With the rapid advancement of information technology, the volume of data transmitted via optical fibers has grown exponentially. However, illegal eavesdroppers can exploit methods such as bending fibers and tapping into adjacent-channel crosstalk to cause slight leakage of optical signals and steal information. Ensuring the security of data transmission in optical fiber networks has thus become a critical issue that demands urgent resolution. Optical covert communication technology conceals information within the optical noise of a public transmission channel. Among noise sources, amplified spontaneous emission (ASE) light, with its broad bandwidth and high noise intensity, serves as an excellent carrier for hiding transmitted signals. Meanwhile, the randomness, sensitivity to initial conditions, and unpredictability of chaotic systems provide a novel approach to enhancing physical-layer information security in optical covert communication. Based on this, we propose an optical covert communication system employing chaotic digital encryption with an ASE light source, featuring a key space of up to 10⁸⁷. Simulation results demonstrate that over a 50 km transmission distance, the covert channel achieves error-free reception at an optical received power of -10 dBm, while the public channel does so at -19 dBm. Moreover, the covert channel introduces only a 0.5 dB power loss to the public channel. Even if an eavesdropper is aware of the covert channel’s existence, the intercepted information exhibits a bit error rate (BER) as high as 0.5 without the encryption key. This scheme effectively ensures communication security and covertness while imposing minimal impact on the public channel.

The structure of the covert communication system is illustrated in Fig. 3, primarily consisting of a covert channel transmitter, receiver, and a public channel. The sender, Alice, and the authorized user, Bob, pre-share the same encryption key. At the transmitter, the covert signal, processed by chaotic digital encryption, undergoes power adjustment via a variable optical attenuator (VOA) to ensure its power is 12.8 dB lower than that of the public channel, thereby achieving signal concealment. The covert signal is then coupled into the public channel using an optical coupler (OC 3) and transmitted over 50 km of single-mode fiber (SMF). At the receiver, Bob employs dispersion-compensating fiber (DCF) to precisely counteract the chromatic dispersion introduced during transmission. The covert signal is then extracted from the public channel using an optical filter, and finally, the original covert signal is recovered by matching the dispersion value with the chaotic digital decryption process.

The simulation results demonstrate that when the covert channel’s power is 12.8 dB lower than that of the public channel, the covert signal becomes completely hidden in the frequency domain [Fig. 7(a)], preventing eavesdroppers from detecting its presence through spectral observation. Without compensating for the dispersion introduced by the covert channel’s encryption or isolating the two branches of the covert signal, the covert channel exhibits noise-like characteristics in the time domain [Fig. 8(a)]. Even if an eavesdropper demodulates the public channel and examines its eye diagram, the comparison of eye patterns reveals no detectable trace of the covert channel (Fig. 9). Furthermore, the covert channel introduces only a 0.5 dB power penalty to the public channel [Fig. 10(a)]. When the received optical power of the covert channel reaches -10 dBm, error-free reception is achieved, and the impact of chaotic digital encryption on transmission performance is negligible [Fig. 10(b)].

Facing these rapidly growing demands for information transmission security and covertness driven by the continuous evolution of information technology, we propose an optical covert communication system based on the integration of chaotic digital encryption and ASE light. Owing to its inherent randomness, sensitivity to initial conditions, and unpredictability, the chaotic system serves as an ideal choice for enhancing information confidentiality. By applying chaotic digital encryption to covert signals and then modulating them onto ASE light for covert transmission, this scheme provides dual-layer security protection concealment and confidentiality thereby strengthening the overall system security. Simulation results demonstrate that by adjusting the power of the chaotic digitally encrypted signal carried by ASE light to be 12.8 dB lower than that of the public channel, the covert signal can be effectively hidden in both frequency and time domains, while introducing only a 0.5 dB power penalty to the public channel. Furthermore, the proposed scheme achieves a key space of up to 10⁸⁷, enabling robust resistance against brute-force attacks by eavesdroppers even when they are aware of the covert channel’s existence. This provides a solid safeguard for information security.