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Abstract
Amplified spontaneous emission (ASE) light is a common noise in optical communication systems with optical amplification, and a suitable optical carrier for optical covert communication. To deeply covert the secure signal, an in-band subnoise optical covert communication scheme is proposed and demonstrated by a proof-of-concept experiment. The power spectral density of optical secure channel is 10 dB less than the optical noise in the public channel. The covert signal is hidden in both optical and electrical domain, and can be transmitted with error-free. The trade-off between covertness and availability is discussed.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Photonic layer security has drawn much attention to deal with the serious information security threat. Various physical properties can be used to protect the privacy of the transmitted optical signal, such as, quantum mechanics for quantum communications [1,2], chaos laser communications [3–7], optical code division multiple access [8,9], quantum noise stream cipher [10–13], optical frequency hopping [14], and optical covert communication [15–20]. Unlike optical encryption methods such as quantum noise stream cipher, optical covert communication can provide imperceptible security by hiding the presence of data transmissions under optical noise in the public transmission channel.
Optical covert communication based on amplified spontaneous emission (ASE) light shows superior performance in covertness, as it shares identical signal characteristics with the optical noise in optical networks. Due to the periodic nature of the coherent optical pulses based covert signal, the signal is easily exposed by clock spectral lines that reveal the presence of the signal in the electrical spectrum. To further improve the covertness, the signal-to-noise ratio of the covert channel can be reduced to a subnoise signal or even a deep subnoise signal. However, ASE-based optical covert communication has poor transmission performance compared to coherent optical carriers, which limits the covert channel’s concealment under optical noise [21]. Subnoise optical covert communication faces the challenge of separating and recovering covert signals from broadband optical noise. Subnoise electrical signal can be detected and denoised by instantaneous spectral cloning [22] and analog spectrum convolution [23], and optical signal can be denoised by Talbot processing [24,25]. However, in these schemes, the basic optical sources are coherent optical pulses, which has different properties from ASE light and not suitable for ASE based optical covert communication.
In this paper, a subnoise optical covert communication scheme based on ASE light is proposed. The power spectrum of optical secure channel is 10 dB less than the optical noise in the public channel. The optical denoising is based on microwave photonic filter. In addition, the reported optical covert communication systems mainly focus on the hiding performance in the optical time-frequency domain, and the signal characterization in the electro-spectral domain has not been investigated. In this work, the covert signal is hidden in both optical and electrical domain. The covert signal is deeply hidden by three layers, one is the covert modulation, the second is the analog optical link, and the third is the optical noise. The transmission performance of optical covert channel is tested, and the trade off between covertness and availability is discussed.
2. Principle
2.1 Hiding optical signals in the electrical spectrum
In an optical covert communication system, optical covert signal is transmitted over existing optical wavelength divisional multiplexing (WDM) optical network. The schematic diagram of the subnoise optical covert transmitter is shown as Fig. 1. The optical carrier of optical covert channel is generated by an ASE light source. The ASE light is filtered by optical filter to two spectral complemented branches, which are connected with two electro-optical modulators and modulated with two complemented signals. Then, the two branches are combined by an optical coupler. To obtain the same signal character as the input ASE light, a variable optical attenuator and an optical tunable delay line are added to adjust the power and time delay. However, since the bias drift of intensity modulator or the imprecise matching of time delays, the electrical spectrum will show the presence of the signal. A dispersion device is added, and the frequency response of the transmitter is shown as Fig. 1(b). Then, the covert signal is further suppressed by the analog optical transmission link in electrical domain.
Fig. 1. (a) Schematic diagram of transmitter of the subnoise optical covert communication, (b) the response of the transmitter. AWG, arbitrary waveform generator; OF, optical filter; EOM, electro-optical modulator; OTDL, optical tunable delay line; VOA, variable optical power attenuator; OC, optical coupler.
For one branch in the spectral complemented branches, the modulated signal with dispersion can be expressed as
Omitting direct current components and sideband-to-sideband beat terms, the detected signal is
As can be seen, the signal apart from the direct current will be suppressed, such as the exposed signal caused by the imprecise matching of time delays.
2.2 Recovering of optical covert signal
The signal is recovered by complementary decoding and microwave photonic filtering. Schematic diagram of receiver of the subnoise optical covert communication is shown as Fig. 2. The optical covert signal is transmitted over the single mode fiber (SMF) in the public channel, then decoded by an optical filter whose passband spectrum coincides with the spectrum of the branch which is modulated with data. In addition to decoding, the public signals are suppressed by the optical filter through notch filtering. An erbium-doped fiber amplifier (EDFA) is used to amplify the decoded optical covert signal. The amplified signal is sent to a Mach–Zehnder interferometer (MZI), which is composed by two optical couplers, two optical tunable delay lines and a polarization controller. Then the optical covert signal is detected by a photodetector.
Fig. 2. Schematic diagram of the receiver of the subnoise optical covert communication. SMF, single mode fiber; PC, polarization controller; PD, photodetector.
After MZI, the decoded optical covert signal is
Then, after photodetection, the spectrum of the detected signal is
In optical domain, the optical signal is hidden under the ASE noise in the public channel. In electrical domain, after photodetection, the optical in-band noise manifests itself as ASE-ASE beat noise, which is a broadband electrical noise. With MZI, the transceiver is equivalent to a microwave photonic filter. The beat noise is suppressed by microwave photonic filter, and a passband is shown at the central frequency of the modulated signal to recover the original signal. Since the optical in-band noise is converted to broadband electrical noise, most of the noise is filtered out by the microwave photonic filter, leaving only a small amount of in-band noise in the electrical domain. The central angular frequency of the passband is ${\Delta \tau }/{\beta _2}$, and the central angular frequency can be varied by tuning the optical tunable delay line in the MZI. The covert signal is recovered, and the original signal is received after down-conversion.
The maximum value of passband out-of-band rejection ratio is more than 60dB by the microwave photonic filter [27]. Then, the power of the optical covert signal can be lower than the optical noise, which can be as low as a subnoise state. Under subnoise state, the signal can be hidden well in waveform and spectrum of optical and electrical domain.
3. Experiment setup
A proof-of-concept experiment is setup to demonstrate the proposed scheme. The experiment setup is as shown in Fig. 3. In the public channel, the transmitter consists of distributed feedback (DFB) semiconductor laser, intensity modulator, and noise. The continuous wave output from the DFB laser is modulated by the signal from pulse pattern generator (PPG), and the noise is the ASE noise generated by optical amplification in public channel. The public signal is transmitted over a 50 km span of single-mode fiber. In the public receiver, the signal is sent to a −841 ps/nm dispersion compensated fiber. Then, the signal is amplified by an EDFA and filtered by an optical filter with bandwidth of 100 GHz. The detected signal is sent to the bit error rate tester.
Fig. 3. The setup of the proof-of-concept experiment. IM, intensity modulator; DCF, dispersion compensation fiber; PPG, pulse pattern generator.
The optical covert signal is generated by the transmitter as described in Section 2, and the signal is injected to the public channel through an optical coupler. In the receiver, a dispersion chromatic device is used to compensate the excess dispersion. The detected optical covert signal is sent to the oscilloscope for analysis.
4. Results and discussions
4.1 Frequency response
The calculated frequency response of the back-to-back transceiver is shown in Fig. 4. The fiber length is set as 100 km, the bandwidth of the optical carrier is set as 3 THz. As can be seen, there is a passband with center frequency around 7.5 GHz when MZI is used. However, when MZI is not used, there is no passband at the same frequency. The fading in the electrical domain can be used to further enhance the covertness of covert signal. In addition, without MZI, null frequency occurs due to the dispersion induced modulation type evolution, which implies that the optical intensity modulation is translated to phase modulation.
With MZI, the frequency response of the back-to-back transceiver is measured and the results are shown in Fig. 5. The dispersion is $+$425 ps/nm. Fig. 5(a) shows the frequency response of covert signal with and without optical noise. The 3-dB bandwidth of the passband is about 150 MHz. The maximum power in the passband decreases in the presence of ASE noise. Fig. 5(b) is the optical spectrum of the covert signal without ASE noise. As can be seen, the signal is sliced by the MZI. In the case of $\Delta \tau$ different from Fig. 5(a), changing the power of the ASE noise, the measured frequency responses are shown in Fig. 5(c). The average optical power of the signal is −16.25 dBm. The magnitude of the passband increases as the signal to noise power ratio increases. As shown in Fig. 5(d), the maximum power in the passband increases approximately linearly with the signal to noise power ratio. The reason is that the ASE-ASE beat noise increases as the ASE noise power increases, and there is power competition in receivers.
Fig. 5. The (a) frequency response, (b) optical spectrum, (c) frequency response under different power ratios, (d) the maximum power of passband with different power ratios.
The electrical spectra for detected decoded signals are shown as Fig. 6. The dispersion is +425 ps/nm. As can be seen, the signal is hidden in the electrical frequency domain. By tuning the optical tunable delay line in the MZI, the central frequency of the passband is matched with the central frequency of the original electrical signal, and then the original electrical signal is recovered. In addition, compared Fig. 6(a) with (c), under subnoise condition, the power of the received signal is decreased. This result is consistent with the results of the frequency response.
Fig. 6. The electrical spectrum of the decoded signal (a) without noise but with MZI, (b) without noise or MZI, (c) with noise and MZI, (d) with noise but without MZI .
4.2 Time and frequency characteristics
The waveform and spectrum of optical covert signal is shown in Fig. 7. Compared Fig. 7(a) with (b), the complementary modulated signal has the same waveform as noise. However, due to distortion of the modulated square wave, there is a burr signal with low power. As shown in Fig. 7(c), after coupling to ASE noise, the residual signal is hidden in both the time and frequency domains. The optical spectrum of the complementary modulated signal is shown in Fig. 7(d), the spectrum is flat. Therefore, the signal can be both in the time, electrical and optical spectral domain. For complementary modulation, the integration may be realized by wavelength selection switch through online inverse design chip [28]. The waveform of the decoded signal is shown in Fig. 7(e), the signal is recovered by complementary decoding and microwave photonic filtering.
Fig. 7. Waveform and spectrum of (a) the original signal, (b) the covert signal with complementary modulation, (c) subnoise covert signal; (d) the optical spectrum of the covert signal with complementary modulation; (e) the waveform of the decoded signal.
4.3 Transmission performance
The BER curves of public channel are shown as Fig. 8. The BER values are measured by the BER tester. The public channel with bit rate of 5 Gb/s has a receiver sensitivity less than the public channel with 10 Gb/s. Under the same signal-to-noise ratio, compared the public channel with and without covert signal, the power penalty is less than 0.3 dB. Therefore, the introduction of covert channel has little effect on the transmission performance of the public channel. This is because the optical carrier of the covert channel is ASE light, which is incoherent and no beat noise is introduced in the public receiver. In addition, the power of optical covert channel is 5 or 10 dB less than the ASE noise in the public channel, the ASE noise is the main noise in the public receiver.
The BER curves of the stealth channel is shown as Fig. 9. The BER values are calculated numerically from the received signal by the oscilloscope. The dispersion is $+$425 ps/nm in the transmitter, the transmission distance is 50 km. The public signal is suppressed by the optical filter at the receiver, the main noise is the ASE noise in the transmitter and the ASE noise induced by EDFA in the transmission link. The covert signal can be transmitted with error-free. As the signal to noise power ratio of covert channel increases, the receiver sensitivity decreases. Therefore, there is a trade-off between the covertness and availability. With a constant signal-to noise ratio, the covertness can be optimized by increasing the bandwidth of the ASE carrier and increasing the dispersion of the dispersion device in the transmitter.
5. Conclusions
Based on amplified spontaneous emission light, a subnoise optical covert communication scheme is proposed and demonstrated by a proof-of-concept experiment. The covert signal is complementary modulated and the covert transmission link is equivalent to a microwave photonic filter. The covert signal can be hidden in optical and electrical domain, and can be transmitted with error-free. There is a trade-off between the covertness and availability. This solution is compatible with wireless communication systems and is expected to realize wired wireless converged covert communication.
Funding
National Natural Science Foundation of China (61901480, 62071487, 62301569).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. S. Pirandola, U. L. Andersen, L. Banchi, et al., “Advances in quantum cryptography,” Adv. Opt. Photonics 12(4), 1012–1236 (2020). [CrossRef]
2. H. Zhang, Z. Sun, R. Qi, et al., “Realization of quantum secure direct communication over 100 km fiber with time-bin and phase quantum states,” Light: Sci. Appl. 11(1), 83 (2022). [CrossRef]
3. Y. Zhang, M. Xu, M. Pu, et al., “Experimental demonstration of an 8-Gbit/s free-space secure optical communication link using all-optical chaos modulation,” Opt. Lett. 48(6), 1470–1473 (2023). [CrossRef]
4. Y. Zeng, Y. Huang, P. Zhou, et al., “Controlling the likelihood of extreme events in an optically pumped spin-VCSEL via chaotic optical injection,” Opt. Express 31(10), 16178–16191 (2023). [CrossRef]
5. J. Ke, L. Yi, G. Xia, et al., “Chaotic optical communications over 100-km fiber transmission at 30-gb/s bit rate,” Opt. Lett. 43(6), 1323–1326 (2018). [CrossRef]
6. Z. Gao, Q. Li, L. Zhang, et al., “32 Gb/s physical-layer secure optical communication over 200 km based on temporal dispersion and self-feedback phase encryption,” Opt. Lett. 47(4), 913–916 (2022). [CrossRef]
7. Z. Yang, J. Ke, Q. Zhuge, et al., “Coherent chaotic optical communication of 30 Gb/s over 340-km fiber transmission via deep learning,” Opt. Lett. 47(11), 2650–2653 (2022). [CrossRef]
8. J. Ji, B. Wu, J. Zhang, et al., “Design and investigation of 10 Gb/s FSO wiretap channel using OCDMA time-diversity reception,” IEEE Photonics J. 12(3), 1–12 (2020). [CrossRef]
9. X. Wang, Z. Gao, X. Wang, et al., “Bit-by-bit optical code scrambling technique for secure optical communication,” Opt. Express 19(4), 3503–3512 (2011). [CrossRef]
10. Y. Li, Y. Li, K. Zhu, et al., “Analysis of the encryption penalty in a QAM-based quantum noise stream cipher,” Opt. Express 31(12), 19006–19020 (2023). [CrossRef]
11. K. Tanizawa and F. Futami, “Ultra-long-haul digital coherent PSK Y-00 quantum stream cipher transmission system,” Opt. Express 29(7), 10451–10464 (2021). [CrossRef]
12. H. Luo, L. Zhong, X. Dai, et al., “DAC/ADC-free 4 × 12.9 Gbit/s 65,536-level quantum noise stream cipher secure optical WDM transmission based on delta-sigma modulation,” Opt. Lett. 47(19), 5104–5107 (2022). [CrossRef]
13. F. Futami, K. Tanizawa, and K. Kato, “Y-00 quantum-noise randomized stream cipher using intensity modulation signals for physical layer security of optical communications,” J. Lightwave Technol. 38(10), 2774–2781 (2020). [CrossRef]
14. Y. Jin, Y. Qi, Y. Chen, et al., “Secure fiber-optic communication system based on Internet-accessible multipath transmission of ciphertext fragments,” Opt. Express 29(16), 24919–24927 (2021). [CrossRef]
15. X. Wang, W. Mou, and H. Zhu, “Effect of laser parameters on optical stealth transmission system performance,” Sensors 21(16), 5358 (2021). [CrossRef]
16. X. Hong, D. Wang, L. Xu, et al., “Demonstration of optical steganography transmission using temporal phase coded optical signals with spectral notch filtering,” Opt. Express 18(12), 12415–12420 (2010). [CrossRef]
17. H. Zhu, R. Wang, T. Pu, et al., “Optical stealth transmission based on super-continuum generation in highly nonlinear fiber over WDM network,” Opt. Lett. 40(11), 2561–2563 (2015). [CrossRef]
18. B. Wu, Y. Tang, C. Qiu, et al., “Secure analysis of optical steganography with spectral signature measurement,” IEEE Photonics Technol. Lett. 33(17), 971–974 (2021). [CrossRef]
19. E. Wohlgemuth, Y. Yoffe, I. Attia, et al., “A field trial of multi-homodyne coherent detection over multi-core fiber for encryption and steganography,” J. Lightwave Technol. 41(9), 2723–2735 (2023). [CrossRef]
20. Y. Wang, W. Shao, J. Wu, et al., “Real-time stealth optical transmission via dither-remodulation in a bias controller of a mach-zehnder modulator,” Opt. Express 31(12), 18773–18789 (2023). [CrossRef]
21. H. Zhu, T. Pu, W. Mou, et al., “Analysis of beat noise in optical stealth transmission system,” Opt. Commun. 501, 127390 (2021). [CrossRef]
22. V. Ataie, D. Esman, B. P. Kuo, et al., “Subnoise detection of a fast random event,” Science 350(6266), 1343–1346 (2015). [CrossRef]
23. X. Yan, X. Zou, P. Li, et al., “Covert wireless communication using massive optical comb channels for deep denoising,” Photonics Res. 9(6), 1124–1133 (2021). [CrossRef]
24. B. Crockett, L. Romero Cortés, R. Maram, et al., “Optical signal denoising through temporal passive amplification,” Optica 9(1), 130–138 (2022). [CrossRef]
25. B. Crockett, L. Romero Cortes, S. R. Konatham, et al., “Full recovery of ultrafast waveforms lost under noise,” Nat. Commun. 12(1), 2402 (2021). [CrossRef]
26. G.-H. Duan and E. Gorgiev, “Non-white photodetection noise at the output of an optical amplifier: theory and experiment,” IEEE J. Quantum Electron. 37(8), 1008–1014 (2001). [CrossRef]
27. V. R. Supradeepa, C. M. Long, R. Wu, et al., “Comb-based radiofrequency photonic filters with rapid tunability and high selectivity,” Nat. Photonics 6(3), 186–194 (2012). [CrossRef]
28. W. Zhang, J. Cheng, H. Zhou, et al., “Online inverse design chip for matrix computation and tunable wavelength selection switch,” in Conference on Lasers and Electro-Optics, (Optica Publishing Group, 2022), p. JTh3B.22.
