectively, the keys preferably being functions of time symmetric with respect to the time axis.