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Journal of Lightwave Technology

Secure Optical Network Transmission of 16QAM Signals Enabled by Phase Coding and Dispersive Stretching

Rui Li, Xiaoxue Gong, Wenshuai Qin, Qihan Zhang, Weigang Hou, Yuxin Xu, Lei Guo

Published Apr 24, 2026• 0 citations
Publisher

Abstract

High-order modulation format optical signals are widely deployed in modern optical networks but remain insufficiently protected at the physical layer, leading to significant security vulnerabilities. This work proposes an all-optical secure transmission scheme for 16QAM signals based on phase coding and dispersive stretching. The scheme employs symmetric cascaded in-phase and quadrature Mach–Zehnder modulators (IQMZMs) to realize phase encoding and decoding, providing a simple structure with good suitability for photonic integration. The universality of constant-envelope key signals within the proposed architecture is investigated, providing references for key-format and device selection in practical applications and important guidance for future research on all-optical digital encryption. To the best of our knowledge, this is the first demonstration of all-optical digital encryption and decryption of 16QAM signals using high-order modulated key signals. Compared with existing schemes of the same category, the key space increases by a factor of$2^{n}$, where$n$denotes the number of symbols. Furthermore, the key signal undergoes dynamic temporal broadening through dispersive processing, enabling steganographic transmission. Simulation results at a bit rate of 100 Gb/s demonstrate significant security enhancement with high stability and reliability. Overall, the proposed scheme provides an effective approach to enhancing physical-layer security in high-order modulation optical transmission systems.

DOI
10.1109/jlt.2026.3687195

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High-order modulation format optical signals are widely deployed in modern optical networks but remain insufficiently protected at the physical layer, leading to significant security vulnerabilities. This work proposes an all-optical secure transmission scheme for 16QAM signals based on phase coding and dispersive stretching. The scheme employs symmetric cascaded in-phase and quadrature Mach–Zehnder modulators (IQMZMs) to realize phase encoding and decoding, providing a simple structure with good suitability for photonic integration. The universality of constant-envelope key signals within the proposed architecture is investigated, providing references for key-format and device selection in practical applications and important guidance for future research on all-optical digital encryption. To the best of our knowledge, this is the first demonstration of all-optical digital encryption and decryption of 16QAM signals using high-order modulated key signals. Compared with existing schemes of the same category, the key space increases by a factor of2n2^{n}2n, wherennndenotes the number of symbols. Furthermore, the key signal undergoes dynamic temporal broadening through dispersive processing, enabling steganographic transmission. Simulation results at a bit rate of 100 Gb/s demonstrate significant security enhancement with high stability and reliability. Overall, the proposed scheme provides an effective approach to enhancing physical-layer security in high-order modulation optical transmission systems.
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.
A novel digital signal processing (DSP)-based scheme for physical layer security in coherent optical communication systems is proposed and numerically investigated. The optical layer signal encryption is accomplished by two dispersive elements and one phase modulator (PM) driven by a DSP-generated encryption key, whilst signal decryption uses similar components but with inverted dispersion values and security keys. A critical aspect of the DSP-based physical layer security is that the security keys, driving the PMs to hide/recover the data signals, must be highly unpredictable and noise-like, thus orthogonal frequency division multiplexing (OFDM) signals are employed as they possess these characteristics, they can also be easily generated and cover a suitably wide range of unique keys. Numerical simulations are conducted to determine optimum system parameters for achieving a high level of security, the key parameters requiring optimization are the dispersion of the dispersive elements and the bandwidth of the security keys. Using these determined optimum parameters, in-depth investigations are undertaken of encryption/decryption induced transmission performance penalties, sensitives to various parameter offsets and operation over various transmission distances. To observe any data signal dependencies, various performance metrics are investigated for different combinations of modulation formats (DQPSK and 16QAM) and baud rates (40 Gbaud and 100 Gbaud) for the transmitted data signals. The proposed DSP-based physical layer security scheme is shown to have the potential to achieve, in a low-cost and highly effective manner, a high level of physical layer security with acceptable performance penalties for existing coherent optical communication systems.
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.
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.
Achieving high speed physical layer security is a constant pursuit but critical challenge for the information society. In this paper, a novel symbol-by-symbol optical phase encryption technique relying on ultra-long, reconfigurable optical phase patterns and commercial off-the-shelf dispersive components is proposed for high speed physical layer security. A record 40Gb/s secure optical communication system with symbol overlapped, optical phase encrypted differential-phase-shift-keying modulated signal is experimentally demonstrated based on the proposed technique. Security robustness against various eavesdropper's attacks has been validated for three optical codes with a chip rate of 40Gchip/s and variable code-length of 128-chip, 512-chip and 1024-chip, respectively. The demonstrated technique exhibits the advantages of supporting high bit rate operation and advanced optical modulation formats, improving code flexibility and cardinality, which makes it very promising for future ultra-fast secure optical communication.
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