Coherent Transceivers Research Articles

Low hardware-complexity IQ skew tolerant finite field and time domain hybrid adaptive equalization for 400G/800G-ZR coherent transmission

In 400G/800G-ZR transmission systems afflicted by substantial inter-symbol interference (ISI) stemming from chromatic dispersion (CD), frequency-domain adaptive equalizers (FD-AEQs) are preferred to time-domain AEQs (TD-AEQs) as the former have relatively lower computation load. However, the reliance of FD-AEQs on computationally expensive fast Fourier transform (FFT)/inverse fast Fourier transform (IFFT) operations poses a major obstacle in the development of low-power 400G/800G-ZR digital coherent transceivers. Furthermore, to mitigate the significant performance degradation due to receiver in-phase and quadrature (IQ) skew, the employment of twice the number of filters becomes imperative to enable individual adjustments of the I and Q tributaries, nearly doubling the number of necessary FFT/IFFT operations and computation load. This prohibitively high computation load poses a formidable obstacle to the realization of low-power and cost-effective transceivers. To address this issue, we introduce a novel low hardware-complexity IQ skew tolerant finite field (FF) and TD hybrid AEQ for 400G/800G-ZR coherent transmission. By optimizing the AEQ design and the Fermat number transform (FNT) based linear convolution scheme, the proposed hybrid AEQ can achieve a remarkable reduction in hardware complexity by more than 70%, compared with the existing IQ skew tolerant FD-AEQ, while preserving the same receiver sensitivity. Both numerical simulations and experiments demonstrate that the performance of the low complexity hybrid AEQ is the same as the FD-AEQ with much higher complexity. The proposed comprehensive and hardware efficient hybrid AEQ scheme is very appealing for 400G/800G-ZR coherent receivers which need to compensate for large ISI and IQ skew under stringent power constraints.

Novel Monolithic All-Silicon Coherent Transceiver Sub-Assembly Based on Ring Modulators

Novel Monolithic All-Silicon Coherent Transceiver Sub-Assembly Based on Ring Modulators

Visible-wavelength and monostatic coherent transceiver with independent second harmonic generation on transmitted and local lights using commercially available lithium niobate phase modulator for fundamental wavelength

We propose what we believe to be a new monostatic coherent transceiver at visible wavelength using a commercially available lithium niobate (LiNbO3) phase modulator (LNPM), which is capable of high-speed, wide-bandwidth, and multi-functional operation. The proposed coherent transceiver needs successful heterodyne detection with independent second harmonic generation (SHG) on transmitted and local lights. Experimental proof of the above-mentioned heterodyne detection is shown. The proof of concept for the application to continuous-waves coherent Doppler lidar (CW-CDL) is shown in both free-space and underwater sensing. Preliminary experiment regarding high-speed and wide-bandwidth modulation is additionally shown.

Endless optical domain polarization demultiplexing in 2000 km simplified coherent transceiver for long-distance transmission

We demonstrate an endless optical domain polarization demultiplexing scheme that was realized in a 96 Gbaud-PS-16QAM coherent transmission system to alleviate the data processing pressure of the DSP in the receiver. It can be effectively employed in the 140 km fiber link (Metro DCI) and extended to the 2000 km optical fiber networks (Long haul DCI or backbone networks transmission). It can track 10 krad/s state of polarization (SOP) changes following Rayleigh distribution which is the general statistical case for real fibers. The proposed method has a noise tolerance equivalent to that induced by a 2000 km transmission, with OSNR as low as 20 dB, with a maximum PMD of 4 ps and a mean DGD of 2.5 ps.

Real-time demonstration of 64 × 200 Gbps UDWDM-PON downstream transmission based on silicon photonic integrated transceiver

Coherent detection, high-order modulation, and dense wavelength division multiplexing (DWDM) technologies provide solutions for increasing bandwidth demand of future access networks. We experimentally demonstrate a real-time 64 × 200-Gb/s coherent ultra-dense wavelength-division (UDWDM) coherent passive optical networks (PONs) at 75-GHz channel spacing. The demonstrated downlink transmission is realized with the dual-polarization QPSK (DP-QPSK) modulation scheme. The cost is reduced by using silicon photonic integrated coherent transceiver modules and wider grid AWGs. The power budget is evaluated when all 200 channels are launched at C band. The power budget is 26 dB after 20-km G.652D standard single mode fiber (SSMF) transmission without amplifier at the receiver side under soft-decision FEC (SD-FEC) threshold of 1 × 10−2, and can achieve 32.5 dB with pre-amplified semiconductor optical amplifier (SOA) used at the receiver side.

Quantum computer-enabled receivers for optical communication

Optical communication is the standard for high-bandwidth information transfer in today’s digital age. The increasing demand for bandwidth has led to the maturation of coherent transceivers that use phase- and amplitude-modulated optical signals to encode more bits of information per transmitted pulse. Such encoding schemes achieve higher information density, but also require more complicated receivers to discriminate the signaling states. In fact, achieving the ultimate limit of optical communication capacity, especially in the low light regime, requires coherent joint detection of multiple pulses. Despite their superiority, such joint detection receivers are not in widespread use because of the difficulty of constructing them in the optical domain. In this work we describe how optomechanical transduction of phase information from coherent optical pulses to superconducting qubit states followed by the execution of trained short-depth variational quantum circuits can perform joint detection of communication codewords with error probabilities that surpass all classical, individual pulse detection receivers. Importantly, we utilize a model of optomechanical transduction that captures non-idealities such as thermal noise and loss in order to understand the transduction performance necessary to achieve a quantum advantage with such a scheme. We also execute the trained variational circuits on an IBM-Q device with the modeled transduced states as input to demonstrate that a quantum advantage is possible even with current levels of quantum computing hardware noise.

DSP-based inter-channel interference monitoring in flexible wavelength-routed networks

The efficiency of optical networks employing flexible wavelength division multiplexing (WDM) can be increased by maximizing the throughput of each individual channel, provided that the position of its neighboring channel is known with sufficient accuracy in order to avoid inter-channel interference. In this paper, we propose a digital signal processing (DSP) algorithm, leveraging the use of an artificial neural network (ANN), to estimate the neighboring channels’ distance by processing raw digital samples from a standard coherent receiver. We present an efficient dataset design approach, based on Latin hypercube sampling (LHS), in order to effectively optimize and validate the algorithm under different assumptions on the optical WDM channels. We investigate the accuracy of the ANN-based DSP scheme through simulation analysis, highlighting its potential in relation to the characteristics of the optical network. Finally, we validate our approach in an experimental setup using standard commercial coherent transceivers. The experimental results show that the distance from the neighboring WDM channels can be estimated with a root mean square error of less than 1.5 GHz for a channel under test with a symbol rate of 52 GBaud.

Planning tools for next-generation DSP-based passive optical networks above 50G [Invited Tutorial

Next-generation optical access networks are evolving towards ultra-high bit rates (above 50 Gbps per wavelength) and extended fiber reach architectures. This trend will likely push the optoelectronics to their limits, thus requiring impairment compensation based on digital signal processing (DSP) techniques in the transceivers. In this paper, which is an invited follow-up of a tutorial given at ECOC 2023, we first present an overview of this evolving scenario and then propose a unified analytical model that is able to predict the performance of these new systems for both direct-detection and coherent transceiver types. We believe that this model can be useful for preliminary scalability studies of new access architectures (as it happens in international standardization bodies). Moreover, when they are deployed, it can be useful as a base for network planning tools, particularly if future transceivers will be, as expected, highly reconfigurable at the DSP level.

Parallel wavelength-division-multiplexed signal transmission and dispersion compensation enabled by soliton microcombs and microrings

The proliferation of computation-intensive technologies has led to a significant rise in the number of datacenters, posing challenges for high-speed and power-efficient datacenter interconnects (DCIs). Although inter-DCIs based on intensity modulation and direct detection (IM-DD) along with wavelength-division multiplexing technologies exhibit power-efficient and large-capacity properties, the requirement of multiple laser sources leads to high costs and limited scalability, and the chromatic dispersion (CD) restricts the transmission length of optical signals. Here we propose a scalable on-chip parallel IM-DD data transmission system enabled by a single-soliton Kerr microcomb and a reconfigurable microring resonator-based CD compensator. We experimentally demonstrate an aggregate line rate of 1.68 Tbit/s over a 20-km-long SMF. The extrapolated energy consumption for CD compensation of 40-km-SMFs is ~0.3 pJ/bit, which is calculated as being around 6 times less than that of the commercial 400G-ZR coherent transceivers. Our approach holds significant promise for achieving data rates exceeding 10 terabits.

ZR 400 Gbit/s and 800 Gbit/s use cases, trials, deployments, and future prospects [Invited

As bandwidth grows operators are moving from 100 Gbit/s to 400 Gbit/s ports on core routers with wavelengths on underlying transmission also increasing to 400 Gbit/s and higher. Developments in pluggable coherent optical transceivers such as ZR optics present different optical layer architectures: pluggable coherent transceivers inserted into the router versus external transponders and “hop-by-hop” architectures, in which packets pass through multiple intermediate routers, versus “optical express” architectures via ROADMs. The paper describes using a bespoke modeling tool to compare cost and power consumption for various 400 Gbit/s architectures, applied to the core network of a major operator as total bandwidth increases from current levels ∼28Tbit/s to a future 220 Tbit/s scenario. The model shows that, as bandwidths increase, “optical express” architectures become lower cost compared to “hop-by-hop” architectures. The paper describes a field trial of pluggable 400 Gbit/s ZR+ transceivers directly deployed in routers and transported over a ROADM network and reviews architectural options likely to arise as bandwidths increase to 800 Gbit/s. For 400 Gbit/s and higher, operators face a range of optical layer architectures: the best outcome gives the lowest cost and power consumption while enabling efficient network operations, and it could be that different solutions are best for different networks.

Analyses of BER-referred noise-to-signal ratio in online measurements of high-baud-rate coherent receivers.

Digital coherent transmission features a very large transmission bandwidth and has played a main role in core optical transmission networks. With the progress of semiconductor technologies, practical coherent transceivers with rates over 100 Gbaud are becoming feasible. With such advances, the transceiver components must have lower power consumption and lower costs, and it becomes important to know how each component contributes to the overall transmission performance. Here, to decompose the effects of noise factors in high-baud-rate DP-16QAM transmissions, we used the theoretical relationship between the bit error rate (BER) and noise-to-signal ratio (NSR) and performed linear analyses. The NSR could be decomposed into individual noise contributions according to dependences on the inverse signal and local photocurrents. The obtained parameters were shown to be useful for predicting required optical signal-to-noise ratio (ROSNR) characteristics.

Power equalization-based disaggregated optical network using switched gain equalization controlled amplifiers.

Optical network disaggregation is an attractive agreement to avoid vendor lock-in in choosing desirable partners based on lead time of delivery and facilitates strong negotiations. Disaggregation of conventional coherent transceivers (XCVRs) with 400G ZR+ technologies (of similar bandwidth density and form factor QSFP-DD) allows operators to enjoy significant cost reduction and substantial power savings in high-capacity IP over a WDM transmission. However, interoperability between low power and high back-to-back required optical signal-to-noise ratio (OSNR) of ZR+ and high power with relatively lower back-to-back required OSNR of conventional coherent XCVRs limits the performance of the latter, which makes disaggregation more complex. This can be avoided by equalizing the power of ZR+ and conventional XCVRs, by wavelength-selective switches (WSSs) in re-configurable optical add/drop multiplexers. However, such power equalization can lead to lower link OSNR, originating from insertion loss and penalty caused by cascaded WSSs. In this Letter, we use a dynamic gain equalizer of a switched gain equalization controlled (SGEC) amplifier at each in-line amplifier site to equalize the powers, without introducing insertion loss or filter penalty. We report a field trial of a low power 400G-ZR+ QSFP-DD-DCO as an alien channel over ten high-power coherent host channels, interoperating with open forward error correction, between two different vendors. We obtain a similar Q-margin in the absence and presence of 400-ZR+ channel, suggesting the equalization by SGEC amplifiers. We further simulate the power and OSNR evolution of channels along the field trial link to verify the equalization. Simulation results are consistent with that of the field trial.

Cost- and energy-efficient filterless architectures for metropolitan networks

Network operators are forced to find cost- and energy-efficient solutions for networks supporting new and emerging services with strict latency and ultra-high-capacity requirements. A disruptive approach for delivering network agility in a cost- and energy-efficient manner is employing filterless optical networking based on broadcast-and-select nodes and coherent transceivers. The filterless network concept has been widely studied for terrestrial and submarine applications. In this paper, we investigate the performance of filterless optical networks in metropolitan core and aggregation networks where agility is required due to service dynamics, customer changes, and service flexibility requirements. We compare our results with a conventional metro network based on active switching. The results show that the filterless metro network based on a hierarchical structure similar to its active switching counterpart has comparable installed first cost and spectrum usage at 11 Tb/s of total traffic, as well as cost and wavelength consumption advantages of 19.5% and 16%, respectively, at 107 Tb/s of total traffic. These results confirm that the filterless architecture is an attractive alternative for metro network deployments.

Optimized design of filterless horseshoe networks exploiting point-to-multipoint coherent transceivers

The horseshoe topology is widely used to realize metro-aggregation networks, since it provides a natural fit to the hub-and-spoke traffic pattern present in the majority of these deployments, while enabling survivability against single link and hub failures. A filterless architecture can also be adopted to further reduce capital expenditure (CapEx) by replacing active elements, such as reconfigurable add/drop multiplexers, with simpler and passive splitters/combiners. Such an architecture can effectively host coherent-based point-to-multipoint (P2MP) transceivers enabled by digital subcarrier multiplexing (DSCM). Importantly, by carefully optimizing the deployment of amplifiers (location and gain) and splitters/combiners (type), it may be possible to reduce the total number of optical amplifiers required, further decreasing CapEx. This paper proposes an integer linear programming framework to optimize metro-aggregation filterless horseshoe networks, taking into account the specific requirements of DSCM-based P2MP coherent transceivers. The results indicate that a considerable reduction in amplifier count is possible while ensuring that end-to-end performance thresholds are met, which include the minimum required input power at the receivers, a maximum subcarrier input power difference at the hub’s receivers, and the minimum optical-signal-to-noise ratio.

Hybrid integrated Si3N4 external cavity laser with high power and narrow linewidth.

We have designed and fabricated a hybrid integrated laser source with full C-band wavelength tunability and high-power output. The external cavity laser is composed of a gain chip and a dual micro-ring narrowband filter integrated on the silicon nitride photonic chip to achieve a wavelength tuning range of 55 nm and a SMSR higher than 50 dB. Through the integration of the semiconductor optical amplifier in the miniaturized package, the laser exhibits an output power of 220 mW and linewidth narrower than 8 kHz over the full C-band. Such a high-power, narrow-linewidth laser diode with a compact and low-cost design could be applied whenever coherence and interferometric resolutions are needed, such as silicon optical coherent transceiver module for space laser communication, light detection and ranging (LiDAR).

Single-Shot Characterization of Frequency-Resolved Imbalance in Coherent Transceivers Based on Inter-Channel Response Ratio

The compensation for in-phase/quadrature (IQ) and polarization imbalance residing at the coherent transceivers is crucial to the overall performance of high-speed transmission systems. To simultaneously characterize the frequency-dependent IQ/polarization imbalance by a single-shot measurement for either the transmitter or receiver, we propose a simple, but efficient and accurate method based on inter-channel response ratio in the frequency domain. The characterization of the transmitter-side frequency-resolved imperfections only requires one single-ended photodetector (PD) for direct detection while the coherent receiver characterization is based on the classical polarization-diversity coherent receiver configuration without modification. Experimental results have demonstrated the excellent performance of our proposed method to estimate these frequency-dependent imperfections. The estimation accuracy for the IQ/polarization skew residing at both transmitter and receiver can reach tens of femtoseconds (fs). The transmission experiment for a 75-Gbaud dual-polarization (DP) probabilistic constellation-shaped (PCS) 256- quadrature amplitude modulation (QAM) signals with 1.05-Tb/s raw data rate confirms the effectiveness of our proposed method for both IQ/polarization skew and frequency-dependent imbalance.

C-Band optical 90-degree hybrid using thin film lithium niobate.

The integrated optical 90-degree hybrid is a crucial component for coherent receivers. Here, we simulate and fabricate a 4 × 4 multimode interference coupler as a 90-degree hybrid using thin film lithium niobate (TFLN). The device features low loss (0.37 dB), high common mode rejection ratio (over 22 dB), compact footprint, and small phase error (below 2°) within the whole C-band experimentally, which is promising for integration with coherent modulators and photodetectors for TFLN-based high-bandwidth optical coherent transceivers.

Simple Single-Laser Coherent Transceiver Based on Low-Cost D-EML for Edge Networks

In this work we demonstrate single-laser, D-EML based coherent transceivers which transmit and simultaneously receive optical single sideband modulations through a phase noise cancelling scheme, leading to phase-noise and frequency-detuning clean signals. We use QPSK and 16-QAM modulations over a 500 Mbaud signal, reaching 1 Gbps and 2 Gbps respectively, with sensitivities as low as −53 and −40 dBm with unmodulated local oscillator, respectively, and −35 dBm for QPSK with modulated local oscillator, achieving high sensitivity coherent UD-WDM for analog radio over fiber and mobile fronthaul over passive optical networks.

Tree-determination, routing, and spectrum assignment using point-to-multipoint coherent optics

In this paper, we investigate a novel problem for dimensioning optical networks with digital subcarrier-based point-to-multipoint (P2MP) coherent transceivers, which we refer to as tree-determination, routing, and spectrum assignment (TRSA). Given a physical optical fiber network with P2MP transceivers and a target IP topology with associated capacity requirements, the TRSA problem involves partitioning capacity demands into logical hub–spoke trees and associated transceiver allocation, along with light-tree routing and spectrum assignment. We propose a heuristic algorithm to solve the TRSA problem with and without fault tolerance for arbitrary network topologies. The algorithms are used to evaluate potential cost benefits of P2MP transceivers for different IP densities and loads in a case study on a metro-network reference scenario. Results indicate that P2MP equipment can bring significant transceiver cost and equipment savings in comparison to point-to-point solutions, although such savings come at a trade-off with increased spectrum occupation.

Two-stage frequency compensation for Doppler shift on BPSK transceiver with LDPC codes for free-space optical communication systems

The deployment of low earth orbits is seen as a promising way of enlarging data capacities as well as high data rates. Catering to these interests, optical communication presents possible ways of larger bandwidth than microwave communication. The current generation of mainstream communication systems are classified as coherent systems and incoherent systems, and in particular, coherent systems have received more attention owing to their high receiving sensitivity. This study investigates a digital coherent transceiver, based on binary phase-shift keying technology. As coherent demodulation will be affected by considering the Doppler shift effect in digital demodulation, Doppler shift of ± GHz can be compensated by adopting a two-stage frequency offset compensation. Moreover, by leveraging a fast filtering algorithm a considerable amount of resource consumption is saved in its engineering implementation, and its sensitivity can be significantly enhanced via a high-speed parallel error-correction codec based on low-density parity-check technology.