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An intelligent optical performance monitoring scheme for simultaneous modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring is proposed and experimentally demonstrated in digital coherent receivers. The proposed scheme introduces convolutional neural network (CNN) to automatically extract and monitor modulation format and OSNR dependent features (the empirical distribution of intensity and differential-phase) which can be obtained after constant modulus algorithm (CMA) equalization. The experiment results show that 100% identification accuracies for all modulation formats (e.g. 28-GBaud PDM-QPSK/−8PSK/ −8QAM/−16QAM/−32QAM/−64QAM) are achieved at OSNR values are lower than the corresponding theoretical 20% FEC limit (BER = 2.4×10−2). Furthermore, under the chromatic dispersion (CD) variation from 0-ps/nm to 1200-ps/nm, the root-mean- square error (RMSE) and mean absolute error (MAE) of OSNR monitoring for all modulation formats are less than 0.0896-dB and 0.0657-dB, respectively. Subsequently, the influence of frequency offset and fiber nonlinearity effect on the intelligent optical performance monitoring scheme is also analyzed. We believe that our proposed multi-parameter monitoring scheme has the potential to be applied in the next-generation intelligent elastic optical networks. [ABSTRACT FROM AUTHOR]

We review the latest developments in Fiber-optic Telecommunications and Networks presented at OFC 2022 and compare the progress accomplished since the previous year conference, OFC 2021. In particular, we note this year's emphasis on the application of Neural Networks (NN) as part of Artificial Intelligence (AI) and Machine Learning (ML), as well as continued research in quantum communications and key distribution. We also expect future conferences to include papers about optical variants of quantum computers, a field of great interest that can exploit many quantum systems, as well as papers on continuing developments in High-Speed Communications beyond 800 Gb/s, and increased use of coherent modulation and detection in Data Centers Interconnect, Cloud and Edge computing, the latter in support of 5 G Internet Of Things (IOT). [ABSTRACT FROM AUTHOR]

Introduction/Context: The use of coherent detection jointly with high-level modulation formats such as 16 and 64-QAM seems to be a convenient strategy to increment capacity of future optical access networks. However, coherent detection requires high complexity digital signal processing to mitigate different impairments. Objective: Mitigate signal distortions using nonsymmetrical demodulation techniques based on Machine Learning (ML) algorithms. Methodology: First, a single channel Nyquist m-QAM system at 28 and 32 Gbps was simulated in VPIDesignSuite software. Then, different signals modulated at 16 and 64-QAM were generated with different laser linewidth, transmission distances and launch powers. Two ML algorithms were implemented to carry out the demodulation of the generated signals. The performance of the algorithms was evaluated using the bit error rate (BER) in terms of different system parameters as laser linewidth, transmission distance, launch power and modulation format. Results: The use of ML allowed gains up to 2 dB in terms of optical signal-to-noise ratio at a BER value of for 16-QAM and 1.5 dB for 64-QAM. Also, the use of ML showed that it is possible to use a lower cost laser (100 kHz linewidth vs 25 kHz) with a better BER performance than using conventional demodulation. Conclusions: We showed that the use of both algorithms could mitigate nonlinear effects and could reduce computational complexity for future optical access networks.

We propose and experimentally demonstrate a converged optical-wireless WDM access network architecture enabled by a highly integrated quantum dot coherent comb laser. The converged optical-wireless WDM network features simultaneous delivery of coherent and millimeter wave (mmWave) / citizens broadband radio service (CBRS) signals over 50-km and 20 km fiber links, respectively.

Historically, undersea systems capacity increases were primarily based on maximizing the total throughput per fiber. Whilst previously designed to operate at optimum launch power of the fiber, subsea systems are now designed for higher optical power efficiency. Next generation of spatial division multiplexed (SDM) cables require operation at lower signal power enabling repeater pump farming and providing higher reliability and efficiency. To adapt to these power constraints, the benefits of adaptive transponders and the application of machine learning to submarine line design are investigated. [ABSTRACT FROM AUTHOR]

Compensation of Kerr nonlinearity-induced distortions has been shown to allow for increasing transmission rate and reach, with optical compensation techniques particularly attractive for broadband wavelength-division multiplexed (WDM) scenarios. However, they normally require additional devices within the link, which is particularly challenging for already deployed systems, and even more so for unrepeatered transmission. In this work, we focus on providing lumped optical nonlinearity suppression, either at the transmitter or at the receiver side, based on optical phase conjugation (OPC). The theory to design scaled-down OPC compensation modules is derived, and it allows to move beyond the standard mid-link OPC approach and to explore compensation modules with fibers types not directly linked to the dispersion properties of the transmission link. This design method is then validated both through numerical investigations and experimental demonstrations for a number of systems with a varying degree of complexity. Ultimately, significant performance improvement is shown by employing short OPC-based compensation structures which are carefully designed to match much longer transmission links. [ABSTRACT FROM AUTHOR]

We report an industry leading optical dense wavelength division multiplexing (DWDM) field trial with line rates per channel $\ge 1.66$ Tb/s using 130 GBaud dual-polarization probabilistic constellation shaping 256-ary quadrature amplitude modulation (DP-PCS256QAM) in a high capacity data center interconnect (DCI) scenario. This research trial was performed on 96.5 km of field-deployed standard single mode G.652 fiber infrastructure of Deutsche Telekom in Germany employing Erbium-doped fiber amplifier (EDFA)-only amplification. A total of 34 channels were transmitted with 150 GHz spacing for a total fiber capacity of 56.51 Tb/s and a spectral efficiency higher than 11bit/s/Hz. In the single-channel transmission scenario 1.71 Tb/s was achieved over the same link. In addition, we successfully demonstrate record net bitrates of 1.88 Tb/s in back-to-back (B2B) using 130 GBaud DP-PCS400QAM. [ABSTRACT FROM AUTHOR]

Introduction--The use of coherent detection jointly with high-level modulation formats such as 16 and 64-QAM seems to be a convenient strategy to increment capacity of future optical access networks. However, coherent detection requires high complexity digital signal processing to mitigate different impairments. Objective--Mitigate signal distortions using nonsymmetrical demodulation techniques based on Machine Learning (ML) algorithms. Methodology--First, a single channel Nyquist m-QAM system at 28 and 32 Gbps was simulated in VPIDesignSuite software. Then, different signals modulated at 16 and 64-QAM were generated with different laser linewidth, transmission distances and launch powers. Two ML algorithms were implemented to carry out the demodulation of the generated signals. The performance of the algorithms was evaluated using the Bit Error Rate (BER) in terms of different system parameters as laser linewidth, transmission distance, launch power and modulation format. Results--The use of ML allowed gains up to 2 dB in terms of optical signal-to-noise ratio at a BER value of for 16-QAM and 1.5 dB for 64-QAM. Also, the use of ML showed that it is possible to use a lower cost laser (100 kHz linewidth vs 25 kHz) with a better BER performance than using conventional demodulation. Conclusions--We showed that the use of both algorithms could mitigate nonlinear effects and could reduce computational complexity for future optical access networks. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a converged optical-wireless WDM access network architecture enabled by a highly integrated quantum dot coherent comb laser. The converged optical-wireless WDM network features simultaneous delivery of coherent and millimeter wave (mmWave) / citizens broadband radio service (CBRS) signals over 50-km and 20 km fiber links, respectively. [ABSTRACT FROM AUTHOR]

We propose an efficient and modulation-format-transparent method to jointly estimate optical signal-to-noise ratio (OSNR) and frequency offset (FO) by using signal spectrum correlations for coherent optical fiber communication systems. Based on the signal spectrum correlation analysis, a coarse FO estimation (FOE) and the corresponding compensation can be conducted after chromatic dispersion compensation (CDC). Then, OSNR monitoring can be accurately operated without deterioration caused by FO. Meanwhile, to realize fine FOE, down-sampling process of signal can be used to reduce the complexity of fast Fourier transform-based FOE (FFT-FOE) without losing FOE resolution. Simulation results show that when the OSNR is in the range of 10 to 30 dB, the proposed scheme presents an absolute OSNR estimation error lower than 0.18 dB. Finally, we experimentally demonstrate our scheme in an optical back-to-back (B2B) transmission link using 28 Gbaud dual-polarization (DP)-4/16/32-quadrature amplitude modulation (QAM) formats, and an absolute OSNR estimation error lower than 0.66 dB is achieved for OSNR ranging from 15 to 30 dB. [ABSTRACT FROM AUTHOR]

In this work, we present a novel strategy to satisfy the nonlinearity compensation criteria by optical-phase-conjugation (OPC). Contrary to the most common approach, which relies on tailoring power profiles using distributed Raman-amplification, we achieve the required OPC propagation symmetry through optimized dispersion management across the link. The method is applied to transmission systems with periodic lumped amplification, and the symmetry enhancement is directly translated into a substantial increase in the OPC compensation gains. This study is based on a numerical analysis combined with experimental validation of the findings. The numerical part provides a comprehensive overview of different dispersion management techniques, and compares them against the symmetric schemes we propose. It is consistently shown that the symmetry-optimized systems provide the best performance when OPC is included, with the signal-to-noise ratio (SNR) gains reaching up to 6.6 and 5.2 dB for transmission of a single and seven wavelength channels, respectively. These results are verified in an experimental investigation, where we implemented and compared a standard dispersion mapping scheme to the optimized design. For all distances considered, the optimized link is demonstrated superior once OPC is included, leading up to 1.9 dB improvement in SNR for seven-channel transmission. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate on-chip modal crosstalk mitigation over two mode-division multiplexing (MDM) integrated circuits supporting four and eleven waveguide modes employing low-coherence matched detection. 30-Gbaud 8-PSK mode-multiplexed signal is successfully transmitted over the circuits without using multiple-in-multiple-output (MIMO) processing under a maximum modal crosstalk of 10 dB. Ten-mode-multiplexed on-chip transmission achieves a total capacity of 900 Gbit/s. This work shows the feasibility of using high-order modes in on-chip MDM transmission for future high-capacity optical interconnects. [ABSTRACT FROM AUTHOR]

Featured Application: In this paper, we put forward an algorithm switching orthogonalization procedure (ASOP) according to the quality of the in-phase and quadrature signal branches based on the Q value of the eye diagram with less computation. The proposed ASOP scheme can contribute to the frequency offset estimation and phase estimation of free-space optical communication (FSO) systems with turbulence disturbance. Thus, it is hoped that the ASOP scheme we proposed will see the practical application of coherent FSO communication. The Gram–Schmidt orthogonalization procedure (GSOP) and Löwdin symmetric orthogonalization procedure (SYOP) are the two mainstream algorithms for the compensation of phase mismatch in an imperfect optical 90° hybrid. In this paper, we put forward an algorithm switching orthogonalization procedure (ASOP) according to the quality of in-phase and quadrature signals based on the Q value of the eye diagram with less computation. If the quality of the in-phase and quadrature signals has a significant difference, we use the GSOP and select the signal branch with better quality as the initial reference vector for orthogonalization. If they are of about the same quality, then we use the SYOP. We present computer simulations for a coherent free-space optical (FSO) quadrature phase-shift keying (QPSK) communication system and demonstrate the system improvement that can be achieved using the ASOP. Finally, we also show that the proposed ASOP scheme can contribute to the frequency offset and phase estimation of the FSO system in the environment of atmospheric turbulence. [ABSTRACT FROM AUTHOR]

We reviewed recent breakthroughs on silicon photonic for 100Gbaud operation. Recent progress on high-speed Ge photodetector and carrier depletion modulator promises 100Gbaud optical transceivers with all-silicon material platform for commercial applications. We achieved high performance high-speed all-silicon photonics carrier-depletion Mach–Zehnder modulation by co-optimization of doping and device design assisted with an accurate electro-optical (EO) model. We reported all-silicon Mach–Zehnder modulator with a measured 6 dB EO bandwidth of >60 GHz by using a medium doping and 2 mm long phase shifter. We experimentally demonstrated 120Gbaud QPSK and 100Gbaud 32QAM operations using a high performance all-silicon in-phase/quadrature (IQ) modulator with extinction ratio of >25 dB, moderate ${{\boldsymbol{V}}_{\boldsymbol{\pi }}}$ of 6.3 V, and 6 dB electro-optic bandwidth of 50 GHz employing practical Nyquist filter and linear compensation in commercial arbitrary wave generator (AWG) and optical modulation analyzer (OMA). We studied both performance optimization and limitation. Our results show that BER performance can be optimized by pre-equalization (Pre-EQ) method for bandwidth-limited silicon photonic modulators. However, the performance on BER and modulation loss is strongly affected by the equalization bandwidth due to peak-to-average-power-ratio (PAPR). The frequency at fast roll-off of transmitter response is more critical than 3 dB electro-optic bandwidth when Pre-EQ is used for bandwidth compensation. [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 generation of optical beams with multiple, mutually coherent orbital-angular-momentum (OAM) modes using phase gratings is analyzed from the perspective of energy distribution and radial mode composition. We show that phase gratings designed with equally weighted Laguerre-Gauss (LG) modes will generate beams with uneven energy distribution among OAM components. This unwanted outcome cannot be corrected by adjusting the width of the illuminating beam. We propose a way to design phase gratings that will produce a uniform energy distribution among the constituent OAM states after illumination while minimizing the content of high radial modes. This method is based on a generalized definition for the LG modes that take advantage of the freedom to select their radial scales.

In this article, for the first time, a full-spectrum periodic nonlinear Fourier transform (NFT)-based communication system with the inverse transformation at the transmitter performed by using the solution of Riemann-Hilbert problem (RHP), is proposed and studied. The entire control over the nonlinear spectrum rendered by our technique, where we operate with two qualitatively different components of this spectrum represented, correspondingly, in terms of the main spectrum and the phases, allows us to design a time-domain signal tailored to the characteristics of the transmission channel. In the heart of our system is the RHP-based signal processing utilised to generate the time-domain signal from the modulated nonlinear spectrum. This type of NFT processing leads to a computational complexity that scales linearly with the number of time-domain samples, and we can process signal samples in parallel. In this article, we suggest the way of getting an exactly periodic signal through the correctly formulated RHP, and present evidence of the analogy between band-limited (in ordinary Fourier sense) signals and finite-band (in RHP sense) signals. Also, for the first time, we explain how to modulate the phases of individual periodic nonlinear modes. The performance of our transmission system is evaluated through numerical simulations in terms of bit error rate and Q $^2$ -factor dependencies on the transmission distance and power, and the results demonstrate the good potential of the approach. [ABSTRACT FROM AUTHOR]

Introduction/Context: The use of coherent detection jointly with high-level modulation formats such as 16 and 64-QAM seems to be a convenient strategy to increment capacity of future optical access networks. However, coherent detection requires high complexity digital signal processing to mitigate different impairments. Objective: Mitigate signal distortions using nonsymmetrical demodulation techniques based on Machine Learning (ML) algorithms. Methodology: First, a single channel Nyquist m-QAM system at 28 and 32 Gbps was simulated in VPIDesignSuite software. Then, different signals modulated at 16 and 64-QAM were generated with different laser linewidth, transmission distances and launch powers. Two ML algorithms were implemented to carry out the demodulation of the generated signals. The performance of the algorithms was evaluated using the bit error rate (BER) in terms of different system parameters as laser linewidth, transmission distance, launch power and modulation format. Results: The use of ML allowed gains up to 2 dB in terms of optical signal-to-noise ratio at a BER value of 1 × 10-2 for 16-QAM and 1.5 dB for 64-QAM. Also, the use of ML showed that it is possible to use a lower cost laser (100 kHz linewidth vs 25 kHz) with a better BER performance than using conventional demodulation. Conclusions: We showed that the use of both algorithms could mitigate nonlinear effects and could reduce computational complexity for future optical access networks. [ABSTRACT FROM AUTHOR]

We introduce and expand on key high speed optical innovations that enable scalable and efficient optical transmission networks; technologies such as advanced modulation formats including Probabilistic Constellation Shaping (PCS), flexible grid and optical superchannels as well as emerging high speed Ethernet client technology. We describe and show examples of our recent industry leading technology trials giving real-world experiences of leading edge technologies in field deployed network environments. We focus on a recent successful trial of real-time 2 × 200G PCS DP-16QAM superchannel transporting 400 GbE over a 727 km live flexible grid link, achieving high spectral efficiency of 5.33 bit/s/Hz. We also show that 200G PCS DP-16QAM has significantly enhanced performance over the conventional DP-16QAM, achieving a distance of 1454 km. These industry leading trials give network operators crucial understanding of the performance, maturity and operational viability of emerging high speed technologies, and therefore confidence that they are fit for purpose and will allow network operators to scale their networks gracefully to meet increasing bandwidth demands. [ABSTRACT FROM AUTHOR]

Addressing the optical communication systems employing the nonlinear Fourier transform (NFT) for the data modulation/demodulation, we provide an explicit proof for the properties of the signals emerging in the so-called $b$ -modulation method, the nonlinear signal modulation technique that provides explicit control over the signal extent. We present details of the procedure and related rigorous mathematical proofs addressing the case where the time-domain profile corresponding to the $b$ -modulated data has a limited duration, and when the bound states corresponding to specifically chosen discrete solitonic eigenvalues and norming constants, are also present. We also prove that the number of solitary modes that we can embed without violating the exact localisation of the time-domain profile, is actually infinite. Our theoretical findings are illustrated with numerical examples, where simple example waveforms are used for the $b$ -coefficient, demonstrating the validity of the developed approach. We also demonstrate the influence of the bound states on the noise tolerance of the b-modulated system. [ABSTRACT FROM AUTHOR]

We report on the spectral characteristics of the phase noise enhancement of continuous-wave (CW) optical signals passing through a semiconductor optical amplifier (SOA) operating in the saturation regime and used for relative intensity noise (RIN) reduction. We show that the known effect of phase noise enhancement, attributed to the linewidth enhancement factor of the device, happens only at a limited band of the phase noise spectrum, and the actual measurable linewidth of the output CW signal may not be affected. While this phase noise enhancement is not shown as an increase of spectral linewidth, it can still affect system performance when coherent detection is used, especially in applications with relatively low symbol rates. Numerical simulations and experimental results are used to support the observation. A single spectral line from a quantum-dot mode-locked laser is used as the light source, which is known to have relatively high RIN (> −120 dB/Hz in the low frequency region). Experimental transmission of 16-QAM modulation with coherent detection has been performed at 5 GBd to assess the implication on system performance. [ABSTRACT FROM AUTHOR]

We report on the use of semiconductor optical amplifiers (SOAs) to extend the optical bandwidth of next generation optical systems to 100 nm and beyond. After discussing the technological progress and the motivation for rekindling the interest in SOAs for line amplification, we describe the innovative approach developed for the realization of ultra-wideband (UWB) SOAs. Leveraging custom design of singly polarized SOAs to provide gain over 100+ nm bandwidth, we developed a polarization diversity architecture to realize UWB-SOA modules. Embedded in a compact package, the UWB amplifier modules have been successfully used to demonstrate 100+ Tb/s transmissions. We subsequently review recent experimental transmission results based on such novel 100+ nm wide semiconductor optical amplifiers, including our first demonstration of 100+Tb/s transmission over 100 km distance, our field trial using real-time traffic, and finally the transmission of 107 Tb/s throughput over three spans of standard single mode fiber (SSMF) using hybrid UWB Raman/SOA amplification technique. [ABSTRACT FROM AUTHOR]

We propose an adaptive phase-diversity method for optical M-ary differential phase-shift keying (MDPSK) homodyne receivers, to achieve coherent optical communication. This method involves eliminating the random fluctuations in the phase differences between the signal light and local oscillator and the in-plane and quadrature signal (IQ) imbalance compensation. For the former, a phase-diversity method, which can theoretically eliminate all phase-related noise, is proposed. The gain and phase imbalances between the I and Q channels can be compensated completely by using automatic phase- and gain-control systems. The core idea is the adaptability of the receiver to the local phase shift of the local oscillator. The performance of the receiver is not sensitive to environmental factors such as temperature and vibrations. Our simulation results show that the imbalances between the IQ channels can be completely compensated over a wide imbalance range, and the baseband signals can be accurately recovered using the proposed scheme. [ABSTRACT FROM AUTHOR]

In this work, we developed an adaptive optical correction module and polarization control module to compensate for the effects of atmospheric turbulence on long-distance free-space coherent optical communications. We established a coherent optical communication system based on heterodyne differential binary phase-shift keying modulation in Xi'an University of Technology, China, to achieve video transmissions over a long distance of 10.2 km. Indeed, the modules resulted in a baseband signal with a more regular pattern with a significant decrease in burrs and the polarization state of the optical signal was effectively controlled, which verifies the usability of the modules. [ABSTRACT FROM AUTHOR]

The In-Phase/Quadrature (I/Q) imbalance is a limiting factor for accurate phase demodulation in coherent communication systems. Recently, the 90° optical/electrical hybrid I/Q demodulator have been introduced into distributed optical fiber sensor (DOFS) system to significantly reduce the phase measurement time but concurrently bring the limitation of I/Q quadrature imbalances that might corrupt the accuracy of the measured phase signal. This paper introduces a novel coherent optical pulse phase rotation reflectometry (COPPRR) system insensitive to I/Q quadrature imbalance. The theoretical basis of the COPPRR system is the optical pulse phase rotation and integration (PRI) technique that stipulates the use of integration of ${N}$ groups of I/Q signals with regular phase-shift to mutually eliminate the effects of quadrature imbalance carried in the individual pair of signals. Experimental results effectively demonstrated a phase recovery of 100 Hz sinusoidal vibration robust against various types of simulated quadrature imbalance where the I/Q demodulation completely failed to restore the phase signal. [ABSTRACT FROM AUTHOR]

We report on the ultra-wideband (UWB) transmission of a 103 nm signal over 3 × 100 km of standard single mode fiber (SSMF), a widely-deployed fiber in terrestrial transmission networks. In such UWB systems, nonlinear effects such as stimulated Raman scattering (SRS) must be taken into account in the system design. We characterize the SRS impact in high power regime for this UWB spectrum. We study the evolution of SRS over the bandwidth when tilting the input spectrum to counter the inter-band power transition. We transmit 254 probabilistically constellation shaped (PCS)-64QAM channels over 300 km of SSMF and demonstrate a 107-Tb/s transmission throughput over a continuous 103 nm optical bandwidth using backward Raman pumping and SOA technology. [ABSTRACT FROM AUTHOR]

This article presents an experimental characterization of the impact of nonlinear impairments induced by semiconductor optical amplifiers (SOA) in a wavelength division multiplexed (WDM) coherent communication system. Motivated by a ceaseless traffic growth in optical transport systems, extending the bandwidth of the fiber link is a promising way to increase the throughput in optical networks. Using specific design, SOA can provide ultra-wideband (UWB) and continuous amplification spreading from S to L bands. For both off-the-shelf C-band SOA and UWB SOA devices, we show that SOA nonlinear impairments are not critical in the WDM regime. We report on the impact of the WDM channel count on the nonlinear distortions induced by an UWB SOA. We also demonstrate that besides having a wider bandwidth, our custom UWB SOA devices have superior performance compared to off-the shelf components even for C-band only transmission, and are well suited for wideband WDM signal amplification. [ABSTRACT FROM AUTHOR]

We achieved high performance high speed silicon photonics carrier-depletion Mach-Zehnder modulation with commercial foundry by co-optimization of doping and device design assisted with an accurate electro-optical (EO) model. We demonstrated high performance IQ modulators operating at 85 Gbaud 16 QAM and 64 Gbaud 64 QAM with extinction ratio of over 25 dB. For the design of the high performance all-silicon carrier depletion modulator, we developed modeling and design tools to provide not only accuracy, but also efficiency in the simulation of distributed optical and electronic characteristics of travelling waveguides with different designs of optical and microwave waveguides under various doping conditions, which allow the co-design of velocity phase match between optical and microwave waveguides and the impedance match between microwave travelling waveguide and terminal impedance. Our experimental characterization test data agreed well with the model simulation data. More recently, with practical Nyquist filter and linear compensation in commercial arbitrary wave generator (AWG) and optical modulation analyzer (OMA), we demonstrated 100 Gbaud 32 QAM with an all-silicon IQ modulator, which has 6 dB electro-optical bandwidth of 50 GHz and BER achieving FEC threshold with a modern FEC, showing the potential for Tb/s applications. [ABSTRACT FROM AUTHOR]

We report the results of 50G, 100G, 200G, 400G un- repeatered transmission aimed at achieving the longest distance without any inline active elements, this system realizes record single-carrier 50 Gb/s (PM-BPSK), 100 Gb/s (PS-QPSK), 200 Gb/s (PM-8QAM) and 400 Gb/s (PM-64QAM) unrepeatered transmission over 670.64 km with 103.95 dB span loss, 653.35 km with 101.27 dB span loss, 601.93 km with 93.3 dB span loss, and 502.13 km with 77.83 dB span loss, respectively. This is achieved using optimized high-order Raman pumps, cascaded RGUs and coherent modulation format with concatenated FEC. G.654.E fiber with ultra-low loss & 130 μm2 effective area is used as span fiber. [ABSTRACT FROM AUTHOR]

Coherent communications aim to support higher data rates and extended connectivity at lower power consumption compared with traditional point-to-point transmissions. The typical setting of coherent communication schemes is based on a single data stream with multiple transmitters and a single receiver. This paper studies the case of multiple concurrent data streams, each with multiple transmitters and multiple receivers, in self-organizing wireless networks. A distributed optimization solution based on joint network formation and beamforming is developed for coherent group communications in a network of nodes that need to communicate over long distances. This solution significantly improves the power gain for a single data stream and the signal-to-interference-ratio for multiple data streams compared to point-to-point communications. These gains are in turn translated to improvements in communication range, power efficiency, reliability, and throughput. In this multi-layer network optimization solution, nodes coordinate with each other in a distributed manner to form transmitter and receiver groups, and communicate with each other coherently over long distances. The coherent beamforming protocol is optimized for given transmitter and receiver groups, whereas the network formation protocol is optimized to determine these groups. By using single-antenna communication nodes, the proposed optimization solution provides a major gain in network communications and outperforms other benchmark combinations of beamforming and network formation protocols. [ABSTRACT FROM AUTHOR]

In this work, deep learning is used to monitor coherent channel performance with eye diagram measurement. Experiments show that the proposed technique can determine the modulation format, optical signal to noise ratio (OSNR), roll-off factor (ROF), and timing skew of a quadrature amplitude modulation (QAM) transmitter with high accuracy. Trained vanilla convolutional neural network (CNN) and MobileNet can be utilized to jointly monitor above four parameters with >98% prediction accuracy for 32 GBd coherent channels with quadrature phase shift keying (QPSK), 8-QAM or 16-QAM formats, under OSNR from 15 to 40 dB, IQ skew from −15 to 15 ps, ROF from 0.05 to 1. Our proposed deep learning approach outperforms many traditional machine learning methods, such as decision tree, k-nearest neighbor algorithm (KNN), and histogram of oriented gradient (HOG) based support vector machine (SVM). Unlike other optical performance monitoring approaches, the use of eye diagram measurement combined with deep learning could enable joint monitoring of multiple system performance parameters with reduced hardware implementation complexity. Comparing with vanilla CNN, MobileNet has relatively simplified iteration algorithm, thus reduces the requirement on the computational power, while still maintaining high accuracy for classification issues. [ABSTRACT FROM AUTHOR]

This letter demonstrates unrepeatered real-time transmission of 200 Gb/s (5 bit per symbol modulation format, 56.8 GBaud) signal over terrestrial 520 km single-span fiber link. This was achieved using ultra low-loss fibers with large effective area, ROPAs with dedicated fibers, and distributed Raman amplifiers with co- and counter-propagating pumps. [ABSTRACT FROM AUTHOR]

In this work, we present a comprehensive experimental and numerical investigation of the impact of system parameters on wavelength converters based on four-wave-mixing, with focus on practical system implementations in addition to the interaction within the nonlinear medium. The input signal power optimization is emphasized according to the trade-off between the linear and the nonlinear impairments, and the origin of the limitations at the optimum is studied. The impact of the input signal quality on the converted idler is discussed, and depending on the dominant noise contribution a varying conversion penalty is demonstrated. The penalty is also shown to scale with increasing number of WDM channels due to additional nonlinear cross-talk between them. Finally, by means of numerical simulations we extend the experimental characterization to high pump powers, showing the impact of parametric noise amplification, and different pump laser linewidths, which lead to increased phase-noise transfer. The experimental characterization employs an integrated AlGaAs-on-insulator waveguide, and the numerical simulations accompany the results to make the analysis general for $\chi ^{(3)}$ materials that satisfy the assumptions of the split-step Fourier method. [ABSTRACT FROM AUTHOR]

Efficient synchronization is of great importance for fiber optical coherent communication system. Here, we propose an accurate and low-complexity timing/frequency synchronization scheme based on sparse fast Fourier transform (S-FFT). The proposed scheme consists of coarse timing/frequency synchronization, fine timing synchronization, and fine frequency synchronization. Inspired by the idea of S-FFT, we take full advantages of the sparse nature of training symbols (TSs) and realize the synchronization in the frequency domain. The proposed scheme enables accurate timing synchronization at low signal-to-noise ratio (SNR) region. A complete frequency offset estimation (FOE) range of [−symbol rate/2, + symbol rate/2] can be realized with high resolution. Finally, the proposed scheme is experimentally verified through 10 Gbaud dual-polarization (DP) 16/32-quadrature amplitude modulation (QAM) transmission, without the BER performance penalty. In particular, the total computation complexity is reduced by nearly 200 times in comparison with that using conventional sliding window correlation scheme. [ABSTRACT FROM AUTHOR]

Coherent fiber-optic communication systems are limited by the Kerr-induced nonlinearity. Benchmark optical and digital nonlinearity compensation techniques are typically complex and tackle deterministic-induced nonlinearities. However, these techniques ignore the impact of stochastic nonlinear distortions in the network, such as the interaction of fiber nonlinearity with amplified spontaneous emission from optical amplification. Unsupervised machine learning clustering (e.g., K-means) has recently been proposed as a practical approach to the blind compensation of stochastic and deterministic nonlinear distortions. In this work, the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm is employed, for the first time, for blind nonlinearity compensation. DBSCAN is tested experimentally in a 40 Gb/s 16 quadrature amplitude-modulated system at 50 km of standard single-mode fiber transmission. It is shown that at high launched optical powers, DBSCAN can offer up to 0.83 and 8.84 dB enhancement in Q-factor when compared to conventional K-means clustering and linear equalisation, respectively. [ABSTRACT FROM AUTHOR]

The Gram–Schmidt orthogonalization procedure (GSOP) and Löwdin symmetric orthogonalization procedure (SYOP) are the two mainstream algorithms for the compensation of phase mismatch in an imperfect optical 90° hybrid. In this paper, we put forward an algorithm switching orthogonalization procedure (ASOP) according to the quality of in-phase and quadrature signals based on the Q value of the eye diagram with less computation. If the quality of the in-phase and quadrature signals has a significant difference, we use the GSOP and select the signal branch with better quality as the initial reference vector for orthogonalization. If they are of about the same quality, then we use the SYOP. We present computer simulations for a coherent free-space optical (FSO) quadrature phase-shift keying (QPSK) communication system and demonstrate the system improvement that can be achieved using the ASOP. Finally, we also show that the proposed ASOP scheme can contribute to the frequency offset and phase estimation of the FSO system in the environment of atmospheric turbulence.

Pre-equalizer as high-pass digital filter compensates for transmitter (Tx) frequency response to improve high-baud rate transmission performances; however, it also increases the signal peak to average power ratio (PAPR) at the digital to analog converter (DAC). The increased PAPR induces performance penalty due to the reduced signal-to-noise-and-distortion-ratio (SINAD) at the DAC. It also reduces the Tx optical power due to the limited peak-to-peak amplitude of the DAC output. As a tradeoff between bandwidth compensation and the PAPR, the pre-equalization can be optimized for the best transmission performance. With the optimized pure digital pre-equalization, 100G Baud 16 QAM and 32 QAM transmission is performed with all-silicon photonics IQ modulator. We also investigate the limitations due to analog bandwidth. Simulations show that the Tx analog frequency response roll-off slope and the analog bandwidth below −15 dB are important for high-baud rate transmission with pre-equalization. [ABSTRACT FROM AUTHOR]

Optical signal-to-noise ratio (OSNR) monitoring is essential to both the operation of reliable and reconfigurable network and the supply of high quality-of-service. Recently, deep learning technique has been proposed for the implementation of OSNR monitoring. However, the potential of deep learning technique for OSNR monitoring has not been fully exploited in terms of the monitoring accuracy and robust operation. Here, we propose an OSNR monitoring scheme with high accuracy and short response time using the long short-term memory neural network (LSTM-NN). The use of LSTM-NN is helpful to identify the relationship between the time-varied data and corresponding OSNR without manual feature extraction. We investigate the optimal number of time steps for the LSTM-NN-based OSNR monitoring and the monitoring accuracy with respect to various modulation formats with variable baud-rates. Finally, we implement an experimental verification for 34.94-GBd polarization division multiplexing-16 quadrature amplitude modulation signal, achieving a mean absolute error of 0.05 dB over the OSNR range from 15 to 25 dB. The advantages of LSTM-NN-based OSNR monitoring include high accuracy and smart response, which is ideal for future agile optical network. [ABSTRACT FROM AUTHOR]

We successfully demonstrate 400 Gbit/s transmission at 1577 nm over an unamplified 40 km standard single-mode fiber (SSMF) link with 11-dB loss target. Commercially available high-bandwidth coherent driver modulator (HB-CDM) and high-bandwidth micro integrated coherent receiver (HB- $\mu $ ICR), designed for $C$ -band are used in combination with two TO-can packaged and thermally tuned low-cost $L$ -band laser sources. With the aid of a memory-less one sample/symbol nonlinear offline digital-signal processing (DSP) correcting for the nonlinear transfer curve of the driver and modulator we can maximize the transmitter optical power, which is critical for unamplified fiber applications. In addition, multistage carrier recovery enables stable performance also with the large phase noise of the used coherent laser source, originally part of an electro-absorption modulated laser module (EML) and designed for extremely price-sensitive passive optical network (PON) applications. [ABSTRACT FROM AUTHOR]

Graphical abstract An integrated 90° hybrid 2 × 4 coupler based on flexible, thermal stable, and low insertion losses organic–inorganic hybrids thin films was fabricated using direct laser writing, resulting in an optical coherent receiver able to extract modulated data IQ components. Its performance was demonstrated for demodulation of 20 Gb/s QPSK transmission (over 40 km), yielding a 2.5 dB power penalty (BER = 3.8 × 10−3) which are the best values known for low-refractive index contrast waveguides. Thus, the presented device is an energy-efficient and low cost alternative to the current coherent receivers, capable of breaking cost-performance barriers and easily incorporated in PICs for the NGOA. Highlights • We present a QPSK demodulator produced in di-ureasil organic-inorganic hybrid films. • The device was patterned by UV-direct laser writing, which is low-cost methodology. • The 90° hybrid coupler was validated with 20 Gbs QPSK transmission in 40 km of fibre. • The demodulator performance was demonstrated revealing a power penalty of 2.5 dB. • The device performance and low-cost constitutes a breakthrough in the PIC production. Abstract The recent advances in coherent optical systems enable the use of efficient modulation formats, such as quadrature phase shift keying (QPSK). In this framework, we present an integrated 90° hybrid 2 × 4 coupler based on self-patternable organic–inorganic hybrid thin films fabricated using direct laser writing. The device was coupled to balanced photodiodes, resulting in an optical coherent receiver with capacity to extract the modulated data. The device performance was demonstrated for a transmission over 40 km (G.652 fiber) with a 20 Gb/s QPSK, yielding a 2.5 dB power penalty (BER = 3.8 × 10−3), being the best figures of merit for waveguides processed by low-cost patterning techniques. Furthermore, the hybrid materials are thermally and mechanically stable than polymers, with lower fiber-to-device insertion losses. Therefore, the proposed optical coherent receiver is an easy processed and cost-effective approach for the next generation of optical access networks. [ABSTRACT FROM AUTHOR]

There is increasing interest in low-complexity coherent optical transceivers for the use in short-reach fiber links. Amongst the simplest configurations is the heterodyne coherent receiver, using a 3-dB coupler to combine the signal with the local oscillator (LO) laser output, and a single photodiode for detection of each polarization. In this paper, through numerical simulations, we investigate the impact of signal–signal beating interference (SSBI) and LO relative intensity noise (RIN) on the performance of such coherent transceivers. We assess the performance of two methods to mitigate the SSBI: first, the use of high LO laser power, and second, the application of digital signal processing-based receiver linearization, specifically, the Kramers–Kronig (KK) scheme. The results indicate that, in the case of a RIN-free LO laser, a strong LO is effective in mitigating SSBI and achieves a similar performance to that of the KK algorithm. However, the required increase in LO-to-signal power ratio (LOSPR) is significant. For example, a 20 dB higher optimum LOSPR was observed in the 28 Gbaud dual polarization 16 QAM system at an optical signal-to-noise power ratio of 22 dB. The drawback of using such a high LOSPR is the increased penalty due to RIN-LO beating terms, which we next investigated. The lower optimum LOSPR and, consequently, the lower impact of LO RIN on the performance of the KK receiver lead to a reduction in the pre-FEC BER by over an order of magnitude for LO RIN levels above −140 dBc/Hz. [ABSTRACT FROM AUTHOR]

In this article, we review the latest progress on data center interconnect (DCI). We then discuss different perspectives on the 400G pluggable module, including form factor, architecture, digital signal processing (DSP), and module power consumption, following 400G pluggable optics in DCI applications. Next, we experimentally investigate the capacity-reach matrix for high-baud-rate and high-order quadrature amplitude modulation (QAM) single-carrier signals in the unamplified single-mode optical fiber (SMF) link. We show that the 64 GBd 16-QAM, and 64-QAM signals can potentially enable 400 Gb/s and 600 Gb/s DCI application for 40 km and beyond of unamplified fiber link. [ABSTRACT FROM AUTHOR]

Abstract Four Wave Mixing (FWM) is one of the main transmission impairments in coherent optical systems and a broad gamut of analytical expressions calculating its power already exists in the literature. Most of these expressions, such as the GN-model, assume that due to the large values of accumulated dispersion, each transmitted signal behaves as stationary circular Gaussian noise, ignoring the dependence of FWM on the employed modulation format. This assumption leads to significant error especially in the case of quadrature-phase-shift keying (QPSK), in systems with small number of spans and in the dispersion managed case. In addition, the GN-model is inaccurate when applied to metro networks, due to the low accumulated dispersion. In this paper, we derive closed-form expressions which incorporate the impact of modulation format on FWM generation, extending and improving the accuracy compared to existing closed-form FWM expressions. Moreover, the proposed method includes terms that make it applicable to systems with small span lengths (<30 km), which is the case in metro networks. The accuracy of the proposed formulas is benchmarked against a numerical method, where the signal transmission is modeled using the split step fourier method (SSFM), for a wide range of system configurations and parameters. [ABSTRACT FROM AUTHOR]

Coherent in-phase quadrature (IQ) transponders are ubiquitous in the long-haul and the metro optical networks. During the transmission, the coherent signal experiences a bandwidth narrowing effect after passing through multiple reconfigurable optical add-drop multiplexers (ROADMs). The coherent signal also experiences a bandwidth narrowing effect when electrical or optical components of the coherent IQ transponder experience aging. A dynamic method to compensate the bandwidth narrowing effect is thus required. In the coherent optical receiver, signal bandwidth is estimated from the raw analog-to-digital converter (ADC) outputs. By adaptively adjusting the tap coefficients of the finite impulse response (FIR) filter, simple post-ADC FIR filters can increase the resiliency of the coherent signal to the bandwidth narrowing effect. The influence of chromatic dispersion, polarization mode dispersion, and polarization dependent loss are studied comprehensively. Furthermore, the bandwidth information of the transmitted analog signal is fed back to the coherent optical transmitter for signal optimization, and the transmitter-side FIR filter thus changes accordingly. [ABSTRACT FROM AUTHOR]

In order to transition multi-core fiber (MCF) technologies from their research state to volume production, the key design specifications have to be broadly agreed upon, first and foremost an acceptable level of inter-core crosstalk per unit length. Against common belief, we show that MCF crosstalk requirements per unit length are fairly independent of transmission distance in the context of modern coherent optical communication systems. As a consequence, a single value for the tolerable inter-core crosstalk per unit length can be used to specify MCFs, valid from metropolitan (~100 km) to trans-pacific (~10 000 km) deployment scenarios. The notion of a universal inter-core crosstalk specification allows for application-independent MCF designs (including a distance-independent optimum core density) and will facilitate the standardization and volume manufacturing of MCF. [ABSTRACT FROM AUTHOR]

We report on the use of semiconductor optical amplifiers (SOAs) to extend the optical bandwidth of next generation optical systems. After discussing the technological progress and the motivation for rekindling the interest in SOAs for line amplification, we point out that the nonlinear gain dynamics of SOA is not a limiting factor for employing SOA as line amplifier. We show that the passage to high wavelength division multiplexed (WDM) channel count tends to substantially reduce nonlinear impairments, both through numerical analysis and experimental investigations based on standard off the shelf SOA devices. We then present the characteristics of our novel ultrawideband SOA-based packaged devices exhibiting high gain over 100+nm optical bandwidth, high output saturation power and low noise figure. We finally review the recent demonstration of the first >100+Tb/s transmission based on such novel 100+ nm wide semiconductor optical amplifiers over 100 km distance. [ABSTRACT FROM AUTHOR]

Laser frequency combs with repetition rates on the order of 10 GHz and higher can be used as multi-carrier sources in wavelength-division multiplexing (WDM). They allow replacing tens of tunable continuous-wave lasers by a single laser source. In addition, the comb's line spacing stability and broadband phase coherence enable signal processing beyond what is possible with an array of independent lasers. Modern WDM systems operate with advanced modulation formats and coherent receivers. This introduces stringent requirements in terms of signal-to-noise ratio, power per line, and optical linewidth which can be challenging to attain for frequency comb sources. Here, we set quantitative benchmarks for these characteristics and discuss tradeoffs in terms of transmission reach and achievable data rates. We also highlight recent achievements for comb-based superchannels, including >10  Tb/s transmission with extremely high spectral efficiency, and the possibility to significantly simplify the coherent receiver by realizing joint digital signal processing. We finally discuss advances with microresonator frequency combs and compare their performance in terms of flatness and conversion efficiency against state-of-the-art electro-optic frequency comb generators. This contribution provides guidelines for developing frequency comb sources in coherent fiber-optic communication systems. [ABSTRACT FROM AUTHOR]

We investigate the performance of single-pilot-tone locked frequency comb-based superchannel transmission for distances up to 1200km. In our scheme, electro-optic transmitter and receiver combs are locked by leaving one of the transmitter carriers unmodulated and regenerating the receiver comb via optical injection locking. This approach significantly reduces carrier offsets and therefore leads to reduced digital signal processing complexity. We experimentally assess how transmission impairments such as noise added by optical amplifiers and fiber nonlinearities affect the quality of the comb regeneration. Our results show that while the operating conditions are more stringent at longer distances, the single pilot is robust to impairments. At optimal launch power, similar performance with respect to an intradyne receiver is observed, showing that the optical pilot tones can be co-transmitted with data channels even at distances spanning hundreds of kilometer. The total superchannel spectral efficiency (throughput), including the pilot tone and guardbands, is 9.6 bits/s/Hz (12 Tbit/s) after 480 km and 8.4 bits/s/Hz (10.5 Tbit/s) after 960km. [ABSTRACT FROM AUTHOR]

Abstract Although there is an urgent need, it is still a significant challenge to establish high-speed laser communication links between satellites to ground stations owing to the influence of atmospheric turbulence. Adaptive optics (AO) is a considerable scheme to solve the problem. In this paper, the primary experiment of satellite-to-ground laser communication with the data rate of 5Gbps and BPSK modulation is carried out through a 500 m horizontal channel with relatively strong turbulence. A 137-element AO unit is used for correcting the phase distortion brought by the turbulence. When the AO unit is working, both the received optical power and bit error rate (BER) (All the eight experimental results of BER at different time are below 10−6.) are improved significantly. The primary results demonstrate that the ground station with AO technology has the capability to establish the satellite-to-ground laser communication links and provide a reference for the design of satellite terminals. [ABSTRACT FROM AUTHOR]