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Optical networks generate a vast amount of diagnostic, control and performance monitoring data. When information is extracted from this data, reconfigurable network elements and reconfigurable transceivers allow the network to adapt both to changes in the physical infrastructure but also changing traffic conditions. Machine learning is emerging as a disruptive technology for extracting useful information from this raw data to enable enhanced planning, monitoring and dynamic control. We provide a survey of the recent literature and highlight numerous promising avenues for machine learning applied to optical networks, including explainable machine learning, digital twins and approaches in which we embed our knowledge into the machine learning such as physics-informed machine learning for the physical layer and graph-based machine learning for the networking layer.

A bidirectional symmetric 100-Gb/s/λ coherent PON with a simplified ONU transceiver is experimentally demonstrated. With only 3-dBm LO power at the ONU side, >30-dB power budget is achieved for both upstream and downstream with 50-km reach.

Digital coherent transceivers have revolutionized optical fiber communications due to their superior performance offered compared to intensity modulation and direct detection based alternatives. As systems employing digital coherent transceivers seek to approach their information theoretic capacity, the use of multilevel modulation formats combined with appropriate forward error correction becomes essential. Given this context, in this tutorial paper, we, therefore, explore the digital signal processing (DSP) utilized in a coherent transceiver with a focus on multilevel modulation formats. By way of an introduction, we open by discussing the photonic technology required to realize a coherent transceiver. After discussing this interface between the analog optical channel and the digital domain, the rest of the paper is focused on DSP. We begin by discussing algorithms that correct for imperfections in the optical to digital conversion, including IQ imbalance and timing skew. Next, we discuss channel equalization including means for their realization for both quasi-static and dynamic channel impairments. Synchronization algorithms that correct for the difference between the transmitter and receiver oscillators both optical and electrical are then discussed and issues associated with symbol decoding highlighted. For most of the cases, we start with polarization division multiplexed quadrature phase-shift keying format as a basis and then discuss the extension to allow for high-order multilevel formats. Finally, we conclude by discussing some of the open research challenges in the field.

Passive optical network (PON) employing coherent receiver is a promising technology for future high-speed optical access network. However, the commonly used phase and polarization diverse coherent receiver for long-haul communication is not suitable for cost stringent access network. Therefore, low-complexity (and thereby low-cost) coherent receiver design for access network becomes an attracting research field in current years. In this paper, we propose a simple heterodyne single-polarization coherent receiver by using a 3-dB coupler and a single-ended photodiode to realize a 25-Gb/s optical network unit (ONU) receiver. To make the receiver polarization-insensitive, Alamouti-coded dual polarization 14-Gbaud QPSK transmitter is used at the optical line terminal (OLT) side. By means of computer simulation, it is confirmed that the proposed ONU receiver allows a high split ratio of 1:512 for 30-km transmission length whereas a lower splitting ratio of 1:16 enable the use of long-reach PON (LR-PON) of 100 km.

A new metric that analytically approximates the maximum a posteriori (MAP) solution is presented. Used with a decision-directed carrier phase estimation algorithm, the linewidth tolerance exceeds the limits achieved when using the conventional Euclidean distance.

Carrier-phase estimation (CPE) is one of the key elements of the digital signal processing (DSP) unit of a coherent optical receiver. CPE is required to eliminate the effect phase noise, mainly stemming from transmitting laser and local oscillator (LO). In this paper, a simple CPE scheme for 16-QAM digital coherent receiver is proposed which is based on coordinate transformation and sign-error modified constant-modulus algorithm (CMA). Such CPE can be achieved with very low implementation complexity. Performance of the proposed CPE scheme is analyzed with intensive computer simulation. For a 32-Gbaud 16-QAM system, the combined laser linewidth tolerance of more than 2 MHz is found for 1-dB sensitivity penalty.

Alamouti space-time block code (STBC) combined with a simple heterodyne coherent receiver enables phase diverse coherent detection without any optical polarization tracking. While such a system consisting of only a 3-dB coupler and a single balanced photodiode has been recently demonstrated using orthogonal frequency-division multiplexed (OFDM) signals, herein we report the first application to single-carrier systems. Applicability of such technique for single-carrier systems is not straightforward since specialized digital signal processing (DSP) algorithms are required for data recovery. In this paper, we address the implementing issues and DSP algorithms applicable for single-carrier (SC) Alamouti STBC based simplified heterodyne receivers. Polarization-insensitive operation of the proposed scheme and its performance are verified by means of simulation for a 12-Gbits/s quadrature phase-shift keying (QPSK) transmission system.

DGD pre-distortion implements a polarization time code enabling a single polarization coherent transceiver to recover the signal for all polarization states. Using a DFE the maximum polarization dependent sensitivity variation is 1.9/3.0dB for BPSK/QPSK respectively.

A novel blind equalization and carrier-phase-recovery technique based on modified constant-modulus algorithm is proposed for polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) systems. The proposed scheme is validated with computer simulations as well as transmission experiments with 40-Gbits/s PDM-QPSK signals.

In this paper, unconstraint adaptive frequency-domain equalizer (FDE) based on the constant-modulus algorithm (CMA) is investigated for equalization of linear transmission impairments in coherent optical receivers. With unrepeated 200-km transmission of 10-Gbaud quadrature phase-shift keying (QPSK) signal, it is verified that, compared with conventional constraint FDE, the proposed unconstraint FDE causes a very small sensitivity penalty; however, can be applied with much less computational complexity.

© 2014 Systematic Paris Region Systems and ICT Cluster. We propose a novel adaptive frequency-domain equalization (FDE) scheme in digital coherent optical receivers, which can work with rationally-oversampled input sequences using the constant-modulus algorithm (CMA). Effectiveness of the proposed scheme is verified with 10-Gbaud dual-polarization QPSK transmission experiments.

Among the wide spectrum of optical performance monitoring (OPM), optical signal-to-noise ratio (OSNR) monitoring is very important as it signify a direct measure of signal quality. In this paper, we propose a novel method of OSNR monitoring from the second- and fourth-order statistical moments of equalized signal in 16-QAM coherent optical receivers. The distinct statistical nature of carrier signal and noise enables such estimation. The proposed scheme is verified using computer simulation of 10-Gsymbols/s 16-QAM optical transmission systems. It is found that our scheme can estimate the OSNR with less that 0.5-dB error over a wide monitoring range.

We propose and demonstrate a new 112 Gbps Polarization division Multiplexing digital optical coherent systems with hexagonal 16-QAM (HEX-16-QAM) modulation. In our proposed system we use decision directed dispersion compensation algorithm which is Least Mean Square (LMS) algorithm and single tap decision directed carrier recovery algorithm. We compare the HEX-16-QAM systems with 16-QAM system for uncompensated transmission. Our analysis result found that HEX-16-QAM can achieve bit error rate (BER) of 1.5×10-4 whereas square 16-QAM BER is 5×10-4 for uncompensated 500-km transmission at 25 dB OSNR. Propose system achieve 0.3 dB OSNR penalty with compare to square 16-QAM system.

We propose a new radial directed joint equalization technique for blind adaptive equalization and Carrier Phase Estimation of Quadrature Amplitude Modulation (QAM) in coherent optical fiber communication. By taking the advantages of radial directed constant modulus algorithm (RD-CMA) and multimodulus algorithm (MMA), we introduced a new equalization technique can be call Radial Directed Multi Modulus Algorithm (RD-MMA) which can compensate the fiber dispersions and phase fluctuations hence the laser phase noise. By comparing with the traditional multi modulus algorithm (MMA) we found MMA can compensate up to 30 KHz Laser phase noise where as our algorithm can compensates 100 KHz for the same power penalty. for less than 100 KHz laser our algorithm performs much better with less amount of OSNR penalty.

We propose and demonstrate a new blind carrier recovering technique, named, Level Discrimination Carrier Recovery Algorithm (LD-CRA) for optical coherent receiver. Our proposed single tap LD-CRA can blindly compensate the laser phase noise without using any training sequence. Our analysis result found that for 300 KHz linewidth laser, the OSNR penalty is below 1.4dB at 10−3 bit error rate (BER) which has same as the decision directed single tap filter.

We propose and demonstrate a low complexity non decision directed blind carrier recovery algorithm (NDD-CRA) for 16-QAM optical coherent receivers. Through optimization of step size parameter (μ), we obtained similar performance compared to decision-directed phase estimation algorithm, however, our propose scheme has less complexity compared to the decision directed carrier recovery algorithm (DD-CRA) as the proposed scheme does not require any decision feedback module. From simulation, it is observed that the proposed scheme has less than 1.7 dB OSNR penalty at 10−3 BER and up to 200-kHz linewidth laser compared with the theoretical performance.

We propose a novel method of compensation for front-end IQ errors in coherent optical receivers. Computer simulations for the dual-polarization 16-QAM format show that IQ errors can be compensated for over a wide range.

We propose and demonstrate a new Non-Decision Aid Modified Least Mean Square (NDA-MLMS) algorithm for joint blind equalization and carrier recovery. The bit error rate (BER) of 10−3 is achieved with less than 1.9dB OSNR penalty for 14Gbaud 4-QAM system after 100km transmission and 500 KHz linewidth laser.

In this paper, we propose a novel hardware-efficient quadrature imbalance compensator circuit for coherent optical receivers. Numerical simulations for 16-QAM format show that it can fully compensate the quadrature imbalance over a wide range.

We propose and demonstrate a low complexity blind Level Discrimination Carrier Recovery Algorithm (LD-CRA) for optical coherent receivers. Through optimization of step size parameter we found that LD-CRA has below 2 dB OSNR penalty for 14Gbaud 16-QAM signal for 600 kHz line width laser.

In this paper, the impact of quadrature imbalance (QI) is analyzed for an optical 16-quadrature amplitude modulation (16-QAM) transmission system that employs digital coherent receivers. A compensation method for QI in digital domain is demonstrated using four real-valued filters connected in butterfly configurations and adapted by decision-directed least-mean-square (DD-LMS) algorithm. The effectiveness of the proposed QI compensation scheme is verified by numerical simulations.

We propose a simple and precise algorithm to monitor multiple impairments such as CD, PMD, and PDL, which are jointly mitigated in the adaptive equalizer of a digital coherent optical receiver. We validate our algorithm by dual-polarization QPSK transmission experiments.

For long-haul fiber-optic transmission systems to support multiple high speed channels wavelength-division multiplexing (WDM), is currently being deployed to achieve high-capacity. It allows information to be transmitted at different channels with different wavelength. But FWM is one of the major problems needed to be taken into account when one designs high-capacity long-haul WDM transmission system. Recently, unequal-spaced channel-allocation technique have been studied and analyzed to reduce four wave mixing (FWM) crosstalk. Finding a solution by this proposed channel allocation technique need two parameters such as minimum channel spacing and number of channel used in WDM system. To get better result minimum channel spacing has to be selected perfectly.

In this paper, crosstalk induced by cross phase modulation (XPM) and stimulated Raman scattering (SRS) in a multiwavelength subcarrier multiplexed transmission system is analyzed. Then combined effect of SRS and XPM induced crosstalk in the system performance is investigated. It is found that SRS dominates at lower modulation frequency with large channel spacing while XPM at higher modulation frequency and lower channel spacing. BER performance is also mentioned in presence of SRS and XPM crosstalk for per wavelength channel capacity of 10 Gb/s. Maximum allowable power for a BER of 10-9 as a function of wavelength channel separation is also presented.

We propose a novel frequency-domain adaptive equalizer in digital coherent optical receivers, which can reduce computational complexity of the conventional time-domain adaptive equalizer based on finite-impulse-response (FIR) filters. The proposed equalizer can operate on the input sequence sampled by free-running analog-to-digital converters (ADCs) at the rate of two samples per symbol; therefore, the arbitrary initial sampling phase of ADCs can be adjusted so that the best symbol-spaced sequence is produced. The equalizer can also be configured in the butterfly structure, which enables demultiplexing of polarization tributaries apart from equalization of linear transmission impairments. The performance of the proposed equalization scheme is verified by 40-Gbits/s dual-polarization quadrature phase-shift keying (QPSK) transmission experiments.

We propose a novel and unified algorithm that estimates linear impairments in optical transmission systems from tap coefficients of an adaptive finite-impulse response (FIR) filter in a coherent optical receiver. Measurable impairments include chromatic dispersion (CD), differential group delay (DGD) between two principal states of polarization, second-order polarization-mode dispersion (second-order PMD), and polarization-dependent loss (PDL). We validate our multi-impairment monitoring algorithm by dual-polarization quadrature phase-shift keying (QPSK) transmission experiments.

We propose a novel method of OSNR estimation in Nyquist-WDM transmission systems based on the measurement of statistical moments of equalized signals in the digital coherent receiver. Its effectiveness is verified with computer simulations.

We propose a straightforward algorithm to estimate second-order PMD from adaptive FIR-filter tap coefficients in a digital coherent optical receiver. The novel algorithm is verified with 100-Gbit/s dual-polarization QPSK experiments.

Aiming at the reduction of computational complexity of conventional time-domain equalizers in digital coherent receivers, we propose and demonstrate a novel frequency-domain adaptive equalization technique based on the constant-modulus algorithm.