End-to-end learning strategy based on a frequency domain feature decoupling network emulator with joint probabilistic shaping and equalization for a 300-Gbit/s OAM mode division multiplexing transmission.

Mode coupling and device nonlinear impairment appear to be a long-standing challenge in the orbital angular momentum (OAM) mode division multiplexing (MDM) of intensity modulation direct detection (IM/DD) transmission systems. In this paper, we propose an end-to-end (E2E) learning strategy based on a frequency domain feature decoupling network (FDFDnet) emulator with joint probabilistic shaping (PS) and equalization for an OAM-MDM IM/DD transmission with three modes. Our FDFDnet emulator can accurately build a complex nonlinear model of an OAM-MDM system by separating the signal into features from different frequency domains. Furthermore, a FDFDnet-based E2E strategy for joint PS and equalization is presented with the aim of compensating the signal impairment for the OAM-MDM IM/DD system. An experiment is carried out on a 300 Gbit/s carrierless amplitude phase-32 (CAP-32) signal with three OAM modes over a 10 km ring-core fiber transmission, and the results show that the proposed FDFDnet emulator outperforms the traditional CGAN emulator, with improvements in the modelling accuracy of 30.8%, 26.3% and 31% for the three OAM modes. Moreover, the receiver sensitivity of the proposed E2E learning strategy is higher than for the CGAN emulator by 3, 2.5, 2.2 dBm and the real channel by 5.5, 5.1, and 5.3 dBm for the three OAM modes, respectively. Our experimental results demonstrate that the proposed FDFDnet emulator-based E2E learning strategy is a promising contender for achieving ultra-high-capacity interconnectivity between data centers.

Dual vector millimeter-wave signal generation based on optical carrier suppression modulation and direct detection with one photodetector.

We propose a novel, to the best of our knowledge, scheme for dual vector millimeter-wave (mm-wave) signal generation and transmission, based on optical carrier suppression (OCS) modulation, precoding, and direct detection by a single-ended photodiode (PD). At the transmitter side, two independent vector radio frequency (RF) signals with precoding, generated via digital signal processing (DSP), are used to drive an in-phase/quadrature (I/Q) modulator operating at the optical OCS modulation mode to simultaneously generate two independent frequency-doubling optical vector mm-wave signals, which can reduce the bandwidth requirement of transmitter's components and enhance spectral efficiency. With the aid of the single-ended PD and subsequent DSP at the receiver side, two independent frequency-doubling vector mm-wave signals can be separated and demodulated without data error. Based on our proposed scheme, we experimentally demonstrate the generation, transmission, and detection of 2-Gbaud 30-GHz quadrature-phase-shift-keying (QPSK) and 2-Gbaud 46-GHz QPSK signals over 10-km single-mode fiber-28 (SMF-28) and 1-m wireless transmission. The results indicate that the bit-error ratio (BER) of the dual vector mm-wave signals can each reach the hard-decision forward-error-correction (HD-FEC) threshold of 3.8 × 10-3.

OAM mode-division multiplexing IM/DD transmission at 4.32†Tbit/s with a low-complexity adaptive-network-based fuzzy inference system nonlinear equalizer.

Stochastic nonlinear impairment is the primary factor that limits the transmission performance of high-speed orbital angular momentum (OAM) mode-division multiplexing (MDM) optical fiber communication systems. This Letter presents a low-complexity adaptive-network-based fuzzy inference system (LANFIS) nonlinear equalizer for OAM-MDM intensity-modulation direct-detection (IM/DD) transmission with three OAM modes and 15 wavelength division multiplex (WDM) channels. The LANFIS equalizer could adjust the probability distribution functions (PDFs) of the distorted pulse amplitude modulation (PAM) symbols to fit the statistical characteristics of the WDM-OAM-MDM transmission channel. Therefore, although the transmission symbols in the WDM-OAM-MDM system are subjected to a stochastic nonlinear impairment, the proposed LANFIS equalizer can effectively compensate the distorted signals. The proposed equalizer outperforms the Volterra equalizer with improvements in receiver sensitivity of 2, 1.5, and 1.3 dB for three OAM modes at a wavelength of 1550.12 nm, respectively. It also outperforms a CNN equalizer, with improvements in receiver sensitivity of 1, 0.5, and 0.3 dB, respectively. Moreover, complexity reductions of 67%, 74%, and 99.9% are achieved for the LANFIS equalizer compared with the Volterra, CNN, and ANFIS equalizers, respectively. The proposed equalizer has high performance and low complexity, making it a promising candidate for a high-speed WDM-OAM-MDM system.

Data rate measurement method with selectable range in linear optical sampling.

Linear optical sampling (LOS) is one of the most powerful techniques for high-speed signal monitoring. To measure the data-rate of signal under test (SUT) in optical sampling, multi-frequency sampling (MFS) was proposed. However, the measurable data-rate range of the existing method based on MFS is limited, which makes it very difficult to measure the data-rate of high-speed signals. To solve the above problem, a range selectable data-rate measurement method based on MFS in LOS is proposed in this paper. Through this method, the measurable data-rate range can be selected to match the data-rate range of SUT and the data-rate of SUT can be measured precisely, independently of the modulation format. What's more, the sampling order can be judged using the discriminant in the proposed method, which is key for plotting eye diagrams with correct time information. We experimentally measure the baud-rates of PDM-QPSK signal from 800 MBaud to 40.8 GBaud in different ranges and judge the sampling orders. The relative error of measured baud-rate is less than 0.17% while the error vector magnitude (EVM) is less than 0.38. Compared with the existing method, under the same sampling cost, our proposed method realizes the selectivity of the measurable data-rate range and the judgment of sampling order, greatly extends the measurable data-rate range of SUT. Hence, the data-rate measurement method with selectable range has great potential for high-speed signal data-rate monitoring.

Bayesian generative adversarial network emulator based end-to-end learning strategy of the probabilistic shaping for OAM mode division multiplexing IM/DD transmission.

Orbital angular momentum (OAM) mode division multiplexing (MDM) has emerged as a new multiplexing technology that can significantly increase transmission capacity. In addition, probabilistic shaping (PS) is a well-established technique that can increase the transmission capacity of an optical fiber to close to the Shannon limit. However, both the mode coupling and the nonlinear impairment lead to a considerable gap between the OAM-MDM channel and the conventional additive white Gaussian noise (AWGN) channel, meaning that existing PS technology is not suitable for an OAM-MDM intensity-modulation direct-detection (IM-DD) system. In this paper, we propose a Bayesian generative adversarial network (BGAN) emulator based on an end-to-end (E2E) learning strategy with probabilistic shaping (PS) for an OAM-MDM IM/DD transmission with two modes. The weights and biases of the BGAN emulator are treated as a probability distribution, which can be accurately matched to the stochastic nonlinear model of OAM-MDM. Furthermore, a BGAN emulator based on an E2E learning strategy is proposed to find the optimal probability distribution of PS for an OAM-MDM IM/DD system. An experiment was conducted on a 200 Gbit/s two OAM modes carrierless amplitude phase-32(CAP-32) signal over a 5 km ring-core fiber transmission, and the results showed that the proposed BGAN emulator outperformed a conventional CGAN emulator, with improvements in modelling accuracy of 29.3% and 26.3% for the two OAM modes, respectively. Moreover, the generalized mutual information (GMI) of the proposed E2E learning strategy outperformed the conventional MB distribution and the CGAN emulator by 0.31 and 0.33 bits/symbol and 0.16 and 0.2 bits/symbol for the two OAM modes, respectively. Our experimental results demonstrate that the proposed E2E learning strategy with the BGAN emulator is a promising candidate for OAM-MDM IM/DD optic fiber communication.

Secure key real-time update and dynamic DNA encryption for CO-OFDM-PON based on a hybrid 5-D chaos.

A double key (DK) real-time update and hybrid five-dimensional (5-D) hyperchaotic deoxyribonucleic acid (DNA) dynamic encryption scheme is proposed, which can ensure the security in the orthogonal frequency division multiplexing passive optical network (OFDM-PON). Chaotic sequences for DNA dynamic encryption are produced using a four-dimensional (4-D) hyperchaotic Lü system and a one-dimensional (1-D) logistic map. In this scheme, the DK consists of an external key set, which is stored locally, and an internal key, which is associated with the plaintext and external key. In addition, a pilot cluster is used as the carrier of key transmission and key embedding is achieved by converting key to phase information of the pilot. To verify the feasibility of the scheme, a simulation validation is performed on a 46.5Gb/s 16 quadrature amplitude modulation (QAM) coherent OFDM-PON system transmitted over an 80 km transmission distance. The results show that the proposed scheme can improve the security performance of OFDM-PON at a low OSNR cost of 0.3 dB and the key space is expanded to (8.514 × 10102)S. When the correlation redundancy (CR) G⩾7, the 0 bit error rate (BER) of key can be achieved and the key can be updated and distributed in real-time without occupying additional secure channels.

400 Gbit/s 4 mode transmission for IM/DD OAM mode division multiplexing optical fiber communication with a few-shot learning-based AffinityNet nonlinear equalizer.

Nonlinear impairment in a high-speed orbital angular momentum (OAM) mode-division multiplexing (MDM) optical fiber communication system presents high complexity and strong stochasticity due to the massive optoelectronic devices. In this paper, we propose an Affinity Network (AffinityNet) nonlinear equalizer for an OAM-MDM intensity-modulation direct-detection (IM/DD) transmission with four OAM modes. The labeled training and testing signals from the OAM-MDM system can be regarded as "small sample" and "large target", respectively. AffinityNet can be used to build an accurate nonlinear model using "small sample" based on few-shot learning and can predict the stochastic characteristic nonlinearity of OAM-MDM with a high level of generalization. As a result, the AffinityNet nonlinear equalizer can effectively compensate the stochastic nonlinearity in the OAM-MDM system, despite the large difference between the training and testing signals due to the stochastic nonlinear impairment. An experiment was conducted on a 400 Gbit/s transmission with four OAM modes using a pulse amplitude modulation-8 (PAM-8) signal over a 2 km ring-core fiber (RCF). Our experimental results show that the proposed nonlinear equalizer outperformed the conventional Volterra equalizer with improvements in receiver sensitivity of 1.7, 1.8, 3, and 3.3 dB for the four OAM modes at the 15% forward error correction (FEC) threshold, respectively. In addition, the proposed equalizer outperformed a convolutional neural network (CNN) equalizer with improvements in receiver sensitivity of 0.8, 0.5, 0.9, and 1.4 dB for the four OAM modes at the 15% FEC threshold. In the experiment, a complexity reduction of 37% and 83% of the AffinityNet equalizer is taken compared to the conventional Volterra equalizer and CNN equalizer, respectively. The proposed equalizer is a promising candidate for a high-speed OAM-MDM optical fiber communication system.

Hidden conditional random field-based equalizer for the 3D-CAP-64 transmission of OAM mode-division multiplexed ring-core fiber communication.

As a key technique for achieving ultra-high capacity optical fiber communications, orbital angular momentum (OAM) mode-division multiplexing (MDM) is affected by severe nonlinear impairments, including modulation related nonlinearities, square-law nonlinearity and mode-coupling-induced nonlinearity. In this paper, an equalizer based on a hidden conditional random field (HCRF) is proposed for the nonlinear mitigation of OAM-MDM optical fiber communication systems with 20 GBaud three-dimensional carrierless amplitude and phase modulation-64 (3D-CAP-64) signals. The HCRF equalizer extracts the stochastic nonlinear feature of the OAM-MDM 3D-CAP-64 signals by estimating the conditional probabilities of the hidden variables, thereby enabling the signals to be classified into subclasses of constellation points. The nonlinear impairment can then be mitigated based on the statistical probability distribution of the hidden variables of the OAM-MDM transmission channel in the HCRF equalizer. Our experimental results show that compared with a convolutional neural network (CNN)-based equalizer, the proposed HCRF equalizer improves the receiver sensitivity by 2 dB and 1 dB for the two OAM modes used here, with l = + 2 and l = + 3, respectively, at the 7% forward error correction (FEC) threshold. When compared with a Volterra nonlinear equalizer (VNE) and CNN-based equalizer, the computational complexity of the proposed HCRF equalizer was found to be reduced by 30% and 41%, respectively. The bit error ratio (BER) performance and reduction in computational complexity indicate that the proposed HCRF equalizer has great potential to mitigate nonlinear distortions in high-speed OAM-MDM fiber communication systems.

Photonic-aided W-band dual-vector RF signal generation and detection enabled by bandpass delta-sigma modulation and heterodyne detection.

We propose a photonic-aided dual-vector radio-frequency (RF) signal generation and detection scheme enabled by bandpass delta-sigma modulation and heterodyne detection. With the aid of the bandpass delta-sigma modulation, our proposed scheme is transparent to the modulation format of the dual-vector RF signals and can support the generation, wireless transmission, and detection of both single-carrier (SC) and orthogonal-frequency-division-multiplexing (OFDM) vector RF signals with high-level quadrature-amplitude-modulation (QAM) modulation. With the aid of the heterodyne detection, our proposed scheme can support up to W-band (75-110 GHz) dual-vector RF signal generation and detection. For the validation of our proposed scheme, we experimentally demonstrate the simultaneous generation of a SC-64QAM signal at 94.5 GHz and a SC-128QAM signal at 93.5 GHz and their error-free high-fidelity transmission over a 20-km single-mode fiber 28 (SMF-28) and a 1-m single-input single-output (SISO) wireless link at the W-band. To the best of our knowledge, this is the first time that delta-sigma modulation has been introduced into a W-band photonic-aided fiber-wireless integration system to achieve flexible and high-fidelity dual-vector RF signal generation and detection.

Fast linear optical sampling with high repetition-frequency using fiber delay lines.

Linear optical sampling (LOS) is one of the most promising techniques for optical modulation analyzers. The LOS system generally adopts a mode-locked fiber laser (MFL) to generate an ultra-stable optical pulse to realize under-sampling for signal under test (SUT). However, it is challenging for MFL to produce a high-repetition-frequency pulse, making more measurement errors of conventional LOS technology, especially for high-speed signals. This paper proposes a dual-pulse mixing (DPM) based LOS system to increase the repetition frequency using fiber delay lines with the multiplied optical pulse. We propose the pulse location and peak extraction algorithms to compensate the time bias and amplitude bias in the DPM-based LOS system, which significantly improves the measurement speed and range. The experiment results show that the DPM-based LOS system can increase the number of sampling points twice compared with the conventional LOS within the same sampling time window. Furthermore, the proposed DPM-based LOS system can achieve less error vector magnitude with a reduction of 9.1% compared with the conventional LOS. Hence, the proposed DPM-based LOS system has great potential for high-speed signal processing.

Chaotic physical layer encryption scheme based on phase ambiguity for a DMT system.

Chaotic encryption is a promising scheme for physical layer security. By solving the multi-dimensional chaotic equations and transforming the obtained results, both bit-level and symbol-level encryption can be realized. One of the mainstream symbol-level encryption solutions is the constellation shifting (CS) scheme, which treats the chaotic sequence as artificial noise and adds it to the QAM signal sequence to achieve encryption. However, this scheme has several technical flaws in practical application, in terms of computational complexity and coexistence with blind equalization algorithm and the probabilistic shaping (PS) technique. In this paper, we propose a novel symbol-level encryption scheme based on phase ambiguity (PA), which converts the two sequences originally used to generate artificial noise into a set of phase rotation keys and complex conjugate keys, so that the encrypted symbols are still on the ideal constellation point coordinates. Simulation verification is carried out in a discrete multi-tone (DMT) system with 64QAM modulation. Results show that the proposed scheme can fully retain the shaping gain brought by the PS technique and avoid the error convergence of the blind equalizer. Moreover, the time required to solve the chaotic equations is only 38% of the CS scheme. Experimental verification is carried out, and the obtained results once again prove the superiority of the proposed encryption algorithm, which is a practical alternative for future physical layer secure optical communications.

Modified low CSPR Kramer-Kronig receivers based on a signal-signal beat interference estimation.

Due to their simple structures and high robustness to fibre dispersion, single-sideband transmission systems using Kramer-Kronig (KK) receivers are an attractive solution for 80 km data centre interconnects. However, a major problem with KK receivers is that the optimal carrier signal power ratio (CSPR) needs to be large, due to the existence of signal-signal beat interference (SSBI), but this results in low receiver sensitivity. We propose two modified KK receivers that can estimate and eliminate SSBI and allow for signal detection when the CSPR is less than 0 dB. After transmission over an 80 km optical fibre, our modified KK receivers were able to increase the receiver sensitivity by 3 dB compared with a traditional KK receiver while the CSPR was reduced by 6 dB, and improvements of 1.16 and 1.60 dB in the OSNR were achieved with a reduction in the CSPR of 8 dB. Our improved KK receivers can greatly reduce the carrier power and CSPR necessary for the system, and thus reduce the cost of the system transmitter.

Experimental demonstration of twin-single-sideband signal detection system based on a single photodetector and without optical bandpass filter.

A twin-single-sideband (twin-SSB) signal single-photodiode (PD) detection system without optical bandpass filter is experimentally demonstrated for the first time. After direct detection by a single-ended PD at the receiver side, we can directly separate the optical left sideband (LSB) and right sideband (RSB) using a simple one-path digital signal processing algorithm without separating the two sideband signals using an optical bandpass filter (OBPF), thus achieving lower complexity and low cost while doubling the spectral efficiency. Using our proposed twin-SSB scheme, we demonstrate 1-, 2-, and 4-Gbaud LSB geometric shaping 4-quadrature amplitude modulation and RSB quadrature phase shift keying signal transmission over 10 km of single-mode fiber (SMF). Our experimental results demonstrate that the bit-error rate (BER) of the 4-Gbaud LSB geometric shaping 4-quadrature amplitude modulation (GS-4QAM) and RSB quadrature phase shift keying (QPSK) transmission system is below the 7% hard decision forward error correction threshold.

Probabilistic neural network equalizer for nonlinear mitigation in OAM mode division multiplexed optical fiber communication.

Orbital angular momentum (OAM) mode-division multiplexing (MDM) is a key technique to achieve ultra-high-capacity optical fiber communications. However, the high nonlinear impairment from optoelectronic devices, such as spatial light modulators, modulators, and photodiodes, is a long-standing challenge for OAM-MDM. In this paper, an equalizer based on a probabilistic neural network (PNN) is presented to mitigate the nonlinear impairment for an OAM-MDM fiber communication system with 32 GBaud Nyquist pulse amplitude modulation-8 (PAM8) intensity-modulation direct-detection (IM-DD) signals. PNN equalizer can calculate the distribution of the nonlinearity using Bayesian decision theory and thus mitigate the stochastic nonlinear impairment of the received signal. Experimental results show that compared with the convolutional neural network (CNN) equalizer, the PNN equalizer improves the receiver sensitivity by 0.6dB and 2dB for two OAM modes with l = + 3 and l = + 4 at the 20% FEC limit, respectively. Moreover, compared with Volterra or CNN equalizers, the PNN equalizer can reduce the computation complexity significantly, which has great potential to mitigate the nonlinear signal distortions in high-speed IM-DD OAM-MDM fiber communication systems.

Fiber optofluidic Coriolis flowmeter based on a dual-antiresonant reflecting optical waveguide.

A microfiber optofluidic flowmeter based on the Coriolis principle and a dual-antiresonant reflecting optical waveguide (ARROW) is proposed and experimentally demonstrated. A hollow hole in a hollow-core fiber is fabricated as an optofluidic channel to move the liquid sample, which forms a dual-ARROW in the hollow-core fiber. Two sides of the hollow-core fiber are used as two adjacent Fabry-Perot resonators based on the refractive index modulation of a CO2 laser, which is used to investigate the vibration signals of the two resonators. The flow rate can be measured based on the Coriolis force by calculating the phase difference between the two ARROWs. The experimental results show that a flow rate sensitivity of 8.04 deg/(µl/s) can be achieved for ethanol solution. The proposed micro Coriolis fiber optic flowmeter can be used in various fields, such as food production, medicine, bioanalysis, etc.

Dynamic routing, modulation, and spectrum assignment based on fuzzy logic control in elastic optical networks.

Considering the constantly changing network resources and randomly generated spectrum fragmentation problem, a dynamic routing, modulation, and spectrum assignment based on the fuzzy logic control (RMSA-FLC) algorithm is proposed in this paper. A fuzzy logic control (FLC) system based on the degree of fragmentation optimization and the degree of link selection is constructed. The path-control weight (PW) is achieved as the output of the FLC system, and the path and spectrum allocation scheme with the maximum PW is selected for the immediate reservation requests. The simulation results show that the proposed RMSA-FLC algorithm can effectively reduce the blocking probability and improve the spectrum resource utilization in a dynamic elastic optical network.

Probabilistic shaped trellis coded modulation with generalized frequency division multiplexing for data center optical networks.

With the requirement for large capacity communication systems, the technology to close the gap between the achievable information rate and the Shannon capacity limit attracts more and more attention. In this paper, we present a novel scheme of trellis coded modulation combined with probabilistic shaping (PS-TCM) in the intensity-modulation/direct detection (IM/DD) system, using generalized frequency division multiplexing (GFDM). The principle of PS-TCM is analyzed with mutual information. We experimentally demonstrate the proposed scheme in the optical GFDM system. The results show that 4 Gb/s PS-TCM-32QAM signal achieves ∼2dB gain over the regular 16QAM signal in the back to back case. After 20 km transmission, the scheme of PS-TCM-32QAM provides 1.8 dB performance gain over that of the regular 16QAM signal. The bandwidth effect of optical filter on the performance of PS-TCM-32QAM signal is analyzed. The proposed scheme has significant gain, flexible spectrum efficiency, and approached Shannon limit, which brings a better trade-off between effectiveness and reliability performance for the multi-carrier optical system.