Fibers For Space Division Multiplexing Research Articles

Transmission of optical communication signals through ring core fiber using perfect vortex beams.

Orbital angular momentum can be used to implement high capacity data transmission systems that can be applied for classical and quantum communications. Here we experimentally study the generation and transmission properties of the so-called perfect vortex beams and the Laguerre-Gaussian beams in ring-core optical fibers. Our results show that when using a single preparation stage, the perfect vortex beams present less ring-radius variation that allows coupling of higher optical power into a ring core fiber. These results lead to lower power requirements to establish fiber-based communications links using orbital angular momentum and set the stage for future implementations of high-dimensional quantum communication over space division multiplexing fibers.

Optimizing graded-index few-mode fiber for space division multiplexing.

We investigate the graded-index few-mode fiber (GI-FMF) to realize a 4-LP-mode (i.e. LP01, LP11, LP21, and LP02) fiber for mode-division-multiplexed transmission. This study optimizes the GI-FMF for both, first, for large effective indices differences (Δneff), and second, for low differential mode delay (DMD) between any two LP modes, for different optimized parameters. Thus, it shows that GI-FMF is suitable for both weakly-coupled few-mode fiber (WC-FMF) as well as strongly-coupled few-mode fiber (SC-FMF) via adjusting the profile parameter (α), refractive index difference between core and cladding (nco - nclad), and core radius (a). We report the optimized parameters for WC-GI-FMF with large effective indices difference (Δneff) of 0.6 × 10-3 and low |DMD| of 5.4 ns/km while the minimum effective mode area (Min.|Aeff|) is 80 µm2 and bending loss (BL) of the highest order mode is 0.005 dB/turn (much lower than 10 dB/turn) at a 10 mm bend radius. Here, we could break down the degeneracy between LP21 and LP02 mode, which remains a challenging task in GI-FMF. To the best of our knowledge, this is the lowest DMD (5.4 ns/km) ever reported for such a weakly-coupled (Δneff = 0.6 × 10-3) 4-LP-mode FMF. Similarly, we optimized the parameters for SC-GI-FMF with Δneff of 0.1 × 10-3 and the lowest DMD of 0.9 ns/km while Min.|Aeff| is ≫ 100 µm2 and BL of higher order mode is 6 dB/turn (< 10 dB/turn) at 10 mm bend radius. Further, we investigate narrow air trench-assisted SC-GI-FMF to reduce the DMD and achieve the lowest DMD of 16 ps/km for a 4-LP-mode GI-FMF with a minimum Δneff of 0.7 × 10-5.

Ultrafast measurement of vector spatial modes by using two-dimensional linear optical sampling.

To the best of the authors' knowledge, we present the first-ever demonstration of the two-dimensional linear optical sampling of fiber spatial modes. The images of the fiber cross sections excited by the LP01 or LP11 modes are directly projected onto a two-dimensional photodetector array and coherently sampled by local pulses with a uniform spatial distribution. Consequently, the spatiotemporal complex amplitude of the fiber mode is observed with a time resolution of a few picoseconds by using electronics with a bandwidth of only a few MHz. Such ultrafast and direct observation of vector spatial modes enables the characterization of the space-division multiplexing fiber itself with high time accuracy and wide bandwidth.

Theoretical investigations of weakly- and strongly-coupled multi-core fibers for the applications of optical submarine communications under power and fiber count limits.

The practical cable design for optical submarine communications has a limited fiber pair count due to the mechanical considerations of cable weight and size. Consequently, multi-core fibers (MCFs) could exhibit higher capacity than conventional single-mode fibers (SMFs) thanks to space division multiplexing (SDM). That is because the power supply to a submarine cable is fed by the voltage difference between shores. Under the power-limited condition, SDM improves the cable capacity by using more paths which outperforms the SMF link whose capacity approximately complies with a logarithmic relationship to optical power. At the same time, fiber nonlinearity can be alleviated by the reduced power density of transmitted light in MCFs, due to the increased spatial diversity and mode coupling among coupled cores. In this work, we theoretically investigate the potentials of MCFs including weakly-coupled multicore fiber (WC-MCF) and strongly-coupled multicore fiber (SC-MCF) as the propagation media for submarine communications across the Atlantic and the Pacific. To fairly compare the performances of SMFs- and MCFs-based submarine cables, the Gaussian noise (GN) model for SDM links is employed to optimize the systematic settings including spatial multiplicity and single span length. Then, we develop an SDM and wavelength division multiplexing (WDM) fiber transmission model based on coupled nonlinear Schrodinger equations (CNSE) to investigate the optical filed coupling effect in MCFs-based cables. The developed transmission model has been self-examined by measuring the inter-core crosstalk (IC-XT) and spatial mode dispersion (SMD), referring to the set values. As indicated by the theoretical analysis, the WC-MCFs cable exhibits a larger capacity than the SMFs cable, when the fiber pair count is limited below 32. Moreover, the SC-MCFs cable outperforms the WC-MCFs cable thanks to the reduced fiber nonlinearity due to the random mode coupling and the assistance of multiple-input and multiple-output digital signal processing (MIMO-DSP). At last, the marginal influences of IC-XT, SMD, and insertion loss of Fan-in and Fan-out couplers are also analyzed for the MCFs cable.

Nonlinear impairment scaling in coupled-core multi-core fibers for space-division multiplexing

We study the nonlinear impairment scaling in coupled-core multi-core fibers (CC-MCFs) with core count by evaluating four typical CC-MCFs with different numbers of cores, which have been demonstrated promising for long-haul optical fiber transmissions, using the Gaussian-noise model. The results show that nonlinear signal degradation in CC-MCFs with more cores becomes significantly weaker, which would be of great interest to ultralong-haul transmission in CC-MCFs. We further analyze how the fiber loss, differential group delay and effective mode field area impact on the trend. Moreover, we compare CC-MCFs and few-mode fibers with the same number of spatial modes in terms of nonlinear impairment and find that CC-MCFs tend to achieve better nonlinear performance. It is also shown that few-mode fibers operated in strong coupling regime are quite promising for long-haul transmission.

Demonstration of a helix side-pumping coupler with ultra-high pump coupling efficiency

The integrated and all-fiberized space division multiplexing fiber amplifier (SDMFA) with high spatial signal gain is urgently needed. Due to the lack of fiber-based multimode wavelength division multiplexing coupling device, the side-pumping coupler is essential to form an all-fiberized SDMFA. Here, a helix side-pumping coupler with ultra-high pump coupling efficiency is proposed by winding a tapered pump fiber to the target fiber. First, we establish the theoretical model of the side-pumping coupler and analyze the influence of the tapered pump fiber parameters on the pump coupling efficiency. Then the coupler is fabricated by arcuate tapering and heating. Finally, a multimode pump source and three signal modes (LP01, LP11a, LP11b) are experimentally applied to verify the pump coupling efficiency and signal transmission efficiencies. Results show that the experimental results are in good agreement with the simulation conclusions, and under the premise that all signal modes transmission efficiencies are more than 94%, the pump coupling efficiency is higher than 97% by introducing a piece of coreless fiber as bridge fiber. This helix side-pumping coupler has advantages of low signal loss, ultra-high pump coupling efficiency and higher-order modes transmittable capability.

Weakly Coupled Multicore Fiber Technology, Deployment, and Systems

Space-division multiplexing (SDM) technology is a promising candidate to achieve massive parallelism in optical fiber cables and overcome the capacity limitation of the conventional single-mode fiber (SMF). Among the several kinds of SDM fiber, a weakly coupled multicore fiber (MCF) is advantageous in that it can be easily upgraded for use in existing equipment and requires no complex signal processing for optical multi-input multi-output (MIMO). In this article, we discuss the progress and standardization activity of MCF technologies from the viewpoint of accelerating the practical application of SDM technology. We first describe the possible application areas of SDM fibers and then show the recent progress of the essential technologies to construct the MCF link: the MCF itself, its splicing and connectorization, and the input/output techniques. We also discuss the availability and deployability of MCF as a large-capacity transmission medium and detail the progress on establishing a new international standard for SDM technology, which includes both historical progress and more recent endeavors on the standardization of optical fiber cables and SDM technology in International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and International Electrotechnical Commission (IEC). We close with a summary of MCF technology and its standardization activity with an eye to next-generation transmission systems and the long-term development of ultralarge capacity SDM transmission technology.

Influence of birefringence- and core ellipticity-induced mode coupling on the gain statistics of forward-pumped Raman amplified few-mode fiber links.

The equations describing light propagation in a few-mode fiber for space-division multiplexing are derived under the presence of linear mode coupling and both Kerr- and Raman-induced nonlinearity. By considering physical models of stress birefringence and core ellipticity, the effect of such fiber imperfections on the gain of a forward-pumped Raman-amplified link is assessed through numerical simulations. The average gain and the variation of signal power at the output of the amplified fiber span is numerically evaluated for different levels of coupling strength in fibers supporting 2 and 4 groups of LP modes, identifying three main propagation regimes and assessing the effect of coupling between different groups of degenerate modes.

Design and Characteristics of Diamond-Assisted Ring-Core Fiber for Space Division Multiplexing

In this paper, a novel diamond-assisted ring-core fiber (DRF) is proposed. With the introduction of a low-refractive-index diamond-shaped region located in the center of the core, the proposed fiber effectively eliminates spatial degeneracy of the LPmn mode groups and maintains a low level of birefringence. Under the fiber structure parameters proposed in this paper, the effective refractive index difference (Δneff) between the spatial modes supported by the fiber in the entire C-band is greater than 2.25 × 10−4, and the Δneff between adjacent modes falls within the scope of (2.11~9.41) × 10−4. The degree of degenerate separation between the two polarization modes of all modes is very low, which is 2~3 orders of magnitude lower than that of the spatial mode. By discussing the mode characteristics of DRF and several other center-assisted ring-core fibers, the method that can be used to manipulate the spatial mode degenerate separation with structural symmetry is obtained, which can be applied to provide guidance for similar fiber designs. The proposed fiber structure is a promising candidate in space division multiplexing systems.

Design and characteristics of ellipse-assisted ring-core fiber for space division multiplexing

• Elliptical-assisted ring-core fiber with central low-refractive-index elliptical region. • 9 effectively separated spatial modes have uniform Δn eff with values greater than 3.14 × 10 −4 at 1550 nm. • The level of polarization separation is lower than 3.33 × 10 −6 . • Good scalability and fabrication error tolerance. • This fiber design could be promising candidates toward high-capacity SDM systems. We propose a novel ellipse-assisted ring-core fiber characterized by an elliptical region of low refractive index in the core. The proposed fiber proves effective in eliminating the spatial degeneracy of the LP mn mode groups and keeping the birefringence properties at a low level. This fiber, with optimized structural parameters, can guide 9 modes over the entire C-band. The simulation results show that the effective refractive index difference (Δn eff ) between adjacent modes at 1530–1565 nm falls within the scope of (3.1 ∼ 6.64) × 10 −4 . At the same time, the polarization separation level of each mode is lower than 3.23 × 10 −6 , which is 2 orders of magnitude lower than the degree of spatial mode degeneracy separation. This fiber design has good application prospects in space division multiplexing systems.

Learning to transmit images through optical speckle of a multimode fiber with high fidelity

Multimode fibers provide a unique opportunity for exploring the spatial degrees of freedom for high throughput light transmission. However, the modal dispersion prevents from the straightforward application of multimode fibers for space division multiplexing, such as image transmission. Herein, we propose and experimentally demonstrate a deep neural network termed multimode fiber inverse-scattering net for overcoming the modal dispersion induced scrambling in multimode fibers. Such a network is capable of transmitting grayscale image through the multimode fiber with high fidelity. 256-level grayscale images with 128 × 128 spatial channels encoded in the input wavefront can be retrieved from the output optical speckle patterns, where the average Pearson correlation coefficient and structural similarity index are as large as 0.97 and 0.95, respectively. Our results demonstrate that the proposed deep neural network has an excellent ability for learning the relationship between the input and output optical fields of a multimode fiber, which might facilitate the realization of high throughput space division multiplexing through multimode fibers.

Reconfigurable Microwave Photonic Filter Based on Space-Division Multiplexing Powered by Artificial Neural Networks

Space-division multiplexing (SDM) techniques bring new approaches for various applications in microwave photonic signal processing. In this work, an all-fiber reconfigurable microwave photonic filter (MPF) is realized based on SDM techniques using multicore fiber (MCF) and multimode fiber (MMF). The multimode interference (MMI) generated from the proposed MCF-MMF-MCF structure can be manipulated in dimensions of space and wavelength simultaneously, thus configuring the power distribution in all the taps of the MPF. The trained artificial neural network can accurately predict the operating wavelength and spatial input core for a target power distribution. In a low-cost, light-weight, and compact way, SDM fibers combined with MMI allow for greater flexibility and diversity in the response of the reconfigurable MPF.

Modulation format recognition with transfer learning assisted convolutional neural network using multiple Stokes sectional plane image in multi-core fibers.

A modulation format recognition (MFR) scheme based on multi-core fiber (MCF) is proposed for the next generation of elastic optical networks (EONs). In this scheme, multiple Stokes sectional planes images are used as signal features which are typed into a transfer learning (TL) assisted convolutional neural network (CNN) to realize MFR. Compared with the traditional Jones matrix, the Stokes space mapping method is insensitive to polarization mixing, carrier frequency skew and phase offset, therefore, it has better feature representation ability. TL is introduced to transfer the model used in standard single-mode fiber (SSMF) to MCF transmission, reducing the required training data and complexity. In addition, multiple Stokes sectional planes images are input simultaneously, which improves the accuracy of the neural network. Experimental verifications were performed for a polarization division multiplexing (PDM)-EONs system at a symbol rate of 12.5GBaud by 5 km MCF. Nine modulation formats, including three standard modulation formats (BPSK, QPSK, 8PSK), three uniformly shaped (US) modulation formats (US-8QAM, US-16QAM, US-32QAM) and three probabilistically shaped (PS) modulation formats (PS-8QAM, PS-16QAM, PS-32QAM), were recognized by our scheme. The experimental results show that the scheme achieves high recognition accuracy even at low optical signal-to-noise ratio (OSNR). Moreover, the required number of training samples is less 40% compared to the traditional CNN. The proposed scheme has a high tolerance to the crosstalk damage of MCF itself and can realize the short training time of large-capacity space division multiplexing (SDM)-EONs. Our findings have the potential to be used in the next generation of a SDM fiber transmission system.

Design and Characteristics of Rectangular-Assistant Ring-Core Fiber for Space Division Multiplexing

Design and Characteristics of Rectangular-Assistant Ring-Core Fiber for Space Division Multiplexing

Design and research of a broadband mode-division multiplexer based on three-core photonic crystal fiber

A broadband mode-division multiplexer based on asymmetric three-core photonic crystal fiber is proposed in this paper. The device is mainly composed of a central core, which can provide the transmission of fundamental mode and higher-order mode, and two side cores providing fundamental mode transmission. According to the theory of optical coupling, the LP&lt;sub&gt;01&lt;/sub&gt; mode light is input to the three fiber cores at the initial port separately, and in the transmission process the LP&lt;sub&gt;01&lt;/sub&gt; mode on the left side core will be coupled and converted into the LP&lt;sub&gt;21&lt;/sub&gt; mode light in the central core gradually. Similarly, the LP&lt;sub&gt;01&lt;/sub&gt; mode of the right side core is transformed into the LP&lt;sub&gt;31&lt;/sub&gt; mode of the center core. By optimizing the structural design and selecting the length of optical fiber, the best conversion from side core into central core can be completed at the output end simultaneously, thereby realizing the multiplexing of LP&lt;sub&gt;01&lt;/sub&gt;, LP&lt;sub&gt;21&lt;/sub&gt; and LP&lt;sub&gt;31&lt;/sub&gt; modes in the central core. In the opposite direction, if the output end of the device is used as the initial port, the demultiplexing of three modes of light from the central core to the three cores can be realized. In thiswork, the finite element method and beam propagation method are used to optimize the simulation, and the optical coupling theory and supermode theory are combined to conduct analysis and calculation. The results show that at wavelength band from 1.49 μm to 1.63 μm, the maximum insertion loss of the device is 0.72 dB, and the lowest insertion loss is 0.543 dB at 1.55 μm, which is far lower than the general evaluation standard of 1 dB insertion loss. The low insertion loss also makes it possible to design cascaded multi-core photonic-crystal-fiber mode-division multiplexer. Compared with the existing mode-division multiplexing scheme, the device is more integrated and less affected by the external environment. When it is used with multi-core space division multiplexing fiber, it can better improve the mode-conversion efficiency and mode purity, reduce the coupling complexity and expand the communication capacity.

Low cross talk homogeneous seven-core five-LP mode fiber based on a high and low refractive index double ring

We propose a what we believe is a novel, to the best of our knowledge, high and low refractive index double-ring (HL-DR) structure in few-mode multicore fibers (FM-MCFs) to simultaneously suppress intercore cross talk (XT) and intermodal cross talk. The fabrication methods of HL-DR seven-core fiber are introduced. A series of intercore XT values are calculated by the average power coupling coefficient, which shows that the HL-DR structure has a great contribution to meet the intercore XT target of less than − 30 d B / 100 k m in FM-MCFs. Intermodal XT is mainly measured by the effective refractive index difference ( Δ n e f f ) between modes, and the Δ n e f f between any two adjacent modes is larger than 2 × 1 0 − 3 . Other main fiber properties, including bending loss and dispersion, are also simulated by the finite-element method. The results imply that the bending loss can satisfy the requirement of five-LP mode operation over the C + L band. In addition, the dispersion comparison of L P 01 mode illustrates that the fiber dispersion performance is not significantly influenced by the addition of the HL-DR structure. Through numerical analyses, an optimal HL-DR seven-core five-LP mode fiber with low XT is obtained, and it has a relative core multiplicity factor (RCMF) of 62.17. The proposed structure targets the design and application of space division multiplexing fibers with high capacity and independent channels.

50.47-Tbit/s Standard Cladding Coupled 4-Core Fiber Transmission Over 9,150 km

Since optical submarine cable systems are a part of the global communications infrastructure, their total capacity must be continuously and dramatically enlarged. Recently, methods how to maximize the transmission capacity under electrical power limitations have been studied, and it has been reported that a single band (C-band only) transmission system with more fiber pairs (FPs) could be a promising technology. This finding has triggered work on submarine cables with more FPs. For a further increase in FPs in optical submarine cable systems, which also have space limitations in existing cable designs, space-division multiplexing (SDM) technologies such as multi-core fibers (MCFs) and multi-mode fibers (MMFs) could be promising solutions. In particular, 125-μm standard cladding SDM fibers are attractive for early deployment in submarine cable systems since they are expected to have high productivity and high mechanical reliability similar to existing single-mode fibers (SMFs) with the same cladding diameter. In this paper, we report transpacific MCF transmission over a 30-nm bandwidth using standard cladding ultralow-loss coupled 4-core fibers, extending our previous work. The Q <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -factors of 608 (4 core × 152 WDM) SDM/WDM channels modulated with 24-Gbaud DP (dual polarization)-QPSK (quadrature phase shift keying) exceeded the assumed forward error correction (FEC) limits after a 9,150-km transmission. As a result, transmission capacity of 50.47 Tbit/s and a capacity-distance product of 461.8 Pbit/s·km were achieved for standard cladding diameter SDM fibers.

Asymmetry core effects in multimode fibers for space-division multiplexing

Here we report a detailed numerical investigation into the effects of asymmetric cores on the modal properties in multimode fibers (MMFs). Three asymmetric core shapes combine with three core structures, respectively, forming nine kinds of common asymmetric cores in our simulation. The discussed modal properties include mode number, birefringence, effective index, power confinement factor, fiber loss, effective mode area, and differential mode delay. An evaluation factor called mode-NSD is proposed to integratedly judge the nonlinearity and compatibility ability in the MMF. Finally, the manufacturing tolerance with the size variation up to 5% is also discussed for practical needs. These numerical results are beneficial for the comprehensive assessment of the space-division-multiplexing optical fibers with asymmetric cores.

Space-division multiplexing for optical fiber communications

Research on space-division multiplexing (SDM) came to prominence in early 2010 being primarily proposed as a means of multiplying the information-carrying capacity of optical fibers at the same time as increasing efficiency through resource sharing. Proposed SDM transmission systems range from parallel single-mode fibers with shared amplifier pump lasers to the full spatial integration of transceiver hardware, signal processing, and amplification around a fiber with over 100 spatial channels comprising multiple cores each carrying multiple modes. In this paper, we review progress in SDM research. We first outline the main classifications and features of novel SDM fibers such as multicore fibers (MCFs), multimode fibers, few-mode MCFs, and coupled-core MCFs. We review research achievements of each fiber type before discussing digital-signal processing, amplifier technology, and milestones of transmission and networking demonstrations. Finally, we draw comparisons between fiber types before discussing the current trends and speculate on future developments and applications beyond optical data transmission.

Peta-bit-per-second optical communications system using a standard cladding diameter 15-mode fiber

Data rates in optical fiber networks have increased exponentially over the past decades and core-networks are expected to operate in the peta-bit-per-second regime by 2030. As current single-mode fiber-based transmission systems are reaching their capacity limits, space-division multiplexing has been investigated as a means to increase the per-fiber capacity. Of all space-division multiplexing fibers proposed to date, multi-mode fibers have the highest spatial channel density, as signals traveling in orthogonal fiber modes share the same fiber-core. By combining a high mode-count multi-mode fiber with wideband wavelength-division multiplexing, we report a peta-bit-per-second class transmission demonstration in multi-mode fibers. This was enabled by combining three key technologies: a wideband optical comb-based transmitter to generate highly spectral efficient 64-quadrature-amplitude modulated signals between 1528 nm and 1610 nm wavelength, a broadband mode-multiplexer, based on multi-plane light conversion, and a 15-mode multi-mode fiber with optimized transmission characteristics for wideband operation.