Spatial Superchannels Research Articles

Demonstration of a 15-Mode Network Node Supported by a Field-Deployed 15-Mode Fiber

This paper extends a recent demonstration of a 2-line side 15-mode spatial division multiplexing network node. The node is based on fifteen 2×2 wavelength cross-connects (WXCs) to direct up to six 5 Tb/s, 15-mode, spatial super channels (SSCs). For this demonstration, we consider full add and drop, full express and partial add and drop network scenarios. The WXCs were made using three commercial 4×16 wavelength cross-connects, which were reprogrammed to each implement five identical 2×2 WXCs. The network node is demonstrated using a 6.1 km, 15-mode, multi-mode fiber (MMF) with a standard cladding diameter of 125 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$µ$</tex-math></inline-formula> m. This is the first field-deployed MMF and is installed in an underground tunnel in the city of L'Aquila, Italy. We further extend the network node to support up to 20 SSCs for a throughput per line side of 100 Tb/s. It is shown that the MIMO requirements for this type of systems is strongly dependent on the differential delays of the different modes propagating within the network node.

Compensation of inter-core skew in multi-core fibers with group velocity dispersion.

This work proposes an approach for the compensation of inter-core skew in homogeneous single-mode multi-core fiber links. We adjust the wavelengths of the transmitted spatial channels in such a way that the skew induced by group velocity counters inter-core skew. This approach is demonstrated experimentally using a 111 Gb/s spatial super channel (4 spatial channels at 27.8 Gb/s) on a 10.1 km 19-core multi-core fiber. It is shown that inter-core skew may be compensated without the need for devices such as variable optical delay lines or electronic buffers.

Intermodal Nonlinear Signal Distortions in Multi-Span Transmission With Few-Mode Fibers

Intermodal nonlinear signal distortions in a graded-index three-mode fiber are experimentally investigated in a recirculating loop transmission over distances up to 684 km. Transmitting a 24.5 GBaud 16-quadrature-amplitude-modulated spatial super channel as channel under test together with an interfering channel band that is swept by more than 20 nm across the C-band, we find a Q-factor penalty of 1 dB for the signals in the LP01 mode when the interfering channels are approximately 17 nm separated from the channel under test. We find that the wavelength separation between strongest interacting spectral components is independent of the transmission distance.

High-Capacity Super-Channel-Enabled Multi-Core Fiber Optical Switching System for Converged Inter/Intra Data Center and Edge Optical Networks

The introduction of the space domain in optical communications, collectively referred as space division multiplexing (SDM), have recently gained a lot of research attention motivated by the capacity exhaustion of conventional wavelength division multiplexing (WDM) systems. SDM not only increases the capacity, but also introduces new challenges and opportunities owing to the properties of the spatial channels in the form of fiber cores, modes or a combination of both. One promising application of SDM systems could be in future edge optical networks, where multi-core fibers (MCFs) with low inter-core crosstalk may be used with packet spatial super-channels (pSSCs) and core-joint SDM optical switches in a time-slotted network. We first briefly review the state-of-the-art of SDM transmission systems and other important technologies for edge networks such as burst-mode transmission. Finally, we focus on two recent experimental demonstration of such a switching system, achieving a capacity of 53.3 Tb/s for 7 spatial channels and QPSK modulation formant and an upgraded version with 8 spatial channels and 8PSK modulation format that achieves 83.33 Tb/s.

1.2 Pb/s Throughput Transmission Using a 160 $\mu$m Cladding, 4-Core, 3-Mode Fiber

This paper extends a recent demonstration of 1.2 Pb/s transmission using a few-mode multi-core fiber with a cladding diameter of 160 μm. The fiber was a 3.37 km, 4-core, 3-mode few-mode multi-core fiber with spatial multiplexing and demultiplexing performed with especially designed mode-selective couplers. We transmitted 368 wavelength division multiplexed spatial super channels across the C and L bands. We selected polarization-division-multiplexed-256-quadrature amplitude modulation format to maximize the generalized mutual information of the transmitted signals, which were decoded using punctured soft-decision forward error correction with a code rate granularity of 0.01. The maximum crosstalk of the system was -31 dB between LP 11 modes of adjacent cores, leading to a Q-factor penalty of approximately 0.5 dB. We estimated the system mode-dependent loss values ranging from 3 dB to 5 dB across C and L bands with negligible wavelength dependence.

Investigation of Intermodal Nonlinear Signal Distortions in Few-Mode Fiber Transmission

We present an experimental investigation of the Q -factor penalty due to intra and intermodal nonlinear signal distortions on 24.5 GBaud 16-quadratic-amplitude modulation based spatial super channel in a 36 km few-mode fiber. The results show that intermodal nonlinear signal degradations, caused by interfering spectral components separated by more than 17 nm, lead to similar Q -factor penalties as intramodal nonlinear signal degradations, originating in spectral components of less than 1 nm separation. We further compare the experimentally observed intra and intermodal nonlinear signal degradations with an analytical nonlinear Gaussian noise model and find a strong agreement between the two, validating the model as a computationally inexpensive tool for the design of few-mode fiber-based transmission systems.

Scaling SDM Optical Networks Using Full-Spectrum Spatial Switching

In today's optical networks, the capacity supported by a single optical fiber is far higher than the demand between source–destination pairs. Thus, to take advantage of the installed capacity, lightpaths allocated between distinct source–destination pairs share the capacity provided by an optical fiber. This sharing is only possible if these lightpaths have different wavelengths. With the exponential evolution of traffic, the demand between source– destination pairs in a network can approach or exceed the capacity of a single fiber. In this scenario, spacedivision multiplexing (SDM) networks appear as a viable option, and new network-sharing technologies become relevant. In this paper, we study different switching approaches for SDM networks with uncoupled spatial superchannels—such as networks with multicore fibers or bundles of single-mode fibers—which result in different network-sharing strategies. We start by comparing three switching architectures considering the evolution of traffic over time: (1) full-spectrum spatial switching (full-spectrum SS); (2) wavelength switching (WS) of uncoupled spatial superchannels; and (3) independent switching (IS) of spatial and wavelength channels. IS provides high network flexibility at the cost of complex switching nodes. On the other hand, full-spectrum SS and WS simplify network nodes but reduce flexibility by switching superchannels. Simulation results indicate that WS offers poor utilization in the long term. Full-spectrum SS, in contrast, exhibits low utilization in the short term, but outperforms IS in the long term. We also propose alternative hybrid switching strategies that start with IS and change to full-spectrum SS after a certain number of spatial channels are activated. The simulations suggest that well-designed strategies for migration from IS to full-spectrum SS delay premature activation investments while saving on switching costs.

Converged Inter/Intradata Center Optical Network With Packet Super-Channels and 83.33 Tb/s/port

We experimentally demonstrate a novel, time-slotted, converged inter/intradata center optical network that employs multi-core fibers (MFCs), packet spatial super-channels (pSSCs), and core-joint optical switches. The fundamental switching subsystem is a high-granularity core-joint electro-absorption optical switch, based on an array of electro-absorption 1×4 switching gates with a switching time of approximately 10 ns, and is able to jointly switch all the wavelength and spatial channels of the pSSCs simultaneously. Lower granularity core-joint switches may be used in the pSSC switching nodes for other tasks, such as network protection or bypass. Previously, we experimentally demonstrated a 2×2 pSSC switching system with a capacity of 53.3 Tb/s/port in a testbed with pSSCs composed of 64 wavelength channels, 7 spatial channels, and PDM-QPSK-modulated payloads. In this paper, we enhance our testbed experimental setup to demonstrate a record switching capacity of 83.33 Tb/s/port with pSSCs composed of 64 wavelength channels, 8 spatial channels, and PDM-8PSK-modulated payloads. We also demonstrate the transparency of the core-joint electro-absorption switch by comparing the new results with the previous PDM-QPSK results in terms of the error-vector magnitude.

Time-Division Packet Spatial Super-Channel Switching System With 53.3 Tb/s/Port for Converged Inter/Intradata Center Optical Networks

This paper presents and experimentally demonstrates a novel high-capacity switching system with several types of optical switches, based on time division multiplexing, space division multiplexing, and coherently modulated packet spatial super-channels (pSSCs), suitable for converged inter/intradata center networks. The fundamental subsystem of this network is the core-joint optical switch which enables efficient switching of all the spatial channels of the multicore fibers (MCFs) used for data transmission. Three novel core-joint optical switches, for 7-core MCFs and varying granularities to accommodate different network functions, are developed: 1) an electroabsorption-based, ns-fast joint switch, 2) an acoustooptic modulator-based, $\mu$ s-fast joint switch, and 3) a free-space mirror-based, ms-fast joint switch. We use these three subsystems in a 2 $\times$ 2 switching node that makes use of 64 wavelength channels, 7-core MCFs, and coherently modulated pSSCs to experimentally demonstrate a record switching capacity of 53.3 Tb/s/port and network protection capabilities in less than 10 ms in the event of an MCF failure.

Cost-effective spatial super-channel allocation in Flex-Grid/MCF optical core networks

Space Division Multiplexing (SDM) is a key technology to cope with the bandwidth limitations of single mode fibers. Multi-Core Fibers (MCFs) are considered as a promising candidate technology to implement SDM, due to their low inter-core crosstalk (ICXT), experimentally proven in laboratory prototypes. Among the different channel allocation options making use of the newly enabled space dimension, the so-called spatial super-channel (Spa-SCh) is the most likely solution to be implemented, given the inherent cost reduction of the joint-switching operation (i.e., jointly switching a spectrum portion in all MCF cores at once). This work targets the cost-effective Spa-SCh allocation over MCF-enabled Flex-Grid optical core networks. To this goal, state-of-the-art 22-core MCFs are assumed, although the proposed solutions are applicable to any MCF type. In particular, we propose and evaluate a partial-core assignment as a cost-effective strategy to improve spectrum utilization and save Capital Expenditure (CapEx) costs by minimizing the number of optical transceivers used per Spa-SCh. Numerical results reveal that reductions up to 44% and 33% in the number of active transceivers in the network can be obtained in national- and continental-wide backbone networks, respectively, without affecting the network Grade-of-Service (GoS), measured in terms of Bandwidth Blocking Probability (BBP). To evaluate the impact of the ICXT, we also compare the performance of the MCF scenarios under study against equivalent Multi-Fiber (MF) ones. From the obtained results, ICXT in MCF scenarios requires the utilization of less efficient modulation formats, which reduces the admissible offered network load by up to 17% for a 1% BBP target. Furthermore, this lower spectral efficiency also demands an increase of the symbol rate per sub-channel up to a 26%, a key indicator of the modulator electronic complexity.

Impact of Spatial and Spectral Granularity on the Performance of SDM Networks Based on Spatial Superchannel Switching

Spatially integrated switching architectures have been recently investigated in an attempt to provide switching capability for networks based on spatial division multiplexing (SDM) fibers, as well as to reduce the implementation cost. These architectures rely on the following switching paradigms, furnishing different degrees of spectral and spatial switching granularity: independent switching, which offers full spatial-spectral flexibility; joint-switching, which treats all spatial modes as a single entity; and fractional-joint switching, whereby subgroups of spatial modes are switched together as independent units. The last two paradigms are categorized as spatial group switching solutions since the spatial resources (modes, cores, or single-mode fibers) are switched in groups. In this paper, we compare the performance (in terms of spectral utilization, data occupancy, and network switching infrastructure cost) of the SDM switching paradigms listed above for varying spatial and spectral switching granularities in a network planning scenario. The spatial granularity is related to the grouping of the spatial resources, whereas the spectral granularity depends on the channel baud rate and the spectral resolution supported by wavelength selective switches (WSS). We consider two WSS technologies for handling of the SDM switching paradigms: 1) the current WSS realization, 2) WSS technology with a factor-two resolution improvement. Bundles of single-mode fibers are assumed across all links as a near-term SDM solution. Results show that the performance of all switching paradigms converge as the size of the traffic demands increases, but finer spatial and spectral granularity can lead to significant performance improvement for small traffic demands. Additionally, we demonstrate that spectral switching granularity must be adaptable with respect to the size of the traffic in order to have a globally optimum spectrum utilization in an SDM network. Finally, we calculate the number of required WSSs and their port count for each of the switching architectures under evaluation, and estimate the switching-related cost of an SDM network, assuming the current WSS realization as well as the improved resolution WSS technology.

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.

High Capacity Transmission Systems Using Homogeneous Multi-Core Fibers

We discuss optical transmission using homogeneous, single-mode, multicore fibers. We first describe the key fiber properties that can affect transmission, such as intercore crosstalk and the variation of propagation delay between cores, and the consequences of these properties on spatial super-channel transmission. We then describe a series of experiments and demonstrations of system features including self-homodyne detection and joint spatial superchannel modulation before discussing their suitability to high-capacity transmission.

Survey of Photonic Switching Architectures and Technologies in Support of Spatially and Spectrally Flexible Optical Networking [Invited

As traffic volumes carried by optical networks continue to grow by tens of percent year over year, we are rapidly approaching the capacity limit of the conventional communication band within a single-mode fiber. New measures such as elastic optical networking, spectral extension to multi-bands, and spatial expansion to additional fiber overlays or new fiber types are all being considered as potential solutions, whether near term or far. In this tutorial paper, we survey the photonic switching hardware solutions in support of evolving optical networking solutions enabling capacity expansion based on the proposed approaches. We also suggest how reconfigurable add/drop multiplexing nodes will evolve under these scenarios and gauge their properties and relative cost scalings. We identify that the switching technologies continue to evolve and offer network operators the required flexibility in routing information channels in both the spectral and spatial domains. New wavelength-selective switch designs can now support greater resolution, increased functionality and packing density, as well as operation with multiple input and output ports. Various switching constraints can be applied, such as routing of complete spatial superchannels, in an effort to reduce the network cost and simplify the routing protocols and managed pathway count. However, such constraints also reduce the transport efficiency when the network is only partially loaded, and may incur fragmentation. System tradeoffs between switching granularity and implementation complexity and cost will have to be carefully considered for future high-capacity SDM–WDM optical networks. In this work, we present the first cost comparisons, to our knowledge, of the different approaches in an effort to quantify such tradeoffs.

Comparison of Spectral and Spatial Super-Channel Allocation Schemes for SDM Networks

We evaluate the advantages of using the extra dimension introduced by space-division multiplexing (SDM) for dynamic bandwidth-allocation purposes in a flexible optical network. In that respect, we aim to compare spectral and spatial super-channel (Sp-Ch) allocation policies in an SDM network based on bundles of SMFs (to eliminate coupling between spatial dimensions from the study) and to investigate the role of modulation format selection in the blocking probability performance with an emphasis on the spectral efficiency (SE)/reach tradeoff for different multiline-rate scenarios, created either by changing the number of sub-channels (Sb-Ch), or by employing different modulation formats. Our network-performance results show that DP-8QAM —in a multichannel (MC) single-modulation-format system assuming ITU-T 50-GHz WDM Sb-Ch spectrum occupation—offers the best compromise between SE and optical reach for both spectral and spatial Sp-Ch allocation policies. They also reveal that an MC multimodulation-format system improves the network performance, particularly for spectral Sp-Ch allocation with Sb-Ch spectrum occupation of 37.5 GHz on the 12.5-GHz grid. Additionally, as another important contribution of the paper, we investigate, for spatial Sp-Ch allocation, the performance of several SDM switching options: independent switching (InS), which offers highest flexibility, joint-switching (JoS), which routes all spatial modes as a single entity, and fractional-joint switching, which separates out the spatial modes into sub-sets of spatial modes which are routed independently. JoS is proved to offer a similar performance to that of InS for particular network load profiles, while allowing a significant reduction in the number of wavelength-selective switches.

Single parity check-coded 16QAM over spatial superchannels in multicore fiber transmission.

We experimentally investigate single-parity check (SPC) coded spatial superchannels based on polarization-multiplexed 16-ary quadrature amplitude modulation (PM-16QAM) for multicore fiber transmission systems, using a 7-core fiber. We investigate SPC over 1, 2, 4, 5 or 7 cores in a back-to-back configuration and compare the sensitivity to uncoded PM-16QAM, showing that at symbol rates of 20 Gbaud and at a bit-error-rate (BER) of 10-3, the SPC superchannels exhibit sensitivity improvements of 2.7 dB, 2.0 dB, 1.7 dB, 1.3 dB, and 1.1 dB, respectively. We perform both single channel and wavelength division multiplexed (WDM) transmission experiments with 22 GHz channel spacing and 20 Gbaud channel symbol rate for SPC over 1, 3 and 7 cores and compare the results to PM-16QAM with the same spacing and symbol rate. We show that in WDM signals, SPC over hl1 core can achieve more than double the transmission distance compared to PM-16QAM at the cost of 0.91 bit/s/Hz/core in spectral efficiency (SE). When sharing the parity-bit over 7 cores, the loss in SE becomes only 0.13 bit/s/Hz/core while the increase in transmission reach over PM-16QAM is 44 %.

Software defined networking (SDN) over space division multiplexing (SDM) optical networks: features, benefits and experimental demonstration

We present results from the first demonstration of a fully integrated SDN-controlled bandwidth-flexible and programmable SDM optical network utilizing sliceable self-homodyne spatial superchannels to support dynamic bandwidth and QoT provisioning, infrastructure slicing and isolation. Results show that SDN is a suitable control plane solution for the high-capacity flexible SDM network. It is able to provision end-to-end bandwidth and QoT requests according to user requirements, considering the unique characteristics of the underlying SDM infrastructure.

Spatial Superchannel Routing in a Two-Span ROADM System for Space Division Multiplexing

We report a two-span, 67-km space-division-multiplexed (SDM) wavelength-division-multiplexed (WDM) system incorporating the first reconfigurable optical add-drop multiplexer (ROADM) supporting spatial superchannels and the first cladding-pumped multicore erbium-doped fiber amplifier directly spliced to multicore transmission fiber. The ROADM subsystem utilizes two conventional 1 × 20 wavelength selective switches (WSS) each configured to implement a 7 × (1 × 2) WSS. ROADM performance tests indicate that the subchannel insertion losses, attenuation accuracies, and passband widths are well matched to each other and show no significant penalty, compared to the conventional operating mode for the WSS. For 6 × 40 × 128-Gb/s SDM-WDM polarization-multiplexed quadrature phase-shift-keyed (PM-QPSK) transmission on 50 GHz spacing, optical signal-to-noise ratio penalties are less than 1.6 dB in Add, Drop, and Express paths. In addition, we demonstrate the feasibility of utilizing joint signal processing of subchannels in this two-span, ROADM system.

1x11 few-mode fiber wavelength selective switch using photonic lanterns

We demonstrate an 11 port count wavelength selective switch (WSS) supporting spatial superchannels of three spatial modes, based on the combination of photonic lanterns and a high-port count single-mode WSS.

Joint Digital Signal Processing Receivers for Spatial Superchannels

We discuss the advantages of spatial superchannels for future terabit networks based on space-division multiplexing (SDM), and demonstrate reception of spatial superchannels by a coherent receiver utilizing joint digital signal processing (DSP). In a spatial superchannel, the SDM modes at a given wavelength are routed together, allowing a simplified design of both transponders and optical routing equipment. For example, common-mode impairments can be exploited to streamline the receiver's DSP. Our laboratory measurements reveal that the phase fluctuations between the cores of a multicore fiber are strongly correlated, and therefore constitute such a common-mode impairment. We implement master-slave phase recovery of two simultaneous 112-Gbps subchannels in a seven-core fiber, demonstrating reduced processing complexity with no increase in the bit-error ratio. Furthermore, we investigate the feasibility of applying this technique to subchannels carried on separate single-mode fibers, a potential transition strategy to evolve today's fiber networks toward future networks using multicore fibers.