RESUMO
Bandwidth limitation in optoelectrical components and the chromatic dispersion-induced power fading phenomenon cause severe inter-symbol interference (ISI) in high-speed intensity modulation and direct detection (IM-DD) optical interconnects. While the equalizer implemented in the receiver's digital signal processing procedure can mitigate ISI, it also inevitably enhances the noise located in the decayed frequency region, known as equalization-enhanced colored noise (EECN). Additionally, the nonlinear impairments of the modulator and photodetector also deteriorate the performance of the IM-DD system, especially for high-order modulation formats. In this work, we propose a gradient-descent noise whitening (GD-NW) algorithm to address EECN and extend it by introducing nonlinear kernels to simultaneously mitigate EECN and nonlinear impairments. The proposed algorithms are compared with conventional counterparts in terms of the achievable baud rate and the receiver optical power sensitivity. As a proof-of-concept experiment, we validate the principles of the proposed algorithms by successfully transmitting 360-GBd on-off-keying (OOK) and 180-GBd 4-level pulse-amplitude-modulation (PAM-4) signals in the back-to-back case under a 62-GHz brick-wall bandwidth limitation. 280-GBd OOK and 150-GBd PAM-4 transmissions are also demonstrated over 1-km standard single-mode fiber with a bit error rate below 7% hard-decision forward error correction aided by the proposed approach.
RESUMO
Physical-layer authentication (PLA) based on hardware fingerprints can safeguard optical networks against large-scale masquerade or active injection attacks. However, traditional schemes rely on massive labeled close-set data. Here, we propose an unsupervised hardware fingerprint authentication based on a variational autoencoder (VAE). Specifically, the triplets are generated through variational inference on unlabeled optical spectra and then applied to train the feature extractor, which has an excellent generalization ability and enables fingerprint feature extraction from previously unknown optical transmitters. The feasibility of the proposed scheme is experimentally verified by the successful classification of eight optical transmitters after a 20â km standard single-mode fiber (SSMF) transmission, to distinguish efficiently the rogue from legal devices. A recognition accuracy of 99% and a miss alarm rate of 0% are achieved even under the interference of multiple rogue devices. Moreover, the proposed scheme is verified to have a comparable performance with the results obtained from supervised learning.
RESUMO
Physical-layer secure key distribution (PLSKD) generally acquires highly correlated entropy sources via bidirectional transmission to share the channel reciprocity. For long-haul fiber links, the non-negligible backscattering noise (BSN) and the challenge of bidirectional optical amplification degrade the key generation performances. Since the channel reciprocity can be precisely mapped using neural networks (NNs), unidirectional PLSKD provides a feasible PLSKD for longer fiber links. Here, a final error-free key generation rate (KGR) in unidirectional PLSKD of 3.07â Gb/s is demonstrated over a 300â km fiber link using NNs. Moreover, the channel mapping is analyzed in terms of fiber distance, chromatic dispersion, the nonlinearity of random source, and BSN.
RESUMO
The digital radio-over-fiber (D-RoF) transmission with two-level coding (TLC) is proposed and demonstrated in this Letter. A joint design considering the importance of quantization bits, the protection ability of forward error correction (FEC), and the bit error ratio of quadrature amplitude modulation (QAM) symbols is realized. In TLC-based D-RoF systems, the more significant bits among quantization bits are protected by a FEC and are assigned to the least reliable bits of modulated QAM symbols. Conversely, the less significant bits, without FEC protection, are allocated to the more reliable bits of QAM symbols. Experiments on an 11-km standard single-mode fiber transmission are conducted to evaluate the performance. The results indicate that, with a maximum iteration number of 2, compared to the conventional bit-interleaved coded modulation (BICM) with all bits encoded, the D-RoF based on TLC attains nearly identical performance under the 0.34% error vector magnitude threshold of 65536QAM wireless signals, specifically achieving complexity reductions of 54.55% and 67.66% for 16QAM and 64QAM optical transmissions, respectively.
RESUMO
The accurate estimation of mutual information (MI) plays a vital role in understanding channel capacity and optimizing the performance of optical communications. While MI computations for the additive white Gaussian noise (AWGN) channel are well-established, they fall short when dealing with the challenges posed by nonlinear optical fiber channels due to an unknown channel model. For the first time, to our knowledge, this Letter introduces a mutual information neural estimator (MINE) for MI estimation in optical fiber communications. We propose an enhanced MINE (E-MINE), achieved by enlarging the training batch size to improve estimation accuracy and stability. Our findings reveal that the E-MINE achieves highly accurate estimations in the AWGN channel and maintains strong consistency with symbol-by-symbol MI estimations, comparable to Monte Carlo (MC) methods based on a Gaussian distribution in long-haul optical fiber channels. Furthermore, with multi-symbol estimation, the E-MINE yields a 0.16â bits/4D-symbol improvement in our experiments. We anticipate that our findings will drive further research in the field, opening new possibilities for enhancing communication systems design and performance using deep learning techniques.
RESUMO
A white-box power-lite Volterra-inspired neural network (VINN) equalizer is proposed to solve the problem of complexity discontinuity in a Volterra nonlinear equalizer (VNLE). By adjusting the granularity of the solution space, it conserves computational resources while maintaining nonlinear compensation capability. The performance of VINN is verified on a field-programmable gate array (FPGA) in a short-reach intensity modulation and direct detection (IMDD) system, and a 240-Gb/s real-time signal processing rate is achieved. Under the 25% overhead soft-decision forward error correction (SD-FEC) bit error rate (BER) threshold, we realize a record net rate of up to 180â Gb/s based on the FPGA.
RESUMO
Broadband amplified spontaneous emission (ASE) light sources are recognized for their cost-effective generation. However, their inherent high-intensity noise and the stringent requirement for time delay matching limits their widespread application in coherent optical telecommunication. Here we propose a broadband ASE source-enabled digital-analog radio-over-fiber (DA-RoF) mobile fronthaul architecture, leveraging semiconductor optical amplifiers (SOAs) and multicore fiber in tandem. Our proposed system uses SOAs to suppress the intensity noise of the ASE carrier and transmits the DA-RoF signal alongside an unmodulated carrier through distinct cores of an 8-core, 1-km fiber. This setup significantly enhances the signal-to-noise ratio (SNR) by 19.4 dB, boosts capacity, and enables self-homodyne detection at the receiver end. We achieve an aggregated bandwidth of 35 GHz (7 cores × 5 GHz), supporting a 2.05-Tb/s CPRI-equivalent data rate with 1024-ary quadrature-amplitude-modulated (1024-QAM) signals. Additionally, we analyze the impact of chromatic dispersion on signal-to-noise ratio for broadband source coherent detection systems. This innovative scheme offers a pragmatic solution for integrating low-cost broadband sources into cost-sensitive fronthaul systems, providing both high capacity and fidelity in massive deployment scenarios.
RESUMO
Clock recovery (CR) algorithms that support higher baud rates and advanced modulation formats are crucial for short-distance optical interconnections, and it is desirable to push CR to operate at baud rate with minimal computing resources and power. In this Letter, we proposed a hardware-efficient and multiplication operation-free baud-rate timing error detector (TED) as a solution to meet these demands. Our approach involves employing both the absolute value of samples and the nonlinear sign operation to emphasize the clock tone, which is deteriorated by severe bandwidth limitation in Nyquist and faster than Nyquist (FTN) systems. Through experimental investigations based on a transceiver system with a 3â dB bandwidth of 30â GHz, the proposed baud-rate TED exhibits excellent performance. The proposed scheme successfully achieves clock synchronization of the received signals with the transmitted signals, including 50â GBaud PAM4/8, 80â GBaud PAM4, and up to 120â GBaud PAM4 FTN signals. To the best of our knowledge, the CR based on the proposed baud-rate TED is the most optimal solution for ultrahigh-speed short-reach IM/DD transmission, comprehensively considering the timing jitter, bit error rate (BER), and implementation complexity.
RESUMO
Self-homodyne coherent transmission has recently received extensive investigation as a coherent lite candidate for high-speed short-reach optical networks. In this Letter, we propose a weakly coupled mode-division-multiplexing (MDM) self-homodyne coherent scheme using a multiple-ring-core few-mode fiber, in which one of the modes transmits a self-homodyne local oscillator (LO) and the rest are utilized for carrying signals. Multiple rings of index perturbations in the fiber core are applied to achieve low modal crosstalk, allowing the signals and the remote LO to be transmitted independently. We experimentally demonstrate a 7.2-Tb/s (5.64-Tb/s net rate) self-homodyne coherent transmission with an 800-Gb/s data rate for each of the nine information-bearing modes formatted in 80-GBaud probabilistic constellation-shaped 64-quadrature-amplitude modulation. To the best of our knowledge, this is the first experimental demonstration of an MDM self-homodyne coherent transmission with up to 10 spatial modes. The proposed scheme may pave the way for future high-capacity data center interconnections.
RESUMO
Neural network (NN)-based equalizers have been widely applied for dealing with nonlinear impairments in intensity-modulated direct detection (IM/DD) systems due to their excellent performance. However, the computational complexity (CC) is a major concern that limits the real-time application of NN-based receivers. In this Letter, we propose, to our knowledge, a novel weight-adaptive joint mixed-precision quantization and pruning approach to reduce the CC of NN-based equalizers, where only integer arithmetic is taken into account instead of floating-point operations. The NN connections are either directly cutoff or represented by a proper number of quantization bits by weight partitioning, leading to a hybrid compressed sparse network that computes much faster and consumes less hardware resources. The proposed approach is verified in a 50-Gb/s 25-km pulse amplitude modulation (PAM)-4 IM/DD link using a directly modulated laser (DML) in the C-band. Compared with the traditional fully connected NN-based equalizer operated with standard floating-point arithmetic, about 80% memory can be saved at a minimum network size without degrading the system performance. Quantization is also shown to be more suitable to over-parameterized NN-based equalizers compared with NNs selected at a minimum size.
RESUMO
Weakly coupled mode-division-multiplexing (MDM) systems based on intensity modulation and direct detection (IM-DD) are a good candidate for further improving the capacity of short-reach optical interconnections. However, restrained by the modal crosstalk of the transmission link and the reception of degenerate mode groups (DMGs) utilizing bandwidth-limited multimode photodetectors (PDs), high-speed MDM IM-DD has encountered a capacity bottleneck. In this Letter, we investigate a high-speed weakly coupled MDM IM-DD transmission system utilizing a degenerate mode diversity receiver scheme adopting high-bandwidth single-mode PDs over a multiple-ring-core (MRC) few-mode fiber (FMF) and a low-crosstalk mode multiplexer/demultiplexer (MUX/DMUX). An MDM IM-DD transmission with four DMGs and eight wavelengths is experimentally demonstrated with 112-GBaud four-level pulse-amplitude modulation (PAM4) and probabilistically shaped PAM8 per lane over 200-m weakly coupled MRC-FMF. To the best of our knowledge, this is the first experimental demonstration of the MDM IM-DD transmission system with up to 112-GBaud baud rate and beyond 6.4-Tb/s net rate. Meanwhile, the experimental results show that the proposed MDM IM-DD transmission link has a superior performance only adopting a low-complexity feedforward equalizer, making it a promising candidate for high-speed optical interconnections.
RESUMO
In this paper, for the first time to the best of our knowledge, we investigate the experiment of polar coded probabilistic shaped 8-ary pulse amplitude modulation (PS-PAM8) in weak turbulence. A systematic interleaver (SIL) is proposed to improve the polar code performance for PS-PAM8, compatible with the 5â G channel coding standard. Considering the effects of turbulence and shaped constellations, the pilot with identical distributions as the transmitted data is used for dynamic channel estimation to avoid demodulation failure. Moreover, the application of hybrid equalization with nonlinear and linear equalizers effectively reduces the receiver sensitivity. In 25 GBd transmission over a 4 m free-space link, the transmission performance of polar coded PAM8 schemes with SIL is better than that of the low-density parity check code by 1.0â dB, and the power budget is further saved by 0.72â¼0.83â dB after linear equalization. Meanwhile, the shaping gains of polar coded PS-PAM8 with SIL and hybrid equalization are up to 2.0â dB at 1.5 bits/channel use. In addition, different weak turbulence conditions can be generated inside a chamber, and the observed channel fading is consistent with the log-normal model. The results show that the proposed polar coded PS scheme can improve the Q-factor by 0.49â¼1.74â dB in different turbulence conditions.
RESUMO
The diverse applications of mode-locked fiber lasers (MLFLs) raise various demands on the output of the laser, including the pulse duration, energy, and shape. Simulation is an excellent method to guide the design and construction of an MLFL for on-demand laser output. Traditional simulation of an MLFL uses the split-step Fourier method (SSFM) to solve the nonlinear Schrödinger (NLS) equation, which suffers from high computational complexity. As a result, the inverse design of MLFLs via the traditional SSFM-based simulation method relies on the design experience. Here, a completely data-driven approach for the inverse design of MLFLs is proposed, which significantly reduces the computational complexity and achieves a fast automatic inverse design of MLFLs. We utilize a recurrent neural network to realize fast and accurate MLFL modeling, then the desired cavity settings meeting the output demands are searched via a deep-reinforcement learning algorithm. The results prove that the data-driven method enables the accurate inverse design of an MLFL to produce a preset target femtosecond pulse with a certain duration and pulse energy. In addition, the cavity settings generating soliton molecules with different target separations can also be located via the data-driven inverse design. With the GPU acceleration, the time consumption of the data-driven inverse design of an MLFL is less than 1.3 hours. The proposed data-driven approach is applicable to guide the inverse design of an MLFL to meet the different demands of various applications.
RESUMO
We propose a novel (to our knowledge) driving scheme to suppress the stimulated Brillouin scattering (SBS) effect in master oscillator power amplification (MOPA) systems based on an external high-order phase modulation. Since seed sources with the linear chirp can uniformly broaden the SBS gain spectrum with a high SBS threshold, a chirp-like signal was designed by applying further editing and processing to the piecewise parabolic signal. Compared with the traditional piecewise parabolic signal, the chirp-like signal has similar linear chirp characteristics and can reduce the driving power and sampling rate requirements, enabling more efficient spectral spreading. The SBS threshold model is constructed theoretically based on the three-wave coupling equation. The spectrum modulated by the chirp-like signal is compared with the flat-top and Gaussian spectra in terms of the SBS threshold and the bandwidth-distribution normalized threshold, and a considerable improvement is demonstrated. Meanwhile, the experimental validation is carried out in a watt-class amplifier based on the MOPA structure. At a 3â dB bandwidth of â¼10â GHz, the SBS threshold of the seed source modulated by the chirp-like signal is improved by 35% compared to the flat-top spectrum and 18% compared to the Gaussian spectrum, respectively, and the normalized threshold is also the highest among them. Our study shows that the SBS suppression effect is not only related to the power distribution of the spectrum but also can be improved by the time domain design, which provides a new idea for analyzing and improving the SBS threshold of narrow-linewidth fiber lasers.
RESUMO
Digital pre-distortion (DPD) is a powerful technique to mitigate transmitter nonlinear distortion in optical transmissions. In this Letter, the identification of DPD coefficients based on the direct learning architecture (DLA) using the Gauss-Newton (GN) method is applied in optical communications for the first time. To the best of our knowledge, this is the first time that the DLA has been realized without training an auxiliary neural network to mitigate optical transmitter nonlinear distortion. We describe the principle of the DLA using the GN method and compare the DLA with the indirect learning architecture (ILA) that uses the least-square (LS) method. Extensive numerical and experimental results indicate that the GN-based DLA is superior to the LS-based ILA, especially in a low signal-to-noise ratio scenario.
RESUMO
Chaotic optical communication encrypts transmitted signals through physical noise; this ensures high security while causing a certain decrease in the signal-to-noise ratio (SNR). Thus, it is necessary to analyze the SNR degradation of decrypted signals after chaotic encryption and the minimum requirements for the SNR of the fiber channel to meet the required bit error rate (BER) performance. Accordingly, an SNR model of decrypted signals for optoelectronic feedback-based chaotic optical communication systems is proposed. Under different channel SNRs, the SNR degradation of 40â Gbit/s phase chaos and intensity chaos models is investigated by simulation and experiment, respectively, with a 15â GHz wideband chaotic carrier. Comparing decrypted signals with original signals, the simulation results show that there is a 2.9â dB SNR degradation for both intensity chaos and phase chaos. Further, in experiments, SNR degradation from 4.5â dB to 5.6â dB, with various channel SNRs for intensity chaos, is analyzed, while there is an SNR degradation from 7.1â dB to 8.3â dB for phase chaos. The simulation and experimental results provide guidance for long-distance transmission chaotic optical communication systems.
RESUMO
The digital-analog radio-over-fiber (DA-RoF) scheme offers a high-fidelity and spectrally efficient solution for future mobile fronthaul. However, to be implemented in the low-cost directly modulated laser with direct detection (DML-DD) link, both the digital and analog parts in DA-RoF modulation would suffer from the composite second-order (CSO) and composite triple beat (CTB) caused by the chirp-dispersion interaction. In this Letter, we propose and experimentally demonstrate a computationally efficient composite triple beat cancellation (CTB-C) algorithm for DA-RoF fronthaul in the dispersion-uncompensated C-band DML-DD link. The CSO and CTB are suppressed at the receiver-side DSP based on the theoretical model of these nonlinear distortions. In the proof-of-concept experiment, a 1.2-dB improvement in the recovered signal-to-noise ratio (SNR) is obtained with 5.5-GHz 1024-QAM orthogonal frequency division multiplexing (OFDM) signal after 10-km standard single-mode fiber (SSMF) transmission. The proposed CTB-C technique does not require the training process and performs close to the Volterra-based feed-forward equalizer (VFE) under the complexity constraint.
RESUMO
High-speed physical-layer secure key generation and distribution (SKGD) schemes via channel reciprocity are achieved using external electro-optical modulation or random source distribution via additional fiber links. Here, we propose and demonstrate an SKGD scheme using the fluctuation of polarization states from an amplified spontaneous emission (ASE) source, without any external electro-optical modulation or additional fiber link. Experimentally, an error-free key generation rate (KGR) of 10.1 Gb/s is achieved over a 10-km standard single-mode fiber (SSMF), with true randomness originating from ASE. Moreover, the single fiber channel can be shared for SKGD as well as data transmission, allowing the integration of the proposed SKGD with the deployed fiber infrastructure.
RESUMO
A low-complexity scheme is proposed to realize irregular uniform quadrature amplitude modulation (QAM) formats with Gray mapping, which are named amplitude-division irregular QAM (AD-Ir-QAM) formats. Compared to conventional probabilistic shaping (PS) with the Maxwell-Boltzmann (MB) distribution (PS-MB), irregular QAM formats show a smaller peak-to-average power (PAPR) and achieve a better performance in systems with the peak-power constraint. Compared with irregular QAM formats realized by PS (PS-Ir-QAM), AD-Ir-QAM formats realize a more flexible rate adaptation and have a lower implementation complexity. Experimental results obtained in an unamplified coherent optical system show that, at a generalized mutual information (GMI) of 4.5 bits/2D-symbol, AD-Ir-100QAM achieves gains of 2.1 and 0.5â dB in the power budget over PS-MB-100QAM and PS-Ir-100QAM, respectively.
RESUMO
Direct detection system is expected to possess the phase and polarization diversity in order to achieve high spectral efficiency and fiber impairment compensation such as chromatic dispersion and polarization rotation. In this Letter, we theoretically extend the concept of the proposed Jones-space field recovery (JSFR) to include a dynamic polarization rotation matrix and experimentally demonstrate the rapid polarization state tracking ability of the JSFR receiver based on a 3 × 3 optical coupler. Under a rotation of the state of polarization at a rate of 1â Mrad/s, we successfully transmit 59-GBd dual-polarization 16-ary quadrature-amplitude-modulation signals over an 80-km standard single-mode fiber based on a decision-directed least mean square (DD-LMS) or a recursive least square (DD-RLS), with a bit-error rate below the 14% hard-decision forward error correction threshold of 1 × 10-2. The experimental results indicate that the legacy polarization tracking algorithms designed for coherent optical communication are also applicable for this direct detection scheme. To our best knowledge, this work demonstrates the first polarization rotation-tolerant direct detection system with phase and polarization diversity, providing a low-cost and high-speed solution for short-reach communications.