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In conventional chaos communication systems based on external-cavity semiconductor lasers (ECSLs), messages are embedded into the chaotic carriers and then directly transmitted to the receiver for recovery. Since the modulated chaotic carrier (chaos + message) is directly transmitted in public link, the eavesdropper can easily access it for interception. It has been proved that when the bit rate is relatively low, the message hidden in the chaotic carrier can be intercepted by using a linear filter with a proper cutoff frequency or reconstructing an illegal receiver system based on the injectionlocking effect. We propose and numerically demonstrate a security-enhanced chaos communication system by introducing an optical time-frequency encryption (OTFE) module to convert the modulated chaotic carrier (chaos + message) as an uncorrelated signal before transmission. At the receiver end, a matching optical time-frequency decryption (OTFD) module is adopted to recover the modulated chaotic carrier for the final message recovery. The results demonstrate that, with the OTFE module the modulated chaotic carrier would be transformed as an uncorrelated chaotic carrier with the time delay signature being perfectly suppressed in the time domain. Simultaneously, in the frequency domain, the spectrum of chaotic carrier would be flattened, and the efficient bandwidth of chaotic carrier can be expanded by several times. With respect to conventional chaos communication systems, the proposed scheme shows obviously higher security under the attack scenarios of direct linear filtering and synchronization utilization. The proposed scheme provides a novel way to implement high-security chaos communication.
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