Long-haul Optical Communication Research Articles

A photonic integrated circuit-based erbium-doped amplifier.

Erbium-doped fiber amplifiers revolutionized long-haul optical communications and laser technology. Erbium ions could provide a basis for efficient optical amplification in photonic integrated circuits but their use remains impractical as a result of insufficient output power. We demonstrate a photonic integrated circuit-based erbium amplifier reaching 145 milliwatts of output power and more than 30 decibels of small-signal gain-on par with commercial fiber amplifiers and surpassing state-of-the-art III-V heterogeneously integrated semiconductor amplifiers. We apply ion implantation to ultralow-loss silicon nitride (Si3N4) photonic integrated circuits, which are able to increase the soliton microcomb output power by 100 times, achieving power requirements for low-noise photonic microwave generation and wavelength-division multiplexing optical communications. Endowing Si3N4 photonic integrated circuits with gain enables the miniaturization of various fiber-based devices such as high-pulse-energy femtosecond mode-locked lasers.

On-Chip Waveguide Amplifiers for Multi-Band Optical Communications: A Review and Challenge

Erbium (Er)-doped fiber amplifier (EDFA) has achieved great success in modern optical communications for decades due to its outstanding gain performance in C-band. Nowadays, people are making efforts to make it monolithic integration due to the imperious demands from the advancement of high-performance and multi-function optoelectronic devices and chips. Except for the Er-doped waveguide amplifier, the other ion-doped on-chip waveguide amplifiers have also attracted much attention for multi-band optical communications, since they have own advantages in gain, bandwidth, selectivity of host materials, and integrability. Herein, we review the transmission characteristics of ion-doped on-chip waveguide amplifiers including gain and bandwidth, combining with the emission mechanisms and fabrication processes in applications of long-haul and short-reach optical communications. To facilitate the practical progress of the integrated waveguide amplifiers, from the perspective of gain medium, we also discuss the challenges and potential solutions for future technology roadmap. We expect that the bismuth (Bi)-doped waveguide amplifier would present impressive advantages in multi-band optical communications.

Tolerance of 16-Channel Dense Wavelength Division Multiplexing System to Fibre Dispersion and Four-Wave Mixing under Varying Capacities and Channel Spacing

An essential feature of a long-haul Optical Communication System (OCS) is to transmit huge volumes of data over long distances. In OCS, fiber dispersion and Four-Wave Mixing (FWM) play negative roles in achieving this, as the propagation of the light signal, associated with the pulse, travels at varying speeds. Dense Wavelength Division Multiplexing (DWDM) system is not spared from these identified limitations. In this work, a mathematical analysis for the dispersion and FWM was presented and validated through simulation software. The research was designed and implemented using an optical simulation software tool, OptiSystem 17.0. Transmission parameters of dispersion and Nonlinear (NL) effects due to refractive index, FWM, were considered. The research was carried out on a 125 km-long fiber link and was investigated under a 16-channel DWDM system. The effects of identified limitations on the performance of the DWDM system with channel arrangement of 100, 50, and 25 GHz were further explored. This work compared the DWDM system under these different channel arrangements. The transmission performance of the DWDM system was evaluated based on signal power, noise power, and eye pattern. The results obtained show that any increase in capacity causes significant decreases in the performance of the DWDM system. It was, however, noted that noise power is independent of an increase in bit rate. The results obtained further revealled that the launched power of the system must be reduced for better performance of the DWDM system and improved immunity against identified limitations.

Optimal power allocation in nonlinear MDM‐WDM systems using Gaussian noise model

Mode-division multiplexing (MDM) using few-mode fibre (FMF) has received increasing attention to address the exponential growth of data traffic in long-haul optical communication systems. Also, combining the MDM with wavelength-division multiplexing (WDM) is a promising approach for dramatically growing the transmission capacity in such systems. However, a major barrier in this regard is the FMF nonlinear effects, which can significantly reduce the link performance. In this paper, in order to alleviate the FMF nonlinear effects, we focus on power allocation in FMF links by optimizing the input power of each optical WDM channel of each spatial mode, which leads to maximizing the total capacity transmission and also the minimum signal to noise ratio (SNR) margin. The FMF nonlinearity has been already modelled as the Gaussian noise (GN) for which no closed-form formulation has been developed so far. Here, we derive a closed-form GN model for this problem and verify it by comparing with the integral-form GN model and split-step Fourier method. In this approach, an optimal power is independently determined for each channel of each mode by optimizing a capacity maximization and a minimum SNR margin maximization problem in convex forms. The performance of different links including the single mode fibre-WDM, MDM-single channel, and MDM-WDM are compared using computer simulations. These systems are comprehensively investigated in equal/non-equal required SNR as well as flat/non-flat amplifier gain scenarios. It is shown that optimized power allocation to each channel of each mode has a significant enhancement in the minimum SNR margin maximization scheme compared to the best equal power allocation. Furthermore, this improvement is much more in non-equal required SNR and the non-flat amplifier gain scenarios, showing the efficiency of the established approach in practical communication links.

Dual-Stage EDFA for Improving the Performance of Long-Haul Optical Systems

A cascade of single-stage erbium doped fiber amplifiers (SS-EDFAs) are known to improve the signal strength and system reach in long-haul optical systems. However, such a cascade introduces an amplified spontaneous emission (ASE) noise accumulation, which results in system performance degradation. To reduce this ASE noise accumulation, we propose the incorporation of a dual-stage EDFA (DS-EDFA) with a narrow bandwidth (typically 3 nm or even smaller depending on the system under consideration) bandwidth optical bandpass filter (OBPF) in the system. We analyze the role of a DS-EDFA, which is formed using two identical SS-EDFAs, for its effectiveness in improving the overall system performance. The gain vs input power characteristic comparison is provided for the DS-EDFA (without interstage OBPF) modelled in OptSim and MATLAB. A system-level numerical model in MATLAB matching the OptSim parameters has been presented to validate the OptSim results. For a cascade incorporating DS-EDFA with interstage OBPF, a system-level analysis is presented with results obtained from a simulated model designed using OptSim. The results demonstrate the effectiveness of DS-EDFA in improving the optical signal-to-noise ratio (OSNR) and bit error rate (BER) performance of the long- haul optical system under consideration. From the obtained results, the optimum placement of a DS-EDFA at the start of the amplifier chain/cascade is recommended for practical implementation in a long-haul optical communication system.

Combined Neural Network and Adaptive DSP Training for Long-Haul Optical Communications

Machine Learning (ML) algorithms have shown to complement standard digital signal processing (DSP) tools in mitigating fiber nonlinearity and improving long-haul transmission performance. However, dynamic transmission impairments such as polarization effects and carrier phase noise corrupt the training data and conventional cost functions for neural network (NN) training become unsuitable. Simple cascade of ML and standard adaptive DSP blocks will also result in suboptimal transmission performance or require impractical training methodologies in presence of such dynamic transmission impairments. We show how the adaptive DSP blocks can be treated as extra stateful NN layers and be combined with the main NN so that standard backpropagation-like training algorithms in ML can be applied. In this case, the adaptive filters are viewed as NN states which are updated in the forward pass of the backpropagation. We study the combined training of linear and nonlinear parameters in the digital backpropagation (DBP) algorithm for fiber nonlinearity compensation (named generalized DBP (GDBP) hereafter), residual impairments, polarization effects, frequency offsets and carrier phase noise compensation filters as a single NN in a 7 × 288 Gb/s polarization multiplexed (PM)-16QAM transmission experiment over 1125 km. We derived the complete set of backpropagation-like gradients and state update equations for the static and dynamic parameters of the combined NN. We further proposed and open-sourced a JAX-based coding framework for their easy and practical implementation. GDBP is more generalized than other DBP variants proposed in literature and for a given total number of steps, GDBP is the first experimental demonstration of optimal single-channel DBP based-fiber nonlinearity compensation algorithm. In addition, for complexity constrained situations with shortened filter taps, GDBP enables a 1 dB performance improvement over other DBP variants.

Performance and Complexity Analysis of Bi-Directional Recurrent Neural Network Models Versus Volterra Nonlinear Equalizers in Digital Coherent Systems

We investigate the complexity and performance of recurrent neural network (RNN) models as post-processing units for the compensation of fibre nonlinearities in digital coherent systems carrying polarization multiplexed 16-QAM and 32-QAM signals. We evaluate three bi-directional RNN models, namely the bi-LSTM, bi-GRU and bi-Vanilla-RNN and show that all of them are promising nonlinearity compensators especially in dispersion unmanaged systems. Αs far as inference is concerned, οur simulations show that the three models provide similar compensation performance, therefore, in real-life systems, the simplest scheme based on Vanilla-RNN units should be preferred. We compare bi-Vanilla-RNN in its many-to-many form with Volterra nonlinear equalizers and exhibit its superiority both in terms of performance and complexity, thus highlighting that RNN processing is a very promising pathway for the upgrade of long-haul optical communication systems utilizing coherent detection.

Nonlinear Coherent Optical Systems in the Presence of Equalization Enhanced Phase Noise

Equalization enhanced phase noise (EEPN) occurs due to the interplay between laser phase noise and electronic dispersion compensation (EDC) module. It degrades significantly the performance of uncompensated long-haul coherent optical fiber communication systems. In this work, a general expression accounting for EEPN is presented based on Gaussian noise model to evaluate the performance of multi-channel optical communication systems using EDC and digital nonlinearity compensation (NLC). The nonlinear interaction between the signal and the EEPN is analyzed. Numerical simulations are carried out in nonlinear Nyquist-spaced wavelength division multiplexing (WDM) coherent transmission systems. Significant performance degradation due to EEPN in the cases of EDC and NLC are observed, with and without the consideration of transceiver (TRx) noise. The validation of the analytical approach has been done via split-step Fourier simulations. The maximum transmission distance and the laser linewidth tolerance are also estimated to provide important insights into the impact of EEPN.

Trading off security and practicability to explore high-speed and long-haul chaotic optical communication.

Recent demonstrations of chaos-based secure communication have proven the feasibility of secured transmission of high-speed (tens of Gbit/s) signals over certain distances (∼100-km), which bring hope for secure communication from theoretical analysis to practical applications. So far, the chaos-based secure communication system with chaos-masking (CMS) encryption is considered as one of the most important and feasible schemes. In this paper, an optical chaotic carrier generated by an opto-electronic oscillator is used to encrypt 112-Gbit/s message by CMS encryption for data transmission over a 1040-km single-mode-fiber. The message is successfully decrypted by combining coherent detection and our proposed blind decryption algorithms, which can successfully separate the chaotic carrier and the message with the bit-error-rate (BER) below the forward error correction (FEC) threshold. Experimental results show that the coherent detection combined digital signal processing algorithms may be a possible way to promote the practical applications of chaotic optical communication in the future. In addition, this paper reveals that the security of the CMS encryption may be not high enough for those systems requiring rigorous confidentiality. Subsequently, we further discuss the bottlenecks encountered in current high-speed chaotic optical communication systems and analyze how to improve and weight the security and practicability.

Probing of nonlinear impairments in long range optical transmission systems

Abstract Optical communication systems represent the backbone of modern long-haul communication networks. Nonlinear impairments such as the Kerr effect pose threat and deteriorate transmission in long-haul optical communication systems. At high launch powers, nonlinear effects become more important and their compensation is entailed to improve the transmission performance. In this paper, one of the major nonlinear effects of four waves mixing (FWM) is addressed using a theoretical model and validated through simulation in Optisystem. Two mechanisms with and without dispersion variation are investigated. The impact of FWM is investigated in terms of various values of launch power. The transmission performances of the proposed model are evaluated on the basis of bit error rate, optical signal to noise ratio, and quality factor using key parameters of effective area, nonlinear refractive index, nonlinear dispersion, and linear dispersion.

Generation of Solitons:The wave of future

Very high speed and data rate optical fiber transmissions over a large distance in thousands of kilometers is turning into a reality. This paper discusses about the evolution of soliton beats along with their generation which is shown with the help of simulations carried out in Opti-system. Stable soliton pulses result from the balance between combination of both linear and non-linear effects i.e. Group Velocity Dispersion (GVD) and Self Phase Modulation (SPM) simultaneously. Thus, Long-haul optical fiber communication can be made possible now using solitons, as they provide high data conveying capacity and require less no of repeaters for transmission making them ideal waves of the future. At the end, some of the issues and challenges related with solitons and their cures have additionally been laid out.

Tunable Optical Demultiplexer for Dense Wavelength Division Multiplexing Systems Using Graphene–Silicon Microring Resonators

An ultracompact tunable optical dense wavelength division demultiplexer (DWDM) compatible with complementary metal–oxide–semiconductor technology is proposed, consisting of a central bus waveguide coupled to two drop waveguides through two ring resonators, all made of silicon (Si). Each Si ring resonator is topped by a graphene microribbon whose chemical potential can be tuned as desired. The center wavelength of each drop channel is determined by the resonant wavelength of the adjacent ring resonator, which in turn is tuned by modifying the conductivity of the graphene. Simulations show that the proposed structure with two graphene–Si ring resonators (GSRRs) having a chemical potential difference of 210 meV demultiplexes a broad optical signal into two narrow channels centered at about 1552.42 nm and 1552.85 nm (with a channel spacing of less than 0.43 nm), each emerging from one drop waveguide, with a transmission efficiency above 64%. The full-width at half-maximum values of the channels are 160 pm and 250 pm, providing quality factors of ∼ 9705 and 6209 with crosstalk of less than − 13.3 dB. By exchanging the chemical potentials of the GSRRs, the drop channels can be swapped dynamically without mechanical manipulation of the device. The proposed ultracompact graphene-based demultiplexer enables electrically tuned optical demultiplexing for DWDM systems as a potential alternative to the thermooptically tuned demultiplexers employed in long-haul optical communications.

Modeling and minimization of FWM effects in DWDM-based long-haul optical communication systems

Optical communication systems (OCSs) mainly represent the backbone of modern long-haul communication networks because of low loss transmission over long distances and ultra-high capacity. However high data-rate transmission through optical fiber suffers from deterioration due to nonlinear impairments, such as four-wave mixing (FWM) in particular. At high launch power levels, which are required for the long-haul transmission over hundreds of km, these nonlinear effects become more severe which imposes a challenge to achieve satisfactory transmission performance. In this paper, a theoretical model for the FWM effects and its mitigation is presented and validated through simulation results. Moreover, two other nonlinear effects, polarization mode dispersion and nonlinear dispersion variations are also investigated for various values of launch power level. The transmission performance of the proposed OCS model is evaluated on the basis of bit error rate, optical signal-to-noise ratio and quality factor using different transmission channel parameters such as effective area, nonlinear refractive index, nonlinear dispersion, and linear dispersion.

Symmetric-type dispersion maps in dispersion-managed optical link with mid-span spectral inversion

In long-haul optical communication systems consisting of single-mode fiber spans and fiber amplifiers, such as an erbium-doped fiber amplifier, signal distortion causes performance to deteriorate because of group velocity dispersion and fiber nonlinearity. A combination of dispersion management and optical phase conjugation is an effective technique of compensating for the distortion. In an optical link configured with this combination, the dispersion map mainly affects the compensation for the distorted optical signals. Improvements in system performance have been reported for various types of dispersion maps. In this study, a symmetric type of dispersion map with respect to the midway optical phase conjugator is proposed. The effect of the proposed dispersion maps on the compensation for the distorted 24 channel × 40 Gbps wavelength-division-multiplexed signals was assessed through numerical simulation. It was confirmed that antipodal-type dispersion maps are most appropriate for the compensation, as well as for the flexibility of the link configuration.

Design of Time-Frequency Packed WDM Superchannel Transmission Systems

We consider time and frequency-packing (TFP) wavelength division-multiplexing (WDM) superchannel systems for longhaul optical fiber communication. Employing very high baud rates in conjunction with higher-order modulation formats is challenging due to practical constraints. TFP superchannels therefore become an attractive choice to facilitate high data rates with improved spectral efficiency. However, TFP introduces inter-symbol interference (ISI) and/or inter-carrier interference (ICI) that necessitate efficient interference handling techniques. Moreover, phase noise (PN) due to wide laser linewidth (LLW) may cause significant signal distortion in WDM transmission, and the presence of ISI and ICI in TFP systems further complicates carrier phase recovery (CPR). In this article, we propose a spectrally efficient TFP superchannel design equipped with powerful interference cancellation and PN mitigation techniques, targeting Terabit-per-second (Tbps) data rates. First, we propose a joint ISI and ICI channel estimation algorithm coupled with polarization-recovery and a coarse PN cancellation method. Second, we investigate two iterative CPR algorithms to mitigate the distortion due to the residual PN. Third, a combination of successive and parallel interference cancellation methods for ICI mitigation is investigated in tandem with turbo ISI equalization. The effectiveness of the proposed algorithms is demonstrated through computer simulations of a coded TFP superchannel system in the presence of linear fiber impairments, targeting a throughput of 1.2 Tbps. Numerical results suggest that the proposed TFP design offers more than 2 dB performance gains and as high as 960 km transmission distance improvement over existing competitive super-Nyquist designs. Simulation results also indicate that the proposed design exhibits excellent tolerance to high LLWs and aggressive optical filtering stemming from cascaded reconfigurable optical add-drop multiplexers in the fiber link.

Advancing theoretical understanding and practical performance of signal processing for nonlinear optical communications through machine learning

In long-haul optical communication systems, compensating nonlinear effects through digital signal processing (DSP) is difficult due to intractable interactions between Kerr nonlinearity, chromatic dispersion (CD) and amplified spontaneous emission (ASE) noise from inline amplifiers. Optimizing the standard digital back propagation (DBP) as a deep neural network (DNN) with interleaving linear and nonlinear operations for fiber nonlinearity compensation was shown to improve transmission performance in idealized simulation environments. Here, we extend such concepts to practical single-channel and polarization division multiplexed wavelength division multiplexed experiments. We show improved performance compared to state-of-the-art DSP algorithms and additionally, the optimized DNN-based DBP parameters exhibit a mathematical structure which guides us to further analyze the noise statistics of fiber nonlinearity compensation. This machine learning-inspired analysis reveals that ASE noise and incomplete CD compensation of the Kerr nonlinear term produce extra distortions that accumulates along the DBP stages. Therefore, the best DSP should balance between suppressing these distortions and inverting the fiber propagation effects, and such trade-off shifts across different DBP stages in a quantifiable manner. Instead of the common ‘black-box’ approach to intractable problems, our work shows how machine learning can be a complementary tool to human analytical thinking and help advance theoretical understandings in disciplines such as optics.

Comprehensive investigation of coherent optical OFDM-RoF employing 16QAM external modulation for long-haul optical communication system

Given the growing need for long haul transmission that requires a high rate of data, the orthogonal frequency division multiplexing scheme (OFDM), is regarded as a technique with high potentials for high-capacity optical networks. OFDM transmits over both optical and wireless channels, with the data distributed over a huge amount of the subcarrier, and the data is distributed over a huge number of subcarriers. OFDM achieves RF signal for a long-haul transmitting by utilizing Radio over Fiber (RoF) system, which is known to produce higher orthogonality of the OFDM modulated signal designed for the wireless network. RoF systems comprise of heterogeneous networks designed through the use of wireless and optical links. The aim of this paper is to carry out an investigation of the performance of the external modulation in RoF links, while analyzing the shortcomings caused by the various elements of the optical system. The Mach–Zehnder modulator (MZM) can be applied in external modulation, and exhibits a more robust performance when implemented with the OFDM modulation technique.

Modeling and performance investigation of 4 $$\times$$ 20 Gbps underwater optical wireless communication link incorporating space division multiplexing of Hermite Gaussian modes

In this work, we report the development of long-haul high-speed underwater optical wireless communication (UOWC) transmission link by incorporating space division multiplexing (SDM) of distinct Hermite Gaussian (HG) modes for enhanced channel-capacity. Four distinct 20 Gbps non-return to zero encoded information signals are transmitted over distinct HG modes of a single wavelength laser at 1800 m underwater link range to realize an 80 Gbps UOWC system. Also, we investigate the performance of the proposed UOWC transmission link under the influence of different operating wavelengths, aperture diameter, attenuation coefficient, and link range. Further, we optimize the proposed UOWC link performance in terms of spatial aperture to stabilize the bit-error rate (BER) performance of the link at high signal attenuation values and varying transmission link reach.

Soft Failure Identification for Long-haul Optical Communication Systems Based on One-dimensional Convolutional Neural Network

With the advance of elastic optical networks, optical communication systems are becoming more flexible and dynamic. In this scenario, soft failures are more likely to occur due to various link impairments. If these soft failures are not handled properly and timely, service disruption may occur. Identifying the cause of soft failure is a key step to restore the degraded links. However, it is difficult for traditional methods to accomplish this task. Fortunately, powerful machine learning (ML) algorithms provide a promising path to address this problem. In this article, a novel two-stage soft failure identification scheme based on a convolutional neural network (CNN) and receiver DSP is proposed. The input of the CNN is the power spectrum density (PSD) extracted from a coherent receiver, and the output contains the identified cause of soft failures together with their probabilities. Extensive simulations are performed to validate the proposed method. Four types of soft failure causes are explored including the offset of optical filter's center frequency (FS), the tightening of optical filter's 3-dB bandwidth (FT), SNR degradation due to the increased amplified spontaneous emission (ASE) noise, and the Kerr nonlinear effect. When only one soft failure cause exists, excellent accuracy is achieved. When multiple soft failure causes exist, the probabilities of these causes provided by the CNN are used to gain insight into their influences on the system. Finally, we investigate the interpretation of the CNN and a reasonable interpretation is given and discussed.

Compact silicon photonic hybrid ring external cavity (SHREC)/InGaSb-AlGaAsSb wavelength-tunable laser diode operating from 1881-1947 nm.

In recent years, the 2 µm waveband has been gaining significant attention due to its potential in the realization of several key technologies, specifically, future long-haul optical communications near the 1.9 µm wavelength region. In this work, we present a hybrid silicon photonic wavelength-tunable diode laser with an operating range of 1881-1947 nm (66 nm) for the first time, providing good compatibility with the hollow-core photonic bandgap fiber and thulium-doped fiber amplifier. Room-temperature continuous-wave operation was achieved with a favorable on-chip output power of 28 mW. Stable single-mode lasing was observed with side-mode suppression ratio up to 35 dB. Besides the abovementioned potential applications, the demonstrated wavelength region will find critical purpose in H2O spectroscopic sensing, optical logic, signal processing as well as enabling the strong optical Kerr effect on Si.