Millimeter-wave over fiber integrated sensing and communication system using self-coherent OFDM.

Orthogonal frequency-division multiplexing (OFDM) waveform is highly preferred as a dual-function candidate for integrated sensing and communication (ISAC) systems. However, the sensitivity to both carrier frequency offset (CFO) and phase noise greatly impedes its applications in millimeter-wave ISAC systems. Here, we propose and experimentally demonstrate a photonic millimeter-wave ISAC system employing the virtual-carrier-aided self-coherent OFDM technique, wherein a digitally-generated local oscillator is transmitted along with the OFDM signal. Then, a compact CFO-immune and phase noise-immune envelope detection method is implemented for down-converting millimeter-wave communication and radar echo signals. In experiments, a V-band ISAC system is successfully implemented with a simplified remote radio unit, using the remote photonic millimeter-wave heterodyning up-conversion for downlink and the envelope detection-assisted down-conversion for uplink (or radar echoes). In the converged transmission link with a 5-km fiber link and 2-m space link, the Kramers-Kronig (KK) receiver supports a communication data rate up to 16-Gbit/s by mitigating signal-signal beat interference (SSBI). More significantly, the SSBI leads to negligible effects on the sensing performance when classic matched filtering is adopted for target identification. Consequently, a 4.8-cm range resolution and a 4-mm range accuracy are obtained for the radar sensing function.

Regulation of cluster synchronization in multilayer networks of delay coupled semiconductor lasers with the use of disjoint layer symmetry.

In real-world complex systems, heterogeneous components often interact in complex connection patterns and could be schematized by a formalism of multilayer network. In this work, the synchronization characteristics of multilayer network composed of semiconductor lasers (SLs) are investigated systematically. It is demonstrated that the interplay between different layers plays an important role on the synchronization patterns. We elucidate that the performance of cluster synchronization could be facilitated effectively with the introduction of disjoint layer symmetry into network topology. Intertwined stability of clusters from different layers could be decoupled into independent, and the parameter spaces for stable synchronization are extended significantly. The robustness of our proposed regulation scheme on operation parameters is numerically evaluated. Furthermore, the generality of presented theoretical results is validated in networks with more complex topology and multiple layers.

Real-time adaptive optical self-interference cancellation for in-band full-duplex transmission using SARSA(λ) reinforcement learning.

Self-interference (SI) due to signal leakage from a local transmitter is an issue in an in-band full-duplex (IBFD) transmission system, which would cause severe distortions to a receiving signal of interest (SOI). By superimposing a local reference signal with the same amplitude and opposite phase, the SI signal can be fully canceled. However, as the manipulation of the reference signal is usually operated manually, it is difficult to ensure a high speed and high accurate cancellation. To overcome this problem, a real-time adaptive optical SI cancellation (RTA-OSIC) scheme using a SARSA(λ) reinforcement learning (RL) algorithm is proposed and experimentally demonstrated. The proposed RTA-OSIC scheme can automatically adjust the amplitude and phase of a reference signal by adjusting a variable optical attenuator (VOA) and a variable optical delay line (VODL) achieved through an adaptive feedback signal, which is generated by evaluating the quality of the received SOI. To verify the feasibility of the proposed scheme, a 5 GHz 16QAM OFDM IBFD transmission experiment is demonstrated. By using the proposed RTA-OSIC scheme, for an SOI at three different bandwidths of 200, 400, and 800 MHz, the signal can be adaptively and correctly recovered within 8 time periods (TPs), which is the required time of a single adaptive control step. The cancellation depth for the SOI with a bandwidth of 800 MHz is 20.18 dB. The short- and long-term stability of the proposed RTA-OSIC scheme is also evaluated. The experimental results indicate that the proposed approach could be a promising solution for real-time adaptive SI cancellation in future IBFD transmission systems.

Numerical and experimental investigation of a dispersive optoelectronic oscillator for chaotic time-delay signature suppression.

Chaos generation from a novel single-loop dispersive optoelectronic oscillator (OEO) with a broadband chirped fiber Bragg grating (CFBG) is numerically and experimentally investigated. The CFBG has much broader bandwidth than the chaotic dynamics such that its dispersion effect rather than filtering effect dominates the reflection. The proposed dispersive OEO exhibits chaotic dynamics when sufficient feedback strength is guaranteed. Suppression of chaotic time-delay signature (TDS) is observed as the feedback strength increases. The TDS can be further suppressed as the amount of grating dispersion increases. Without compromising bandwidth performance, our proposed system extends the parameter space of chaos, enhances the robustness to modulator bias variation, and improves TDS suppression by at least five times comparing to the classical OEO. Experimental results qualitatively agree well with numerical simulations. In addition, the advantage of dispersive OEO is further verified by experimentally demonstrating random bit generation with tunable rate up to 160 Gbps.

Predicting nonlinear multi-pulse propagation in optical fibers via a lightweight convolutional neural network.

The nonlinear evolution of ultrashort pulses in optical fiber has broad applications, but the computational burden of convolutional numerical solutions necessitates rapid modeling methods. Here, a lightweight convolutional neural network is designed to characterize nonlinear multi-pulse propagation in highly nonlinear fiber. With the proposed network, we achieve the forward mapping of multi-pulse propagation using the initial multi-pulse temporal profile as well as the inverse mapping of the initial multi-pulse based on the propagated multi-pulse with the coexistence of group velocity dispersion and self-phase modulation. A multi-pulse comprising various Gaussian pulses in 4-level pulse amplitude modulation is utilized to simulate the evolution of a complex random multi-pulse and investigate the prediction precision of two tasks. The results obtained from the unlearned testing sets demonstrate excellent generalization and prediction performance, with a maximum absolute error of 0.026 and 0.01 in the forward and inverse mapping, respectively. The approach provides considerable potential for modeling and predicting the evolution of an arbitrary complex multi-pulse.

Photonic super-resolution millimeter-wave joint radar-communication system using self-coherent detection.

A millimeter-wave (MMW) joint radar-communication (JRC) system with super-resolution is proposed and experimentally demonstrated, using optical heterodyne upconversion and self-coherent detection downconversion techniques. The point lies in the designed coherent dual-band constant envelope linear frequency modulation-orthogonal frequency division multiplexing (LFM-OFDM) signal with opposite phase modulation indexes for the JRC system. Then the self-coherent detection, as a simple and low-cost means, is accordingly facilitated for both de-chirping of MMW radar and frequency downconversion reception of MMW communication, which circumvents costly high-speed mixers along with MMW local oscillators and, more significantly, achieves the real-time decomposition of radar and communication information. Furthermore, a super-resolution radar range profile is realized through the coherent fusion processing of dual-band JRC signals. In experiments, a dual-band LFM-OFDM JRC signal centered at 54 GHz and 61 GHz is generated. The two bands feature an identical instantaneous bandwidth of 2 GHz and carry an OFDM signal of 1 Gbaud, which helps to achieve a 6-Gbit/s data rate for communication and a 1.76-cm range resolution for radar.

Photonic reservoir computing using a self-injection locked semiconductor laser under narrowband optical feedback.

Photonic time-delay reservoir computing (TDRC) using a self-injection locked semiconductor laser under optical feedback from a narrowband apodized fiber Bragg grating (AFBG) is proposed and numerically demonstrated. The narrowband AFBG suppresses the laser's relaxation oscillation and provides self-injection locking in both the weak and strong feedback regimes. By contrast, conventional optical feedback provides locking only in the weak feedback regime. The TDRC based on self-injection locking is first evaluated by the computational ability and memory capacity, then benchmarked by the time series prediction and channel equalization. Good computing performances can be achieved using both the weak and strong feedback regimes. Interestingly, the strong feedback regime broadens the usable feedback strength range and improves robustness to feedback phase variations in the benchmark tests.

Ultrahigh extinction ratio and ultralow insertion loss for polarization beam splitter based on two folded asymmetrical directional couplers with dual-stage etching.

We propose and experimentally demonstrate a polarization beam splitter (PBS) with excellent performance in terms of ultrahigh extinction ratio and ultralow insertion loss. The PBS consists of two dual-stage etched asymmetrical directional couplers, which are cascaded by a bend waveguide to form a folded structure. In the PBS, the fundamental transverse magnetic (T M 0) mode is efficiently cross-coupled to the cross-port, while the fundamental transverse electric (T E 0) mode outputs at the through-port. Thanks to the cascaded structure and dual-stage etching, a silicon-on-insulator-based PBS with ultrahigh extinction ratio and ultralow insertion loss is achieved. The measurement results reveal an extinction ratio beyond 25 dB and an insertion loss less than 0.7 dB within a wide bandwidth of 175 nm for the T E 0 and T M 0 modes.

Adaptive polarization control for a fiber system based on the optimized AdamSPGD algorithm.

In this work, an adaptive control scheme based on the optimized AdamSPGD algorithm is proposed to maintain the stable state of polarization (SOP) of the optical signal in a fiber system. The search space can be reduced by half with the guidance of the physical equation of optical intensity that passes through a liner polarizer, leading to an increase in the speed and stability. Moreover, the robustness is guaranteed by the adoption of AdamSPGD as the optimization object. In the experiment, the input optical signals with random SOPs are successfully controlled to a stable output SOP. Compared to the original algorithm, the speed is increased by 44.73%, and the standard deviation of the required number of iterations is reduced by 21.27%.

Strong cluster synchronization in complex semiconductor laser networks with time delay signature suppression.

Cluster synchronization is a state where clusters of nodes inside the network exhibit isochronous synchronization. Here, we present a mechanism to realize the strong cluster synchronization in semiconductor laser (SL) networks with complex topology, where stable cluster synchronization is achieved with decreased correlation between dynamics of different clusters and time delay signature concealment. We elucidate that, with the removal of intra-coupling within clusters, the stability of cluster synchronization could be enhanced effectively, while the statistical correlation among dynamics of each cluster decreases. Moreover, it is demonstrated that the correlation between clusters can be further reduced with the introduction of dual-path injection and frequency detuning. The robustness of strong cluster synchronization on operation parameters is discussed systematically. Time delay signature in chaotic outputs of SL network is concealed simultaneously with heterogeneous inter-coupling among different clusters. Our results suggest a new approach to control the cluster synchronization in complex SL networks and may potentially lead to new network solutions for communication schemes and encryption key distribution.

Digitized radio-over-fiber transmission based on probabilistic quantization codeword shaping.

To improve the receiver sensitivity of the digitized radio-over-fiber (DRoF) transmission system, a vector quantization scheme based on probabilistic quantization codeword shaping (PQCS) is proposed. The PQCS performs quantization bits (QBs) rematching on the original codebook to optimize the proportion distribution of level '0' and level '2' in 4-Pulse Amplitude Modulation (PAM-4) for improving system sensitivity. A 16-Quadrature Amplitude Modulation (16-QAM) DRoF transmission system using intensity-modulation/direct-detection is employed to experimentally verify the proposed scheme. The experimental results indicate that, compared to the traditional vector quantization scheme, the PQCS method offers 1.45 dB shaping gain for system sensitivity at a bit error rate (BER) of 5 × 10-4. Nevertheless, the error vector magnitude (EVM) can be obtained below 2% when 6 and 7 QBs are adopted for 64-QAM and 256-QAM, respectively.

RoF distributed antenna architecture and reinforcement learning empowered real-time EMI immunity for highly reliable railway communication: publisher's note.

The publisher's note contains a correction to [Opt. Express2932333 (2021)10.1364/OE.438439]. The article was corrected on 17 June 2022.

Millimeter-wave joint radar and communication system based on photonic frequency-multiplying constant envelope LFM-OFDM.

The joint radar and communication (JRC) system providing both large-capacity transmission and high-resolution ranging will play a pivotal role in the next-generation wireless networks (e.g., 6G and beyond) and defense applications. Here, we propose and experimentally demonstrate a novel photonics-assisted millimeter-wave (mm-wave) JRC system with a multi-Gbit/s data rate for communication function and centimeter-level range resolution for radar function. The key is the design of the intermediate-frequency (IF) JRC signal through the angle modulation of the linear frequency modulation (LFM) radar carrier using orthogonal frequency division multiplexing (OFDM) communication signal, inspired by the idea of constant-envelope OFDM (CE-OFDM). This IF angle-modulated waveform facilitates the broadband photonic frequency (phase)-multiplying scheme to generate mm-wave JRC signal with multiplied instantaneous bandwidth and phase modulation index for high-resolution LFM radar and noise-robust CE-OFDM communication. It is with fixed low power-to-average power ratio to render robustness against the nonlinear distortions. In proof-of-concept experiments, a 60-GHz JRC signal with an instantaneous bandwidth over 10-GHz is synthesized through a CE-LFM-OFDM signal encoded with a 2-GBaud 16-QAM OFDM signal. Consequently, a 1.5-cm range resolution of two-dimension imaging and an 8-Gbit/s data rate are achieved for both radar and communication functions, respectively. Furthermore, the proposed JRC system is able to achieve higher radar range resolution and better anti-noise communication, when using higher-order photonic frequency multiplying.

Optical frequency comb assisted denoising for multiple access and capacity enhancement of covert wireless communication.

An optical frequency comb (OFC)-assisted covert wireless communication system with multiple access and enhanced capacity is proposed and experimentally demonstrated. In the scheme, signals in multiple channels are spread and mixed together to use a single transmitter and then received by individual receivers according to multiple access channels. The mixed signal is highly contaminated by noise to achieve high concealment in both the time and frequency domains, and then effectively recovered as different channels using the OFC assisted analog deep denoising technique. In experiments, mixed signals of 16 access channels with a signal-to-noise ratio (SNR) from -18 to -5 dB are accommodated, showing high covertness and 16× capacity enhancement (16×10 Mbit/s). Mutual interference among different channels is also analyzed and greatly eliminated by phases optimization in the spectral-spreading process. This scheme can greatly improve the time and spectrum utilization efficiency, which will be of great significance for enabling multiple access, large capacity, and high security for wireless communications.

Hierarchical-dependent cluster synchronization in directed networks with semiconductor lasers.

Cluster synchronization in complex networks with mutually coupled semiconductor lasers (SLs) has recently been extensively studied. However, most of the previous works on cluster synchronization patterns have concentrated on undirected networks. Here, we numerically study the complete cluster synchronization patterns in directed networks composed of SLs, and demonstrate that the values of the SLs parameter and network parameter play a prominent role on the formation and stability of cluster synchronization patterns. Moreover, it is shown that there is a hierarchical dependency between the synchronization stability of different clusters in directed networks. The stability of one cluster can be affected by another cluster, but not vice versa. Without loss of generality, the results are validated in another SLs network with more complex topology.

Physics-based deep learning for modeling nonlinear pulse propagation in optical fibers.

A physics-based deep learning (DL) method termed Phynet is proposed for modeling the nonlinear pulse propagation in optical fibers totally independent of the ground truth. The presented Phynet is a combination of a handcrafted neural network and the nonlinear Schrödinger physics model. In particular, Phynet is optimized through physics loss generated by the interaction between the network and the physical model rather than the supervised loss. The inverse pulse propagation problem is leveraged to exemplify the performance of Phynet when in comparison to the typical DL method under the same structure and datasets. The results demonstrate that Phynet is able to precisely restore the initial pulse profiles with varied initial widths and powers, while revealing a similar prediction accuracy compared with the typical DL method. The proposed Phynet method can be expected to break the severe bottleneck of the traditional DL method in terms of relying on abundant labeled data during the training phase, which thus brings new insight for modeling and predicting the nonlinear dynamics of the fibers.

Fading-free Φ-OTDR evaluation based on the statistical analysis of phase hopping.

In phase-sensitive optical time domain reflectometry (Φ-OTDR), false phase peaks caused by interference fading have been observed experimentally; however, the statistical law has not yet been disclosed. In this work, after clarifying that the false phase peaks originate from the phase hopping of demodulated phase noise during the unwinding process, we define the phase hopping rate (PHR) to evaluate the degree of fading and study the quantitative relationship between the PHR and signal-to-noise ratio (SNR) of the measured signal through theoretical derivation and experimental verification. In addition, a moving rotated-vector-average (MRVA) method is proposed to suppress the fading and eliminate the false phase peaks. In the experiment, after MRVA processing with a 25 ns sliding window, the lowest SNR is pulled from 0.003 to 14.9, and the corresponding PHR is reduced from 0.237 to less than 0.0001, which is consistent with the theoretical analysis.

Photonic arbitrary waveform generation based on the temporal Talbot effect.

In this paper, we propose a novel photonic approach for generating arbitrary waveform. The approach is based on the property of real-time Fourier transform in the temporal Talbot effect, where the spectrum of the modulating analog signal is converted into the output time-domain waveform in each period. We present a concise and strict theoretical framework to reveal the relationship of real-time Fourier transform between the optical signals before and after the dispersion. A proof-of-concept experiment is implemented to validate the presented theoretical model. We propose to generate symmetrical or asymmetrical arbitrary waveforms by using double-sideband or single-sideband modulation, respectively, which is verified by simulation results. It is shown that the given approach can be used to generate a repetition-rate multiplied optical pulse train with arbitrary waveform by simply using a multi-tone RF signal with appropriate frequencies and powers.

Improving spectral efficiency of digital radio-over-fiber transmission using two-dimensional discrete cosine transform with vector quantization.

Radio-over-fiber (RoF) transmission is a quite reliable technology to support the current and future demands of rapidly progressing broadband wireless network with large capacity and high spectral efficiency. In this paper, we report and demonstrate a digital RoF transmission system using two-dimensional discrete cosine transform with vector quantization (2D-DCT-VQ). By employing the 2D-DCT-VQ technique, the spectral efficiency can be greatly improved, while the system performance is comparable to the traditional approach without compression. The proposed method is experimentally demonstrated in a 20-km 5-Gbaud/λ four-level pulse modulation intensity-modulation/direct-detection optical link. In the orthogonal frequency-division multiplexing -modulated downlink illustrated experimentally, the transmission rate rises by 69.49% on account of the compressed samples by using the proposed method.

RoF distributed antenna architecture- and reinforcement learning-empowered real-time EMI immunity for highly reliable railway communication.

Highly reliable wireless train-ground communication immune to the electromagnetic interferences (EMIs) is of critical importance for the security and efficiency of high-speed railways (HSRs). However, the rapid development of HSRs (>52,000 km all over the world) brings great challenges on the conventional EMIs mitigation strategies featuring non-real-time and passive. In this paper, the convergence of radio-over-fiber distributed antenna architecture (RoF-DAA) and reinforcement learning technologies is explored to empower a real-time, cognitive and efficient wireless communication solution for HSRs, with strong immunity to EMIs. A centralized communication system utilizes the RoF-DAA to connect the center station (CS) and distributed remote radio units (RRUs) along with the railway track-sides to collect electromagnetic signals from environments. Real-time recognition of EMIs and interactions between the CS and RRUs are enabled by the RoF link featuring broad bandwidth and low transmission loss. An intelligent proactive interference avoidance scheme is proposed to perform EMI-immunity wireless communication. Then an improved Win or learn Fast-Policy Hill Climbing (WoLF-PHC) multi-agent reinforcement learning algorithm is adopted to dynamically select and switch the operation frequency bands of RRUs in a self-adaptive mode, avoiding the frequency channel contaminated by the EMIs. In proof-of-concept experiments and simulations, EMIs towards a single RRU and multiple RRUs in the same cluster and towards two adjacent RRUs in distinct clusters are effectively avoided for the Global System for Mobile communications-Railway (GSM-R) system in HSRs. The proposed system has a superior performance in terms of circumventing either static or dynamic EMIs, serving as an improved cognitive radio scheme to ensuring high security and high efficiency railway communication.