Space Division Multiplexing Research Articles

Performance analysis of multiple radio-access provision in a multicore-fibre optical fronthaul

Abstract In this work we report a comprehensive experimental study targeting the dimensioning of the next-generation multicore-fibre (MCF) optical fronthaul employing space-division multiplexing (SDM). This fronthaul is capable of simultaneous provision of multiple radio-access technologies (multi-RATs) with advanced multi-antenna MIMO capabilities per RAT. The different parameters required for fronthaul dimensioning are evaluated considering state-of-the-art 4G LTE-Advanced altogether other legacy wireless standards in operation nowadays. In particular, the modulation characteristics, the antenna quality requirements (in terms of EVM, phase error or rho) and the signal-to-noise ratio (SNR) thresholds are evaluated employing fully-standard cellular signals transmitted on a multicore fibre (MCF) fronthaul. The study includes bi-directional signal transmission and multi-antenna MIMO multiplexing. The MCF optical fronthaul is evaluated with a multiplexed transmission of 2G, 3G, 3.9G and 4G MIMO signals in radio-over-multicore-fibre (RoMCF) employing a commercially available four-core MCF. The SNR requirements at the transmitter antenna are obtained for each cellular signal considering GSM, EDGE, EGPRS2-A, cdma2000 1xEV-DO, UMTS HSPA+ and LTE-Advanced standards. LTE-Advanced single-antenna and two-antenna systems implementing 2 × 2 MIMO transmission can be accomplished with SNR levels over 25 dB. In the case of LTE-Advanced 4 × 4 MIMO multiplexing over four cores of MCF media, 32 dB SNR is needed to achieve almost four times provided bitrate per user.

Vector beam generation from vertical cavity surface emitting lasers.

Radially and azimuthally polarized beams in a single transverse mode are generated from a commercially available vertical cavity surface emitting laser (VCSEL) in an external cavity with a birefringent rutile lens, of which the c axis is parallel to the optical axis of the cavity, to select favorable polarization. Additionally, a vector Bessel-Gaussian beam is generated from a VCSEL, which is fabricated to oscillate with a linear polarization in a fixed direction in free running, in the same way. These results clearly show the potential ability of VCSELs to generate vector beams, which will be essential to space-division multiplexing in the future optical communication.

Novel fragmentation-aware algorithms for multipath routing and spectrum assignment in elastic optical networks-space division multiplexing (EON-SDM)

Abstract An innovative technology to expand transmission capacity of optical backbone networks and to add flexibility to conventional WDM networks is Elastic Optical Networks with Space Division Multiplexing (EON-SDM). In EON-SDM, connection requests can be established in a much efficient manner than WDM networks and even EON network without SDM capability. The notion of increasing fiber capacity with Space Division Multiplexing (SDM) is almost as old as fiber communications itself. One of the important problems in EON-SDM is fragmentation created by dynamically establishing and releasing lightpaths over time, thus increasing connection blocking probability. In this article, three novel algorithms are introduced to resolve fragmentation problem and improve blocking probability in EON-SDM; called Fragmentation Measure Metric Aware with Routing and Spectrum and Core Assignment (FMMA-RSCA), Most Fragmented Path per Core (MFPC), and Spectrum Block Multipathing per Core (SBMC). These algorithms can reduce blocking probability by controlling fragmentation in cores since they try different ways in order to assign spectrum for a given connection request. By providing this opportunity in spectrum allocation and assignment, connection blocking probability is improved as proved based on our performance evaluation results.

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.

Efficient Grating Couplers for Space Division Multiplexing Applications

Grating couplers are developed as one of the building blocks in silicon photonics to couple light between on-chip devices and optical fibers. For emerging high-capacity datacom and telecom systems enabled by space division multiplexing, it is essential to couple light together with mode conversion functions. In this work, we show the feasibility of combining conventional grating couplers and mode converters, both with a high efficiency. The proposed devices are very compact, built on standard silicon-on-insulator wafers, with no extra fabrication efforts required. It is shown that the LP01, LP11a, LP11b, and LP21b modes in a few-mode optical fiber can be successfully excited directly from the proposed grating couplers, which have the fundamental mode as an input from an integrated waveguide. The overall coupling efficiency, including mode conversion efficiency, can be as high as 50%. We also analyze the performance of the grating couplers in terms of bandwidth, efficiency, and tolerance to device fabrication errors.

Multicore-Fiber Receptacle With Compact Fan-In/Fan-Out Device for SDM Transceiver Applications

We describe a new multicore fiber (MCF) receptacle for space division multiplexing (SDM) based transceiver applications, in which we integrated a compact fiber-bundle type fan-in/fan-out device for directly connecting MCF to LDs and PDs. We design the length of the fan-out fibers taking their bending stress into account to achieve a configuration with a smaller footprint. With the proposed receptacle, we develop a 100 Gb/s SDM transmitter based on a four-channel, 1.3 μm membrane laser array on Si, in which spot-size convertor (SSC) using inverse taper InP and SiON waveguides is integrated. Since the core size of SiON waveguide is much smaller than that of MCF, we utilize different fibers with optimized modal fields at each connection point and introduce thermally-expanded-core (TEC) fusion splicing to realize a low-loss connection between them. We achieve a pluggable connection between the lasers and 125 μm-cladding 4-core MCF in an LC connector. The developed receptacle maintained satisfactory laser characteristics including sufficient output power and clear eye openings for directly modulated 28 Gb/s NRZ signals. We also achieve an error-free transmission at a bit-error rate of less than 10−12 with a 500-m MCF transmission link for all 4 channels with simultaneous 4-channel operation.

Enabling Simultaneous DAS and DTS Through Space-Division Multiplexing Based on Multicore Fiber

We have proposed and demonstrated a hybrid optical-fiber sensor that enables simultaneous distributed acoustic sensing (DAS) and distributed temperature sensing (DTS). The hybrid fiber sensor is realized through space-division multiplexed (SDM) reflectometers in a multicore fiber (MCF), where Raman optical time-domain reflectometry (ROTDR) for DTS is implemented simultaneously with phase-sensitive optical time-domain reflectometry (Φ-OTDR) for DAS through space-division multiplexing. The SDM reflectometers share an identical pulse source, but use separate interrogation fiber cores, allowing simultaneous measurement of ROTDR and Φ-OTDR. The proposed hybrid sensor based on MCF does not suffer from the incompatible pump power levels issue existing in its counterpart based on single mode fiber thanks to the SDM implementation. Thus it effectively eliminates the restriction imposed by fiber nonlinear effects (e.g., modulation instability). Wavelet transform denoising method is employed to reduce the noise of temporal ROTDR traces; as a result, the worst temperature uncertainty is reduced from 4.1 to 0.5 °C over 5.76 km sensing range. The proposed SDM hybrid fiber sensor can realize simultaneous distributed intrusion detection and temperature monitoring. It offers great potential in long-term real-time pipeline monitoring for oil and gas industry.

Distributed Vibration Sensor Based on Space-Division Multiplexed Reflectometer and Interferometer in Multicore Fiber

The maximum detectable vibration frequency by phase-sensitive optical time-domain reflectometer (Φ-OTDR) is limited by the repetition rate of the pump pulse. Additionally, the intensity-demodulation based Φ-OTDR sensor suffers from the generally nonlinear dependency of the backscattered optical intensity on vibration. This makes quantitative measurement of vibration frequency difficult. In order to mitigate these limitations, we propose and demonstrate a multicore fiber (MCF) based space-division multiplexed (SDM) Φ-OTDR and Mach–Zehnder interferometer (MZI) hybrid sensor, enabling truly uninterrupted distributed vibration sensing with broad vibration frequency response range and high spatial resolution. By taking advantage of highly integrated multiple spatial cores offered by an MCF, Φ-OTDR implemented in one of the cores is used to locate the vibration. Meanwhile, an MZI is constructed using another two parallel spatial cores with sufficient continuous-wave optical power from a coherent narrow linewidth laser, which is used to retrieve the vibration frequency by processing the acquired interference signal. Compared with the single mode fiber based hybrid systems, the proposed SDM configuration allows implementation of Φ-OTDR and MZI independently without any crosstalk between them. Consequently it offers many unique advantages, such as simple data processing procedure, good signal-to-noise ratio of the demodulated vibration frequency spectrum, no frequency dead zone, and single-end access. This cost-effective SDM hybrid sensor provides significant potential for distributed and accurate monitoring of vibrations.

Mode coupling and its influence on space division multiplexing in step-index plastic-clad silica fibers

Abstract The state of mode coupling in step-index plastic-clad silica fibers is investigated in this article by numerical solving of the time-independent power flow equation. The coupling length Lc at which the equilibrium mode distribution is achieved and the length (zs) of the fiber required for achieving the steady-state mode distribution are obtained. It was found that the plastic clad silica fiber investigated in this work showed a much weaker mode coupling than the plastic-clad silica fibers investigated in earlier works. We have also shown that mode coupling significantly limits the fiber length at which the space division multiplexing (SDM) can be realized with a minimal crosstalk between the neighbor optical channels.

Spatial multiplexing of soliton microcombs

Dual-comb interferometry utilizes two optical frequency combs to map the optical field's spectrum to a radio-frequency signal without using moving parts, allowing improved speed and accuracy. However, the method is compounded by the complexity and demanding stability associated with operating multiple laser frequency combs. To overcome these challenges, we demonstrate simultaneous generation of multiple frequency combs from a single optical microresonator and a single continuous-wave laser. Similar to space-division multiplexing, we generate several dissipative Kerr soliton states - circulating solitonic pulses driven by a continuous-wave laser - in different spatial (or polarization) modes of a $\mathrm{MgF_2}$ microresonator. Up to three distinct combs are produced simultaneously, featuring excellent mutual coherence and substantial repetition rate differences, useful for fast acquisition and efficient rejection of soliton intermodulation products. Dual-comb spectroscopy with amplitude and phase retrieval, as well as optical sampling of a breathing soliton, is realised with the free-running system. Compatibility with photonic-integrated resonators could enable the deployment of dual- and triple-comb-based methods to applications where they remained impractical with current technology.

Joint Assignment of Spatial Granularity, Routing, Modulation, and Spectrum in SDM-EONs: Minimizing the Network CAPEX Considering Spectrum, WSS, and Laser Resources

In space division multiplexing (SDM) systems, the spatial dimensions can be divided into one or several group(s). Spatial dimensions in the same group can be jointly switched based on joint switching (J-Sw)/fractional joint switching (FrJ-Sw), resulting in less wavelength selective switch (WSS) usage. Additionally, optical carriers on the same frequency of these spatial dimensions in the same group can share a single laser source. Therefore, a coarser spatial (switching) granularity can reduce the usage of both WSSs and lasers. However, compared to the case of a finer spatial granularity, this scenario leads to more guard bands (GBs), i.e., more spectrum resources, being required. Consequently, spatial granularity is a key factor of minimizing the cost in SDM network design. In this paper, we present a problem called routing, modulation, spatial granularity, and spectrum assignment (RMGSA). Our objective is to minimize the network CAPEX, i.e., minimize the total cost related to the network resources containing the spectrum, WSS, and laser. We first present a joint ILP-RMGSA formulation to solve this problem. Next, we present a decomposition method that divides RMGSA into two substituent subproblems, i.e., first, routing, modulation, and spatial granularity and, second, spectrum assignment, and then solve them sequentially. Because of the limitation of ILP formulation in larger scale problem instances, a rerouting-based heuristic algorithm is proposed. We examine the performances of the proposed approaches via simulation experiments and find that the best spatial granularity is related to the network resources to which network operators attached more importance. Moreover, as another contribution of this paper, we analyze the impact of GB width and the size of the connection requests on the decision regarding the best spatial granularity.

Greedy randomized adaptive search procedure for joint optimization of unicast and anycast traffic in spectrally-spatially flexible optical networks

The volume of data traffic in backbone networks is increasing at a speed that exceeds the growth rates of capacities offered in currently used wavelength division multiplexing (WDM) networks. As a short term solution to the potential capacity crunch, the concept of flexible-grid elastic optical networks is nowadays implemented in many backbone networks. However, in a longer term the space division multiplexing (SDM) technology is the most promising solution for satisfying the capacity requirements in future optical networks. The combination of both technologies allows for realization of spectrally-spatially flexible optical networks (SS-FONs). In this paper, we focus on a network scenario in which two different types of traffic flows are carried over an SS-FON, namely, unicast and anycast flows. Unicast flows are used for basic one-to-one communication, while anycast flows — defined as one-to-one-of-many communication — are related to the network traffic generated in data centers and the fact that data centers placed in different network locations can provide the same service or content to network users. For provisioning of lightpaths for both types of traffic, we address the basic optimization problem in SS-FONs, which is routing, space and spectrum allocation (RSSA). The aim of this study is threefold. First, we propose an effective metaheuristic method based on the greedy randomized adaptive search procedure (GRASP) to solve the RSSA problem. Second, we compare performance of the proposed GRASP method against reference optimization approaches. Finally, we present and discuss a wide range of experiments focused on analysis of SS-FONs with joint unicast and anycast traffic. The main conclusions are: (i) the proposed GRASP algorithm provides results very close to optimal (only 0.5% optimality gap) for smaller problem instances and significantly outperforms other heuristics for larger problem instances (in average around 6.8% better than the reference simulated annealing algorithm); (ii) the use of anycast connections in SS-FONs allows to improve the network performance in terms of spectrum usage.

Vector mode based optical direct detection orthogonal frequency division multiplexing transmission in short-reach optical link

As one solution to implement the large-capacity space division multiplexing (SDM) transmission systems, the mode division multiplexing (MDM) has gained much attention recently. The vector mode (VM), which is the eigenmode of the optical fiber, has also been adopted to realize the optical communications including the transmission over free-space optical (FSO) and optical fiber links. Considering the concerns on the short-reach optical interconnects, the low cost and high integration technologies should be developed. Direct detection (DD) with higher-order modulation formats in combination of MDM technologies could offer an available trade-off in system performance and complexity. We review demonstrations of FSO and fiber high-speed data transmission based on the VM MDM (VMDM) technologies. The special VMs, cylindrical vector beams (CVB), have been generated by the q-plate (QP) and characterized accordingly. And then they were used to implement the VMDM transmission with direct-detection orthogonal frequency division multiplexing (DD-OFDM). These demonstrations show the potential of VMDM-DD-OFDM technology in the large-capacity short-reach transmission links.

Interconnecting data based on vortex beams by adjusting the ellipticity of a ring-core fiber.

In order to exchange data in a space-division multiplexing (SDM) system, a novel vortex-beam-based data interconnection concept, which is achieved by adjusting the ellipticity of a ring-core fiber, is proposed. A new ring-core fiber is also designed and fabricated for exchanging and propagating the data carried by first- or second-order vortex (orbital angular momentum) beams. The proposed scheme is not only analyzed and simulated in principle, but is also verified through experiments. The numerical results demonstrate that the vortex beams can be exchanged by appropriately adjusting the phase difference (with respect to the ellipticity of a ring-core fiber) between the even and odd vector modes. A new experimental platform is designed and established for the sake of investigating the feasibility of the proposed scheme. The experimental results are consistent with the results of the simulation, and demonstrate that the data carried by the first- or second-order vortex beams can be successfully switched with acceptable bit error rates (BERs) between the first-order vortex beams (L=1 or -1) or between the second-order vortex beams (L=2 or -2, left or right circular polarization), respectively. The measured BERs and constellation diagrams of 16-QAM are employed to evaluate the data exchange performance with respect to different cases (i.e., data exchange once or twice, and data exchange with or without crosstalk). The measured BERs and constellation diagrams also demonstrate that the performance degrades with increase in topological charge or crosstalk. The proposed scheme is flexible, simple, and reliable for data exchange in a SDM system.

Protection, Routing, Modulation, Core, and Spectrum Allocation in SDM Elastic Optical Networks

Space division multiplexing is a promising solution, which has been proposed for elastic optical networks to deal with the anticipated exhaustion of the capacity of single core networks. This letter introduces the PERFECTA algorithm, a novel algorithm to provide failure-independent path protecting p-cycle for path protection in elastic optical networks using modulation and space division multiplexing.

On the Cost Minimization in Space Division Multiplexing Based Elastic Optical Networks

Abstract The required upgradation of the single-mode fibre (SMF)’s network capacity constrained by the non-linear Shannon’s limit, and the capacity provisioning which is needed by the future diverse Internet traffic can be resolved by the space division multiplexing (SDM) based elastic optical networks (EONs) (SDM-b-EONs). Recently, the multiple core fibre (MCF) technology has gained momentum over the current multi-fibre (MF) technology after laboratory experiments conducted on MCF models established much lower inter-core crosstalk values. In view of channel assignment, the spatial-super-channel (Spat-Sup-Chn) method has evolved as a prime candidate owing to its use of the joint switching (JoSw) mechanism which minimizes the cost. In the current work, in a JoSw based MCF enabled SDM-b-EON, for provisioning the demands; we aim at cost minimization while assigning both, the spectral and the spatial resources. Initially, we establish that in specific cases; use of the complete core allotment (CCA) technique, which uses all the MCF cores to provision the demands, leads to spectrum underutilization following which; we introduce a novel core adapting allotment (CAA) technique which adapts the cores amounts. We then investigate the possibility of using the CAA technique to provision the Spat-Sup-Chns simultaneously ensuring the same spectral requirements as needed when the CCA technique is used. For the performance evaluations, we consider the 37 core trench-assisted MCFs and our results show that by using the CAA technique, indeed there is a possibility of the transceivers amount being minimized in effect resulting in the Spat-Sup-Chn method being an economically practical solution.

Transmission of wireless signals using space division multiplexing in few mode fibers

Evolution of next generation wireless networks brings challenges to efficiently transmit a large amount of data from a base station to a remote antenna unit. We investigate a space division multiplexing technique that employs few mode fibers (FMFs) to transmit 3 × 3 MIMO wireless signals, aiming to employ a common digital signal processing (DSP) unit to equalize both the fiber and wireless channel. We optimize system parameters and obtain above 28 dB and 23 dB signal-to-interference and noise ratio (SINR) for 3 meters wireless systems with 500 m and 2 km FMF, which correspond to the transmission capacity of 578 Mb/s and 468 Mb/s using a 20 MHz bandwidth, respectively. Moreover, we analyze that the nonlinear spectrum distortion due to the combined effect of nonlinearity in the directly modulated laser and the differential mode delay in multimode fibers and validate it by simulations.

High-order encoding schemes for floodlight quantum key distribution

Floodlight quantum key distribution (FL-QKD) has realized a 1.3\,Gbit/s secret-key rate (SKR) over a 10-dB-loss channel against a frequency-domain collective attack [Quantum~Sci.~Technol. {\bf 3}, 025007 (2018)]. It achieved this remarkable SKR by means of binary phase-shift keying (BPSK) of multiple optical modes. Moreover, it did so with available technology, and without space-division or wavelength-division multiplexing. In this paper we explore whether replacing FL-QKD's BPSK modulation with a high-order encoding can further increase that protocol's SKR. First, we show that going to $K$-ary phase-shift keying with $K = 32$ doubles---from 2.0 to 4.5\,Gbit/s---the theoretical prediction from [Phys.~Rev.~A {\bf 94}, 012322 (2016)] for FL-QKD's BPSK SKR on a 50-km-long fiber link. Second, we show that $2d\times 2d$ quadrature amplitude modulation (QAM) does not offer any SKR improvement beyond what its $d=1$ case---which is equivalent to quadrature phase-shift keying---provides.

Route, Modulation Format, MIMO, and Spectrum Assignment in Flex-Grid/MCF Transparent Optical Core Networks

In this paper, we target an optimal multiple-input multiple-output digital signal processing (MIMO-DSP) assignment to super-channels affected by intercore crosstalk (ICXT) in multicore fiber (MCF) enabled transparent optical core networks. MIMO-DSP undoes ICXT effects, but can be costly with high core density MCFs. Hence, its implementation in the network must be carefully decided. We address our objective as a joint route, modulation format, MIMO and spectrum assignment (RMMSA) problem, for which integer linear programming formulations are provided to optimally solve it in small network scenarios. Moreover, several heuristic approaches are also proposed to solve large-scale problem instances with good accuracy. Their goal is to minimize both network spectral requirements and the amount of MIMO equalized super-channels, taking a crosstalk-free space division multiplexing (SDM) solution as a reference, for example, based on parallel single mode fibers [i.e., a multifiber (MF) scenario]. For our evaluation, we consider several state-of-the-art MCF prototypes and different network topologies. The obtained results, with the considered MCFs, disclose that in national backbone networks, the desirable percentage of super-channels with MIMO equalization to match the performance of an equivalent crosstalk-free SDM solution ranges from 0% to 36, while in continental-wide networks this range raises from 0% to 56%. In addition, in the case of a nonideal MIMO (with a 3 dB/km of crosstalk compensation), such percentages range from 0% to 28% and from 0% to 45% in national and continental-wide backbone networks, respectively, experimenting a performance gap up to 12% with respect to the MF reference scenario.

Silicon chip-scale space-division multiplexing: from devices to system

Space-division multiplexing (SDM) technique has attracted increasing attentions recently, because it provides an effective way to increase transmission capacity. With the continuous and exponential increase in data demands, high-density integration of silicon photonic components is of significant interest in terms of link price, performance and power consumption. The multimode/mutlicore devices applied to achieve diverse functionalities are key building blocks to construct a chip-scale SDM system based on a silicon on insulator (SOI) platform. This study reviews the recent progress of multimode/multicore devices, which enable coupling, multiplexing/demultiplexing, transmitting switching, as well as modulation and detection. Based on these devices, a complete on-chip SDM system is constructed and discussed.