Optical Superchannel Research Articles

High Capacity Innovations Enabling Scalable Optical Transmission Networks

We review and introduce key optical innovations that enable scalable and efficient high capacity optical networks, such as advanced modulation formats, flexible grid, optical superchannel, 400GE and emerging 400G ZR/ZR+. We show results from recent industry leading technology trial of real-time 2 × 200G Probabilistic Constellation Shaping (PCS) DP-16QAM superchannel transporting 400GE over a live long haul flexible grid link, achieving long term error free performance with a high spectral efficiency of 5.33 bit/s/Hz and significantly enhanced performance over conventional 200G DP-16QAM. We also present results from successful demonstration of emerging 400G ZR/ZR+ technologies and IP over WDM applications, where different network scenarios and use cases are investigated including single span WDM link of 125 km and multi-span WDM link of 252 km as well as unamplified 40 km fiber link. 400G ZR/ZR+ IP over WDM wavelengths have shown robust performance with good optical margin for practical network applications as well as potential for longer optical reach. In comparison, 400G ZR+ has enhanced performance over 400G ZR with additional margin of ∼3 dB. We have also demonstrated streaming telemetry, a new network monitoring approach, for monitoring of 400G ZR/ZR+ IP over WDM wavelengths giving more performance insights. These industry leading trials and demonstration give network operators critical understandings of the performance, maturity and operational viability of emerging optical technologies which will allow them to scale their networks gracefully to meet continuing bandwidth growth.

Electro-Optical Modulator Requirements for 1 Tb/s per Channel Coherent Systems

The electro-optical bandwidth and extinction ratio requirements of in-phase and quadrature modulators to enable 1 Tb/s per channel optical transmission rates are evaluated through numerical simulations. Employing sixty-four quadrature amplitude modulation (64QAM), the simulation results indicate that the best trade-off between the number of optical carriers per channel and the technological complexity of designing and manufacturing an integrated transceiver is achieved for a superchannel composed by 2 optical carriers, since it allows modulator requirements in terms of electro-optical bandwidth and extinction ratio equal to 27 GHz and 18 dB, respectively. The systemic optimization of the carrier launch power employing the approximate enhanced Gaussian noise model indicates that this configuration could achieve transmission reaches up to around 1080 km. These results point out to 1 Tb/s optical superchannels as a viable solution for next generation coherent systems in metropolitan and long-haul scenarios.

Effects of Receiver-Side Optical Filtering On Optical Superchannel System Performance

Optical superchannels provide a route to obtaining the highest achievable data rates in optical fiber systems. Here, we investigate the role of optical filtering on system performance and required optical signal-to-noise ratio, when receiving a single sub-band from an optical superchannel, where the dominant noise source changes from optical to transceiver noise. We find that optical filtering can provide improvement up to 3 dB in signal quality and around 1 b/symbol in information rate, in an experiment where transceiver noise is dominant.

High Spectral Efficiency Coherent Superchannel Transmission With Soliton Microcombs

Spectral efficiency (SE) is one of the key metrics for optical communication networks. An important building block for its maximization are optical superchannels, channels that are composed of several subchannels with an aggregate bandwidth larger than the bandwidth of the detector electronics. Superchannels which are routed through the network as a single entity, together with flex-grid routing, allow to more efficiently utilize available bandwidth and eliminate the guard-bands between channels, thus increasing spectral efficiency. In contrast to traditional wavelength division multiplexing (WDM) channels, subchannel spacing and thus superchannel SE is governed by the linewidth and stability of the frequency spacing of the transmitter lasers. Integrated optical frequency combs, particulary the parametrically generated so-called microcombs, which provide optical lines on a fixed frequency grid are a promising solution for low power superchannel laser sources that allow to minimize the SE loss from suboptimal channel spacing. However, it is extremely challenging to realize microcombs with sufficient line power, coherence and line spacing that is compatible with electronic bandwidths. Because the line-spacing generated by most devices is above 40 GHz, demonstrations often rely on additional electro-optic frequency shifter or divider stages to avoid digital-to-analog-converter (DAC) performance degradation when operating at high symbol rates. Here we demonstrate a 52-line superchannel from a single 22 GHz line spacing soliton microcomb. We demonstrate 12 Tb/s throughput with >10 bits/s/Hz SE efficiency after 80 km transmission and 8 Tb/s throughput (SE >6 bits/s/Hz) after 2100 km, proving the feasibility and benefits of generating high signal quality, broadband waveforms directly from the output of a micro-scale device with a symbol rate close to the comb repetition rate.

Frequency comb distillation for optical superchannel transmission

Optical frequency combs can potentially provide an efficient light source for multi-terabit-per-second optical superchannels. However, as the bandwidth of these multi-wavelength light sources is increased, it can result in low per-line power. Optical amplifiers can be used to overcome power limitations, but the accompanying spontaneous optical noise can degrade performance in optical systems. To overcome this, we demonstrate wideband noise reduction for comb lines using a high-Q microring resonator whose resonances align with the comb lines, providing tight optical filtering of multiple combs lines at the same time. By distilling an optical frequency comb in this way, we are able to reduce the required comb line OSNR when these lines are used in a coherent optical communications system. Through performance tests on a 19.45-GHz-spaced comb generating 71 lines, using 18 Gbaud, 64-QAM sub-channels at a spectral efficiency of 10.6 b/s/Hz, we find that noise-corrupted comb lines can reduce the optical signal-to-noise ratio required for the comb by ~ 9 dB when used as optical carriers at the transmitter side, and by ~ 12 dB when used as a local oscillator at the receiver side. This demonstration provides a method to enable low power optical frequency combs to be able to support high bandwidth and high-capacity communications.

Practical Innovations Enabling Scalable Optical Transmission Networks: Real-World Trials and Experiences of Advanced Technologies in Field Deployed Optical Networks

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.

QD-MLL-Based Single-Sideband Superchannel Generation Scheme With Kramers–Kronig Direct Detection Receivers

For their capability of electronic dispersion compensation, transmission systems based on direct detection of single-sideband (SSB) signals are attractive candidates as energy-efficient and cost-effective alternative solutions to intradyne digital coherent systems for interdata center and metro applications. The Kramers–Kronig (KK) receiver scheme has been shown to provide superior performance compared to other schemes in signal-to-signal beat interference (SSBI) cancelation in these direct-detection systems. In this paper, we propose a low-complexity and cost-effective scheme of generating an optical superchannel comprising multiple SSB channels, based on a single quantum-dot mode-locked laser source. The proposed system does not require additional photonic or RF components at the transmitter to generate the required SSB signal with a continuous wave (CW) carrier. It also preserves the full digital-to-analog converters’ bit resolution for data modulation, in contrast to other methods based on digital generation of the CW component. Simulations of system performance with KK receiver, based on measured laser output field, show that the proposed system can achieve bit-error ratio below the hard-decision forward error correction threshold for 16-QAM Nyquist SSB signals after transmission through three amplified spans of single-mode fiber in a 240-km link. Using 8 KK channels at 23 GBaud each, the proposed scheme will be able to achieve a transmission rate of 736 Gb/s with noncoded spectral efficiency of 2.45 b/s/Hz. The impacts of carrier-to-signal power ratio, per channel launch power into the fiber, and component frequency drifting on transmission system performance are also discussed.

Integrated demultiplexing and amplification of coherent optical combs.

The explosive growth of the internet during the last few decades has been enabled by two complementary innovations in optical communications: the use of multiple optical channels within a single optical fibre, and the increase in the bandwidth of individual channels to hundreds of Gbps. Further increases in overall bandwidth look to be provided by more spectrally efficient optical superchannels that use coherent sub-carriers generated using optical orthogonal frequency division multiplexing (OFDM). Yet, a cost effective way of generating these signals has not been demonstrated. One crucial, but missing piece is an effective means to separate the closely frequency spaced optical sub-carriers from the coherent optical comb before placing information on each sub-carrier, and thus creating the OFDM signal. Here, we demonstrate a flexible strategy implemented in a compact photonic integrated circuit (PIC) that is used to separate and amplify these sub-carriers using on-chip injection locking.

Field Trial of Real-Time 400GbE Superchannel Using Configurable Modulation Formats

We report a successful field trial of real-time optical superchannel transport of 400Gb/s Ethernet client traffic over a 216-km flexible grid G.652 fiber link using 64GBaud flexible rate optical transponders. We demonstrated, for the first time, error free transport of 400 GbE client traffic over an ultra-efficient optical superchannel of $2\times 200 \text{G}$ using modulation formats DP–16QAM, DP–8QAM, and DP–QPSK. Error free performance of 400GbE ultra-efficient optical superchannel was achieved at different channel separations as narrow as 37.5 GHz for DP–16QAM, 39 GHz for DP–8QAM, and 50 GHz for DP–QPSK. We also successfully demonstrated non-contiguous 400GbE optical superchannel, where the optical sub-channels are in non-adjacent spectral slots across the whole ${C}$ -band (3.85 THz apart). Stable long term (overnight) error free optical performance and end-to-end 400 GbE full throughput were measured.

Enabling Technologies for Fiber Nonlinearity Mitigation in High Capacity Transmission Systems

Fiber nonlinearity has become a major limiting transmission impairment factor. In this paper, we discuss various nonlinearity mitigation techniques in electrical and optical domains. In electrical domain, multiple reduced complexity digital back-propagation algorithms were developed, including perturbation back-propagation. One drawback is that they can compensate for intra-channel self-phase modulation (SPM) effects only. The inter-channel fiber nonlinearity mitigation can be done in optical domain. For example, digital subcarrier multiplexing (SCM) can effectively mitigate both SPM and cross-phase modulation (XPM) effects. It can become even more beneficial for higher symbol rate systems (e.g., 64 Gbaud and 128 Gbaud) and has a capability to deliver longer reach than single carrier 32 Gbaud DP-16QAM signals and only slightly shorter reach (by up to 7%) compared to 32 Gbaud SCM systems. Another technique, the total intensity directed phase modulation, can provide per-span SPM and XPM compensation. However, this technique is limited to compensation of a few channels only. On the other hand, the proposed enhanced pre-dispersed spectral inversion can effectively compensate for SPM/XPM effects of multiple channels and become more practical by removing limitation to link symmetry even in nonuniform transmission links. Finally, subcarrier power pre-emphasis in optical superchannels can equalize performance of all subcarriers by mitigating intersubcarrier XPM and extend transmission reach by 20%.

Digital Phase Noise Compensation for DSCM-Based Superchannel Transmission System With Quantum Dot Passive Mode-Locked Laser

We propose a simplified digital phase noise compensation technique for a Nyquist pulse-shaped digital subcarrier multiplexed (DSCM) coherent optical transmission system, employing an optical frequency comb based on Quantum dot passive mode-locked laser (QD-PMLL). Our results show that the impact of dominant common mode phase noise can be efficiently compensated at the receiver by digitally mixing the data sideband with the complex conjugate of the residual carrier component. This digital mixing technique resulted in better bit error rate performance compared to the conventional mth power Viterbi-Viterbi algorithm for QPSK and blind phase noise compensation for 16-quadratic-amplitude modulation formats, especially in the presence of large phase noise. To this end, exploiting the mutual coherence between the mode-locked comb lines of QD-PMLL, we numerically demonstrate its potential applicability as a transmission source for coherent optical superchannel transmission.

Frequency Comb-Based WDM Transmission Systems Enabling Joint Signal Processing

We review the use of optical frequency combs in wavelength-division multiplexed (WDM) fiber optic communication systems. In particular, we focus on the unique possibilities that are opened up by the stability of the comb-line spacing and the phase coherence between the lines. We give an overview of different techniques for the generation of optical frequency combs and review their use in WDM systems. We discuss the benefits of the stable line spacing of frequency combs for creating densely-packed optical superchannels with high spectral efficiency. Additionally, we discuss practical considerations when implementing frequency-comb-based transmitters. Furthermore, we describe several techniques for comb-based superchannel receivers that enables the phase coherence between the lines to be used to simplify or increase the performance of the digital carrier recovery. The first set of receiver techniques is based on comb-regeneration from optical pilot tones, enabling low-overhead self-homodyne detection. The second set of techniques takes advantage of the phase coherence by sharing phase information between the channels through joint digital signal processing (DSP) schemes. This enables a lower DSP complexity or a higher phase-noise tolerance.

PDL Impact on Linearly Coded Digital Phase Conjugation Techniques in CO-OFDM Systems

We investigate the impact of polarization-dependent loss (PDL) on the linearly coded digital phase conjugation (DPC) techniques in coherent optical orthogonal frequency-division multiplexing superchannel systems. We consider two DPC approaches: one uses orthogonal polarizations to transmit the linearly coded signal and its phase conjugate, while the other uses two orthogonal time slots of the same polarization. We compare the performances of these DPC approaches by considering both aligned- and statistical-PDL models. The investigation with an aligned-PDL model indicates that the latter approach is more tolerant to PDL-induced distortions when compared with the former. Furthermore, the study using the statistical-PDL model shows that the outage probability of the latter approach tends to zero at a root mean square PDL value of 3.6 dB. On the other hand, the former shows an outage probability of 0.63 for the same PDL value.

Software-Defined Silicon-Photonics-Based Metro Node for Spatial and Wavelength Superchannel Switching

Due to the growing popularity of optical superchannels and software-defined networking, reconfigurable optical add-drop multiplexer (ROADM) architectures for superchannel switching have recently attracted significant attention. ROADMs based on micro-electro-mechanical system (MEMS) and liquid crystal-on-silicon (LCoS) technologies are predominantly used. Motivated by requirements for low power, high-speed, small area footprint, and compact switching solutions, we propose and demonstrate spatial and wavelength flexible superchannel switching using monolithically integrated silicon photonics (SiP) micro-ring resonators (MRRs). We demonstrate the MRRs’ capabilities and potential to be used as a fundamental building block in ROADMs. Unicast and multicast switching operation of an entire superchannel is demonstrated after transmission over 50 km of standard single mode fiber. The performance of each sub-channel from the 120 Gb∕s QPSK Nyquist superchannel is analyzed, and degradation in error vector magnitude performance was observed for outer sub-channels due to the 3 dB bandwidth of the MRRs, which is comparable with the superchannel bandwidth. However, all sub-channels for all switching cases (unicast, multicast, and bi-directional operation) exhibit performance far below the 7%FEClimit. The switching time of the SiPMRRchip is such that high-capacity superchannel interconnects between users can be set up and reconfigured on the microsecond time scale.

Nonlinear Interference Mitigation: Methods and Potential Gain

We explore the potential benefits of digital nonlinearity compensation (NLC) techniques in fully loaded coherent wavelength-division multiplexed (WDM) transmission systems. After providing an overview of the various classes of nonlinear interference noise (NLIN) and digital signal processing approaches for their mitigation, we consider the two practically most relevant digital NLC methods known today: back-propagation (BP) and equalization of nonlinear phase and polarization rotation noise (PPRN). We consider a wide range of system configurations including a variety of modulation formats from quadrature phase-shift keying (QPSK) to 256-ary quadrature amplitude modulation (QAM), single-carrier and digital subcarrier multiplexed (SCM) optical superchannels, as well as both point-to-point line systems and optically routed networks (ORNs). Using theoretical predictions from the time-domain model for NLIN, we show that the gain in peak signal-to-noise ratio in fully loaded WDM systems using single-channel and three-channel joint BP is typically limited to 0.5 and 1 dB. The additional gain provided by increasing the number of jointly back-propagated channels beyond three is limited to 0.1 dB per additional back-propagated channel. The remarkably slow growth of the BP gain with the bandwidth of the back-propagated signal is shown to apply also for systems in which the receiver employs PPRN removal. We additionally explore the potential benefits of SCM across a wide range of system configurations, including the impact of BP and PPRN removal on SCM systems. We show that while the BP gain is similar to single-carrier systems, PPRN removal can have a much stronger effect, particularly for higher order QAM systems, implying that SCM can be advantageous not only for constant modulus formats such as QPSK, but also for high spectral efficiency systems. In the context of ORNs, we find that SCM not only improves system tolerance to nonlinearities, but also induces significantly smaller performance variations in various ORN scenarios.

Enhanced intercarrier interference mitigation based on encoded bit-sequence distribution inside optical superchannels

In the context of gridless optical multicarrier systems, we propose a method for intercarrier interference (ICI) mitigation which allows bit error correction in scenarios of nonspectral flatness between the subcarriers composing the multicarrier system and sub-Nyquist carrier spacing. We propose a hybrid ICI mitigation technique which exploits the advantages of signal equalization at both levels: the physical level for any digital and analog pulse shaping, and the bit-data level and its ability to incorporate advanced correcting codes. The concatenation of these two complementary techniques consists of a nondata-aided equalizer applied to each optical subcarrier, and a hard-decision forward error correction applied to the sequence of bits distributed along the optical subcarriers regardless of prior subchannel quality assessment as performed in orthogonal frequency-division multiplexing modulations for the implementation of the bit-loading technique. The impact of the ICI is systematically evaluated in terms of bit-error-rate as a function of the carrier frequency spacing and the roll-off factor of the digital pulse-shaping filter for a simulated 3×32-Gbaud single-polarization quadrature phase shift keying Nyquist-wavelength division multiplexing system. After the ICI mitigation, a back-to-back error-free decoding was obtained for sub-Nyquist carrier spacings of 28.5 and 30 GHz and roll-off values of 0.1 and 0.4, respectively.

Field Trial Demonstration of Real-Time Optical Superchannel Transport up to 5.6 Tb/s Over 359 km and 2 Tb/s Over a Live 727 km Flexible Grid Optical Link Using 64 GBaud Software Configurable Transponders

We report the first successful real-time optical superchannel field trials of 5.6 Tb/s over 359 km and 2 Tb/s over a live 727 km flexible grid link using production grade 64 GBaud flexible rate optical transponders. Up to 5.6 Tb/s optical superchannels were successfully demonstrated over a 359 km fully managed flexible grid optical link, where the superchannel comprises a maximum of 28 × 64 GBaud 200 G DP – QPSK subchannels. A 100 GE client signal was transported over all the subchannels, demonstrating stable, long term end-to-end error free performance. A record net spectral efficiency of 3.92 bit/s/Hz was achieved for the 64 GBaud 200 G DP – QPSK, with channel spacing as narrow as 51 GHz. We also successfully demonstrated fast superchannel capacity turn-up, via software configuration of the flexible rate transponders, from 2.6 to 5.2 Tb/s including 100 GE client mappings, which is equivalent to a capacity up-lift of approximately 2 Tb/min. In a further trial over a 727 km live flexible grid optical link between London and Dublin, we successfully demonstrated real-time 2 Tb/s superchannel transport using long reach 64 GBaud 200 G DP – QPSK operating together with 100 G DP – QPSK and 200 G DP – 16 QAM channels enabled by the flexible rate transponders. Stable, long term error free performance was measured of the terabit optical channels alongside live customer traffic.

Demonstration of 400 Gb/s optical PDM-OFDM superchannel unrepeatered transmission by all optical phase-conjugated copy

Unrepeatered transmission delivers data over a few hundred kilometers without in-line active elements and it can effectively reduce the line complexity and the overall system cost. In this paper, we propose all optical phase-conjugated copy (OPC) to improve optical signal noise ratio (OSNR) margin and nonlinear tolerance for unrepeatered transmission of polarization division multiplexing (PDM) optical superchannel with EDFA only amplification. Orthogonal pumps FWM scheme is utilized to generate the optical phase-conjugated copy. The original superchannel and the phase-conjugated copy are simultaneously transmitted and received. The phases of the copy symbols are conjugated and summed with the original superchannel symbols to suppress both linear noise and nonlinear phase noise at the receiver. The proposed OPC scheme is simple and effective in phase-conjugated copy generation and digital signal processing (DSP). What is more, it is transparent to signal bit rate and modulation format, which applies to optical superchannel transmission. We experimentally verify the proposed scheme on a 400Gb/s optical polarization division multiplexing orthogonal frequency division multiplexing (PDM-OFDM) superchannel. A Q-factor improvement of 2.1dB is achieved after 180km SSMF unrepeatered transmission. The optimum launch power in OPC scheme increases from −3dBm to −2dBm. To verify the maximum reach, we extend fiber length and realize 240km SSMF unrepeatered transmission.

Sub-GHz-resolution C-band Nyquist-filtering interleaver on a high-index-contrast photonic integrated circuit.

Modern optical communications rely on high-resolution, high-bandwidth filtering to maximize the data-carrying capacity of fiber-optic networks. Such filtering typically requires high-speed, power-hungry digital processes in the electrical domain. Passive optical filters currently provide high bandwidths with low power consumption, but at the expense of resolution. Here, we present a passive filter chip that functions as an optical Nyquist-filtering interleaver featuring sub-GHz resolution and a near-rectangular passband with 8% roll-off. This performance is highly promising for high-spectral-efficiency Nyquist wavelength division multiplexed (N-WDM) optical super-channels. The chip provides a simple two-ring-resonator-assisted Mach-Zehnder interferometer, which has a sub-cm2 footprint owing to the high-index-contrast Si3N4/SiO2 waveguide, while manifests low wavelength-dependency enabling C-band (> 4 THz) coverage with more than 160 effective free spectral ranges of 25 GHz. This device is anticipated to be a critical building block for spectrally-efficient, chip-scale transceivers and ROADMs for N-WDM super-channels in next-generation optical communication networks.

4 Tb/s Transmission Reach Enhancement Using 10 × 400 Gb/s Super-Channels and Polarization Insensitive Dual Band Optical Phase Conjugation

In this paper, we experimentally demonstrate the benefit of polarization insensitive dual-band optical phase conjugation for up to ten 400 Gb/s optical super-channels using a Raman amplified transmission link with a realistic span length of 75 km. We demonstrate that the resultant increase in transmission distance may be predicted analytically if the detrimental impacts of power asymmetry and polarization mode dispersion are taken into account.