Nonlinear Transmission Impairments Research Articles

Optical cable deployment versus fiber leasing: an operator’s perspective on CapEx savings for capacity upgrade in an elastic optical core network

Multiband (MB) and multifiber (MF) technologies get significant attention along with their individual pros and cons to solve the optical fiber capacity crunch problem. The MB system is more susceptible to nonlinear transmission impairments such as interchannel stimulated Raman scattering. On the contrary, MF solutions are costly due to the requirement of parallel fiber deployment. Therefore, it is necessary to find a hybrid solution using MB and MF technologies. This paper performs a techno-economic comparison between MF and MB technologies while taking a robust cost model. We propose, for the first time to our knowledge, a domain-knowledge-assisted, cost-effective network upgrade algorithm to enhance the overall network capacity while including minimum capital expenditure (CapEx). The proposed algorithm prioritizes the targeted deployment of cost-effective upgrade solutions in different links of the network to minimize the overall cost of the upgrade. The comparison between optical cable deployment and fiber leasing is captured to upgrade the overall network capacity with minimum cost-per-bit. Reported results show that a single fiber C+L band system can provide approximately 66.67% gain in terms of traffic admissibility compared to MF C band systems in smaller networks such as BT-UK, while considering fiber leasing. If operator owned dark fibers are not available in any of the links, numerical results show that the MF C+L band system in the BT-UK network can still provide CapEx savings of approximately up to 40.5% compared to the MF C band systems while minimizing the cost-per-bit of the network by approximately 21.9% to achieve the targeted network capacity of 150 Tbps. The effect of longer link length on the network upgrade cost is also studied in this paper. Reported result shows that, for larger geography such as the pan-Europe network, the channel launch power needs to be tuned to achieve the benefit of the MF C+L band compared to the MF C band system.

Ultralow Complexity Long Short-Term Memory Network for Fiber Nonlinearity Mitigation in Coherent Optical Communication Systems

Fiber Kerr nonlinearity is a fundamental limitation to the achievable capacity of long-distance optical fiber communication. Digital back-propagation (DBP) is a primary methodology to mitigate both linear and nonlinear impairments by solving the inverse-propagating nonlinear Schrödinger equation (NLSE), which requires detailed link information. Recently, the paradigms based on neural network (NN) were proposed to mitigate nonlinear transmission impairments in optical communication systems. However, almost all neural network-based equalization schemes yield high computation complexity, which prevents the practical implementation in commercial transmission systems. In this paper, we propose a center-oriented long short-term memory network (Co-LSTM) incorporating a simplified mode with a recycling mechanism in the equalization operation, which can mitigate fiber nonlinearity in coherent optical communication systems with ultralow complexity. To validate the proposed methodology, we carry out an experiment of ten-channel wavelength division multiplexing (WDM) transmission over 1600 km standard single-mode fiber (SSMF) with 64 Gbaud polarization-division-multiplexed 16-ary quadrature amplitude modulation (16-QAM) signals. A 0.51 dB Q² factor gain is observed with the Co-LSTM equalization, which is comparable to that of DBP. The complexity of the Co-LSTM equalization is only 5.2% of that of the conventional bi-directional LSTM, and 28.4% of that of the DBP method with a single step per span. In principle, the complexity of the Co-LSTM with a simplified mode is almost independent of the transmission distance, which shows an essential benefit over the DBP method that determined by the optical signal evolution along the fiber link. The proposed Co-LSTM methodology presents an attractive approach for low complexity nonlinearity mitigation with neural networks.

Mitigation of Nonlinear Impairments by Using Support Vector Machine and Nonlinear Volterra Equalizer

A support vector machine (SVM) based detection is applied to different equalization schemes for a data center interconnect link using coherent 64 GBd 64-QAM over 100 km standard single mode fiber (SSMF). Without any prior knowledge or heuristic assumptions, the SVM is able to learn and capture the transmission characteristics from only a short training data set. We show that, with the use of suitable kernel functions, the SVM can create nonlinear decision thresholds and reduce the errors caused by nonlinear phase noise (NLPN), laser phase noise, I/Q imbalances and so forth. In order to apply the SVM to 64-QAM we introduce a binary coding SVM, which provides a binary multiclass classification with reduced complexity. We investigate the performance of this SVM and show how it can improve the bit-error rate (BER) of the entire system. After 100 km the fiber-induced nonlinear penalty is reduced by 2 dB at a BER of 3.7 × 10 − 3 . Furthermore, we apply a nonlinear Volterra equalizer (NLVE), which is based on the nonlinear Volterra theory, as another method for mitigating nonlinear effects. The combination of SVM and NLVE reduces the large computational complexity of the NLVE and allows more accurate compensation of nonlinear transmission impairments.

A novel QoS oriented RoF system employing Bessel filter and external modulation to overcome non-linear transmission impairments

In optical WDM transport networks, the significant challenges for the transmission are non-linear impairments such as four-wave mixing (FWM), self-phase modulation and cross-phase modulation. These fiber nonlinearities result in deterioration of the performance of WDM systems. In this paper, the effects of FWM has been investigated on radio over fiber (RoF) links and a new RoF simulation model has been developed and discussed for wireless applications. The proposed model utilizes the Mach–Zehnder modulator in conjunction with Bessel filter to reduce the effect of FWM in RoF system. The simulation results (using OptiSystem 14.0) shows a reasonable reduction in FWM effects by considering performance metrics such as channel spacing, input source power and fiber dispersion. In comparison with existing RoF system, it is concluded that the proposed model provides the reduction in FWM sideband power by 10 dBm using channel spacing (10 nm), dispersion (16.75 ps/nm-km) and input power (− 10 dBm). This seems to be a viable QoS (quality of service) oriented RoF model for RoF system supporting wireless applications.

Demonstration of Bidirectional PON Based on Mode Division Multiplexing

Recently, mode division multiplexing (MDM) has been introduced into optical transmission systems and networks for capacity enhancement. In this letter, we propose a bidirectional passive optical network (PON) architecture based on MDM. The bidirectional optical distribution network of the MDM-PON consists of few-mode fiber, passive mode multiplexer/demultiplexer, and few-mode circulators, all of which are characterized by low modal-crosstalk. Signals in different modes are able to be transmitted and received independently for both downstream and upstream transmission. We experimentally demonstrate the bidirectional MDM-PON transmission over 10-km two-mode optical fiber. The 10-Gb/s ON–OFF-keying signals can be independently transmitted and directly detected without coherent receiver and multiple-input multiple-output processing. Moreover, nonlinear transmission impairments are experimentally investigated.

A New All-Optical Signal Regeneration Technique for 10 GB/S DPSK Transmission System

<p>The transmission of high power inside the optical fiber, produce amplitude noise, phase noise and other transmission impairments that degrade the performance of optical communication system. The signal regeneration techniques are used to mitigate these nonlinear impairments in the electrical or in the optical domain. All-optical signal regeneration techniques are one of the solutions to mitigate these nonlinear transmission impairments in the optical domain without converting the signal from optical to electrical domain. The existing techniques are not capable enough to attain the Bit Error Rate (BER) less than 10<sup>-10</sup> with the power penalty less than – 9dBm. In this paper, a new all-optical signal regeneration technique is developed that mitigate amplitude and phase noises in the optical domain. The new optical signal regeneration technique is developed by combining the two existing technique one is 3R (Reshaping, Reamplification and Retiming) regeneration and other is Phase Sensitive Amplification (PSA). The 10Gb/s Differential Phase shift Keying (DPSK) noisy transmission system is used to verify the features of developed technique. The developed technique successfully mitigates the nonlinear impairments from the noisy DPSK system with significant improvement in BER at low power penalty with the additional feature of high Q-factor and an eye open response for the regenerated signal. It is determined that BER of 10<sup>-12</sup> is achieved at the power penalty of -14 dBm with Q-factor of 42 and an eye opened response. The developed technique in the DPSK system is realized using commercial software package Optisystem. The designed technique will be helpful to enhance the performance existing high-speed optical communication by achieving the minimum BER at low power penalty.</p>

Effects of Adding/Dropping Nyquist WDM Superchannels on Linear and Nonlinear Transmission Impairments in an Elastic Optical Network

Effects of Adding/Dropping Nyquist WDM Superchannels on Linear and Nonlinear Transmission Impairments in an Elastic Optical Network

The error statistics analysis of the QPSK-modulated signal in the high-rate optical link

A model of an optical fibre communication line operating at 80Gb/s with QPSK modulation format is studied numerically. The error statistics and patterning effects are analyzed and nonlinear transmission impairments of the optical signal are quantified. Also the advanced detection scheme is proposed.

Evaluation of correlative coding and DP-16QAM n-channel 112Gbit/s coherent transmission: digital non-linear compensation perspective

We numerically report on the complexity reduction of digital backward propagation (DBP) by utilizing correlative encoded transmission (dual-polarization quadrature duobinary) at a bit-rate of 112Gbit/s over 1640km fiber link. The single channel (N=1) and multi-channel (N=10) transmission performances are compared in this paper. In case of multi-channel system, 10 transmitters are multiplexed with 25GHz channel spacing. The fiber link consists of Large A(eff) Pure-Silica core fiber with 20 spans of 82km each. No in-line optical dispersion compensator is employed in the link. The system performances are evaluated by monitoring the bit-error-ratio and the forward error correction limit corresponds to bit-error-ratio of 3.8×10(-3). The DBP algorithm is implemented after the coherent detection and is based on the logarithmic step-size based split-step Fourier method. The results depict that dual-polarization quadrature duobinary can be used to transmit 112Gbit/s signals with an spectral efficiency of 4-b/s/Hz, but at the same time has a higher tolerance to nonlinear transmission impairments. By utilizing dual-polarization quadrature duobinary modulation, comparative system performance with respect to dual-polarization 16-quadrature amplitude modulation transmission can be achieved with 60% less computations and with a step-size of 205km.

On nonlinear distortions of highly dispersive optical coherent systems

We investigate via experiments and simulations the statistical properties and the accumulation of nonlinear transmission impairments in coherent systems without optical dispersion compensation. We experimentally show that signal distortion due to Kerr nonlinearity can be modeled as additive Gaussian noise, and we demonstrate that its variance has a supra-linear dependence on propagation distance for 100 Gb/s transmissions over both low dispersion and standard single mode fiber. We propose a simple empirical model to account for linear and nonlinear noise accumulation, and to predict system performance for a wide range of distances, signal powers and optical noise levels.

On the Tolerance of 111-Gb/s POLMUX-RZ-DQPSK to Nonlinear Transmission Effects

We experimentally investigate the tolerance of 111-Gb/s polarization multiplexed-return to zero-differential quadrature phase-shift keying (POLMUX-RZ-DQPSK) to the different nonlinear transmission impairments in different links structures. We analyze the effect of dispersion maps for four different fiber types and show that with sufficiently large per-span under-compensation in the double periodic dispersion map, a dispersion managed (DM) link can nearly have a similar performance to a non-DM link. Furthermore, we confirm that the super large effective area fiber family can provide around 3 dB advantage in the tolerance to nonlinear effects compared to standard single-mode fiber. Finally, we demonstrate that interleaving the two polarization tributaries of the 111-Gb/s POLMUX-RZ-DQPSK signal provides only a negligible advantage in terms of tolerance to nonlinear effects.

Stochastic BER estimation for coherent QPSK transmission systems with digital carrier phase recovery

We propose a stochastic bit error ratio estimation approach based on a statistical analysis of the retrieved signal phase for coherent optical QPSK systems with digital carrier phase recovery. A family of the generalized exponential function is applied to fit the probability density function of the signal samples. The method provides reasonable performance estimation in presence of both linear and nonlinear transmission impairments while reduces the computational intensity greatly compared to Monte Carlo simulation.

Equalization of nonlinear transmission impairments by maximum-likelihood-sequence estimation in digital coherent receivers

We describe a successful introduction of maximum-likelihood-sequence estimation (MLSE) into digital coherent receivers together with finite-impulse response (FIR) filters in order to equalize both linear and nonlinear fiber impairments. The MLSE equalizer based on the Viterbi algorithm is implemented in the offline digital signal processing (DSP) core. We transmit 20-Gbit/s quadrature phase-shift keying (QPSK) signals through a 200-km-long standard single-mode fiber. The bit-error rate performance shows that the MLSE equalizer outperforms the conventional adaptive FIR filter, especially when nonlinear impairments are predominant.

Alternate-phase return-to-zero transmitter based on integrated dual-parallel Mach-Zehnder modulator

A simple 20 Gbit/s alternate-phase return-to-zero (APRZ) transmitter is demonstrated using a 10 Gbit/s integrated dual-parallel Mach-Zehnder modulator (DPMZM). Two RZ streams are time-multiplexed with stable and adjustable phase relation. Simulation results show that the APRZ generated by this transmitter is robust to nonlinear transmission impairments.

Digital self-coherent detection

We review recent progresses on digital self-coherent detection of differential phase-shift keyed (DPSK) signal using orthogonal differential direct detection followed by high-speed analog-to-digital conversion and digital signal processing (DSP). Techniques such as data-aided multisymbol phase estimation for receiver sensitivity enhancement, unified detection scheme for multi-level DPSK signals, and optical field reconstruction are described. The availability of signal field information brings the possibility to compensate for some linear and nonlinear transmission impairments through further DSP. An adaptive DSP algorithm for simultaneous electronic polarization de-multiplexing and polarization-mode dispersion compensation is also presented.

Electronic compensation of chromatic dispersion using a digital coherent receiver

Digital signal processing (DSP) combined with a phase and polarization diverse coherent receiver is a promising technology for future optical networks. Not only can the DSP be used to remove the need for dynamic polarization control, but also it may be utilized to compensate for nonlinear and linear transmission impairments. In this paper we present results of a 42.8Gbit/s nonlinear transmission experiment, using polarization multiplexed QPSK data at 10.7GBaud, with 4 bits per symbol. The digital coherent receiver allows 107,424 ps/nm of chromatic dispersion to be compensated digitally after transmission over 6400km of standard single mode fiber.

Design of multi-terabit/s terrestrial transmission systems facilitated by simple analytical tools

For 30 years, the capacity distance product of lightwave communication systems has continuously doubled every 16 months, to reach more than 10 Pbit/s.km in 2002. Along with this growth, the complexity of system engineering has increased as linear and nonlinear transmission impairments have become more stringent. Dispersion management is widely accepted as a powerful technique to mitigate some of them, but requires numerous experiments or time-consuming numerical simulations to be performed optimally. We exhibit two analytical rules that provide meaningful insight on this matter. Then we show how these rules helped us to improve the transmission distance of a recent 5 Tbit/s transmission experiment from 1,200 km to 1,500 km.

Computer simulation of a metro WDM interconnected ring network

The impact of nonlinear propagation impairments, amplifier gain ripple, and inband crosstalk on the performance of an interconnected-ring metropolitan-scale wavelength-division-multiplexed (WDM) network is investigated. Despite the apparently short fiber spans typically encountered in metro networks, longer paths may arise in interconnected ring architectures. Within these, nonlinear transmission impairments may become significant even at relatively low launched powers. These paths should be properly engineered to balance the effects of both receiver electrical noise as well as fiber nonlinearities. Common-channel crosstalk will also accumulate in typical worst-case paths introducing additional performance deterioration and as a result crosstalk level performance requirements need to be set for each elementary component in the path.