Mode Division Multiplexing Transmission Research Articles

Research on mode-coupled multi-mode Fiber mode-division in multiplexing transmission system

The mode coupling and random loss between different modes have important effects on the reliability of the multimode fiber-based mode division multiplexing transmission system. Using the VPI optical communication simulation platform, a short-distance analog-division multiplexing transmission system based on multimode fiber was designed and built. By setting the relevant parameters of mode coupling and random loss caused by bending, the quality of the eye diagram of the received signal is analyzed by the photoelectric signal analyzer at the receiving end. The experimental results show that the mode coupling has great influence on the anti-noise performance and anti-inter-symbol interference performance of the multimode fiber-based transmission system, and the random loss has great influence on the anti-noise performance of the system, this effect is more obvious in high-order linear polarization mode fiber.

Prototype of DSP-Free IM/DD MDM Transceiver for Datacenter Interconnection

New transmission techniques for short-reach optical transmission applications especially datacenter interconnections are highly expected to break current bandwidth bottleneck, among which mode division multiplexing (MDM) based on few-mode fiber (FMF) is promising by exploring different linearly-polarized (LP) modes or mode groups as spatial transmission channels. However, it has been a great challenge to deal with degenerate modes in FMFs. In this paper, a FMF design utilizing multiple ring perturbations in fiber core to suppress modal crosstalk during fiber transmission is firstly proposed and a 6-LP-mode double-ring-core (DRC) FMF is fabricated with a high minimum refractive index difference (min|neff|) of 1.4910-3 among all the modes. Secondly, a novel degenerate-mode-selective coupler (DMSC) converting each degenerate LPmn mode into the LP11 mode in a two-mode fiber is proposed. Then intensity-modulation/direct-detection (IM/DD) MDM transmission scheme without any digital signal processing (DSP) can be realized utilizing mode multiplexer/demultiplexer (MUX/DEMUX) consisting of cascaded mode-selective couplers (MSC) and DMSCs, in which each degenerate or non-degenerate LP mode is considered as an independent transmission channel. Then based on the proposed DRC-FMF and mode MUX/DEMUX, we demonstrate a 4-LP-mode prototype system using commercial single-mode 10 Gbps SFP + modules and 4K video transceivers without any hardware or software modifications. Finally, the intra-LP-mode dispersion (ILMD) effect in a weakly-coupled FMF is analyzed and a modified fixed-analyzer method is proposed for its measurement. Experimental results show that it may be one of the major impairments for IM/DD MDM transmission.

Fully degeneracy-lifted PANDA few-mode fiber based on the segmented ring-core

The PANDA segmented ring-core (SERC) few-mode fiber (FMF) can further enhance its characteristic of separating the eigenmodes by dividing the RC into several ring-sectors (RSs) with variable angles and decreasing refractive indices in the azimuthal direction. The novel PANDA SERC FMF with an appropriate choice of parameters can support 10 fully separated eigenmodes, where minimum effective index differences min(∆neff) between adjacent eigenmodes can reach 3.5 × 10−4 over the whole C + L band. Moreover, the group velocity dispersion (GVD), effective mode areas and losses of the fiber are within the range of (−42, 20) ps/nm/km, (31, 75) μm2 and (0.4, 0.56) dB/km over the whole C + L band. Numerical results show that the designed fiber has excellent bending resistance, while the splice losses between standard step-index fiber and PANDA SERC FMF are within the range of 0.62–1.05 dB at 1550 nm. Consequently, the proposed fiber can provide a promising way to design the fully degeneracy-lifted polarization-maintaining (PM) FMF for the mode-division multiplexing (MDM) transmission.

Long-haul intermodal-MIMO-free MDM transmission based on a weakly coupled multiple-ring-core few-mode fiber.

Mode-division multiplexing (MDM) technique based on few-mode fibers (FMFs) can achieve multiplicative growth in single-fiber capacity by using different linearly polarized (LP) modes or mode groups as spatial channels. However, its deployment is seriously impeded because multiple-input multiple-output digital signal processing (MIMO-DSP) with huge computational load must be adopted to combat intermodal crosstalk for long-haul FMF transmission. In this paper, we present an intermodal-MIMO-free MDM transmission scheme based on weakly coupled multiple-ring-core FMF, which achieves ultralow distributed modal crosstalk (DMC) so that the signal in each LP mode can be independently received by single-LP-mode MIMO-DSP even after hundreds-of-kilometer transmission. Evaluation method for the required DMC levels is proposed and different transmission reaches are investigated by simulation. By adopting an improved method for quantitative DMC measurement, we show that the required DMC level for long-haul transmission is feasible. Finally, we experimentally demonstrate 1800-km LP01/LP02 multiplexed transmission and 525-km LP01/LP21/LP02 multiplexed transmission only adopting 2×2 or 4×4 MIMO-DSP. The proposed scheme may pave the way to practical applications of long-haul MDM techniques for the first time.

Design of an all-fiber mode power splitter for mode-division-multiplexing transmission.

An all-fiber mode power splitter (MPS) for mode-division multiplexing (MDM) transmission is proposed and numerically investigated. The MPS is based on the configuration of the symmetric few-mode fiber (FMF) coupler, with the operation of four modes (LP01, LP11, LP21, and LP02). By comprehensively optimizing the FMF coupling parameters, including both the coupling area distance D and the coupling length L, MPSs with various power splitting ratios from 90%:10% to 50%:50% under the condition of different single guided mode injection are obtained. Then, the MPS with a 90%:10% power splitting ratio under the simultaneous injection of LP01, LP11, LP21, and LP02 modes is reported, which can be used as the mode tap for the optical performance monitoring. Moreover, based on the proposed MPS structure, selective mode power strippers with more than 25 dB mode extinction ratio for LP01, LP11, LP21, and LP02 modes are numerically verified. The arbitrary single guided mode arising in the FMF can be selectively coupled into another FMF, together with an insertion loss of less than 1 dB. Owing to the flexible setting of the mode power splitting ratio, the proposed MPS may become a potential solution to realizing system performance monitoring and mode power routing in MDM systems.

Broadband mode-selective couplers based on tapered side-polished fibers.

We propose the broadband mode-selective coupler (MSC) formed with a side-polished six mode fiber (6MF) and a tapered side-polished small core single-mode fiber (SC-SMF) or an SMF. The MSCs are designed to allow the LP01 mode in the SC-SMF and SMF to completely couple to the LP01, LP11, LP21, LP02, LP31, LP12 modes in the 6MF over a broadband wavelength range. The phase-matching conditions of the MSCs are satisfied by tapering the SC-SMF and SMF to specific diameters. The tapered fibers are side-polished to designed residual fiber thickness using the wheel polishing technique. The effective indices of the side-polished fibers are measured with the prism coupling method. The MSCs provide high coupling ratio and high mode purity. High coupling efficiencies in excess of 81% for all the higher-order modes are obtained in the wavelength range 1530-1600 nm. For the LP01, LP11, LP21, LP02, LP31, LP12 MSCs at 1550 nm, the coupling ratios are 96.2%, 99.8%, 89.5%, 85.0%, 90.9%, 96.1%, respectively, and the mode purity of the MSCs is higher than 88.0%. The loss of the MSCs is lower than 1.8 dB in the wavelength range 1530-1600 nm. This device can be applied in broadband mode-division multiplexing transmission systems.

Few-Mode Gain-Flattening Filter Using LPFG in Weakly-Coupled Double-Cladding FMF

Recently, few-mode Erbium-doped fiber amplifiers (FM-EDFA) for mode-division multiplexing (MDM) transmission have attracted much interest. In this paper, we propose a novel few-mode gain-flattening filter (FM-GFF) based on long-period fiber gratings (LPFG) in double-cladding few-mode fiber (DC-FMF). Due to weak coupling among all the core modes and limited number of inner-cladding modes, the phase matching condition for a specific FM-LPFG can be satisfied only between one core mode and one inner-cladding mode each time and independent gain flattening can be realized for each core mode without influencing the other ones. Moreover, both DC-FMF and transmission FMF have the same core refractive index and radius, while the inner-cladding of DC-FMF and the cladding of transmission FMF have the same refractive index, which can effectively maintain the weakly-coupled condition for their connections. We show by simulation that the output gain spectra for all the 4 or 6 linearly-polarized (LP) core modes in two kinds of DC-FMFs can be independently flattened to less than 0.6-dB in the whole C-band by the cascading LPFGs in the DC-FMF. The proposed FM-GFF scheme is beneficial for the practical design of FM-EDFAs.

Silicon-Based TM0-to-TM3 Mode-Order Converter Using On-Chip Shallowly Etched Slot Metasurface

Mode-order converters drive the on-chip applications of multimode silicon photonics. Here, we propose a TM0-to-TM3 mode-order converter by leveraging a shallowly etched slot metasurface pattern atop the silicon waveguide, rather than as some previously reported TE-polarized ones. With a shallowly etched pattern on the silicon waveguide, the whole waveguide refractive index distribution and the corresponding field evolution will be changed. Through further analyses, we have found the required slot metasurface pattern for generating the TM3 mode with high conversion efficiency of 92.9% and low modal crosstalk <−19 dB in a length of 17.73 μm. Moreover, the device’s working bandwidth and the fabrication tolerance of the key structural parameters are analyzed in detail. With these features, such devices would be beneficial for the on-chip multimode applications such as mode-division multiplexing transmission.

Superregular solutions for a coupled nonlinear Schrödinger system in a two-mode nonlinear fiber

For the increase of the transmission capacity in optical communication systems, the so-called few-mode fibers are used for people to design the mode division multiplexing transmission. In this paper, we analytically obtain and graphically display the superregular solutions for a coupled nonlinear Schrödinger (NLS) system which describes the wave evolution in a two-mode nonlinear fiber, where the superregular solutions are the analogue of superregular breathers for certain scalar NLS-type equations. On the nonzero-zero (or proportional nonzero-nonzero) background, regular solutions describe the regular nonlinear waves which are located in a finite t domain but do not perturb the background with t being big enough, and superregular solutions are a subset of regular solutions which describe the nonlinear superposition of breathers and dark-bright (or breather-like) solitons developing from the perturbations on the dark-bright (or breather-like) solitons at a certain z, where z and t denote the evolution dimension and temporal distribution dimension, respectively. On the nonzero-zero background, superregular solutions are constructed in three cases: trivial case, a pair of breathers case and single breather case, and then other superregular solutions could be constructed according to the analyses for such three cases. Superregular solutions on the proportional nonzero-nonzero background are derived via the superregular solutions on the nonzero-zero background and an orthogonal transformation.

IM/DD mode division multiplexing transmission enabled by machine learning-based linear and nonlinear MIMO equalization

We demonstrate two mode division multiplexing transmission systems through few-mode fiber by using two degenerate LP11 modes in high speed and low cost optical interconnection scenarios. Two novel MIMO equalizers based on machine learning are proposed and utilized to eliminate the impairments. In the ultra-low cost optical interconnection, the linear equalizer based on the elastic net can effectively reduce the bit error rate to the hard-decision forward error correction threshold with an extremely fast convergence speed. With the help of the nonlinear equalizer based on the neural network, high speed short-reach transmission has been achieved. By multiplexing two degenerate modes with the bit rate of 100 Gb/s/λ, 2 × 100 Gb/s transmission of 30 G-class 1550 nm optical devices has been realized. Without the optical amplifier, the receiver sensitivity could be around -11 dBm in 2 × 50 Gb/s transmission. This provides potential solutions for the future evolution of short-reach optical links.

Multiple-Ring-Core FM-EDF for Weakly-Coupled MDM Amplification With Low Differential Modal Gain

Recently weakly-coupled mode-division multiplexing (MDM) transmission systems and networks based on ultralow-modal-crosstalk few-mode fiber (FMF) have attracted much interest, for which optical amplification technologies with low modal crosstalk and two-dimension mode-wavelength gain flattening are highly desired. In this paper, we propose for the first time a design method for few-mode Erbium-doped fiber (FM-EDF) with multi-ring-core (MRC) structures of both index and Erbium doping, which has high compatibility with the transmission MRC-FMF and low differential modal gain (DMG) with simple pump injection requirement. By sharing the boundaries of ring areas for both index and Erbium doping, the design method also helps to reduce the complexity of preform fabrication. We show by simulation that at 1550-nm signal wavelength, a 6-mode MRC-EDF can achieve a minimum DMG of 0.35-dB with only LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> pump light injection and a 10-mode MRC-EDF can achieve a minimum DMG of 0.60-dB with LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">03</sub> pump lights injection. Besides, the tolerance of DMG to the variation of fiber parameters induced by fabrication is analyzed. The proposed FM-EDF is beneficial for the practical applications of weakly-coupled MDM transmission systems and networks.

MIMO Equalization Technology Based on Neural Network in High-Speed IM-DD Mode Division Multiplexing Transmission System

MIMO Equalization Technology Based on Neural Network in High-Speed IM-DD Mode Division Multiplexing Transmission System

Near-ML Detection Based on Multilevel SINR Thresholding for Mode Division Multiplexing Transmission With Mode-Dependent Loss

The probability distribution of the signal-to-interference plus noise ratio (SINR) is analyzed and approximated by the generalized gamma distribution for mode-dependent loss (MDL)-impaired fiber links. Based on the characteristics of SINR, an improved reduced-search algorithm (IRS) based on multilevel SINR thresholding (IRS-MST) is proposed. IRS-MST dynamically determines the size of the search space for each mode based on multiple SINR thresholds. Furthermore, a compounding function is used to balance the size of the search space and performance. Simulation results show that IRS-MST can achieve near ML performance for different MDLs. Compared to the conventional IRS algorithm, the complexity of IRS-MST is approximately 5.4 times smaller than that of the IRS.

On-Chip Mode-Division Multiplexing Transmission With Modal Crosstalk Mitigation Employing Low-Coherence Matched Detection

We experimentally demonstrate on-chip modal crosstalk mitigation over two mode-division multiplexing (MDM) integrated circuits supporting four and eleven waveguide modes employing low-coherence matched detection. 30-Gbaud 8-PSK mode-multiplexed signal is successfully transmitted over the circuits without using multiple-in-multiple-output (MIMO) processing under a maximum modal crosstalk of 10 dB. Ten-mode-multiplexed on-chip transmission achieves a total capacity of 900 Gbit/s. This work shows the feasibility of using high-order modes in on-chip MDM transmission for future high-capacity optical interconnects.

Weakly coupled 10-mode-division multiplexed transmission over 48-km few-mode fibers with real-time coherent MIMO receivers

For weakly coupled mode-division multiplexed (MDM) transmission systems, we design and implement optical coherent receiver prototypes with real-time multiple-input multiple-output (MIMO) digital signal processing to equalize two degenerate linearly polarized modes with dual polarization. Using field programmable gate array circuits, we implement real-value 8 × 2 MIMO adaptive equalization with externally separated phase compensators based on the least mean square algorithm, which enables not only training equalization but also fast carrier-phase tracking. With the optical coherent MIMO receiver prototype, we demonstrate real-time weakly coupled 10 × MDM wavelength-division multiplexed dual-polarization quadrature phase shift keying transmission over 48-km few-mode fibers. This report shows a record number of multiplexed spatial modes, namely, 10 modes with dual polarization, in real-time MDM transmission experiments.

Weakly-Coupled MDM-WDM Amplification and Transmission Based on Compact FM-EDFA

Recently optical amplification technologies for mode-division multiplexing (MDM) transmission systems and networks have attracted great interest. In this article, we first propose a compact structure of few-mode Erbium-doped fiber amplifier (FM-EDFA) for weakly-coupled MDM amplification, which is characterized by weakly-coupled few-mode Erbium-doped fiber (EDF), cascaded mode-selective couplers (MSCs) for spatial mode control of pump lights, and few-mode wavelength multiplexer (FM-WMUX) for pump/signal coupling. Compared to previous studies, the proposed FM-EDFA has a similar structure with commercial single-mode EDFA (SM-EDFA) and can support weakly-coupled MDM transmission without inducing large modal crosstalk. Then related components are designed and fabricated for a 2-mode proposed FM-EDFA. The mode-field mismatching between transmission FMF and EDF under central alignment is investigated by simulation. Thanks to the effective suppression of modal crosstalk for the optical components, the transmission FMF, the EDF and their coupling, we experimentally demonstrate the first weakly-coupled MDM-WDM amplification and transmission over 15-km 2-mode FMF with 10-Gb/s optical on-off keying (OOK) modulation and direct detection. The proposed FM-EDFA can be extended to MDM transmission systems and networks supporting more modes.

Bend-Insensitive Grapefruit-Type Holey Ring-Core Fiber for Weakly-Coupled OAM Mode Division Multiplexing Transmission

We propose a bend-insensitive grapefruit-type holey ring-core fiber to support orbital angular momentum (OAM) modes. The large air domain in the cladding, results in a large refractive index contrast between the ring-core and the cladding, which makes this fiber bend-insensitive and makes the vector eigenmodes well separated. Therefore, the proposed fiber can be adopted for the OAM mode division multiplexing transmission without the use of any multiple-input multiple-output digital signal processing (MIMO-DSP). It is also proved that the thickness of the outer silica layer around the ring-core plays an important role in confining the light. If the OAM modes in the ring-core and the cladding modes in the outer silica layer are not phase-matched, the OAM modes in the ring-core will not be coupled into the outer layer of silica and can be well confined. We fabricate such fiber in two steps by using the “stack-and-draw” method. Two kinds of fibers with different thicknesses of the outer silica layer are fabricated under different pressures and temperatures. The testing results of the fabricated fiber demonstrate the mode purity and illustrate that the loss of the OAM $_{+1,1}$ and OAM $_{+2,1}$ modes are 0.095 dB/m and 0.25 dB/m, respectively.

On-chip silicon shallowly etched TM0-to-TM1 mode-order converter with high conversion efficiency and low modal crosstalk

Ever-increasing capacity requirements of optical interconnects drive the emergence and fast development of mode-division-multiplexing (MDM) transmission on-chip, where efficient mode control and conversion components become indispensable. Here, we propose an on-chip silicon TM 0 - to - TM 1 mode-order converter by leveraging shallowly etched rectangular slots on the top surface of silicon nanowire. The mode conversion region consists of two rectangular slots on the same side of a silicon nanowire and a smaller one between them to realize the efficient mode-order conversion from input TM 0 to output TM 1 mode with the help of multimode interference and accumulated phase difference. By studying the etching pattern on the silicon nanowire in detail, we have achieved an on-chip TM 0 - to - TM 1 mode-order converter with a high conversion efficiency of 97.5% and low modal crosstalk &lt; − 23 dB in a conversion length of 11.8 µm; further, the insertion loss is only 0.29 dB at the wavelength of 1.55 µm. Moreover, the device working bandwidth and fabrication tolerance are also analyzed. Note that the proposed shallowly etched slots on the silicon nanowire can also be further developed to achieve the on-chip TM 0 - to - TM 2 mode-order conversion. With these characteristics, such a device could boost the development of MDM transmission on-chip with more TM-polarized mode channels.

402.7-Tb/s MDM-WDM Transmission Over Weakly Coupled 10-Mode Fiber Using Rate-Adaptive PS-16QAM Signals

The mode-division-multiplexing (MDM) transmission technique using a few-mode fiber (FMF) has been investigated to overcome the theoretical limitation of the transmission capacity of a single-mode fiber. In MDM transmission, multiple-input multiple-output (MIMO) digital signal processing (DSP) is generally required on the receiver side to compensate for the modal crosstalk (XT) between the spatial modes. However, the computational cost is high, especially for high-order MDM systems that use many spatial modes. To reduce the MIMO matrix size, MDM transmission with partial MIMO-based DSP has been proposed using weakly coupled FMFs. The modal XT in such fibers can be suppressed by a special fiber design; however, the modal XT still remains and strongly limits the signal-to-noise-ratio (SNR) and capacity. To further increase the capacity for a given SNR, an optimization of the modulation format is significant. Recently, probabilistic shaping (PS) has received attention since it can gradually change the entropy of a modulation format and maximize the capacity for a given SNR. In this article, we demonstrate weakly coupled 10-mode-multiplexed transmission with four 4 × 4 MIMOs and two 2 × 2 MIMOs using rate-adaptive PS-16QAM signals over 48-km FMF, achieving a record transmission capacity of 402.7 Tb/s in 125-μm cladding single-core fiber transmission experiments. Furthermore, we also show the possibility of a further capacity increase by adjusting the entropies of the PS-16QAM signals depending on the spatial mode.

Impact of indirect mode coupling on planar lightwave circuit based mode converters/multiplexers

To improve fiber transmission capacity, space division multiplexing has been proposed and demonstrated, in which mode division multiplexing (MDM) is one option. To implement MDM transmission, mode converters/multiplexers are essential, which can be designed using planar lightwave circuits (PLC). In this work, we propose indirect mode coupling for design of mode converters/multiplexers to improve the performance. It is found that the performance of conversion efficiency, extinction ratio, and crosstalk can be significantly improved by using indirect mode coupling.