Impact Of Fiber Nonlinearity Research Articles

A Non-Data-Aided OSNR Estimation Algorithm for Coherent Optical Fiber Communication Systems Employing Multilevel Constellations

The performance of existing moments-based non-data-aided (NDA) optical signal-to-noise ratio (OSNR) estimation approaches degrades greatly for coherent optical systems employing multilevel constellations. We propose a novel NDA OSNR estimation algorithm, which provides enhanced performance for such systems. The proposed algorithm utilizes the empirical cumulative distribution function of the signal's amplitude to extract the information on the noise variance. Analytical and extensive simulation results show the feasibility and advantages of the algorithm. For the studied systems employing multilevel constellations such as 8-quadrature amplitude modulation (QAM), 16-QAM, 32-QAM, and 64-QAM, the proposed algorithm attains the derived Cramer–Rao lower bound. Furthermore, it achieves a lower mean square error with significantly lower complexity when compared to the conventional moments-based NDA estimation approach. Moreover, the impact of fiber nonlinearity is investigated with a five-channel Nyquist wavelength division multiplexing system, and the proposed algorithm outperforms the moments-based counterpart.

Non-Orthogonal Signal Transmission Over Nonlinear Optical Channels

The performance of spectrally efficient frequency division multiplexing (SEFDM) in optical communication systems is investigated considering the impact of fiber nonlinearities. Relative to orthogonal frequency division multiplexing (OFDM), sub-carriers within SEFDM signals are packed closer at a frequency spacing less than the symbol rate. In order to recover the data, a specially designed sphere decoding detector is used at the receiver end to compensate for the self-created inter carrier interference encountered in SEFDM signals. Our research demonstrated the benefits of the use of sphere decoding in SEFDM and also demonstrates the performance improvement of long-haul optical communication systems using SEFDM compared to the use of conventional OFDM, when fiber nonlinearities are considered. Different modulation formats ranging from 4QAM to 32QAM are studied and it is shown that, for the same spectral efficiency and information rate, SEFDM signals allow a significant increase in the transmission distance compared to conventional OFDM signals.

Impact of Fiber Nonlinearity on the Performance of Mode Division Multiplexing Systems

In this paper, an analytical model is developed to estimate a nonlinear phase noise (NPN) due to Kerr fiber nonlinearity and its interaction with amplifier noise in mode division multiplexing (MDM) systems. Our analysis uses generalized coupled multimode nonlinear Schrödinger equations (MM- NLSE) that describe the propagation of the mode superimposing in the optical fiber. The nonlinear phase noise versus channel power and transmission distance is evaluated by implementing our analytical model for LP01, LP11a, and LP11b spatial modes. Each mode carries a 4-QAM signal at a symbol rate of 20 Gsymbol/s. The results reveal that LP11 mode has a lower phase noise variance than LP01 mode over entire transmission distances. Finally, the error vector magnitude (EVM) versus channel power is quantified using an analytical model.

Frequency-Domain Hybrid Modulation Formats for High Bit-Rate Flexibility and Nonlinear Robustness

Since the deployment of the first commercial polarization-multiplexed (PM) QPSK 100G systems, the evolution of coherent optical communications in the last decade has been largely dominated by single-carrier quadrature amplitude modulation (QAM) based modulation of increasing constellation size. However, as the data traffic becomes more dynamic and heterogeneous, an efficient use of the optical link requires more flexible modulation schemes, capable of adapting data-rate and distance with fine granularity. While typical multi-carrier modulation schemes composed of hundreds of subcarriers may be inadequate for optical transmission, namely due its high peak-to-average power ratio and increased nonlinearity penalties, it has been recently shown that few-carrier modulation (with symbol rate in the order of 2–4 GBaud) can provide an increased robustness to nonlinear propagation impairments. In this paper, we exploit the concept of frequency-domain hybrid modulation formats (FDHMF) based on electronic subcarrier multiplexing with the use of different QAM formats on each subcarrier to simultaneously enhance the data-rate flexibility and the nonlinear propagation performance of the optical link. In addition, by properly designing the FDHMF signal, an increased tolerance against optical filtering can also be achieved. Using the enhanced Gaussian noise model, we report a comprehensive theoretical study on the performance of FDHMF, considering independent- and joint-subcarrier forward-error correction strategies, optimization of power ratio between subcarriers, and corresponding impact of fiber nonlinearities. These theoretical insights are then validated by a broad range of wavelength-division multiplexing experiments with per-channel bit-rates in the range of 150G to 250G. The obtained results demonstrate that FDHMF is an effective solution for the implementation of flexible transponders capable of adapting the data rate to the lightpath quality of transmission: An enabling technology for the introduction of future elastic optical networks.

Mitigation of nonlinear fiber distortion using optical phase conjugation for mode-division multiplexed transmission

Abstract We have investigated and proposed the use of optical phase conjugation (OPC) technique to mitigate the impact of fiber nonlinearities in mode-division multiplexed transmission systems. Numerical simulations are performed for three wavelengths, each loaded with 200 Gb/s dual-polarization 16-level quadrature amplitude modulation (DP-16QAM) format, in weakly guided two-mode fiber. It is known that differential mode group delay (DMGD) in mode-division multiplexed (MDM) transmission systems could be beneficial for system performance of MDM system with MIMO compensation in place. On the other side, for MDM system with OPC in place, the presence of DMGD may limit the overall benefits since signal power evolution per spatial modes should be symmetrical at the system midpoint in order to realize an effective compensation of the nonlinear effects. Our simulation results show that in the reference case (in the absence of DMGD), the employment of OPC module would lead to an average Q-factor improvement of approximately 10 dB. At the same time, in the presence of DMGD, an average Q-factor improvement would be ∼2.8 dB for WDM case. In addition, due to asymmetrical signal power map, the penalties induced by a periodic amplification process cannot be ideally compensated by the midpoint insertion of OPC. However, by accounting the impacts of both DMGD and asymmetrical signal power map, the insertion of the OPC system will still lead to an average Q-factor improvement of ∼1 dB for WDM channel arrangement.

MLSE-Based DQPSK Transmission in 43$~$Gb/s DWDM Long-Haul Dispersion-Managed Optical Systems

We analyze by simulation the performance of the differential quadrature phase-shift keying (DQPSK) modulation format in dispersion-managed multi-span WDM optical systems at 43 Gb/s, with 50 GHz channel spacing, in the presence of fiber nonlinearity. We show that the use of maximum-likelihood sequence-estimation (MLSE) receivers significantly helps to increase tolerance to dispersion map design. We also show that the use of an MLSE algorithm which takes into account the correlation of samples allows to improve system performance, partially mitigating the impact of fiber non-linearity. Some of the results presented here hint at a possible use of similar techniques in the context of coherent systems as well.

Electronic Dispersion Compensation

The performance of different electronic equalization and processing schemes for 40- and 10-Gb/s optical transmission over single-mode fiber (SMF) are discussed, from the point of their ability to compensate chromatic dispersion (CD) and polarization mode dispersion (PMD). In addition, the impact of fiber nonlinearity and modulation format on equalization is also investigated. The main objective of this paper is to present an overview and a comparison of the performances rather than a detailed explanation of the principles of the different equalization schemes. The equalizers which will be covered are analog equalizer (feedforward and decision feedback type), maximum likelihood sequence estimator (MLSE), electronic precompensation, coherent/intradyne detection with digital signal processing (DSP) equalization, DSP-based optical orthogonal frequency division multiplexing (OFDM), and turbo equalization.

Dispersion Management of Electrically Precompensated RZ Single-Sideband Signals at 10 Gb/s Without Inline Dispersion Compensation

An alternative approach is presented to minimize the impact of fiber nonlinearities in optical single-sideband systems at 10 Gb/s using electrical dispersion precompensation by adding optical postcompensation while maintaining low accumulated dispersion. Numerical simulation is used to show that the dependence of the system performance on the dispersion compensation scheme resembles a pseudolinear transmission regime, although it is not considered as such in the strictest sense. A launched power increase of 3 dB is achieved when compared to full electrically precompensated systems. A feasible implementation of the optical transmitter is considered, which imposes a maximum transmitted modulation depth. It is shown that low modulation depths result in an asymmetric optimum dispersion compensation map.

Impact of fiber nonlinearity on PMD penalty in DWDM transmission systems

Polarization-mode dispersion (PMD) penalty measurement on a 10-Gb/s deployment-ready dense wavelength-division-multiplexing (DWDM) long-haul transmission system is reported in this letter. We investigate the impact of self-phase modulation (SPM) and cross-phase modulation (XPM) on PMD penalty in the DWDM system. It is confirmed that SPM can suppress the PMD penalty, and XPM-induced polarization-scattering in DWDM systems can reduce the PMD impairment a little bit further. Our measurements suggest that it is necessary to carry out the PMD penalty test on DWDM systems rather than back-to-back systems in order to accurately allocate transmission margin of the systems.

Impact of Fiber Non-linearity in High Capacity WDM Systems and in Cross-Connected Backbone Networks

The rapidly increasing data traffic volumes will demand for very high transmission capacity and network nodes’ throughput. Wavelength division multiplexing (WDM) technology will be asked to support many channels on the same fiber, both in point-to-point links and in WDM optical networks. The transmission of a high number of wavelength channels in all these systems is a key issue. This paper analyzes this topic, in both high capacity links and optical networks, highlighting the impact of fiber non-linearity, and addressing the main source of impairments. This is done through the use of a semi-analytical model recently upgraded to account for all the contributions deriving from Kerr effects, particularly four-wave mixing and cross-phase modulation. The analysis reveals that more than one hundred of channels at 2.5 Gbit/s can be transmitted in point-to-point links whose length can span until the order of 1000 km, and 32 channels per fiber, at the previous bit rate, can be handled in WDM networks, without dispersion compensation. For a higher number of channels (e.g., 64) dispersion compensation is needed.