Secure key generation and distribution scheme based on historical fiber channel state information with LSTM.

In this paper, a scheme to realize unclonable physical-layer security key generation and distribution (PL-SKGD) based on historical fiber channel state information (HFCSI) is proposed. PL-SKGD schemes based on channel characteristics for enhancing the physical-layer security of optical networks have been proposed in recent years. However, there are potential disadvantages in these schemes, such as 1) low key generation rate (KGR): the slow frequency of the analog waveform change of the channel characteristic leading to low KGR; 2) incompatibility with existing infrastructure: active scrambling to increase the frequency of channel characteristic changes, or tracking changes of channel characteristics requires additional devices; 3) easy to be cloned: all of the optical channel state information is reflected in the signal transmitted inside the fiber, which makes it easy to reproduce by illegal eavesdropper through features analysis and other methods. In order to solve the above problems, a PL-SKGD scheme is designed which uses the chain structure composed of long short-term memory neural network (LSTM-NN) units to learn and store the unique mapping relationship between historical channel time series and provides unclonability based on the fundamental fact that the eavesdropper Eve can never obtain the full HFCSI. The simulation conducted in a quadrature phase shift keying point-to-point optical link system verified successfully that KGR = 0.82 Gbit/s error-free SKGD. The loss function of LSTM-NN drops sharply in the early stages of training and remains a small value. The security of the SKGD system is analyzed, which effectively improves the unclonability of the system. Finally, it is verified that the optimal fiber channel length for error-free SKGD of the proposed scheme is 150 km considering the error correction capability of information reconciliation and weighing key sequence error rate and valid bit generation rate.

Design and experimental demonstration of underwater wireless optical communication system based on semantic communication paradigm.

Underwater wireless optical communication (UWOC) has been widely studied as a key technology for ocean exploration and exploitation. However, current UWOC systems neglect semantic information of transmitted symbols, leading to unnecessary consumption of communication resources for transmitting non-essential data. In this paper, we propose and demonstrate a deep-learning-based underwater wireless optical semantic communication (UWOSC) system for image transmission. By utilizing a deep residual convolutional neural network, the semantic information can be extracted and mapped into the transmitted symbols. Moreover, we design a channel model based on long short-term memory network and employ a two-phase training strategy to ensure that the system matches the underwater channel. To evaluate the performance of the proposed UWOSC system, we conduct a series of experiments on an emulated UWOC experimental platform, in which the effects of different turbidity channel environments and bandwidth compression ratios are investigated. Experimental results show that the UWOSC system exhibits superior performance compared to the conventional communication schemes, particularly in challenging channel environments and low bandwidth compression ratios.

Fingerprint construction of optical transmitters based on the characteristic of electro-optic chaos for secure authentication.

In this paper, an optical transmitter authentication method using hardware fingerprints based on the characteristic of electro-optic chaos is proposed. By means of phase space reconstruction of chaotic time series generated by an electro-optic feedback loop, the largest Lyapunov exponent spectrum (LLES) is defined and used as the hardware fingerprint for secure authentication. The time division multiplexing (TDM) module and the optical temporal encryption (OTE) module are introduced to combine chaotic signal and the message to ensure the security of the fingerprint. Support vector machine (SVM) models are trained to recognize legal and illegal optical transmitters at the receiver. Simulation results show that LLES of chaos has the fingerprint characteristic and is highly sensitive to the time delay of the electro-optic feedback loop. The trained SVM models can distinguish electro-optic chaos generated by different feedback loops with a time delay difference of only 0.03ns and have a good anti-noise ability. Experimental results show that the recognition accuracy of the authentication module based on LLES can reach 98.20% for both legal and illegal transmitters. Our strategy can improve the defense ability of optical networks against active injection attacks and has high flexibility.

Optical transmitter fingerprint construction and identification based on chaotic phase space reconfiguration.

An optical transmitter identification scheme based on optical chaotic phase space reconfiguration for secure communication is proposed to target injection attacks in the physical layer of optical networks. First, a feature fingerprint construction method based on reconfigured phase space of optical chaos is proposed. Then the fingerprint is controlled by the feedback intensity and filtering bandwidth of chaos. The in-phase and quadrature-phase encryption (IQE)/decryption (IQD) ensures the loading of fingerprints and realizes the confidential communication. In the experiment, the recognition rate of three transmitters is up to 99.3%. In the simulation, the recognition rate of five optical transmitters reaches 100% after 600 km transmission. The bit error rate of 25 GBaud QPSK signal after 300 km transmission at 25 dB OSNR is 1.6 × 10-3. Compared with the traditional optical transmitter identification methods, the fingerprint of this scheme is controllable. The IQE and IQD not only realize the chaotic fingerprint loading but also ensure the secure transmission of the signal avoiding the synchronization and time delay exposure problems in traditional chaotic communication systems. It is robust to device parameters, with low implementation difficulty and low cost. Therefore, this scheme has research and application value for secure communication in the physical layer of optical networks.

Physical-layer encryption and authentication scheme based on SKGD and 4D hyper-chaos.

In this paper, a scheme to realize encryption and digital identity authentication at the same time is proposed for enhancing the physical-layer security of point-to-point optical links (PPOL). Exploiting identity code encrypted by the key as authentication information effectively resists passive eavesdropping attacks in fingerprint authentication. The proposed scheme theoretically realizes secure key generation and distribution (SKGD) by phase noise estimation of the optical channel and the generation of identity codes with good randomness and unpredictability by the four-dimensional (4D) hyper-chaotic system. The local laser, erbium doped fiber amplifier (EDFA), and public channel provide the entropy source of uniqueness and randomness to extract symmetric key sequences for legitimate partners. The simulation conducted in a quadrature phase shift keying (QPSK) PPOL system over 100km standard single mode fiber verify successfully that 0.95Gbit/s error-free SKGD. The unpredictability and high sensitivity to the initial value and control parameters of the 4D hyper-chaotic system provide a huge space of ~10125 for identity codes, which is sufficient to resist exhaustive attack. With the proposed scheme, the security level of key and identity can be increased markedly.

Bubbles-induced turbulence channel prediction mechanism based on machine vision in underwater wireless optical communication.

Bubbles-induced turbulence poses a significant challenge to the stability of underwater wireless optical communication (UWOC) system. Existing methods for understanding channel characteristics rely on the pilot information from the feed-back channel, which are ineffective and inaccurate due to the rapidly changing nature of the underwater channel. We propose a machine-vision-based channel prediction mechanism which contains three modules of motion judgment module, image processing module and scintillation index (SI) prediction module. The mechanism captures images of bubbles and calculates the bubble density. Subsequently, a relational function is applied to acquire the predicted SI which quantifies the impacts of bubbles on the channel. Experimental results validate the effectiveness of the proposed mechanism.

Unified statistical thermocline channel model for underwater wireless optical communication.

Understanding the effect of ocean turbulence on optical beam propagation is critical to the design and performance evaluation of underwater wireless optical communication systems. In this Letter, we propose a unified Weibull-generalized gamma distribution to characterize the laser beam irradiance fluctuations of turbulent underwater thermocline wireless optical channels. The proposed model shows an excellent agreement with the measured data under various experimental emulated channel conditions that cover turbulences induced by temperature, salinity, and air bubbles. To the best of our knowledge, this is the first model that comprehensively describes the statistics of the laser beam irradiance fluctuations in underwater wireless optical channels due to both thermohaline gradient and air bubbles.

Channel characteristics estimation based on a secure optical transmission system with deep neural networks.

Optical transmission security has attracted much attention. In recent years, many secure optical transmission systems based on channel characteristics are proposed. However, there are many drawbacks with these systems, such as separated plaintext and key transmission, low key generation rate (KGR), insecurity when the eavesdropper has acquired the lengths of the local fibers utilized by legal parties. To solve the above problems, we propose a novel secure optical transmission system based on neural networks (NNs), which are employed to estimate channel characteristics. By training NNs locally and transmitting pseudo-keys, the proposed system can transmit the plaintext together with key, transforming the key dynamically. Moreover, since the channel characteristics for legal parties and eavesdropper are not completely identical, the NNs trained by legal parties and eavesdropper are inconsistent. Even though the eavesdropper has attained the lengths of local fibers wielded by legal parties, the NN model trained by the legal parties is still unavailable to illegal eavesdropper. The final key is generated by the trained NN and pseudo-key, so the keys generated by legal parties and eavesdropper are dissimilar. The simulation results prove the feasibility of the proposed system with the transmission distance of 100 km and the bit rate of 100 Gbps. Meanwhile, if plaintext and key have equivalent code length, the KGR of 50 Gbps for legal parties and the key disagreement rate (KDR) of 50% for illegal eavesdropper will be realized.

A time-delay signature elimination and broadband electro-optic chaotic system with enhanced nonlinearity by deep learning.

In this paper, a novel electro-optic chaotic system with enhanced nonlinearity by deep learning (ENDL) is proposed to achieve time-delay signature (TDS) elimination. A long-short term memory network (LSTM) is trained by a specially designed loss function to enhance the nonlinear effect that can hide the TDS of the system. For the first time, the trained deep learning module is put into a single feedback loop to participate in chaos generation. Simulation results show that the ENDL system can eliminate TDS and increase the bandwidth to more than 31GHz when the feedback intensity is very low (α = 4V). Moreover, the complexity of the chaotic output can be improved with permutation entropy (PE) reaching 0.9941. The synchronization result shows that the ENDL system has high sensitivity to TDS but has low sensitivity to the feedback intensity, thus the system has both high security and high robustness. This system has an uncomplicated synchronization structure and high flexibility, and it opens up a new direction for high-quality chaos generation.

Optical format interconversion nodes between OOK and QPSK enabled by a reconfigurable two-dimensional vector mover.

An optical format interconversion scheme between on-off keying (OOK) and quadrature phase shift keying (QPSK) is proposed and verified in this paper. The conversion system mainly consists of a coherent vector combiner and a reconfigurable two-dimensional (2D) vector mover. As a key element of the proposed conversion system, the 2D vector mover is implemented by a non-degenerate phase-sensitive amplifier (PSA). The operating principle and theoretical derivations of the PSA-based 2D vector mover are fully introduced. The reconfigurable transfer characteristics of the vector mover are analyzed under different parameter settings to exhibit the flexible 2D moving function. The signal constellations, eye diagrams, spectrum, error vector magnitudes, and bit error ratios are estimated and depicted to validate the proposed idea. With the input signal-to-noise ratios of 20 dB and 25 dB, error-free conversions are achieved between 50G Baud OOK and QPSK. The scheme proposed in this paper fills the lack of the one-to-one interconversion between OOK and QPSK, and has potential applications in optical interconnect nodes, across-dimensional optical transmissions, and flexible optical transceivers.

Electro-optic chaotic system based on time delay feature hiding and key space enhancement based on chaotic post-processing.

To improve the output performance of the classical all-optical chaotic system and solve the security problems of its key exposure and small key space, a new chaotic system, to the best of our knowledge, based on logistic map post-processing is proposed. In terms of the general output performance of the system, the spectrum of the proposed system is flatter than the classical system. Through a bifurcation diagram and permutation entropy analysis, it is found that the output of the system is extremely complex. In terms of security, the simulation results show that, with a reasonable selection of system parameters, key hiding can be achieved under a large parameter range. Moreover, through the sensitivity analysis of logistic parameters, it can be seen that the introduction of logistic parameters can improve the key space of the system and further improve the security of the system.

Recent Progress in Identifying Bacteria with Fluorescent Probes.

The development of new techniques to rapidly and accurately detect bacteria has drawn continuous attention due to the potential threats posed by bacteria to human health and safety. Recently, a novel strategy based on fluorescent probes has drawn considerable interest for the detection of bacteria due to its high selectivity, fast response, and simple operation. In this review, we summarize the recent progress on fluorescent probes for the specific recognition and discrimination of Gram-negative and Gram-positive bacteria. In particular, we outline current design strategies, such as targeting of the differences in surface components, cell wall components, endogenous enzymes, surface charge, and hydrophobicity of various kinds of bacteria to develop various fluorescent sensors (organic small-molecule fluorescent probes, nanoprobes, and metal ion probes). We also emphasize the application of organic molecules in probe recognition elements. We hope that this review can stimulate this research area in bacterial detection and imaging in the future.

Electro-optic chaos system with time delay signature concealment based on XOR operation and multi-bit PRBS.

A novel chaos system with XOR operations and multi-bit PRBS is proposed to improve the sequence complexity and the security of the classic electro-optic intensity chaos system. Through the bifurcation diagram and permutation entropy analysis, the PE can be increased to 0.99. The key space is enlarged because additional DSP parameters and PRBS are introduced. The impacts of ADC/DAC characteristics and PRBS characteristics are analyzed in detail. The simulation results show that the time delay signature can be concealed with the appropriate DSP parameters.

Time-delay signature concealing electro-optic chaotic system with multiply feedback nonlinear loops.

A novel time-delay signature (TDS) concealing electro-optic (EO) chaotic system with multiply feedback nonlinear loops is proposed and analyzed by numerical simulation. The proposed system employs mutual injection structure implemented by two asymmetric branches named as multiply feedback nonlinear loop which introduces an extra nonlinear factor to the system dynamic equation. The complexity of the chaos system is increased by introducing this multiply feedback nonlinear loop. The permutation entropy (PE) of the proposed system is improved to higher than 0.96 when feedback strength (ß) equals 5. The proposed system can enter to chaos regime with a small ß (ß = 0.8). The TDS is concealed effectively due to the extra nonlinear factor introduced by multiply feedback nonlinear loop. Meanwhile, key-space of the proposed system is about 1012 times that of the classical EO system because more tunable time delay parameters are introduced. Furthermore, the performance of a secure communication system based on the proposed chaotic system is discussed, and the simulation results show that the system is sensitive to time delay parameters and robust to feedback strength, which proves the proposed system is suitable for secure communication.

2D-to-1D constellation reforming using phase-sensitive amplifier-based constellation squeezing and shifting.

In this paper, a phase-sensitive amplifier (PSA)-based two dimensional (2D)-to-one dimensional (1D) constellation reforming system is proposed and analyzed in detail. The proposed system theoretically realizes seven kinds of 10 GBaud quadrature amplitude modulation (QAM)-to-pulse amplitude modulation (PAM) conversions, including quadrature phase shift keying-to-PAM4 and 8QAM-to-PAM8 conversions. The constellation reforming system consists of a constellation squeezing PSA and a multi-level vector moving PSA. The operating principle and formula derivations of constellation squeezing and vector moving processes are fully explained, including the PSA transfer characteristics and PSA gain axis angle analytical solutions. When implementing QAM-to-PAM conversions, the constellations, spectra, eye diagrams, error vector magnitudes and bit error ratio (BER) performances of the QAM and PAM signals are measured. For 8QAM-to-PAM8 conversion, with the input OSNR of 25 dB and 30 dB, at the BER of 10-3, the converted PAM8 shows the receiver OSNR of 38.9 dB and 35.2 dB, respectively. The proposed and verified 2D-to-1D constellation reforming system builds an optical bridge connecting long-haul and short-reach networks, which can be employed in the format conversion, high-order format signal generation and shaping, and flexible information aggregation/de-aggregation.

Key space enhancement of a chaos secure communication based on VCSELs with a common phase-modulated electro-optic feedback.

In this paper, a novel chaotic secure communication system based on vertical-cavity surface-emitting lasers (VCSEL) with a common phase-modulated electro-optic (CPMEO) feedback is proposed. The security of the CPMEO system is guaranteed by suppressing the time-delay signature (TDS) with a low-gain electro-optic (EO) feedback loop. Furthermore, the key space is enhanced through a unique secondary encryption method. The first-level encrypted keys are the TDS in the EO feedback loop, and the second-level keys are the physical parameters of the VCSEL under variable-polarization optical feedback. Numerical results show that, compared to the dual-optical feedback system, the TDS of the CPMEO system is suppressed 8 times to less than 0.05 such that they can be completely concealed when the EO gain is 3, and the bandwidth is doubled to over 22 GHz. The error-free 10 Gb/s secure optical transmission can be realized when the time-delay mismatch is controlled within 3 ps. It is shown that the proposed scheme can significantly improve the system performance in TDS concealment, as well as bandwidth and key space enhancement, which has great potential applications in secure dual-channel chaos communication.

Phase-sensitive amplifier-based optical conversion for direct detection of complex modulation format to bridge long-haul transmissions and short-reach interconnects.

An optical conversion node scheme for direct detection of complex modulation format is proposed to bridge long-haul transmissions and short-reach interconnects. A noisy 10G Baud quadrature phase shift keying signal is converted into a 10G Baud normal 4-level pulse amplitude modulation (PAM4) signal by the node. The conversion node is realized mainly relies on four-wave mixing-based phase-sensitive amplifiers. The power ratio and constellation shape of the converted PAM4 both can be flexibly designed based on network demands and five kinds of uniform or non-uniform PAM4s are generated to verify the shaping functionality. With the input optical signal-to-noise ratio range of (10 dB∼30 dB), the key indicators of the signals went through every part are measured, includes constellations, eye diagrams, error vector magnitudes, bit error rates, normalized impact factors of phase and amplitude. The proposed node scheme has great application potential in intermediate nodes for bridging long-haul transmissions and short-reach interconnects, hierarchical modulation and flexible constellations design for advanced format signals.

Multi-resonance and ultra-wideband terahertz metasurface absorber based on micro-template-assisted self-assembly method.

As a promising platform for multi-functional terahertz devices, metasurface absorbers have received widespread attention in recent years. However, due to the existence of manufacturing difficulties, high cost, fragility, single or narrow absorption and other disadvantages, their application ranges are severely limited. Therefore, to effectively solve these problems, we have designed a flexible and high-precision terahertz metasurface absorber based on the micro-template assisted self-assembly method. Free from high cost, complicated process and time-consumption, the sandwich structure terahertz metasurface absorber consisting of a ceramic microspheres layer, a dielectric spacer layer, and a metal copper film is fabricated economically. On the one hand, through assembling the microspheres on the dielectric spacer in a periodic pattern arrangement, multiple resonances can be observed with a maximum absorption rate of up to 92.5% at 0.745 THz and are insensitive to the polarization of incident light. On the other hand, by attaching the microspheres to the dielectric layer in a compact configuration, 90% absorption bandwidth beyond 1.2 THz can be observed with a central frequency of 1.8 THz. The theoretical model of multiple reflection and interference is employed to explain these absorption characteristics. Considering the flexible design and high-throughput manufacturing processes, this work provides a promising platform for the development of high-efficiency and multi-functional terahertz devices.

All-optical aggregation and de-aggregation between 3 × BPSK and 8QAM in HNLF with wavelength preserved.

All-optical aggregation and de-aggregation with wavelength preserved play an important role in a flexible optical network. In addition, it is also expected to apply in a network node that connects different networks and increases the channel utilization by freeing up the low-speed channel. In this paper, we proposed an aggregation and de-aggregation setup between three binary phase shift keying signals and 8-ary quadrature amplitude modulation signal using the nonlinear effect in high nonlinear optical fiber. Moreover, the bit-error rate of the signal is analyzed to evaluate the performance of the system by numerical simulation.

Wideband complex-enhanced bidirectional phase chaotic secure communication with time-delay signature concealment.

A novel bandwidth-enhanced bidirectional phase chaotic secure communication system with time-delay signature (TDS) concealment is proposed and analyzed by numerical simulation. This bidirectional system based on two mutually coupled electro-optic (MCEO) phase feedback loops is driven by a common all-optical (AO) chaotic source. The AO driving source makes the amplitude and phase terms in the Ikeda-based MCEO equation chaotic. Two mutually coupled optoelectronic delayed feedback loops also greatly increase the complexity of the chaotic carrier. By replacing the semiconductor laser in the existing bidirectional communication scheme with an electro-optic feedback loop, the problems of narrow carrier bandwidth and poor synchronization performance can be compensated. Compared to the single MCEO system, the permutation entropy of the AO-MCEO cascaded system with a bit rate of 10 Gbit/s is improved by 0.13 to 0.98. The TDS of the AO-MCEO system is suppressed 35 times to less than 0.01 to be completely hidden when the EO gain is reduced by half to 2.75. The chaos effective bandwidth is increased by 5 GHz to 32.05 GHz, and the spectrum flatness is reduced by 0.33 dB/Hz to 0.82 dB/Hz. Meanwhile, the security is further enhanced by reducing the cross-correlation coefficient to 0.001 between the AO driving source and the electro-optical chaotic carrier. The results show that the proposed model has potential applications in bandwidth-enhanced bidirectional secure chaotic systems.