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Optical Engineering

Optical encryption of parallel quadrature phase shift keying signals based on nondegenerate four-wave mixing in highly nonlinear fiber

Yue Cui, Min Zhang, Yueying Zhan, Danshi Wang, Shanguo Huang

Published Aug 1, 2016• 3 citations
Publisher

Abstract

A scheme for optical parallel encryption/decryption of quadrature phase shift keying (QPSK) signals is proposed, in which three QPSK signals at 10 Gb/s are encrypted and decrypted simultaneously in the optical domain through nondegenerate four-wave mixing in a highly nonlinear fiber. The results of theoretical analysis and simulations show that the scheme can perform high-speed wiretapping against the encryption of parallel signals and receiver sensitivities of encrypted signal and the decrypted signal are −25.9 and −23.8 dBm, respectively, at the forward error correction threshold. The results are useful for designing high-speed encryption/decryption of advanced modulated signals and thus enhancing the physical layer security of optical networks.

DOI
10.1117/1.oe.55.8.086101

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A scheme for optical parallel encryption/decryption of quadrature phase shift keying (QPSK) signals is proposed, in which three QPSK signals at 10 Gb/s are encrypted and decrypted simultaneously in the optical domain through nondegenerate four-wave mixing in a highly nonlinear fiber. The results of theoretical analysis and simulations show that the scheme can perform high-speed wiretapping against the encryption of parallel signals and receiver sensitivities of encrypted signal and the decrypted signal are −25.9 and −23.8 dBm, respectively, at the forward error correction threshold. The results are useful for designing high-speed encryption/decryption of advanced modulated signals and thus enhancing the physical layer security of optical networks.
Aiming at the current eavesdropping security issues in the physical layer of optical networks, a new hybrid optical encryption scheme based on Highly Nonlinear Fiber (HNLF) and Super Structured Fiber Bragg Grating (SSFBG) is proposed in this paper. In the scheme, degenerate Four Wave Mixing (FWM) is used to encrypt and decrypt user information, and optical coding technology is used to encode and decode the key information. Therefore, the encoding and secret transmission of the key signal can be realized simultaneously. The security of the system is measured by comparing the waveform of the encrypted signal transmitted in the channel with the waveform of the original transmitted signal, and the effectiveness of the system is measured by the bit error rate at the system receiver side. The simulation results show that the frequency and phase of the user data information are disturbed, and the optical power of the encoded key signal is as low as -34.4 dBm. Therefore, the eavesdropping channel cannot obtain effective data information nor detect the key signal, which demonstrates the safe transmission of Quadrature Phase Shift Keying (QPSK) signal.
The optical network supporting emerging bandwidth-intensive services has become the cornerstone of the 5G era. However, the development of optical fiber eavesdropping and quantum computing has brought huge security challenges to traditional cryptography. As an effective approach to improving optical layer transmission security, all-optical logic gate encryption has important research significance. This paper proposes a high-speed coherent optical encryption and decryption system based on phase encoding with all-optical logic gates. The system ingeniously designs the phase encoding and decoding scheme between two quadrature phase-shift keying (QPSK) signals, which doubles the encryption and decryption efficiency. In addition, the integration of physical-layer key distribution (PLKD) and physical-layer encryption (PLE) contributes to a reduction in overall system complexity. By adopting real-time signal processing in an all-optical domain, the single-wavelength rate of system security communication can reach 100 Gbps. The system is also applicable to other phase-modulated signals and provides a feasible scheme to enhance transmission security in wavelength division multiplexing (WDM) optical networks featuring high speed, low latency, and high flexibility.
This study investigates a multi-stage digital phase-encryption scheme designed for secure coherent optical communication. While the proposed method is implemented in DSP and can be modeled as a software-level stream cipher, it is explicitly co-designed with and tightly coupled to the physical-layer modulation structure. Unlike conventional XOR-based bitwise schemes, the encryption is applied to the symbol phase domain. It is integrated into the transmitter's modulation process, enabling symbol-level security evaluated in conjunction with transmission physics. By applying K-level phase-shift secret keys toM-ary quadrature amplitude modulation (QAM) signals, this method increases the number of secret-key bits per symbol without altering the signal constellation in its baseline configuration. This approach enhances encryption strength while maintaining signal integrity and reducing quantization overhead in phase-only operation. Experimental evaluations on nonlinear optical transmission channels confirm that baseline digital phase encryption preserves Q-factor stability under self-phase modulation, cross-phase modulation, and four-wave mixing. The proposed scheme ensures error-free transmission at forward error correction thresholds, effectively preventing eavesdroppers from reconstructing symbols under worst-case observation conditions. Additionally, simulations analyzing generalized mutual information and quantization errors demonstrate that the encryption scheme can minimize signal-to-noise ratio penalties, even when employing high-order QAM formats with limited digital-to-analog and analog-to-digital conversion resolutions. As an optional extension, we also analyze pseudo-quantum-noise-based IQ spreading and explicitly quantify the associated trade-off between additional noise masking and increased sensitivity to quantization. These findings position multi-stage digital phase encryption as a practical and power-efficient modulation-layer encryption technique whose behavior is systematically characterized under realistic nonlinear channel and hardware constraints, rather than as an entirely new cryptographic primitive.
Quantum-noise randomized cipher (QNRC) is an anti-interception communication technology, which can realize high-speed encryption for data stream in true randomness. In this paper, the characteristics of the phase shift keying (PSK) and quadrature phase shift keying(QPSK) QNRC systems are verified by the simulation system with digital coherent demodulation. The decryption process of the QNRC coherent system is carried out in the optical domain for the first time. Specifically, at the quadrature coupler, the modulated local light is coupled with the received encrypted optical signal. The multi-symbol phase estimation(MSPE) module is employed to eliminate the influence of the phase deviation on the signal decision. Finally, error-free secure transmission is realized at 2.5Gsymbol/s over 500km optically amplified links. We reveal the new fact that the mesoscopic power of QPSK-QNRC system is higher than that of PSK-QNRC system.
Data security plays an increasingly important role in modern telecommunications. The advent of quantum computational processors presents a significant threat to today's widely employed public key encryption algorithms, necessitating the adoption of new approaches to data encryption. Whilst quantum key distribution guarantees unconditional security for cryptographic key exchange in optical communication networks, the data rate is slow (Mbit/s), especially when compared to conventional optical communication. Here we present a highly secure encryption approach in which the encryption key, generated by quantum key distribution at a rate of up to 2.9 Mbit/s, was used to seed physical layer encryption performed using time domain spectral phase encoding (TDSPE). This allowed us to demonstrate encrypted 40 Gbit/s quadrature phase shift keyed data communications over 52.3 km of installed optical fiber, which cannot be eavesdropped using brute force computational attacks. Any attempt to eavesdrop the encrypted signal in the physical layer is highly time-sensitive – the phase states must be measured and decrypted prior to optical signal attenuation, which means that the attack procedure typically needs to be completed within a few milliseconds. This work represents the first example of quantum-enhanced physical layer encryption at realistic optical data rates that is fully secure from brute force computational attacks and the first demonstration of TDSPE using continuous-wave laser source and quadrature phase shift key modulation.