Standard Single-mode Fiber Transmission Research Articles

Fractionally spaced nonlinear Tomlinson-Harashima precoding with weight clustering for C-band 100 Gbaud/λ PAM-4 transmission.

A nonlinear Tomlinson-Harashima precoding (NTHP) scheme has been verified for its capability to effectively address both the linear and nonlinear inter-symbol interferences (ISIs) arising in the intensity-modulation direct-detection (IM-DD) fiber optics transmission. Nevertheless, the application of the NTHP scheme may significantly increase the number of levels for the intensity modulated signals, resulting in the reduction of both eye width and receiver sensitivity. Here, we propose a fractionally spaced NTHP with a weight clustering (FS-NTHP-WC) scheme. Consequently, an accurate ISI feedback can be obtained to enlarge the eye width; meanwhile a hardware-efficient implementation without the equalization penalty can be achieved by weight clustering and pruning. When the C-band 100 Gbaud/λ PAM-4 signals are transmitted, our proposed FS-NTHP-WC scheme not only can achieve 0.25 dB and 0.5 dB gains of receiver sensitivity under back-to-back (B2B) and 2-km standard single-mode fiber (SSMF) transmission conditions, respectively, but can also cut down the computational complexity by 90% and 76% in terms of the number of multiplications and additions, respectively, in comparison with the NTHP scheme.

Variational autoencoder-assisted unsupervised hardware fingerprint authentication in a fiber network.

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 20 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.

Total net-rate of 27.88 Tb/s full C-band transmission over 4,550 km using 150 km span length and high-gain EDFA amplification

We experimentally demonstrate a total net-rate of 27.88 Tb/s for C-band wavelength-division multiplexing (WDM) transmission over an ultralong span-length of 150 km. It is the largest net capacity × span-length product of 4182 Tb/s·km for C-band, single-core, standard single-mode optical fiber transmission over a length of more than 3,000 km. A total of 99 channels, spaced at 50 GHz intervals, are employed for transmitting 32 GBaud probabilistically constellation-shaped (PCS) 64QAM signals with an information entropy of 5.5. High gain amplifiers can achieve wavelength-division multiplexing (WDM) transmission with a bandwidth of 6.25 THz, at a noise figure below 4.3 dB, without the assistance of distributed Raman amplification.

Pairwise Tomlinson-Harashima precoding for multiple-lane IM-DD transmissions.

As for the photonic interconnection based on the multiple-lane intensity modulation direct detection (IM-DD) transmission, both intra-channel inter-symbol-interference (ISI) originating from bandwidth constraint, and inter-channel performance discrepancy emerging from inter-channel component differences are the major bottleneck for the throughput enhancement. Here, we propose a pairwise Tomlinson-Harshima precoding (P-THP) scheme, in order to simultaneously deal with both intra-channel ISI and inter-channel performance discrepancy. The effective function of the proposed P-THP scheme is experimentally evaluated by transmitting 4-channel 81-GBaud PAM4 signals over 2 km standard single-mode fiber (SSMF). Compared with the conventional scheme with only applying THP on individual wavelength channel, the required optical received power (ROP) under the back-to-back (B2B) transmission can be reduced by 0.75∼1 dB with the help of proposed P-THP in different experimental component configurations, at the 7% hard decision forward error correction (HD-FEC) threshold of BER = 3.8 × 10-3. After the 2 km SSMF transmission, only the use of proposed P-THP can guarantee to reach the designated HD-FEC threshold, leading to a net rate of >600 Gbit/s.

Fast fiber nonlinearity compensation method for PDM coherent optical transmission systems based on the Fourier neural operator.

Fiber nonlinearity compensation (NLC) is likely to become an indispensable component of coherent optical transmission systems for extending the transmission reach and increasing capacity per fiber. In this work, we introduce what we believe to be a novel fast black-box neural network model based on the Fourier neural operator (FNO) to compensate for the chromatic dispersion (CD) and nonlinearity simultaneously. The feasibility of the proposed approach is demonstrated in uniformly distributed as well as probabilistically-shaped 32GBaud 16/32/64-ary quadrature amplitude modulation (16/32/64QAM) polarization-division-multiplexed (PDM) coherent optical communication systems. The experimental results demonstrate that about 0.31 dB Q-factor improvement is achieved compared to traditional digital back-propagation (DBP) with 5 steps per span for PDM-16QAM signals after 1600 km standard single-mode fiber (SSMF) transmission at the optimal launched power of 4 dBm. While, the time consumption is reduced from 6.04 seconds to 1.69 seconds using a central processing unit (CPU), and from 1.54 seconds to only 0.03 seconds using a graphic processing unit (GPU), respectively. This scheme also reveals noticeable generalization ability in terms of launched power and modulation format.

Mode multicasting without parasitic wavelength conversion.

Optical multicasting, which involves delivering an input signal to multiple different channels simultaneously, is a key function to improve network performance. By exploiting individual spatial modes as independent channels, mode-division-multiplexing (MDM) can solve the capacity crunch of traditional standard single-mode fiber (SSMF) transmission system. In order to realize mode multicasting with high flexibility in future hybrid wavelength-division-multiplexing (WDM) and MDM networks, we propose a mode multicasting scheme without parasitic wavelength conversion, based on the inter-modal four-wave mixing (FWM) arising in the few-mode fiber (FMF). The operation mechanism including nonlinear phase shift for efficient mode multicasting is analytically identified. Then, based on the derived operation condition, we numerically investigate the impact of the dual-pump power and the FMF length on the performance of mode multicasting. By properly setting the pump wavelength and the dual-pump power, mode multicasting performance, in terms of mode multicasting efficiency, 3-dB bandwidth, and destination wavelength, can be tuned according to various application scenarios. After the performance optimization, mode multicasting of 25-Gbaud and 100-Gbaud 16-quadratic-amplitude modulation (16-QAM) signals is numerically demonstrated. The proposed reconfigurable mode multicasting is promising for future WDM-MDM networks.

Diffusion-assisted quantum noise stream cipher for physical layer security in UFMC

Universal filtered multicarrier (UFMC) is a promising multicarrier technologies due to its ability to suppress inter symbol interference, increase frequency band utilization and provide flexible adaptation. However, it is vulnerable to illegal eavesdropping in the point-to-multipoint access transmission. Quantum noise stream cipher (QNSC), a classical physical layer encryption scheme, is inherent matched with the UFMC system. However, its security mainly depends on the length of the basis states, which is limited by the performance of commercial high-speed digital-to-analog converters (DACs). In this work, we propose a diffusion-assisted quantum noise stream cipher (DA-QNSC) scheme to enhance the security. Here, we introduce artificial noise generated by hyperchaotic system to pseudo expand the distribution of quantum noise, which will make it more difficult for eavesdroppers to crack plaintext information violently. We experimentally demonstrate the encrypted 16/4096-QAM, 16/16384-QAM, 16/65536-QAM DA-QNSC signals over 30-km standard single mode fiber (SSMF) transmission. Compared to the conventional QNSC systems, the proposed method achieves a 3.6 times improvement in the security with the same DAC resolution and has negligible impact on bit error ratio (BER) performance. For the 16/65536-QAM signal, the number of masked symbols (NMS) are increased from 910 to 3270.

214.7 Tbit/s S, C, and L-band transmission over 50 km SSMF based on silicon photonic integrated transceivers

We experimentally demonstrate a 214.7 Tbit/s generalized mutual information (GMI) estimated throughput by ultra-wideband wavelength division multiplexing (WDM) transmission in standard single-mode fiber (SSMF). With 50-GHz grid, 396 transmission channels are used to deliver 49 GBaud probabilistically constellation-shaped (PCS) 256 quadrature amplitude modulation (QAM) and PCS-64QAM signals. Silicon photonic integrated transceiver is employed to complete electro-optic and optic-electro conversion of the modulated signals. S, C, and L-band rare-earth-doped amplifiers enable the 19.8 THz bandwidth WDM transmission without the assistance of distributed Raman amplification. The measured data rate shows great potential for Silicon photonic devices deployed in ultra-wideband WDM transmission.

Dynamic Secure Key Distribution Based on Dispersion Equalization and Cellular Automata for Optical Transmission

This paper proposes a dynamic secure key distribution scheme based on dispersion equalization and cellular automata (CAs). The scheme effectively eliminates the key inconsistency problem caused by imperfect channel reciprocity, and dynamic key sequences can be conveniently generated with large key space in long-haul optical transmission. In the process of communication, the legitimate parties obtain the secure core parameter from the frequency domain equalizer algorithm, and a final key sequence is generated through CA iterations on the basis of the core parameter. The randomness and reciprocity characteristics of the channel ensure the security and uniqueness of the core parameter and final key sequence. With 10G Baud 16 quadrature amplitude modulation over 400 km standard single-mode fiber transmission, the proposed scheme is verified with a free key error rate and an unlimited key generation rate. The security robustness of this scheme was theoretically analyzed and verified by sweeping the eavesdropper’s tapping position and improving CA operation processing. The proposed key distribution scheme is compatible with the existing transmission system for different signal modulation formats.

Computationally efficient composite triple beat cancellation for DML-based DA-RoF fronthaul.

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.

Weighted Decision Enabled Phase Retrieval Receiver With Adaptive Intensity Transformation

Phase-retrieval (PR) receivers can reconstruct complex-valued signals from two de-correlated intensity measurements by the use of iterative PR algorithm, without the assistance of any optical carriers. However, both the computational complexity with hundreds of iterations and the limited PR accuracy prevent it from being applied to high-speed photonic interconnections. In current submission, we propose a weighted decision enhanced PR receiver with adaptive intensity transformation (WD_AIT_PR). The weighted decision based on a compressed sigmoid nonlinear function is helpful to avoid the hard-decision (HD) errors, leading to the performance enhancement of the PR receiver, in terms of both convergence speed and PR accuracy. When the 56 GBaud 16QAM signal after transmission over 80 km standard single mode fiber (SSMF) is reconstructed, the WD_AIT_PR receiver is numerically verified to possess the faster convergency speed with only 70 iterations and the best steady BER performance by reducing the required optical signal-to-noise ratio (OSNR) by 0.8 dB, 1.8 dB and 1.8 dB in order to reach 7% HD forward error correction (HD-FEC), in comparison with the HD_AIT_PR receiver by replacing WD with HD, the PR receiver with the phase reset (PR_PR) and the PR receiver with adaptive intensity transformation (AIT_PR), respectively. Meanwhile, the WD_AIT_PR receiver is robust to transmission impairments, including transmitter-side I/Q imbalance, receiver-side skew, the noise imbalance between two intensity measurements, laser phase noise, and laser wavelength drift. Finally, the enhanced performance of the WD_AIT_PR receiver is experimentally verified through recovering 25 GBaud 16QAM signals under the condition of 40 km SSMF transmission.

Triple-Convex Probabilistic Constellation Shaping PAM8 in IM/DD System

A novel triple-convex 8-ary pulse amplitude modulation (TC-PAM8) probabilistic constellation shaping (PCS) scheme with bipolar symmetric alphabet is proposed in the intensity modulation/direct detection (IM/DD) system. Maintaining the basic implementation structure of the probabilistic amplitude shaping (PAS), the target entropy of TC-PAM8 is adjusted by setting the one-degree-of-freedom shaping factor accordingly. The performance of the proposed scheme is verified by an O-band 20-GBaud experimental system over 20-km standard single-mode fiber (SSMF) transmission, where the entropy is set to 2.2, 2.5, and 2.8 bit/symbol. The experimental results show that the proposed TC-PAM8 exhibits a better transmission performance compared with Maxwell-Boltzmann PAM8 (MB-PAM8) and Reverse-Maxwell-Boltzmann PAM8 (Rev-MB-PAM8) shaping scheme. Specifically, TC-PAM8 achieves a maximum gain of up to 1 dB and 2.2 dB in receiver sensitivity compared with the MB-PAM8 and Rev-MB-PAM8, respectively when the entropy is 2.2 bit/symbol.

Q-Band MMW Transmission Enabled by Joint Probabilistic Shaping and Precoding With MZM-Based OCS Modulation

In this letter, we propose a novel and straightforward scheme for Q-band (33-50GHz) millimeter-wave signal generation enabled by a Mach-Zehnder modulator (MZM)-based joint algorithm, which combines probabilistic shaping (PS) and precoding technology, to enhance the anti-interference ability of the transmission system. Based on the proposed scheme, we experimentally demonstrate the generation and transmission of 1.4-Gbaud 46-GHz PS-16-quadrature-amplitude-modulation (PS-16QAM) vector signals, showcasing improved bit-error-ratio (BER) in both standard single-mode fiber (SSMF) and wireless transmission cases. The proposed scheme exhibits a clear advantage in terms of nonlinear interference suppression and satisfies the hard-decision forward-error-correction (HD-FEC) threshold with a BER of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> in radio-over-fiber (RoF) systems.

Direct detection system based on a single photodiode receiving the independent quadruple-SSB signal.

We propose and verify a direct detection (DD) system based on a single photodiode (PD) receiving the independent quadruple-single-sideband (quadruple-SSB) signal. At the transmitter side, an I/Q modulator is utilized to modulate the independent quadruple-SSB signal, the signal is received via one PD without optical bandpass filters (OBPFs). Then, the independent quadruple-SSB signal is separated into four sideband signals by subsequent digital signal processing (DSP). In the scheme of back-to-back (BTB), 1-km and 5-km standard single-mode fiber (SSMF) transmission, the four sideband signals are extensively studied and analyzed. The simulation results show that the bit error rate (BER) of 1Gbaud, 2Gbaud and 4Gbaud independent quadruple-SSB signal can reach the 7% hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3 when the received optical power (ROP) is -21, -20 and -17.2 dBm in 5-km SSMF transmission. Meanwhile, as the frequency interval gets wider, the crosstalk in the sideband signal reception can be mitigated and the BER decreases. This scheme for the first time demonstrates that the independent quadruple-SSB signal can further expand the system transmission capacity and enhance the spectrum efficiency. Our simplified independent quadruple-SSB signal direct detection system has a simple structure and high spectral efficiency, which will have a promising future in high-speed optical communication.

Low-complexity optimized detection with cluster assisting for C-band 64-Gb/s OOK transmission over 100-km SSMF.

In this paper, a low-complexity optimized detection scheme consisting of a post filter with weight sharing (PF-WS) and cluster-assisted log-maximum a posteriori estimation (CA-Log-MAP) is proposed. Besides, a modified equal-width discrete (MEWD) clustering algorithm is proposed to eliminate the training process during clustering. After channel equalization, optimized detection schemes improve performance by suppressing the in-band noise raised by the equalizers. The proposed optimized detection scheme was experimentally performed in a C-band 64-Gb/s on-off keying (OOK) transmission system over 100-km standard single-mode fiber (SSMF) transmission. Compared with the optimized detection scheme with the lowest complexity, the proposed method saves 69.23% required number of real-valued multiplications per symbol (RNRM) at 7% hard-decision forward error correction (HD-FEC). In addition, when the detection performance reaches saturation, the proposed CA-Log-MAP with MEWD saves 82.93% RNRM. Compared with the classic k-means clustering algorithm, the proposed MEWD has the same performance without a training process. To the best of our knowledge, this is the first time clustering algorithms have been applied to optimize decision schemes.

Fourier Neural Operator for Accurate Optical Fiber Modeling With Low Complexity

In this paper, a novel deep learning architecture, Fourier neural operator (FNO), has been introduced to ap-proximate the nonlinear Schrodinger equation which characterizes fiber transmission impairments such as fiber attenuation, chromatic dispersion, nonlinear impairments, etc. The proposed scheme is firstly verified via numerical simulation in 28-GBaud 4QAM/16QAM/64QAM systems over 1200-km standard single mode fiber (SSMF) under different launch powers. The simulation results show that the mean square errors (MSEs) are less than 0.002 and the effective SNRs differences between the proposed FNO and SSFM are all within 1 dB at 1200 km SSMF with the launching power from −5 dBm to 5 dBm. The performance of the proposed FNO model is further evaluated by a conceptual experiment. The experimental results show that the Q-factor performance of the FNO-based channel model is close to that of 1200 km experimental SSMF transmission link with the launching power from −3 dBm to 5 dBm.

1.57 Tb/s PDM-layered 80QAM-OFDM in super-channel transmission based on self-homodyne comb

In this paper, polarization division multiplexing-layered 80-ary quadrature amplitude modulation-orthogonal frequency division multiplexing (PDM-layered 80QAM-OFDM) is proposed and demonstrated in super-channel system based on self-homodyne comb. The composition of the super-channel is derived from the optical comb generated by a single laser-driven dual polarized Mach-Zehnder modulator (DP-MZM) modulated by odd sideband suppression. Since the optical comb generated by the transmitter is used as a local oscillator (LO) signal for self-homodyne coherent detection. This scheme effectively reduces the system cost and alleviates the performance constraints caused by phase noise. Moreover, the bit error rate (BER) and generalized mutual information (GMI) of layered 24QAM/48QAM/80QAM-OFDM are evaluated to obtain the highest transmission rate and the laser linewidth tolerance of the system. The results show that in Additive White Gaussian Noise (AWGN) channel, layered QAM signal realized by low-complexity constellation layering technique can significantly improve the BER performance at a small entropy cost compared with ordinary QAM signals. Meanwhile, the layered 80QAM can achieve a signal-to-noise ratio (SNR) gain of 2.7 dB. In addition, after 40 km standard single mode fiber (SSMF) transmission, the total data rate of the system can reach 1.57 Tbit/s and the corresponding spectral efficiency (SE) is 12 bit/s/Hz when the BER of the super-channel system meets soft decision-forward error correction (SD-FEC) threshold.

Impact of the input OSNR on data-driven optical fiber channel modeling

The data-driven approach is promising for computationally efficient modeling of optical fiber channels. Here, for the first time to our knowledge, we investigate the impact of the input optical signal-to-noise ratio (OSNR) on the accuracy of data-driven modeling for optical fiber channels, based on a conditional generative adversarial network (cGAN). Initially, considering a single span of 80 km of standard single mode fiber (SSMF), we vary the launched optical power for the ease of emulating both linear and nonlinear transmission scenarios. When the input OSNR of a root raised cosine (RRC)-shaped 16 quadrature amplitude modulation signal with a roll-off factor of 0.1 varies, we identify that there occurs an OSNR threshold for accurately modeling a single span of 80 km of SSMF in order to reach the normalized mean square error of less than 0.02 in relevance to the traditional split-step Fourier method. The OSNR thresholds are 19 dB and 21 dB, respectively, under scenarios of linear and nonlinear transmissions. Moreover, we verify that the OSNR threshold of accurate modeling is insensitive to both the modulation format and the RRC shaping of the input signal. Furthermore, we find that the cascaded use of cGAN well-trained for the single SSMF span is capable of modeling the multiple-span SSMF transmission in the case that the input OSNR threshold of 22.6 dB is satisfied. We believe that the identification of the OSNR threshold for data-driven optical fiber channel modeling is useful for the accurate and efficient emulation of long-haul fiber optical transmission.

Hardware-efficient and accurately frequency offset compensation based on feedback structure and polar coordinates processing

Compared with off-line digital signal processing (DSP) verification, achieving the expected performance and hardware efficiency in application specific integrated circuit (ASIC) is more critical for DSP to reduce power consumption and cost. The performance of traditional frequency offset compensation algorithms based on feedforward structure is strongly dependent on the accuracy of frequency offset estimation. Therefore, frequency offset calculation and loop filtering implemented in ASIC or field programmable gate array (FPGA) usually choose high word-width fixed-point or floating-point operations, which increase the consumption of logical resources and reduce the clock frequency that hardware logic can achieve. This work proposes a frequency offset compensation scheme based on polar coordinate processing and feedback structure, and a pre-decision-based angle differential estimator is used to estimate the residual frequency offset. In the proposed feedback structure, the frequency offset estimator is realized by a simple accumulator, and the input of the accumulator is the residual frequency offset or its scaling, which will reduce the requirement for the accuracy of the residual frequency offset estimator. The offline verification results show that the performance of the proposed algorithm is close to that of the traditional algorithm, but the proposed algorithm has lower logic resource consumption and higher clock frequency in hardware implementation based on FPGA. The performance of the proposed method is evaluated in real-time through 10-Gbps data rate polarization-multiplexed quadrature phase shift keying modulation after 20-km standard single-mode fiber transmission. Compared with offline processing in MATLAB®, no hardware implementation penalty is observed.

Ultra-Wide Free-Spectral-Range Silicon Microring Modulator for High Capacity WDM

Silicon microring modulators (MRM) are a promising technology for next generation datacenter interconnects, where migration to higher data capacities will require the increasing use of wavelength division multiplexing (WDM). However the periodic repetition of wavelength resonances, and the limited free-spectral range (FSR) of MRMs, can limit the number of WDM channels when we use the wavelength selectivity of the MRM to modulate a particular wavelength from frequency comb sources or other multi-wavelength sources propagating in the bus waveguide. In this paper, we realize an ultra-compact, high bandwidth, CMOS compatible MRM which has an ultra-wide FSR of 4 THz, 3-dB electro-optic (EO) bandwidth of >67 GHz and which can operate at high datarates for standard single mode fiber transmission in the C-band. Using the modulator, we demonstrate 100 Gbaud PAM-4 intensity modulation-direct detection (IM-DD) with 4 wavelengths within a single FSR in the wavelength range of 1540 nm–1570 nm. The bit-error-rate (BER) of the transmission was below the general 7% hard-decision forward-error correction (HD-FEC) of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> .