Launch Power Research Articles

Nonlinear transmission performance comparison employing 60 × 416.7-Gb/s TPS-64QAM and UD-16QAM systems with limited bandwidth.

The impacts of limited bandwidth on the nonlinear transmission performance are investigated by employing a truncated probabilistic shaped 64-ary quadrature amplitude modulation (TPS-64QAM) and a uniformly distributed 16-ary quadrature amplitude modulation (UD-16QAM) over a bandwidth-limited 75-GHz spaced 25-Tb/s (60 × 416.7 Gb/s) 6300-km transmission system. In terms of nonlinear performance measured by optimal launch power, theoretical analyses show that a 0.4-dB improvement could be introduced by UD-16QAM with respect to TPS-64QAM over a 6300-km transmission without limited bandwidth. However, contrary results would be observed that TPS-64QAM would outperform UD-16QAM by about 0.8 dB in terms of optimal launch power when the impacts of limited bandwidth are considered. Besides, numerical simulations and experimental results could both validate that about 1.0-dB optimal launch power improvement could be obtained by TPS-64QAM under bandwidth-limited cases, which is roughly similar to the results of theoretical analyses. Additionally, WDM experimental results show that all 60 tested channels could agree with the BER requirements by employing TPS-64QAM, further validating the superiority of TPS-64QAM compared to UD-16QAM under bandwidth-limited cases.

Comparison of fast quality of transmission estimation methods for C + L + S optical systems

Due to the high potential of multi-band transmission (MBT) systems as a short- to medium-term solution to the ever-increasing demand for fiber capacity, one of the recent areas of research concerns the development of fast but still accurate models to estimate the quality of transmission (QoT) in these systems, since this capability will be paramount for efficient and cost-effective network planning and operation. However, accurately evaluating the QoT of MBT systems with moderate computational complexity is challenging, mainly because stimulated Raman scattering (SRS) becomes a major transmission impairment. Although several models have been proposed in the literature, to the best of the authors’ knowledge, there is a lack of a comprehensive comparison of the advantages and main limitations of the available methodologies. Therefore, this work analyzes the computational time and accuracy of several QoT estimation methods suitable for MBT systems [closed-form interchannel SRS Gaussian noise (ISRS-GN) and four-wave mixing (FWM) models], focusing on transmission systems comprising a total transmission bandwidth of up to 20 THz, which includes not only the C- and L-bands but also a part of the complete S-band. We also propose a modification to the FWM model to take into account the SRS effect in the calculation of the nonlinear interference, referred to as the enhanced FWM (eFWM) model. We show that the closed-form ISRS-GN model has the best trade-off between computational complexity (speed) and accuracy for launch powers per channel up to 4 dBm in a system with 10 spans and with span lengths varying from 50 km to 100 km. However, this method shows generalized signal-to-noise ratio estimation errors as high as 1.5 dB for higher launch powers. Similar conclusions were drawn when using the models to optimize the launch powers in the reference Italian backbone network, i.e., using the closed-form ISRS-GN model leads to more accurate optimized launch powers. However, in this case, the impact on the network-wide average channel capacity of using the launch power optimized using the different approaches is negligible.

A Point-to-Multipoint Flexible Transceiver for Inherently Hub-and-Spoke IMDD Optical Access Networks

Point-to-multipoint (P2MP) transceivers offer a promising solution to transform present point-to-point optical access networks into scalable and flexible P2MP networks capable of dynamically meeting, in a cost-effective and high energy consumption efficiency manner, the requirements associated with 5G-Advance and beyond networks, including large signal transmission capacity, fast and dense connection, high network flexibility/adaptability and low latency. However, the previously reported P2MP optical transceivers based on either coherent XR optics or IMDD digital filter multiplexing (DFM) techniques are not suitable for implementing in low-latency and highly cost-sensitive IMDD-dominated optical access networks. This paper proposes, experimentally demonstrates and optimises a novel P2MP flexible transceiver incorporating a new cascaded IFFT/FFT-based multi-channel aggregation/de-aggregation technique and an orthogonal digital filtering technique. The performances of the proposed technique are extensively evaluated experimentally in an upstream 55.3Gb/s @25km IMDD PON. It is shown that in comparison with the conventional DFM transceivers, the proposed transceivers can reduce the transmitter DSP complexity by a factor that approximates to aggregated channel count, and simultaneously offer additional physical layer network security, without requiring long digital filter lengths and greatly compromising upstream transmission performances/spectral efficiencies as well as differential ONU launch power dynamic ranges.

Spectral power profile optimization of a field-deployed wavelength-division multiplexing network enabled by remote EDFA modeling

We propose a technique for modeling erbium-doped fiber amplifiers (EDFAs) in optical fiber networks, where the amplifier unit is located at a distant node outside the laboratory. We collect data on an optical point-to-point link with the amplifier as the only amplification stage. Different amplifier operating points are modeled using probe signals and by adjusting the settings of the amplifier through a control network. The data are used to train a machine learning algorithm integrated within a physical EDFA model. The obtained mathematical model for the amplifier is used to model all amplifiers of a network and links with multiple amplification stages. To confirm the modeling accuracy, we thereafter predict and optimize launch power profiles of two selected links in the network of 439.4 km and 592.4 km lengths. Maximum/average channel optical signal-to-noise ratio prediction errors of 1.41/0.68 dB and 1.62/0.83 dB are achieved for the two multi-span systems, respectively, using the EDFA model trained on the single span system with margin-optimized launch power profiles. Up to 2.2 dB of margin improvements are obtained with respect to unoptimized transmission.

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.

Proof-of-Concept Real-Time Implementation of Interleavers for Optical Satellite Links

Ground-satellite optical links are a promising technology for increasing capacity and security over radio frequency links, while reducing weight and power consumption. However, atmospheric turbulence generates power fadings at the reception terminal. This effect can be mitigated by combining adaptive optics, forward error correction and data interleaving. While adaptive optics has been extensively studied, forward error correction together with long interleaving (e.g., multiple 10 ms) has not yet been demonstrated in this context. In this article, we present a proof of concept of an optical satellite uplink, precompensated by adaptive optics, which leverages a long interleaver in an external memory and operates at 10 Gbps with SFP+ transceivers. We account for the issues associated with real-time implementation and interfacing to a high-speed memory. To validate our proof of concept, we generate time-series for a typical ground to geostationary satellite communication link, using a field-validated simulator. Using these time series, we calculate the required interleaver time as a function of the launch power. Finally, we show the impact of different transmitter pupil diameters and of different wind speed profiles, on the interleaver duration.

First Demonstration of 25λ × 10 Gb/s C+L Band Classical / DV-QKD Co-Existence Over Single Bidirectional Fiber Link

As quantum key distribution has reached the maturity level for practical deployment, questions about the co-integration with existing classical communication systems are of utmost importance. To this end we demonstrate how the co-propagation of classical and quantum signals can benefit from the development of novel hollow-core fibers. We demonstrate a secure key rate of 330 bit/s for a quantum channel at 1538 nm in the presence of 25 × 10 Gb/s classical channels, transmitted at an aggregated launch power of 12 dBm, spanning over the C+L-band in the same hollow-core fiber link. Furthermore, we show the co-integration of the classical key-distillation channel onto this fiber link, turning it into a bidirectional fiber link and thereby mitigating the need for multiple fibers. We believe this to be an important step towards the deployment and integration of hollow-core fibers together with DV-QKD for the inherently secure telecom network of the future.

A Closed-Form Expression for the Gaussian Noise Model in the Presence of Inter-Channel Stimulated Raman Scattering Extended for Arbitrary Loss and Fibre Length

A closed-form formula for the nonlinear interference (NLI) estimation using the Gaussian noise (GN) model in the presence of inter-channel stimulated Raman scattering (ISRS) is derived. The formula enables accurate estimation of the NLI evolution along any portion of the fibre span together with arbitrary values of optical fibre losses. The formula also accounts for wavelength-dependent fibre parameters, variable modulation formats and launch power profiles. The formula is suitable for ultra-wideband (UWB) optical transmission systems and its accuracy is assessed for a system with 20 THz optical bandwidth over the entire S-, C-, and L- band through comparison with numerical integration of the ISRS GN model and split-step Fourier method (SSFM) simulations in point-to-point transmission and inline NLI estimation scenarios.

Performance Investigation of DP-16QAM Ultra-wideband- Wavelength-Division Multiplexing Communication System: Optimum Power Consideration

Recently, there is increasing interest in using the 18 THz bandwidth offered by S+C+L band to increase the transmission capacity of fiber communication systems. This leads to the generation of ultra-wideband (UWB) wavelength-division multiplexing (WDM) optical communication systems. In these advanced systems, stimulated Raman scattering (SRS) causes a power transfer from high-frequency channels to low-frequency channels. This effect leads to an increase in the nonlinear interference (NLI) between the UWB-WDM channels. Power optimization techniques are required to balance transfer power between band channels, thus increasing the maximum transmission reach (MTR) along with increasing system capacity. In this paper, the transmission performance of S+C+L band system operating with dual-polarization 16-QAM signaling is investigated using enhanced Gaussian noise model. The transmitter and receiver for each DP channel use a -polarized laser and incorporate two identical configurations, one for x- and the other for y-state of polarization (SOP). The results are presented for two values of symbol rate, 40 and 80 GBaud, where the system carries 360 (=160+80+120) and 180 (=80+40+60) channels, respectively. The results revel that the MTR of both cases is equal to 12 100 km-spans when the channel lunch power equals to -4 and -2 dBm, respectively. This work also shows the effect of NLI components as a function of the number of spans, channel spacing, and channel launch power. The results show that the cross-phase modulation component of the NLI has high accumulated value with transmission distance, while the self-phase modulation component is almost constant.

Practical Considerations in Direct Detection Under Tukey Signalling

The deliberate introduction of controlled intersymbol interference (ISI) in Tukey signalling enables the recovery of signal amplitude and (in part) signal phase under direct detection, giving rise to significant data rate improvements compared to intensity modulation with direct detection (IMDD). The use of an integrate-and-dump detector makes precise waveform shaping unnecessary, thereby equipping the scheme with a high degree of robustness to nonlinear signal distortions introduced by practical modulators. Signal sequences drawn from star quadrature amplitude modulation (SQAM) formats admit an efficient trellis description that facilitates codebook design and low-complexity near maximum-likelihood sequence detection in the presence of both shot noise and thermal noise. Under the practical (though suboptimal) allocation of a 50% duty cycle between ISI-free and ISI-present signalling segments, at a symbol rate of 50 Gbaud and a launch power of -10 dBm the Tukey scheme has a maximum theoretically achievable throughput of 200 Gb/s with an (8,4)-SQAM constellation, while an IMDD scheme achieves about 145 Gb/s using PAM-8. Note that the two mentioned constellations have the same number of magnitude levels and the difference in throughput is resulting from exploiting phase information under using a complex-valued signal constellation.

Power-Efficient Voronoi Constellations for Fiber-Optic Communication Systems

Voronoi constellations (VCs) are considered as an effective geometric shaping method due to their high power efficiencies and low complexity. In this paper, the performance of 16- and 32-dimensional VCs with a variety of spectral efficiencies transmitted in the nonlinear fiber channel are investigated. Both single-channel and wavelength-division multiplexing systems are considered for the transmission of the VCs, as well as different signal processing schemes, including chromatic dispersion compensation and digital backpropagation. Multiple performance metrics including the uncoded bit error rate, mutual information (MI), and generalized mutual information (GMI) of VCs are evaluated. Compared with quadrature amplitude modulation (QAM) formats, the VCs provide 1.0–2.4 dB launch power gains, up to 0.50 bits/symbol/dimension-pair MI gains, up to around 30% potential reach increase at the same MI, and up to 0.30 bits/symbol/dimension-pair GMI gains in a limited launch power range. The observed performance gains over QAM are found higher than in the back-to-back case. Moreover, a general GMI estimation method for very large constellations using importance sampling is proposed for the first time.

Meta-Learning Assisted Source Domain Optimization for Transfer Learning Based Optical Fiber Nonlinear Equalization

Transfer learning (TL) has been demonstrated its feasibility on fast remodeling for fiber nonlinearity equalization. It will be very efficient with fine-tuning rather than retraining when the channel parameters have been changed, e.g., the optical launch power and fiber distance. However, the performance is dependent on the highly correlation between the source and target tasks. An improper source domain cannot improve the efficiency or even negative transfer. In this paper, we propose a meta-learning assisted source domain optimization scheme in single- and multi-channel transmissions. We construct the support dataset and query dataset from the information of the inner-task and the across-task to train the neural network so that we can find a unified source domain and efficiently adapt to different tasks with fewer epochs. We conduct an experiment to transmit 120-Gb/s dual polarization (DP) 16-QAM signal over 800-km standard single mode fiber (SSMF). The required epochs for convergence can be significantly reduced and there is slight Q-factor improvement with meta-learning algorithm compared with retraining and conventional TL. We also analyze the TL strategy in multi-channel system. The simulation results show that meta-learning can also help to optimize the source domain with lower complexity.

Real-Valued Neural Network Nonlinear Equalization for Long-Reach PONs Based on SSB Modulation

Real-valued neural network nonlinear equalization for complex-valued signals combined with single sideband (SSB) modulation is proposed to be a promising scheme for long-reach passive optical networks (LR-PONs). With the assist of neural network equalizer (NNE), fiber nonlinearity induced by large launch power can be mitigated. We experimentally demonstrate a transmission of 120 Gbps 16-QAM SSB signal in C-band over an 80-km single mode fiber (SMF) without optical inline amplifier and pre-amplifier.

Detailed performance investigation and BER characterization of S+C+L band-WDM transmission system

Recently, ultra-wideband (UWB) wavelength-division multiplexing (WDM) technique has been introduced to increase the data transmission capacity of optical system using the same infrastructure fibers. This capacity increase will raise the effect of fiber nonlinearity and stimulated Raman scattering (SRS) due to the increase of total launch power and operating wavelength band, respectively. This paper presents comprehensive investigation of UWB spectral characteristics of the fiber nonlinear interference (NLI) in dual-polarization (DP)− 16QAM S+C+L system. The investigation is based on enhanced Gaussian noise (EGN) model and takes into account the wavelength dependence of various fiber parameters. The results are used as a guide line to assess the transmission performance of the system under different values of symbol rate Rs (50–80 GBaud). The results reveal that a maximum transmission reach of 13 100 km-spans is achieved for Rs= 50 GBaud and 75 GHz channel spacing.

Enhanced Compensation of Nonlinear Signal Distortion by Optical Phase Conjugation in Step-Parameter Profiled Fiber Links

Improving the efficacy of optical phase conjugation (OPC) for compensating nonlinear signal distortion in long distance optical fiber links from the Kerr effect is demonstrated. This is based on tailoring the link spans as step-parameter profiled fiber (SPF) for progressively increasing nonlinearity and decreasing dispersion with distance to mitigate the impact of propagation loss on unbalancing the phase distortion induced before and after OPC. With addition of a dispersion element after OPC, the SPF design also targets compensation in a shorter subsection of spans where induced nonlinear distortion is strongest. This enables performance benefit with less required parameter change in spans comprising just two or three segments of different regular fibers as 2-SPF and 3-SPF, respectively. Numerical simulations of 5 channel WDM dual polarization (DP) 32 Gbaud 64-QAM signals in a 12 × 130 km fiber link shows use of OPC and a 58 km 3-SPF in spans achieves 2.1 dB higher <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q²</i> -factor at 5 dB higher launch power than without. Similarly, measurements of 5 channel WDM DP 12 Gbaud 16-QAM signals in a 2 × 155 km link shows use of OPC and a 41 km 2-SPF in spans achieves 1.2 dB higher <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q²</i> -factor at 6 dB higher launch power and with low bit error rate compared to without. The improvements highlight the OPC advancements achievable by link design with regular fibers.

The DNN-based DBP scheme for nonlinear compensation and longitudinal monitoring of optical fiber links

In this paper, a double-effect DNN-based Digital Back-Propagation (DBP) scheme is proposed and studied to achieve the Integrated Communication and Sensing (ICS) ability, which can not only realize nonlinear damage mitigation but also monitor the optical power and dispersion profile over multi-span links. The link status information can be extracted by the characteristics of the learned optical fiber parameters without any other measuring instruments. The efficiency and feasibility of this method have been investigated in different fiber link conditions, including various launch power, transmission distance, and the location and the amount of the abnormal losses. A good monitoring performance can be obtained while the launch optical power is 2 dBm which does not affect the normal operation of the optical communication system and the step size of DBP is 20 km which can provide a better distance resolution. This scheme successfully detects the location of single or multiple optical attenuators in long-distance multi-span fiber links, including different abnormal losses of 2 dB, 4 dB, and 6 dB in 360 km and serval combinations of abnormal losses of (1 dB, 5 dB), (3 dB, 3 dB), (5 dB, 1 dB) in 360 km and 760 km. Meanwhile, the transfer relationship of the estimated coefficient values with different step sizes is further investigated to reduce the complexity of the fiber nonlinear damage compensation. These results provide an attractive approach for precisely sensing the optical fiber link status information and making correct strategies timely to ensure optical communication system operations.

Sparse Volterra Equalizer to Mitigate the Nonlinear Distortion in High Power Budget IM-DD OFDM PON

To improve the power budget for intensity-modulation direct-detection (IM-DD) orthogonal frequency division multiplexing passive optical network (OFDM PON), a high power budget downstream scheme by using the Volterra equalizer in digital signal processing (DSP) processing is proposed in this paper. When the launch power is increased, the subcarrier-to-subcarrier beating interference (SSBI) and other nonlinear distortion are well mitigated by the Volterra equalizer. Moreover, to reduce the complexity of receiver DSP unit, the sparse Volterra equalizer is obtained through optimizing full Volterra equalizer employing orthogonal search method. Simulation and experiment results are presented to evaluate the transmission performance and analyze the distribution of taps of Volterra equalizer under different respects. The results show that the number of tap coefficient of sparse Volterra equalizer is reduced by 60% compared with that of conventional second-order full Volterra equalizer.

Joint Estimation of Linear and Nonlinear Coherent Optical Fiber Signal-to-Noise Ratio

This letter proposes an estimator based on the neural network (NN) to jointly estimate the linear and nonlinear signal-to-noise ratios. The proposed NN-based estimator utilizes new input features based on the entropy extracted from the received signal. Moreover, the computational complexity of the proposed estimator is analyzed. The dataset utilized for training and testing is constructed from dual-polarization 16-ary quadrature amplitude modulation format over different system configurations of the standard single-mode fiber, such as launch power, transmission distances, and the number of wavelength division multiplexed channels. Numerical results reveal the superiority of the proposed NN-based estimator in terms of accuracy and computational complexity compared to the existing NN-based estimators in the literature.

Transformer-based High-fidelity Modeling Method for Radio over Fiber Link

High-fidelity modeling is of great importance for digital twin (DT) technology. A Transformer-based modeling method for radio over fiber (RoF) link is developed and experimentally demonstrated, in which the Transformer is a kind of deep learning network. The modeling performance of the constructed model by using the proposed method is evaluated, which is better than the ones based on fully connected neural network (FCNN), convolution neural network (CNN), and generative adversarial network (GAN). In addition, the modeling consistency with different laser launch power, fiber length, carrier frequency, and signal-to-noise ratio (SNR) is also investigated. Experimental results show that the fitting waveforms predicted by the proposed method agree well with the actual waveforms at the receiver end. The normalized mean square errors of the fitting waveforms are lower than the acceptable bound of 0.02.

Capacity Region Bounds for Optical WDM Channels Based on First-Order Regular Perturbation

It is known that fiber nonlinearities induce crosstalk in a wavelength division multiplexed (WDM) system, which limits the capacity of such systems as the transmitted signal power is increased. A network user in a WDM system is an entity that operates around a given optical wavelength. Traditionally, the channel capacity of a WDM system has been analyzed under different assumptions for the transmitted signals of the other users, while treating the interference arising from these users as noise. In this paper, we instead take a multi-user information theoretic view and treat the optical WDM system impaired by cross-phase modulation and dispersion as an interference channel. We characterize an outer bound on the capacity region of simultaneously achievable rate pairs, assuming a simplified <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K$</tex-math></inline-formula> -user perturbative channel model using genie-aided techniques. Furthermore, an achievable rate region is obtained by time-sharing between certain single-user strategies. It is shown that such time-sharing can achieve better rate tuples compared to treating nonlinear interference as noise. The achievable rates using the latter strategy are known to exhibit a maximum for an optimum launch power, beyond which a further increase of power leads to a reduction in achievable rates. For the single-polarization single-span system under consideration and a power 3.5 dB above the optimum launch power, treating nonlinear interference as noise results in a rate of 4.25 b/sym, while time-sharing gives a rate of 7.99 b/sym.