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Reconfigurable intelligent surfaces (RIS) have been recently introduced to optical wireless communication (OWC) networks to resolve skip areas and improve the signal-to-noise ratio at the user’s end. In OWC networks, RIS are based on mirrors or metasurfaces. Metasurfaces have evolved significantly over the last few years. As a result, coding, digital, programmable, and information metamaterials have been developed. The advantage of these materials is that they can enable digital signal processing (DSP) techniques. For the first time, this paper proposes the use of digital RIS (DRIS) in OWC systems. We discuss the concept of DRIS and the application of DSP methods to the physical material. In addition, we examine metamaterials for optical DRIS with liquid crystals serving as the front row material. Finally, we present a design example and discuss future research directions. [ABSTRACT FROM AUTHOR]

A digital linearization algorithm is proposed to reduce the third-order intermodulation distortion (IMD3) for the microwave photonic (MWP) orthogonal frequency-division multiplexing (OFDM) transmission system with intensity modulation and direct detection. Forward error correction (FEC) is added to the OFDM signal to suppress the IMD3 in conjunction with an iterative operation. The distorted OFDM signal from the MWP system goes through iterative operations including fast Fourier transform, demodulation, error correction, signal reconstruction, and interference cancellation until the output converges. Compared with the non-iterative method and the iterative method without FEC mechanism, the proposed method has a significant advantage in linearizing signals with large nonlinearity and low signal-to-noise ratio. 200-Msym/s 64 quadrature-amplitude modulation (64-QAM) OFDM signals with a large peak-to-peak voltage of 650 mV are amplified by a power amplifier and then input to the MWP link and linearized by the FEC-based iterative method. The results show that the error vector magnitudes (EVMs) of the OFDM signals can be optimized from 13.1% to 2.9% and from 13.4% to 6.3% when the output Eb/N0 from the MWP link is about 35 dB and 13.5 dB, respectively. Besides, a 3-GHz 64-QAM OFDM signal is successfully transmitted through a 25-km standard single-mode fiber with an EVM of 3.9% after linearization. [ABSTRACT FROM AUTHOR]

Volterra equalizations (VE) have delivered significant performance improvement on optical signals, but their extremely high demands on computation complexity severely impede the possibility of large-scale deployment in resource-constrained optical interconnects. In this paper, we propose and experimentally demonstrate a novel sparsity tuned elastic net-pruned Volterra equalization (SENVE) technique to remove the redundancy in the structure of conventional VE (baseline VE) to save complexity without degrading system performance. With a three-stage configuration: pretraining, pruning, and retraining, our SENVE scheme can: (1) realize the compressed structure from a larger VE to reduce computation complexity and (2) accomplish budget-conscious and hardware- friendly structured sparsity of VE to efficiently accelerate the VE's evaluation. We experientially demonstrate the proposed scheme on an 80-Gbps four-level pulse-amplitude modulation (PAM4) signal in O-band for 40-km single-mode fiber (SMF) transmission with a 30-GHz externally modulated laser (EML). Under optimal pruning but without retraining scenarios, the experimental results show that, compared with baseline VE at the KP4-FEC limit, the proposed SENVE can achieve up to 68.7% complexity reduction without degrading performance over 40 km transmission at an received optical power (ROP) of −5 dBm. Moreover, we also find our proposed SENVE with retraining can achieve up to 90.8% and 35.8% complexity reduction, with respect to baseline VE and L1-regularization VE, without sacrificing system performance under the same over-pruning scenarios. [ABSTRACT FROM AUTHOR]

As higher symbol rates are utilized in the intensity modulation and direct detection (IM/DD) scheme to meet the unrelenting growth of data traffic, overcoming the inter-symbol interference (ISI) induced by the limited bandwidth has become increasingly crucial. Channel equalization based on digital signal processing (DSP) is an effective solution, where least-mean squares (LMS) algorithm is adopted to adjust tap coefficients. However, the LMS algorithm usually has slow rate of convergence and requires lots of training symbols. This work proposes a novel training sequence to accelerate the LMS-based equalization. A first-order Markov chain (MC) is employed for sequence generation, which introduces correlation between samples and shapes signal spectrum. Compared with the conventional training sequence that consists of independent and identically distributed (i.i.d.) samples and has a white spectrum, the MC sequence enables faster convergence of tap coefficients and mean-squared error (MSE). Moreover, an experimental demonstration of a 43 Gbaud PAM-4 signal shows that the proposed sequence can achieve a lower pre-forward-error-correction (pre-FEC) bit error rate (BER) than that of the i.i.d. sequence with the same length. When the PAM-4 signal is transmitted over a 5-km standard single mode fiber (SSMF) with 6-dB system bandwidth of 10 GHz, more than 70% training sequence length reduction can be attained. When the fiber length is increased to 10 km and the signal suffers from severe power fading, more than 48% reduction can be achieved. [ABSTRACT FROM AUTHOR]

To meet the upcoming 6G communications challenges, terahertz-wave (THz-wave) wireless data transmission over 100 Gb/s is urgently needed in the short term. However, advanced digital signal processing (DSP) technologies suitable for millimeter-wave (mm-wave) bands cannot satisfy the requirements; increasing the carrier frequencies for more spectra is essential. Photonics-aided THz-wave generation (PTG) technology has become a valid way to solve the bandwidth bottleneck. New progress has been made in PTG by using optical frequency combs and optical modulators. The application of DSP technology for heterodyne detection can not only overcome the nonlinear effect in photoelectric devices but also enhance the receiver sensitivity and spectral efficiency. Multidimensional multiplexing technology can reduce the baud rate of each subchannel to realize the long-distance transmission of THz-wave signals. In this tutorial, we introduce the key technologies and principles of ultrahigh speed and large capacity THz-wave signal transmission. These technologies can effectively improve the transmission capacity and distance and reduce the bandwidth required by optical and electronic equipment. [ABSTRACT FROM AUTHOR]

Advanced digital signal processing (DSP) is viewed as an inevitable technology to deal with nonlinear distortions in cost-sensitive optical interconnection systems. Classic methods such as equalization based on Volterra series turn out to be quite effective solutions to compensate signal impairments. In practical imple-mentations, a great deal of efforts focus on reducing the com-putational complexity. Several model selection and dimensionality reduction approaches have been proposed to simplify the struc-ture, which employ backward strategies. However, the threshold settings for these pruning approaches are environment-sensitive. In this paper, we propose a least angle regression-based nonlinear equalization method (LaNLE). We extend the non-intrusive least angle regression technique to generalized linear models. Through an incremental process by stagewise feature selection and weight fitting, we obtain the low-complexity nonlinear structure with significant feature basis. Experimental verification is performed based on 120 Gbps PAM8-modulated 850 nm VCSEL plus MMF optical link. The results show that, compared to fully-connected Volterra nonlinear equalizer structure (VNLE), up to 60.4% and 70.1% reduction in complexity can be achieved with similar BER performance in back-to-back and 100 m transmission scenarios, respectively. In addition, we experimentally demonstrate 225 Gbps per lane PAM-8 signal transmission over 100 m MMF in a simple and cost-effective implementation manner based on mass-production 850 nm VCSEL and proposed LaNLE method. [ABSTRACT FROM AUTHOR]

A successful migration from current C-band based optical networks to a multiband scenario primarily depends on the development of solutions that can reliably measure physical properties of optical links over broad spectral transmission windows. Additionally, these solutions must be capable of delivering wavelength-dependent and spatially-resolved indicators that can empower network operators to identify faults before they lead to severe service disruptions. Recently, the exploitation of receiver based digital signal processing as a tool for optical performance monitoring has gained tremendous popularity. One successful example is the so-called in-situ power profile estimator, which can reconstruct the per-channel longitudinal power profile along the optical fiber link solely processing the received signal samples. In this work, we propose a novel application for the in-situ power profile estimator by harnessing it on multiple wavelengths to accurately estimate the spectral gain profile of C+L-band in-line Erbium-doped fiber amplifiers deployed in a 280-km single mode fiber link. Furthermore, we show how this scheme can be efficiently used to detect amplification-related anomalies, such as gain tilt and narrowband gain compression. In our measurements, we achieved a sub-dB estimation accuracy by comparing the proposed gain extraction approach with the back-to-back characterization obtained from an optical spectrum analyzer. [ABSTRACT FROM AUTHOR]

A roadmap for future wireless communications is expected to exploit all transmission-suitable spectrum bands, from the microwave to the optical frequencies, to support orders of magnitude faster data transfer with much lower latency than the deployed solutions nowadays. The currently under-exploited mid-infrared (mid-IR) spectrum is an essential building block for such an envisioned all-spectra wireless communication paradigm. Free-space optical (FSO) communications in the mid-IR region have recently attracted great interest due to their intrinsic merits of low propagation loss and high tolerance of atmospheric perturbations. Future development of viable mid-IR FSO transceivers requires a semiconductor source to fulfill the high bandwidth, low energy consumption, and small footprint requirements. In this context, quantum cascade laser (QCL) appears as a promisingtechnological choice. In this work, we present an experimental demonstration of a mid-IR FSO link enabled by a 4.65-μm directly modulated (DM) QCL operating at room temperature. We achieve a transmission data rate of up to 6 Gbps over a 0.5-m link distance. This achievement is enabled by system-level characterization and optimization of transmitter and receiver power level and frequency response and assisted with advanced modulation and digital signal processing (DSP) techniques. This work pushes the QCL-based FSO technology one step closer to practical terrestrial applications, such as the fixed wireless access and the wireless mobile backhaul. Such a QCL-based solution offers a promising way towards the futuristic all-spectra wireless communication paradigm by potentially supporting the whole spectrum from the MIR to the terahertz (THz). [ABSTRACT FROM AUTHOR]

We demonstrate an optical arbitrary waveform measurement (OAWM) system that exploits a bank of silicon photonic (SiP) frequency-tunable coupled-resonator optical waveguide (CROW) filters for gapless spectral slicing of broadband optical signals. The spectral slices are coherently detected using a frequency comb as a multi-wavelength local oscillator (LO) and stitched together by digital signal processing (DSP). For high-quality signal reconstruction, we have implemented a maximum-ratio combining (MRC) technique based on precise calibration of the complex-valued opto-electronic transfer functions of all detection paths. In a proof-of-concept experiment, we demonstrate the viability of the scheme by implementing a four-channel system that offers an overall detection bandwidth of 140 GHz. Exploiting a femtosecond laser with precisely known pulse shape for calibration along with dynamic amplitude and phase estimation, we reconstruct 100 GBd QPSK, 16QAM and 64QAM optical data signals. The reconstructed signals show improved quality compared to that obtained with a single high-speed intradyne receiver, while the electronic bandwidth requirements of the individual coherent receivers are greatly reduced. [ABSTRACT FROM AUTHOR]

Neural networks (NNs) are attractive for nonlinear impairment compensation applications in communication systems, such as optical fiber nonlinearity, nonlinearity of driving amplifiers, and nonlinearity of semiconductor optical amplifiers. Without prior knowledge of the transmission link or the hardware characteristics, optimal parameters are completely constructed from a data-driven approach by exploring training datasets, once the NN structure is given. On the other hand, due to computational power and energy consumption, especially in high-speed communication systems, the computational complexity of the optimized NN needs to be confined to the hardware, such as FPGA or ASIC without sacrificing its performance improvement. In this paper, two approaches are presented to accommodate the NN-based algorithms for high-speed communication systems. The first approach is to reduce computational complexity of the NN-based nonlinearity compensation algorithms on the basis of weight pruning (WP). WP can significantly reduce the computational complexity, especially because the nonlinear compensation task studied here results in a sparse NN. The authors have studied an enhanced approach of WP by imposing an additional restriction on the selection of non-zero weights on each hidden layer. The second approach is to implement NNs onto a silicon-photonic integrated platform, enabling power efficiency to be further improved without sacrificing the high-speed operation. [ABSTRACT FROM AUTHOR]

Optical communication has been employed in a wide range of applications, including terrestrial, submarine, inter-satellite, and even space communication. It is particularly successful in fiber-based communication networks that vastly reshape modern life through the Internet. With the intensified activities such as undersea resource exploration, ecosystem monitoring, and recreation, underwater is an exciting new arena for optical wireless communication (OWC). In this paper, we review the recent progress of underwater and water-air OWC systems. Channel characterization, communication system design, and performance investigations are given. Critical limitations and effective mitigation methods to overcome the influence of bubbles and waves are presented. We also address the current issues for proper performance comparison under different wave conditions. With further research in channel modeling, device innovation, and system design optimization, practical and robust underwater and water-air OWC systems can be realized. [ABSTRACT FROM AUTHOR]

Kramers-Kronig (KK) receiver shows strong feasibility since the phase can be extracted from the detected intensity through the Hilbert transform (HT). To reduce the complexity and make the HT more suitable for the practical KK system, the HT can be approximated as the finite impulse response (FIR) filter, i.e., the HT-FIR filter. However, the finite Fourier series expansion of the HT-FIR filter suffers from oscillations around the discontinuities and further results in an unsatisfactory in-band flatness, which is the so-called Gibbs phenomenon. In the experiment and the simulation, we found that the performance of the KK system using the HT-FIR filter is strongly impaired due to the Gibbs phenomenon, by the roll-off factor of the shaping filter, frequency gap between the direct-current (DC) component and one edge of the signal spectrum, carrier-to-signal power ratio (CSPR), tap number of the HT-FIR filter, and the digital upsampling rate. We have systematically investigated the practical KK phase retrieval performance and the computational complexity considering the Gibbs phenomenon due to the adoption of the HT-FIR filter, rather than the HT used in the previous works without the Gibbs phenomenon. We conclude that there is an optimal upsampling rate when the HT-FIR filter is used, and the optimal upsampling rate is closely related to the five aspects mentioned above. We have carried out a transmission experiment using a 112-Gbit/s 16-QAM system over 1920-km with a Raman fiber amplifier to investigate the limit transmission performance of the practical KK receiver. The experimental results show that by using the 9-tap HT-FIR filter with a digital upsampling rate of 84-GHz (3-sps), the signals can be transmitted the same distance as the HT, up to 1440-km with a bit error ratio (BER) below the 20% soft-decision forward error correction threshold. [ABSTRACT FROM AUTHOR]

High-symbol-rate coherentoptical transceivers suffer more from the critical responses of transceiver components at high frequency, especially when applying a higher order modulation format. We recently proposed a neural network (NN)-based digital pre-distortion (DPD) technique trained to mitigate the transceiver response of a 128 GBaud optical coherent transmission system. In this paper, we further detail this work and assess the NN-based DPD by training it using either a direct learning architecture (DLA) or an indirect learning architecture (ILA), and compare performance against a Volterra series-based ILA DPD and a linear DPD. Furthermore, we deliberately increase the transmitter nonlinearity and compare the performance of the three DPDs schemes. The proposed NN-based DPD trained using DLA performs the best among the three contenders. In comparison to a linear DPD, it provides more than 1 dB signal-to-noise ratio (SNR) gains at the output of a conventional coherent receiver DSP for uniform 64-quadrature amplitude modulation (QAM) and PCS-256-QAM signals. Finally, the NN-based DPD enables achieving a record 1.61 Tb/s net rate transmission on a single channel after 80 km of standard single mode fiber (SSMF). [ABSTRACT FROM AUTHOR]

Passive Optical Networks (PON), able to operate at 50 Gbps per wavelength (λ), are under development and standardization, based on intensity-modulation (IM) and direct-detection (DD) systems. The next step in PON evolution will be driven by 5G/6G fronthauling capacity demands, and will require the development of 100 Gbps/λ (and beyond) systems, which poses big challenges if maintaining the DD-format. In this contribution, we analyze a 100 Gbps/λ PON architecture able to preserve the IM-DD approach at the Optical Network Unit (ONU), placing the complexity at the Optical Line Terminal (OLT), thanks to Digital Signal Processing (DSP). We experimentally demonstrate a 100 Gbps/λ transmission using this architecture in the downstream (DS) direction. Chromatic dispersion digital pre-compensation (CD-DPC) in combination with an IQ Mach-Zehnder Modulator (IQ-MZM) is used at the transmitter (TX). Keeping the ONU DSP as simple as possible, as compared with current DSP proposals for 50 Gbps/λ PON, is another main goal of this work. Adaptive equalization (AEQ) is used to correct for linear impairments, in addition to digital non-linear correction (NLC) at the receiver (RX). We compare two NLC approaches: a full Volterra Non-Linear Equalizer (VNLE) and a simpler NLC technique based on a square-root like function (SQRT). Operation over standard single-mode fiber (SMF) in C-band, achieving reaches from 0 km to 100 km and Optical Path Loss (OPL) values higher than 29 dB, are shown. The analyzed proposal is directly applicable to Terabit-capable wavelength division multiplexing (WDM)-PON, and can be extended to very high-speed Time Division Multiplexing (TDM)-PON and TWDM-PON, with some modifications discussed here. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a 3 × 3 full multiple-input multiple-output (MIMO) fiber–wireless system in the W-band with wavelength-division multiplexing (WDM) and polarization-division multiplexing (PDM) radio-over-fiber (RoF) transmission capability. The system is realized using two highly stable RoF links based on the optical self-heterodyne method and PDM transmission in one of the RoF links. We experimentally demonstrate and confirm satisfactory performances for a 3 × 3 MIMO offset quadrature amplitude modulation-based filter-bank multicarrier signal. Total capacities of approximately 110 and 132 Gb/s, corresponding to spectral efficiencies of approximately 8.5 and 10.2 bits/s/Hz, respectively, are achieved for the system using antennas placed in the same and alternately different polarizations, respectively. The system is applicable to WDM and combined WDM/PDM RoF transmissions. It provides a scalable solution for facilitating large-scale MIMO signal transportation and can be a promising solution for future mobile transport and radio access networks in high-frequency bands. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present the results of the comparative performance-versus-complexity analysis for the several types of artificial neural networks (NNs) used for nonlinear channel equalization in coherent optical communication systems. The comparison is carried out using an experimental set-up with the transmission dominated by the Kerr nonlinearity and component imperfections. For the first time, we investigate the application to the channel equalization of the convolution layer (CNN) in combination with a bidirectional long short-term memory (biLSTM) layer and the design combining CNN with a multi-layer perceptron. Their performance is compared with the one delivered by the previously proposed NN-based equalizers: one biLSTM layer, three-dense-layer perceptron, and the echo state network. Importantly, all architectures have been initially optimized by a Bayesian optimizer. First, we present the general expressions for the computational complexity associated with each NN type; these are given in terms of real multiplications per symbol. We demonstrate that in the experimental system considered, the convolutional layer coupled with the biLSTM (CNN+biLSTM) provides the largest Q-factor improvement compared to the reference linear chromatic dispersion compensation (2.9 dB improvement). Then, we examine the trade-off between the computational complexity and performance of all equalizers and demonstrate that the CNN+biLSTM is the best option when the computational complexity is not constrained, while when we restrict the complexity to some lower levels, the three-layer perceptron provides the best performance. [ABSTRACT FROM AUTHOR]

In this tutorial, we discuss the evolution of the technology deployed for optical interconnects and the trade-offs in the design of low complexity, low power DSP and implementation for direct detect and coherent, pluggable optical modules for data center applications. The design trade-offs include the choice of modulation format, baud rate, optical link design, forward error correction, signal shaping and dispersion compensation. [ABSTRACT FROM AUTHOR]

DSP-enhanced intensity-modulation direct-detection (IM/DD) systems can support up to 56 Gb/s over 100 km signal transmissions at C-band. To achieve higher data rates and longer transmission distances, we propose data-aided iterative algorithm (DIA) and decision-directed DIA (DD-DIA) to digitally mitigate signal-signal beating interference (SSBI) without requiring any modifications to physical layer structures. DIA utilizes pilot symbols with uniformly spaced insertions to relax the modified Gerchberg-Saxton (G-S) algorithms that suffer from the local optimum problem. To further improve the symbol error rate (SER) performance and convergence speed, DD-DIA introduces a decision process to generate pseudo-pilots. We numerically compare DIA with other algorithms and find that DIA can recover signals subject to large fiber dispersions corresponding to which conventional IA and Volterra filter (VF) fail, while DD-DIA significantly accelerates the convergence speed and improves the reconstruction performance compared with DIA, it can support 100-Gb/s PAM4 over 400-km IM/DD transmissions within just 50 iterations. Two orders of magnitude reductions in SER is observed for 100 Gb/s PAM4 signal transmission over 100-km SSMFs. Compared with conventional IA, the proposed techniques have higher convergence speeds, better global optimum features and large tolerances to physical model errors. In particular, DD-DIA results in larger optical signal-to-noise ratio (OSNR)/ received optical power (ROP) improvement, higher transmission capacities and less computational complexity. DD-DIA is a promising algorithm for efficiently reconstructing 2-dimensional optical field for conventional IM/DD optical transmission systems. [ABSTRACT FROM AUTHOR]

A photonic wireless bridge operating at a carrier frequency of 250 GHz is proposed and demonstrated. To mitigate the phase noise of the free-running lasers present in such a link, the tone-assisted carrier recovery is used. Compared to the blind phase noise compensation (PNC) algorithm, this technique exhibited penalties of 0.15 and 0.46 dB when used with aggregated Lorentzian linewidths of 28 and 359 kHz, respectively, and 20 GBd 16-quadrature amplitude modulation (QAM) signals. The wireless bridge is also demonstrated in a wavelength division multiplexing (WDM) scenario, where five optical channels are generated and sent to the Tx remote antenna unit (RAU). In this configuration, the full band from 224 to 294 GHz is used. Finally, a 50 Gbit/s transmission is achieved with the proposed wireless bridge in single channel configuration. The wireless transmission distance is limited to 10 cm due to the low power emitted by the uni-travelling carrier photodiode used in the experiments. However, link budget calculations based on state-of-the-art THz technology show that distances > 1000 m can be achieved with this approach. [ABSTRACT FROM AUTHOR]

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, of which the filter impairment caused by WSS is a major one. If these soft failures are not handled properly and timely, the quality-of-transmission (QoT) will degenerate, even leading to service disruption. During this process, it is important to know the accurate location and the magnitude of soft failure. However, it is difficult for traditional methods to accomplish this target. Fortunately, with the fast progress of the powerful machine learning (ML) algorithms, a new promising way is provided to address this problem. In this article, we propose to use artificial neural network (ANN) and Gaussian process regression (GPR) to localize the soft failure location and estimate anomaly value. The input of the ANN and GPR is extracted from the power spectrum density (PSD) and the equalizer taps, which can be easily obtained from a coherent receiver. We explore two types of soft failure caused by the WSS, including the offset of WSS's center frequency, i.e., frequency shift (FS), and the tightening of WSS's 3-dB bandwidth, i.e., frequency tightening (FT). To validate the proposed method, we perform extensive simulations. The localization accuracy of the ANN can reach 95%, and the mean-absolute-error (MAE) of the GPR can reach 0.1-GHz, demonstrating the effectiveness of the proposed method. Besides, we validate the generalization of the proposed method under different link conditions. Finally, the importance of each input feature is explored, showing the effectiveness of the selected features. [ABSTRACT FROM AUTHOR]

With the increasing demand of bandwidth, a higher capacity communication system is required for short-reach optical link. Intensity modulation/direct detection (IM-DD) is considered as a reasonable method, due to its low cost and power consumption. In the aspect of digital signal processing (DSP), discrete Fourier transform-spread (DFT-s) precoding can be used to reduce peak to average power ratio (PAPR) and average the noise among the subcarriers. In addition, probabilistic shaping (PS) turns out to further increase the system capacity. We experimentally demonstrated a 200 Gbit/s single λ dual band DMT transmission with a spectrum efficiency (SE) of 6.29 bit/s/Hz. The achievable information rate (AIR) of the proposed dual band signal increases about 30 Gbit/s compared with that of single band signal. Moreover, single-sideband (SSB) signal and signal with guard band are implemented to mitigate the frequency selective fading of optical fiber chromatic dispersion (CD). The performance of these two methods are quite closed for short reach transmission. Considering the signal-signal beat interference (SSBI) introduced by the SSB signal, an extra Volterra filter is applied in DSP. Besides, an extra optical filter is required to generate the SSB signal. [ABSTRACT FROM AUTHOR]

We developed an all-fiber-based optical demodulator for the signal interrogation of low-coherence fiber-optic Fabry–Perot interferometric sensors. The optical demodulator consists of a Michelson interferometer implemented by using a 3 × 3 fiber coupler and two fiber-coupled Faraday reflectors with tunable fiber delay lines. The demodulator's output contains two optical interference signals with a constant phase shift and the output shows no sensitivity to the polarization variation in the light source or fibers. A digital phase recovery algorithm is used to extract the measurand information from the phase-shifted signals at high accuracy, high dynamic range, and high stability. The optical demodulator has been applied to the measurement of high-frequency vibrations and strains using fiber-optic sensors. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a dual-polarization (DP) transceiver imbalance monitoring scheme, which is applicable for the coherent system using square QAM signals. The scheme can separate/monitor various kinds of transceiver imbalances and has technical advantages of multi-dimension, wide monitoring range, and modulation-format-transparency. In the proposed scheme, a polarization scrambler is configured between transmitter (Tx) and receiver (Rx). After coherent detection, the proposed digital signal processing (DSP) modules are implemented to monitor transceiver imbalance based on the received signals. To simplify the DSP modules, the implementation sequence of DSP modules is in reverse order compared to the order in which the sequence of impairments is introduced. Firstly, Godard timing error detection (TED) and Gram–Schmidt orthogonalization procedure (GSOP) is used to monitor and compensate Rx imbalances. After compensating the Rx imbalances, Tx imbalance monitoring is implemented. Complex/real maximum likelihood independent component analysis (ML-ICA) and Godard-TED are used to monitor Tx imbalance. Finally, the amplitude ratios of the final output signal and Tx in-phase/quadrature (IQ) amplitude imbalance are fed back to mitigate the interaction of Tx imbalances on the signal power and improve monitoring accuracy. The transceiver IQ and x-polarization/y-polarization (XY) imbalances are separated for the first time by using the frequency offset (FO) naturally existing in the system and the continuous polarization rotation induced by a polarization scrambler. The separation method combined with the reverse algorithm design and amplitude ratio feedback mechanism makes the proposed scheme achieve the most multi-dimensional imbalance monitoring by far. The simulation and experimental results verify that the proposed scheme is modulation-format-transparent and can separate/monitor multi-dimensional transceiver imbalances within a wide range. [ABSTRACT FROM AUTHOR]

Short-time Fourier transform (STFT) is a fundamental concept that enables the observation of dynamic spectral evolution for non-stationary signals. Broadband STFT based on digital signal processing (DSP) usually has limited acquisition frame rate, which is inherently constrained by the performance limitations of present DSP engine. The further extended acquisition frame rate can be achieved by all-optical spectrum analysis approaches, while the frequency resolution is usually worse than 1 GHz. In this article, STFT based on bandwidth-scaled microwave photonics is proposed, where the requirement for dispersion is relaxed and the attendant transmission delay in dispersive media is greatly reduced. Moreover, the frequency resolution towards MHz level could be expected in the proposed STFT. We prove that “bandwidth scaling” combined with periodic wavelength-to-time mapping (WTM) mathematically agrees with the definition of STFT. The method of customizing system parameters according to the requirements of ultrafast spectrum measurement is carefully clarified. Experimentally, joint time-frequency analysis for a variety of ultrafast non-stationary RF signals is demonstrated, where dynamic frequency resolution of 60 MHz, dynamic observation bandwidth of 1.98 GHz and acquisition frame rate of 160 MHz are simultaneously realized. The scheme we proposed is promising for the dynamic spectrum monitoring of broadband non-stationary RF signals. [ABSTRACT FROM AUTHOR]

We present a unique carrier phase recovery algorithm which permits the softwarization of optical coherent receiver digital signal processing (DSP). The proposed algorithm with decision-aided phase unwrapping overcomes the serial dependency issue that has, up to now, prevented the softwarization of optical coherent systems. Its performance is verified through extensive Monte Carlo (MC) simulations and hardware experiments on quadrature phase-shift keying (QPSK) signaling. The proposed algorithm is implemented on a general-purpose graphic processing unit (GPU) and achieves 5-Gbit/s throughput for real-time streaming signals, as well as −44.5-dBm receiver sensitivity. The softwarization of carrier phase recovery function is demonstrated for the first time; our enhancement of disaggregated optical coherent receivers will significantly improve system flexibility and scalability, towards the realization of a new access networking that exploits general purpose hardware and virtualized transport layer functions. [ABSTRACT FROM AUTHOR]

In this article, we present a W-band (3, 1) millimeter-wave vector signal generation based on frequency-quadrupled modulation and nonlinearities tolerant modulation by one Mach–Zehnder modulator (MZM). Despite the phase changes, the constellation and sequence of the received (3, 1) signal is consistent with the original regular signal after the detection of the photo-detector (PD). The obtained (3, 1) constellation has better performance compared to pre-coded quadrature phase shift keying (QPSK) constellation at the receiver end, and effectively reduces system complexity and cost. The frequency-quadrupled scheme is experimentally completed with different date rates at 2 and 4-Gbaud. The bit-error-ratios (BERs) results show that the BER performance of (3, 1) signal is superior to the pre-coded QPSK, and it is below the 7% hard-decision forward-error-correction (HD-FEC) threshold of 3.8 × 10−3 over 5 and 10 km single-mode-fiber (SMF) transmission, which demonstrates that the generation of (3, 1) vector mm-wave signal based on proposed frequency-quadrupled scheme and nonlinearities tolerant modulation can be used in our system. [ABSTRACT FROM AUTHOR]

In this article, we propose a model-based machine-learning approach for dual-polarization systems by parameterizing the split-step Fourier method for the Manakov-PMD equation. The resulting method combines hardware-friendly time-domain nonlinearity mitigation via the recently proposed learned digital backpropagation (LDBP) with distributed compensation of polarization-mode dispersion (PMD). We refer to the resulting approach as LDBP-PMD. We train LDBP-PMD on multiple PMD realizations and show that it converges within 1% of its peak dB performance after 428 training iterations on average, yielding a peak effective signal-to-noise ratio of only 0.30 dB below the PMD-free case. Similar to state-of-the-art lumped PMD compensation algorithms in practical systems, our approach does not assume any knowledge about the particular PMD realization along the link, nor any knowledge about the total accumulated PMD. This is a significant improvement compared to prior work on distributed PMD compensation, where knowledge about the accumulated PMD is typically assumed. We also compare different parameterization choices in terms of performance, complexity, and convergence behavior. Lastly, we demonstrate that the learned models can be successfully retrained after an abrupt change of the PMD realization along the fiber. [ABSTRACT FROM AUTHOR]

Directly modulated lasers (DML) dominate the short reach optical interconnect market. For years, the bandwidth of commercial DMLs has been limited to a level of 25 GHz, which can support a speed up to around 100 Gb/s via advanced modulations like Nyquist-shaped 4-ary pulse amplitude modulation (PAM-4), carrier-less amplitude phase (CAP) modulation, subcarrier multiplexing, and discrete multitone (DMT). To scale with the capacity evolution of interconnect applications, it is in urgent demand to develop DMLs operated at 50G class and even beyond. In this article, we exploit a newly designed 50G-class short-cavity distributed Bragg reflector (DBR) laser, called the two-κ DBR laser, and demonstrate its capability of supporting 200 Gb/s per wavelength direct modulation and direct detection transmissions. Combining high-speed electronics with the 50G-class DML, we demonstrate a variety of advanced digital modulations including Nyquist-shaped 100-GBaud PAM-4, 80-GBaud PAM-8, and 50-GHz DMT signals, with the maximum net bit rate of 236 Gb/s. The DML exhibits an ultra-low chirp parameter of 1.0, which enables isolator-free operation and dispersion-tolerant transmission over tens of kilometers at O-band. [ABSTRACT FROM AUTHOR]

In this article, we propose a geometric shaping (GS) strategy to design 8, 16, 32, and 64-ary modulation formats for the optical fibre channel impaired by both additive white Gaussian (AWGN) and phase noise. The constellations were optimised to maximise generalised mutual information (GMI) using a mismatched channel model. The presented formats demonstrate an enhanced signal-to-noise ratio (SNR) tolerance in high phase noise regimes when compared with their quadrature amplitude modulation (QAM) or AWGN-optimised counterparts. By putting the optimisation results in the context of the 400ZR implementation agreement, we show that GS alone can either relax the laser linewidth (LW) or carrier phase estimation (CPE) requirements of 400 Gbit/s transmission links and beyond. Following the GMI validation, the performance of the presented formats was examined in terms of post forward error correction (FEC) bit-error-rate (BER) for a soft decision (SD) extended Hamming code (128, 120), implemented as per the 400ZR implementation agreement. We demonstrate gains of up to 1.2 dB when compared to the 64-ary AWGN shaped formats. [ABSTRACT FROM AUTHOR]

Network architectural changes to satisfy all the 5G+ mobile network specifications and requirements are necessary due to the popularization of streaming and cloud applications on omnipresent portable devices. The combination of massive installation of micro-cell antenna sites with the cloud access radio network (C-RAN) architecture has recently been nominated as a promising technology for high-capacity mobile fronthaul links, albeit at a high cost. An alternative approach for next-generation fronthaul networks is to utilize the already deployed passive optical networks (PONs) where wireless and wired services may coexist in a converged manner. Non-orthogonal multiple access (NOMA) modulation with multi-band carrierless amplitude and phase modulation (NOMA-CAP) has recently been investigated as a promising 5G+ modulation format candidate to increase the capacity and flexibility of future mobile networks. Here, we experimentally demonstrate the convergence of a NOMA-CAP wireless waveform with a single-carrier wired signal in a PON scenario using radio-over-fiber (RoF) technology. Specifically, fifteen NOMA-CAP bands, with two NOMA power levels to double the capacity, transmit 15 Gb/s multiplexed with a digital 10 Gb/s four-level pulse amplitude modulation (PAM-4) signal for downlink application. Two converged system implementations have been considered, first using electrical frequency division multiplexing (EFDM) and secondly using the hybrid EFDM-wavelength division multiplexing (EFDM-WDM). Successful transmission through a 25 km span of standard single-mode fiber is achieved with negligible transmission penalty for both proposed converged solutions. [ABSTRACT FROM AUTHOR]

We introduce circuit implementations of one- and two-stage carrier phase recovery (CPR) for 256QAM coherent optical receivers. We describe in detail the optimizations of algorithms, such as modified Viterbi-Viterbi (mVV), blind phase search (BPS), and principal component-based phase estimation (PCPE), that are required to develop energy-efficient CPR circuits and show how design parameter settings and limited fixed-point resolution affect the SNR penalty. 30-GBaud CPR circuit netlists synthesized in a 22-nm CMOS process technology allow us to study trade-offs between energy per bit and SNR penalty. We show that it is possible to reach an energy dissipation of around 1 pJ/bit at an SNR penalty of 0.6 dB for two-stage PCPE+BPS and mVV+BPS implementations, and that PCPE+BPS is the preferred choice thanks to its smaller area. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate a single-wavelength 100 Gbps downstream PON transmission aided by chromatic dispersion digital pre-compensation (CD-DPC) using simple digital signal processing (DSP) finite impulse response (FIR) filters in combination with an IQ Mach–Zehnder modulator (IQ-MZM) at the transmitter side and direct-detection receiver at the optical network unit (ONU). A reach of 50 km over standard single-mode fiber in C-band and an optical distribution network (ODN) loss of 28.5 dB are achieved. Transmission of 50 and 125 Gbps over 50 km of fiber is also tested, achieving 32 dB and 24 dB of ODN loss, respectively. The complexity of the filters, the optimization of the main design parameters, and the tolerance of the CD-DPC to the uncertainty of the exact accumulated link dispersion are analyzed in detail. [ABSTRACT FROM AUTHOR]

Theoretically, for continuous signals, Kramers–Kronig (KK) receivers should be able to precisely reconstruct a band-limited signal's phase from its envelope if the minimum phase (MP) condition is satisfied. However, simulations have shown that even if the MP condition is satisfied, for sampled signals the reconstructed signal can contain large errors. In this article, we explore the effectiveness of DSP-based KK receivers close to the MP condition using single-tone and multi-tones. For a single-tone signal, we use a precise mathematical analysis to demonstrate large distortions in the reconstructed signal even when the MP condition is clearly satisfied and illustrate this on the complex-plane. We then study multi-tone signals spectrally, and show that there are strong intermodulation distortion tones when MP is satisfied that either directly fall in-band, or could be aliased back in-band during down-sampling. Satisfying MP does, however, eliminate signal-signal beat interference, as expected with KK processing. High up-sampling ratios reduce the spectral density of these tones, which explains the improved performance seen in experimental systems. We then explore the error-vector magnitude close to the boundary of MP for various carrier-to-signal power ratios and up-sampling ratios, to illustrate that there is a soft transition around the MP condition. Overall, this work shows that the MP condition is not sufficient to guarantee error-free performance in noiseless conditions. [ABSTRACT FROM AUTHOR]

We present an intermediate frequency-over-fiber (IFoF) transmission system that uses cascaded connections between a broadband link and multiple narrowband links for future mobile fronthaul (MFH). The downlink MFH system employs analog and digital signal processing after broadband and narrowband IFoF transmissions, respectively, for extractions and frequency conversions of IF signals. The bandwidth of the optical components for the downlink MFH system is investigated and compared with that for the subcarrier multiplexed passive optical network (SCM-PON). The results support the satisfactory performance of the MFH system for bandwidth reduction. Two types of our developed digital signal processors for the MFH system are also described in detail: One is for an output of a single radio frequency (RF) stream, whereas the other is for simultaneous outputs of multiple RF streams. In addition, using either of the digital signal processors, we experimentally demonstrate the downlink MFH system's transmission of 64-QAM filtered OFDM (f-OFDM) signals with 360-MHz signal bandwidth and 3.6-MHz guard bands between adjacent IF signals and 64-QAM OFDM signals with 380.16-MHz signal bandwidth and 19.84-MHz guard bands between IF signals. In both experiments, eighteen IF signals are successfully transmitted over 20-km and 1-km fibers, which includes analog and real-time digital signal processing for IF channel extractions and frequency conversions. The results obtained in this study demonstrate the potential of cascaded IFoF links using hybrid signal processing for 5G and future MFH systems. [ABSTRACT FROM AUTHOR]

Modern optical communication systems transmit multiple frequency channels, each operating very close to its theoretical limit. The total bandwidth can reach 10 THz limited by the optical amplifiers. Maximizing spectral efficiency, the throughput per bandwidth is thus crucial. Replacing independent lasers with an optical frequency comb can enable very dense packing by overcoming relative drifts. However, to date, interference from non-ideal spectral shaping prevents exploiting the full potential of frequency combs. Here, we demonstrate comb-enabled multi-channel digital signal processing, which overcomes these limitations. Each channel is detected using an independent coherent receiver and processed at two samples-per-symbol. By accounting for the unique comb stability and exploiting aliasing in the design of the dynamic equalizer, we show that the optimal spectral shape changes, resulting in a higher signal-to-noise ratio that pushes the optimal symbol rate towards and even above the channel spacing, resulting in the first example of frequency-domain super-Nyquist transmission with multi-channel detection for optical systems. The scheme is verified both in back-to-back configuration and in single span transmission of a 21 channel superchannel originating from a 25 GHz-spaced frequency comb. By jointly processing three wavelength channels at a time, we achieve spectral efficiency beyond what is possible with independent channels. At the same time, one significantly relaxes the hardware requirements on digital-to-analog resolution and bandwidth, as well as filter tap numbers. Our results show that comb-enabled multi-channel processing can overcome the limitations of classical dense wavelength division multiplexing systems, enabling tighter spacing to make better use of the available spectrum in optical communications. [ABSTRACT FROM AUTHOR]

The design of application-specific integrated circuits (ASIC) is at the core of modern ultra-high-speed transponders employing advanced digital signal processing (DSP) algorithms. This manuscript discusses the motivations for jointly utilizing transmission techniques such as probabilistic shaping and digital sub-carrier multiplexing in digital coherent optical transmissions systems. First, we describe the key-building blocks of modern high-speed DSP-based transponders working at up to 800G per wave. Second, we show the benefits of these transmission methods in terms of system level performance. Finally, we report, to the best of our knowledge, the first long-haul experimental transmission – e.g., over 1000 km – with a real-time 7 nm DSP ASIC and digital coherent optics (DCO) capable of data rates up to 1.6 Tb/s using two waves (2 × 800G). [ABSTRACT FROM AUTHOR]

Chaotic optical communication was originally proposed to provide high-level physical layer encryption. For high-speed and long-distance transmission, chaotic signal is very sensitive to channel impairments such as dispersion and Kerr fiber nonlinearity. In the traditional chaotic optical communications, these impairments must be compensated in optical domain before chaos synchronization. However completely compensating the high-order dispersion and fiber nonlinearity in the optical domain has great challenges, therefore limiting the transmission distance of high-speed chaotic optical communications less than 150 km. Here we propose a method aiming to break the limit. Thanks to coherent detection, channel impairments can be compensated in the digital domain using various algorithms. Digital back propagation algorithm is used for jointly compensating linear and nonlinear impairments of the chaotic signals, constant modulus algorithm is used for channel equalization and extended Kalman filter is adopted for carrier phase recovery. After digital processing, the recovered chaos signal is converted back to the optical domain for chaos synchronization. By this means, we demonstrate a 10 Gb/s phase-modulation signal encrypted by phase chaos transmission over record-breaking 1000 km single-mode fiber with bit-rate error less than 1.0 × 10−3 by simulation, and support the results with extensive numerical analysis. [ABSTRACT FROM AUTHOR]

The Kramers–Kronig (KK) receiver has recently drawn a great deal of attention due to its capability to extract the complex-valued electric field of optical signal from the directly detected signal. One practical issue is the recovery of the DC component lost upon the reception using an AC-coupled photo-detector. Due to the nonlinear operation of KK algorithm, it is necessary to recover the DC value of the signal before executing the algorithm. A common method to estimate the DC component was to sweep the DC value added to the detected signal, while measuring the system performance metrics such as bit-error ratio (BER). In this article, we propose and demonstrate a simple one-step DC recovery method for KK receiver. By inserting a zero-padded preamble in the modulation data and measuring a couple of statistical averages of the detected AC-coupled signal, we can estimate the DC component accurately even when the carrier-to-signal power ratio (CSPR) is low. Thus, this method estimates the DC component rapidly without time-consuming and power-hungry signal processing such as demodulation and BER calculation. We carry out a series of computer simulations and experiments to measure the accuracy of the proposed method. The results show that the proposed method incurs negligible sensitivity penalties compared with the conventional DC-sweep method. Also, provided in this paper is the design guideline on the proposed method. [ABSTRACT FROM AUTHOR]

We propose a bend-insensitive grapefruit-type holey ring-core fiber to support orbital angular momentum (OAM) modes. The large air domain in the cladding, results in a large refractive index contrast between the ring-core and the cladding, which makes this fiber bend-insensitive and makes the vector eigenmodes well separated. Therefore, the proposed fiber can be adopted for the OAM mode division multiplexing transmission without the use of any multiple-input multiple-output digital signal processing (MIMO-DSP). It is also proved that the thickness of the outer silica layer around the ring-core plays an important role in confining the light. If the OAM modes in the ring-core and the cladding modes in the outer silica layer are not phase-matched, the OAM modes in the ring-core will not be coupled into the outer layer of silica and can be well confined. We fabricate such fiber in two steps by using the “stack-and-draw” method. Two kinds of fibers with different thicknesses of the outer silica layer are fabricated under different pressures and temperatures. The testing results of the fabricated fiber demonstrate the mode purity and illustrate that the loss of the OAM $_{+1,1}$ and OAM $_{+2,1}$ modes are 0.095 dB/m and 0.25 dB/m, respectively. [ABSTRACT FROM AUTHOR]

A method for reducing the complexity of estimating multiple timing offset in time interleaved analog to digital converter based on Fourier series decomposition is proposed. The timing offset of different slices of the time interleaved analog to digital converter can be understood as points on a curve, which represents the timing offset per slice. Thus, the estimation of the timing offset can also be achieved by fitting of the timing-offset-curve. The timing offset curve can be represented by a kernel regression coefficient which are orthonormal on a limited set, thus replacing the estimation of timing offset per slice with the estimation of the regression kernel coefficients. In this paper, the idea of estimating kernel coefficients by using Fourier series analysis is presented and analyzed. When the timing offset of different slices are highly correlated, it is shown that an improvement of 3 dB can be achieved with ∼75% reduction in complexity, 6 dB improvement with ∼60% reduction and 10 dB with ∼40% reduction in respect to similar linear methods. [ABSTRACT FROM AUTHOR]

Conventional chirped-pulse (CP) phase-sensitive optical time-domain reflectometry (CP φ-OTDR) allows the interrogation of tens of kilometers of optical fiber with high accuracies of millikelvin or nanostrain. With respect to standard coherent-detection φ-OTDR, it shows increased robustness to coherent fading and allows a linear and quantitative monitoring of the perturbations acting on the fiber. Its spatial resolution, however, remains a critical parameter and new techniques allowing its improvement without reducing significantly other performances (or increasing the setup complexity) are constantly being researched. In this paper, we propose a method to improve the spatial resolution of CP φ-OTDR without reducing the input pulse width, by means of sub-bands processing. The method is based on adding an optical carrier to the input pulse. Using digital filtering, the spectrum of the fiber backscatter can be split into multiple sub-bands. Each of these sub-bands corresponds to the fiber response generated by a short optical pulse, chirped over a smaller frequency range. This way each sub-band results in φ-OTDR measurements with high spatial resolution, but with a reduced SNR. A dedicated post-processing methodology is proposed to mitigate the SNR reduction obtained from each sub-band, while securing high-resolution measurements of the perturbations acting on the fiber. Experimental results demonstrate the possibility of achieving CP φ-OTDR measurements with a 15-fold spatial resolution improvement over the conventional CP analysis, at the expense of an SNR reduction lower than a factor 2. [ABSTRACT FROM AUTHOR]

We present circuit implementations of blind phase search (BPS) carrier phase recovery (CPR) for M-QAM coherent optical receivers and highlight some BPS algorithm modifications necessary to obtain efficient VLSI circuits. In addition, we show how three key design parameters (input word length, number of test phases, and type and size of averaging window) affect the resulting implementation. To study design tradeoffs, we develop BPS CPR circuit netlists for a 32-GBaud system, using a 22-nm CMOS process technology: our implementations reach energy efficiencies of around 1 pJ/bit for 16QAM up to 3 pJ/bit for 256QAM, at an SNR penalty of approximately 0.25 dB at a BER of 10−2. Furthermore, we present a circuit implementation of pilot-symbol-aided CPR, reaching 0.38 pJ/bit and 0.34 pJ/bit for 16QAM and 256QAM, respectively, at a slightly higher SNR penalty. The two CPR methods are also evaluated in terms of silicon area and scaling to higher-order modulation formats. [ABSTRACT FROM AUTHOR]

Nonlinear interactions between neighboring pulses has always been a fundamental bottleneck in soliton transmission systems. Recently, coherent transceivers, digital signal processing (DSP) and the new nonlinear Fourier transform (NFT) theoretical framework has revived and generalized the field of soliton transmissions into nonlinear frequency division multiplexing (NFDM). We hereby demonstrate analytically and experimentally that one can considerably improve soliton transmission performance by intentionally allowing neighboring solitons to interact and collide during propagation and exchange positions at the receiver followed by standard NFT processing. This can be achieved by designing neighboring solitons’ eigenvalues $\boldsymbol {\lambda }$ to have opposite signs in the real part while the magnitude $| {{\bf \Re}(\boldsymbol {\lambda })} |$ is optimized for a given transmission distance so that neighboring transmitted pulses would have swapped their timing positions at the receiver. Experimental results for 6.13 Gbaud 1-soliton systems demonstrate a transmission reach improvement of 100% for 16APSK and 60% for 8PSK modulated on the b-coefficients. The proposed scheme eliminated a long-standing fundamental limitation in soliton transmissions, opened up new dimensions in transmitter signal design and receiver signal processing for nonlinear optical communication systems. [ABSTRACT FROM AUTHOR]

Efficient nonlinearity compensation in fiber-optic communication systems is considered a key element to go beyond the “capacity crunch”. One guiding principle for previous work on the design of practical nonlinearity compensation schemes is that fewer steps lead to better systems. In this paper, we challenge this assumption and show how to carefully design multi-step approaches that provide better performance–complexity trade-offs than their few-step counterparts. We consider the recently proposed learned digital backpropagation (LDBP) approach, where the linear steps in the split-step method are re-interpreted as general linear functions, similar to the weight matrices in a deep neural network. Our main contribution lies in an experimental demonstration of this approach for a 25 Gbaud single-channel optical transmission system. It is shown how LDBP can be integrated into a coherent receiver DSP chain and successfully trained in the presence of various hardware impairments. Our results show that LDBP with limited complexity can achieve better performance than standard DBP by using very short, but jointly optimized, finite-impulse response filters in each step. This paper also provides an overview of recently proposed extensions of LDBP and we comment on potentially interesting avenues for future work. [ABSTRACT FROM AUTHOR]

DSP techniques that enhance the effective resolution of digital-to-analog converters (DACs) are presented. First, DACs that are limited by physical number of bits (PNOB) are addressed. An improved digital-resolution-enhancer (DRE) algorithm, which incorporates an optimized non-uniform quantization scheme is analytically analyzed and experimentally demonstrated. Improvement of up to 8 dB is shown, that enables 16QAM and 64QAM transmission using only 2 and 3 bits DACs, respectively. Furthermore, the DRE concept is elaborated to include additional practical DAC impairments such as timing jitter. Based on a generalized DAC model, we suggest an improved algorithm that increases the tolerance to jitter noise, termed as jitter-aware DRE (JA-DRE). The method is validated in numerical simulations, indicating substantial OSNR gain of $\sim \!5$  dB over non DSP enhanced DACs, and up to 2 dB over the previously proposed DRE algorithm. [ABSTRACT FROM AUTHOR]

Efficient fiber-optic communication requires precise knowledge of the fiber coefficients, but these often change over time due to factors such as aging or bending. We propose a novel algorithm that identifies the average second-order dispersion and Kerr nonlinearity coefficient of a fiber, without employing any special training signals. Instead, ordinary input and output data recorded during normal operation is used. To the best of our knowledge, this is the first such algorithm. The algorithm is based on the nonlinear Fourier spectrum of the signal, which is known to evolve trivially as the signal propagates through an idealized model of the fiber. The algorithm varies the values of the fiber coefficients until the corresponding nonlinear Fourier spectrum at transmitter and receiver match optimally. We test the algorithm on simulated transmission data over a 1600 km link, and accurately identify the fiber coefficients. The identification algorithm is in particular well suited for providing a fiber model for nonlinear Fourier transform-based communication. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present an additive noise model for the signal-to-noise ratio prediction in optical systems employing low-resolution (4 bits and below) digital-to-analog and analog-to-digital converters. Firstly, the expected ASIC power consumption saving by employing low-resolution digital-to-analog and analog-to-digital converters rather than their high-resolution version (8 bits) is assessed and found to be up to 20% for a 4-bit physical resolution. Secondly, to assess the achievable data rate with low-resolution, we present a prediction model, which relies on two steps. First, the mean square error at the low-resolution quantizer output due to quantization and clipping is computed. Second, the mean square error is converted into a signal-to-noise ratio, carefully accounting for the operating parameters (sampling frequency, symbol rate, clipping factor). The domain of validity of this model is assessed for both quantization and clipping noises separately. The predictions are confronted with numerical simulations and a set of experiments in a back-to-back optical set-up. The results show that the signal-to-noise ratio can be predicted with an error of less than 0.5 dB within the domain of validity of the model. Finally, we use this model to establish the resolution requirements for the next generation of metropolitan optical systems (>400 Gbit/s) and compare them with most recent experiments using low-resolution digital-to-analog converters. [ABSTRACT FROM AUTHOR]

High throughput signals with high spectral efficiency (SE) are attracting interest for their application to data center interconnection application. For such application, a high baud rate and high-order modulation are desirable. This article describes each of the three steps in designing a high SE signal with a net rate of 1-Tbps/λ in detail. First, the baud rate dependency of the signal-to-noise ratio (SNR) is obtained by applying methods such as precise digital calibration technique. Test signals of PDM -16 quadrature amplitude modulation (QAM), PDM-64QAM, and PDM-256QAM were compared to illustrate the modulation format dependence on signal SNR. Clipping ratio optimization and joint digital equalization and optical equalization were performed to maximize the SNR. Second, the achievable capacity is calculated from the obtained baud rate dependency of the SNR. Symbol rate is selected to maximize the achievable capacity. WDM-grid and the Nyquist pulse shaping roll-off factor is selected to achieve a high SE. Third, the rate parameter of probabilistically shaped (PS) signal are determined to generate 1-Tbps/λ net rate signal. A high fiber capacity of 41-Tbps is successfully demonstrated with C-band WDM transmission over 100 km with 41-WDM 96-Gbaud PDM-PS-256QAM signals in a 100-GHz grid. [ABSTRACT FROM AUTHOR]