Mode Division Multiplexing Transmission Research Articles

Time-frequency dual-dimension deep neural network (TF-DD-DNN)-based OSNR and XT monitoring in mode-division multiplexing systems

We propose a time-frequency dual-dimension deep neural network (TF-DD-DNN)-based optical performance monitoring (OPM) scheme to jointly monitor the optical signal-to-noise ratio (OSNR) and the mode-coupling induced crosstalk strength (XT) in mode-division multiplexing (MDM) systems, by revealing the diversity of the amplified spontaneous emission (ASE) and XT distortions on optical signals in the time- and frequency-domains. We numerically quantify the predictions of OSNR and XT for a coherent MDM transmission link, achieving the OSNR prediction accuracy of more than 0.99 and the XT prediction accuracy of up to 1 with the root mean square error (RMSE) of 0.11 dB at OSNR = 30 dB @0.1 nm for the case of quadrature-phase-shift keying (QPSK) signals. We evaluate the impacts of the chromatic dispersion (CD) of fibers and the low-pass filter bandwidth on the prediction procedures. The dependency of the modulation formats is also discussed, confirming the robust operation of the proposed scheme. Finally, the TF-DD-DNN scheme is experimental verified. High consistency is obtained between the simulation and experimental results.

MPLC-based low-modal-crosstalk nine-mode-group demultiplexer for DSP-free IM/DD MDM transmission over MMF

Weakly coupled mode-division multiplexing (MDM) transmission over legacy laid multimode fiber (MMF) has great economic efficiency and can enormously enhance the capacity of short-reach optical interconnections. In order to be compatible with cost-efficient intensity-modulation/direct-detection (IM/DD) transceivers, weakly coupled mode-group demultiplexers that can simultaneously receive each mode group of MMFs are highly desired. In this paper, we propose a scalable low-modal-crosstalk mode-group demultiplexer over MMF based on multiplane light conversion (MPLC). Multiple input Hermite–Gaussian (HG) modes of MMF are first converted to bridging modes that are composed of HG00 modes distributed as a right-angle triangle in Cartesian coordinates, and then each HG00 mode belonging to a degenerate mode group is mapped to different overlapped HGn0 modes with vertical orientation for simultaneous detection. With the help of bridging modes, the MPLC-based mode-group demultiplexer can efficiently demultiplex all mode groups in standard MMFs with less than 20 phase masks. A nine-mode-group demultiplexer is further designed for demonstration, and simulation results show that the MPLC-based demultiplexer achieves low modal crosstalk of lower than −22.3dB at 1550 nm and lower than −17.9dB over the C-band for all the nine mode groups with only 16 phase masks.

End-to-end learning strategy based on a frequency domain feature decoupling network emulator with joint probabilistic shaping and equalization for a 300-Gbit/s OAM mode division multiplexing transmission

Mode coupling and device nonlinear impairment appear to be a long-standing challenge in the orbital angular momentum (OAM) mode division multiplexing (MDM) of intensity modulation direct detection (IM/DD) transmission systems. In this paper, we propose an end-to-end (E2E) learning strategy based on a frequency domain feature decoupling network (FDFDnet) emulator with joint probabilistic shaping (PS) and equalization for an OAM-MDM IM/DD transmission with three modes. Our FDFDnet emulator can accurately build a complex nonlinear model of an OAM-MDM system by separating the signal into features from different frequency domains. Furthermore, a FDFDnet-based E2E strategy for joint PS and equalization is presented with the aim of compensating the signal impairment for the OAM-MDM IM/DD system. An experiment is carried out on a 300 Gbit/s carrierless amplitude phase-32 (CAP-32) signal with three OAM modes over a 10 km ring-core fiber transmission, and the results show that the proposed FDFDnet emulator outperforms the traditional CGAN emulator, with improvements in the modelling accuracy of 30.8%, 26.3% and 31% for the three OAM modes. Moreover, the receiver sensitivity of the proposed E2E learning strategy is higher than for the CGAN emulator by 3, 2.5, 2.2 dBm and the real channel by 5.5, 5.1, and 5.3 dBm for the three OAM modes, respectively. Our experimental results demonstrate that the proposed FDFDnet emulator-based E2E learning strategy is a promising contender for achieving ultra-high-capacity interconnectivity between data centers.

All-Fiber Low-Modal-Crosstalk Demultiplexers for DSP-Free IM/DD LP-Mode MDM Transmission

Weakly coupled mode-division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission are promising methods of enhancing the capacity of short-reach scenarios in which low-modal-crosstalk-mode demultiplexers for degenerate linear polarized (LP) modes are highly desired. In this paper, we review two degenerate-mode reception schemes. Firstly, a low-modal-crosstalk orthogonal combined reception method for degenerate modes is proposed based on all-fiber mode-selective couplers, in which signals in both degenerate modes are demultiplexed into the LP01 mode of single-mode fibers and then are multiplexed into the mutually orthogonal LP01 and LP11 modes of a two-mode fiber (TMF) for simultaneous detection. Secondly, a novel degenerate-mode-selective coupler consisting of an input few-mode fiber and an output TMF is proposed, which could demultiplex degenerate LP modes without any digital signal processing (DSP). Both demultiplexers are achieved based on the taper and polish process. The fabricated devices are characterized and compared. The results show that the proposed schemes can pave the way to the practical implementation of DSP-free IM/DD LP-mode MDM transmission systems.

Design and Investigation of Modal Excitation in Tapered Optical Fiber for a Mode Division Multiplexed Transmission System

One of the main advantages of optical communication is its potential to transmit messages through the same channel via multiplexing. The concept of Mode Division Multiplexing (MDM) technology was first suggested to the telecommunication industries as a substantial solution to decrease the cost per bit of optical fiber transmission. This article aims to transmit information with less possible noise. To achieve this, multiple transverse modes are used to deliver information from the transmitter to the receiver. Our design evaluated a tapered nature that excites a total of 20 modes. In the evaluation, we applied Fourier transformation to the modes. We combined two, three and four modes to test the different complex mode mechanisms they produce in amplitude and phase.

Mode-Independent Optical Switch Based on Graphene-Polymer Hybrid Waveguides

Mode-division multiplexing (MDM) is a promising multiplexing technique to further improve the transmission capacity of optical communication and on-chip optical interconnection systems. Furthermore, the multimode optical switch is of great importance in the MDM system, since it makes the MDM system more flexible by directly switching multiple spatial signals simultaneously. In this paper, we proposed a mode-independent optical switch based on the graphene–polymer hybrid waveguide platform that could process the TE11, TE12, TE21 and TE22 modes in a few-mode waveguide. The presented switch is independent of the four guided modes, optimizing the buried position of graphene capacitors in the polymer waveguide to regulate the coplanar interaction between the graphene capacitors and spatial modes. The TE11, TE12, TE21 and TE22 modes can be regulated simultaneously by changing the chemical potential of graphene capacitors in a straight waveguide. Our presented switch can enable the independent management of the spatial modes to be more flexible and efficient and has wide application in the MDM transmission systems.

A practical in-line FM-EDFA based on cladding pumping for MIMO-free MDM amplification and transmission

The amplification of the long-haul mode division multiplexing (MDM) transmission systems is currently mainly implemented by three steps due to the lack of high-performance in-line few-mode erbium-doped fiber amplifiers (FM-EDFA): firstly, the multiplexed signals need to be demultiplexed. Secondly, multiple single-mode EDFAs are used to amplify the modes separately. Finally, the signals are re-multiplexed into the few-mode fiber for transmission. The multi-stage multiplexing and demultiplexing bring significant complexity and additional modal crosstalk, resulting in degraded transmission performance. To address this issue, an in-line FM-EDFA based on cladding pumping is proposed for simultaneous amplification and transmission in MDM systems. The direct connection of the in-line FM-EDFA to the transmission FMFs reduces the need for mode multiplexing/demultiplexing devices, which provides excellent stability and applicability for MDM transmission systems. The experimental results show that the modal gains of in-line FM-EDFA can reach 21 dB with a differential modal gain (DMG) of less than 0.9 dB. Moreover, the in-line FM-EDFA is applied for simultaneous amplification and transmission in a weakly-coupled MDM system with MIMO-free direct detection. The modal crosstalk of the in-line FM-EDFA is less than −11 dB. Therefore, the in-line cladding-pumped FM-EDFA provides good integration to the fiber links, showing much potential in practical long-haul transmission systems.

Multi-step-index fiber with a large number of weakly coupled OAM mode groups for IM/DD systems in data centers: design, fabrication, and characterization

Mode division multiplexing (MDM) technique based on weakly coupled few-mode fibers (FMF) is promising to enhance the capacity of short-reach transmission. We design and fabricate a multi-step-index FMF (MSIF), which supports weakly coupled first-order radial orbital angular momentum mode group (OAMl,1 MG) for MDM transmission. We use three layers of core to regulate the minimum effective refractive index difference (min|Δn eff |) between OAMl,1 MG and the adjacent MGs. In experiments, we demonstrate that the fabricated MSIF can support up to OAM6,1 with the interferometric method, and the loss measured by an optical time-domain reflectometer (OTDR) can achieve <0.5 dB/km for the OAMl,1 with an order from |l| = 0 to |l| = 6. The inter-mode-group cross talk (XT) is tested by the power measurement, and the system-level XT after 20 km fiber transmission in the worst case is about −11.1 dB.

MIMO-free OAM-MDM transmission with a ring-core fiber recirculation loop

We experimentally demonstrate the record long-haul orbital angular momentum (OAM) mode-division multiplexed (MDM) transmission over 300-km assisted by a ring-core fiber (RCF) recirculating loop. The recirculation loop consisting of 50-km RCF is controlled mainly by two acousto-optic switches with opposite states, and the signals are cyclically transmitted in the single-span 50-km RCF to achieve long-haul transmission. The RCF features largely separated OAM mode groups, where the effective refractive index difference between adjacent high-order OAM mode groups is larger than 1.0 × 10-3. Due to the low-crosstalk feature of RCF incorporated into fiber optic links, multiple-input multiple-output (MIMO) equalization technology is not used in the OAM MDM system. We transmit 20-Gbit/s quadrature phase-shift keying signals over two high-order OAM modes, and the transmission performance is characterized. The measured average optical signal-to-noise ratio penalties at a bit-error rate of 1.5 × 10-2 are about 2 dB between single OAM mode transmission and OAM modes multiplexing transmission. The obtained experimental results indicate favorable OAM-based MDM transmission performance in a 300-km recirculating loop link.

Ultra‐High Bandwidth Density and Power Efficiency Chip‐To‐Chip Multimode Transmission through a Rectangular Core Few‐Mode Fiber

AbstractOptical interconnects have emerged as promising solutions for assisting electrical interconnects in short‐reach scenarios, where high bandwidth density and energy efficiency are of particular importance. Mode‐division multiplexing (MDM), capable of enhancing the bandwidth density and energy efficiency of optical systems, has drawn tremendous interest. However, the chip‐to‐chip MDM optical interconnects are impeded by the multimode chip‐fiber interfaces, where the transverse modes on the MDM chip mismatch with the linear polarization modes in the circular core few‐mode fiber (CCF). Moreover, the high differential group delays (DGDs) of a conventional CCF make the digital signal processing (DSP) computation complex thus consuming high power. To overcome these bottlenecks, a new multimode coupling solution based on a rectangular core few‐mode fiber with ultra‐low DGDs and an integrated multimode coupler is proposed. Based on this coupling scheme, a chip‐to‐chip MDM transmission experiment is performed on the TE00, TM00, TE10, and TM10 modes, and the highest bandwidth density of 19.4 Tb s mm −1 is realized at the chip facet. The DSP power consumption is ≈6% of a conventional CCF‐based system. It is believed that this work will pave the way for ultra‐compact, high density, low cost, and low computation‐complexity optical interconnects, such as data centers, cloud computing, and telecommunications.

Low-CD weakly-coupled FMF for short-reach IM/DD MDM transmission.

Weakly-coupled mode division multiplexing (MDM) technique is a promising candidate for capacity enhancement of short-reach optical interconnections, for which the multiple-ring-core few-mode fiber (MRC-FMF) has been proven to be an effective design method to suppress distributed modal crosstalk. Similar to low chromatic-dispersion (CD) O-band transmission based on single-mode fibers (SMF), all the mode channels in a weakly-coupled FMF for short-reach applications should achieve low CD to support intensity-modulation/direct-detection (IM/DD) transmission. In this paper, we propose, for the first time to the best of our knowledge, an index perturbation method to adjust both effective index and CD of each mode in an MRC-FMF. Firstly, an accurate modeling method to model the relationship between SiO2-GeO2 material index and the germanium concentration at different wavelengths is proposed by analyzing the index characteristics of 4 kinds of germanium-doped fused silica SMFs at the same fabrication processing, which could be utilized to calculate the CD characteristics for an MRC-FMF with perturbed index profile. Then, based on the perturbation method considering the influences on both effective index and CD, a weakly-coupled low-CD MRC-FMF supporting 4 linearly-polarized (LP) modes is designed and fabricated. The measured minimum effective index difference min|Δneff| among all modes is larger than 1.3 × 10-3, and the CD values of all the modes lie between -6 and +6 ps/km/nm ranging from 1280 to 1320 nm, which agree well with the design. The 2-km transmission experiment indicates that the fabricated MRC-FMF could support stable digital-signal-processing (DSP)-free IM/DD transmission for all the 4 LP modes. This work is beneficial to the application of short-reach weakly-coupled MDM systems.

Adaptive Bayes-Adam MIMO Equalizer With High Accuracy and Fast Convergence for Orbital Angular Momentum Mode Division Multiplexed Transmission

In this paper, a novel Bayes-Adam-based multiple-input multiple-output (MIMO) equalizer was proposed and experimentally demonstrated for an orbital angular momentum (OAM) mode-division multiplexed (MDM) optical fiber communication system. In general, MIMO equalization is required to compensate the crosstalk of the random intra-group mode coupling in the OAM mode. However, due to the time-varying characteristics of the OAM-MDM transmission, it is a long-standing challenge for MIMO equalization to achieve the high accuracy and fast convergence. In this work, a Bayes-Adam MIMO equalizer based on a penalty term and the Bayesian principle is developed for MDM transmission using eight OAM modes over a 20 km ring-core fiber, which smooths the cost function to achieve global optimization with fast convergence and high accuracy. Experiments show that the proposed adaptive Bayes-Adam MIMO equalizer outperforms two other conventional equalizers in terms of convergence speed and accuracy. The proposed approach achieved fast convergence with only 3,000 iterations for the <+4, left> OAM mode at an OSNR of 22 dB, which is much faster than that of Adam MIMO equalizer (11000 iterations) and SGD MIMO equalizer (16000 iterations). Furthermore, compared with a conventional Adam MIMO equalizer, the proposed scheme gains the OSNR improvement of 4 dB and 3 dB for OAM mode of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = <-4, right> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = <-5, left> at the hard-decision forward error correction (HD-FEC) threshold, respectively, indicating its strong potential for OAM-MDM transmission.

Single-wavelength polarization- and mode-division multiplexing free-space optical communication at 689 Gbit/s in strong turbulent channels.

We experimentally demonstrated, for, it is believed, the first time, high-capacity polarization- and mode-division multiplexing free-space optical transmission with adequate strong turbulence resiliency. A compact spatial light modulator-based polarization multiplexing multi-plane light conversion module was employed to emulate strong turbulent links. By employing advanced successive interference cancellation multiple-input multiple-output decoder and redundant receive channels, the strong turbulence resiliency was significantly improved in a mode-division multiplexing system. As a result, we achieved a record-high line rate of 689.2 Gbit/s, channel number of 10, and net spectral efficiency of 13.9 bit/(s Hz) in a single-wavelength mode-division multiplexing system with strong turbulence.

Five-LP-Mode IM/DD MDM Transmission Based on Degenerate-Mode-Selective Couplers With Side-Polishing Processing

Weakly-coupled mode division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) have attracted great attention for capacity enhancement of short-reach applications such as optical interconnections, for which mode multiplexers/demultiplexers (MMUX/MDEMUX) with low insertion loss (IL) and modal crosstalk are the key components. In this paper, we firstly propose the design method for degenerate–mode-selective couplers (DMSC) with side-polishing processing. Three kinds of DMSCs for different degenerate linearly-polarized (LP) modes (LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> , LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31</sub> ) in a weakly-coupled few-mode fiber (FMF) are designed. Then a pair of MMUX and MDEMUX consisting of cascaded mode-selective couplers (MSC) and DMSCs for all the 5 LP modes of the FMF are fabricated with side-polishing processing. The measurement results show that the MMUX and MDEMUX achieve low back-to-back IL lower than 5.3 dB for all the 5 modes. Finally, stable digital-signal-processing (DSP)-free 5-mode IM/DD MDM transmission utilizing conventional optical modules over 10-km FMF are successfully demonstrated. The proposed low-IL MMUX/MDEMUX is scalable to support more modes and can promote the practical application of MDM techniques.

Design of a highly mode-selective photonic lantern for IM/DD MDM transmission

Recently mode multiplexers and demultiplexers (MMUX/MDEMUX) based on photonic lantern (PL) techniques for few-mode fiber transmission have attracted great interest. However, the design of PLs with high modal selectivity has become a great challenge for weakly-coupled mode-division-multiplexing (MDM) applications. In this paper, we propose a highly mode-selective PL-MMUX/MDEMUX design for intensity-modulation/direct-detection MDM transmission adopting each non-degenerate mode or each pair of degenerate modes as an independent channel. For the PL-MMUX, differential-phase injection (DPI) scheme is adopted for each pair of degenerate modes and a corresponding low-modal-crosstalk dissimilar core arrangement method is proposed considering the interleaving positive and negative phase characteristics of modal lobes of degenerate linearly-polarized (LP) modes, which achieve very high modal selectivity. For the PL-MDEMUX, simultaneous reception of degenerate modes is achieved by combining the optical power of both degenerated modes utilizing non-mode-selective PLs. Numerical simulations for a 6-LP-mode PL-MMUX/MDEMUX are performed. Results show that the PL-MMUX with DPI scheme and proposed cores distribution achieve much lower modal crosstalk than other cores distributions and the overall modal crosstalk of the PL-MMUX and MDEMUX in back-to-back case is lower than -20.63 dB over the C-band for all modes. The tolerance of our design to imperfect fabrication of differentiator or environmental impact are also discussed. The proposed DPI and dissimilar core arrangement method have high expansibility and provide a new way for the design of highly selective PLs and are beneficial to the practical applications of weakly-coupled MDM transmission systems and networks.

Design and analysis of a weakly coupled few-mode hollow-core U-shaped tube nested antiresonant fiber

In this work, we proposed a weakly coupled few-mode hollow-core U-shaped tube nested antiresonant fiber (FM-UARF) for the potential large-capacity mode-division multiplexing transmission without multiple-in-multiple-out (MIMO) digital signal processing. Through theoretical analysis and numerical simulation, the six-tube FM-UARF can be a good candidate for two-mode and three-mode transmission with better performance. After parameter optimization, the weakly coupled condition of effective refractive index difference (Δneff) larger than 5×10−4 can be achieved. Under the two-mode case, the confinement loss (CL) of the LP01 and LP11 modes is less than 0.005 dB/km and 0.1 dB/km in the 700 nm bandwidth range (0.9−1.6µm), respectively. In addition, LP01 achieves the lowest CL of 0.00038 dB/km at a wavelength of 1.06 µm, which gives it potential applications in high-power laser transmission. Under the three-mode case, the CL of the LP01, LP11, and LP21 modes are less than 0.006 dB/km, 0.1 dB/km, and 10 dB/km, respectively, in the wavelength range of 0.95 to 1.65 µm. Both the CL ratios are larger than 150 (23 dB), which ensures the high purity of the supported fiber modes. The results show that the proposed FM-UARF with optimized parameters has the potential in the MIMO-less large-capacity data transmission and so on.

3D Hybrid Constellation With a Regular Octahedral Structure for Few-Mode Transmission

This paper proposes a three-dimensional (3D) geometric shaping (GS) and probabilistic shaping (PS) constellation based on a regular octahedral structure in mode-division multiplexing transmission. The regular octahedron structure is adopted to improve the utilization rate of 3D space effectively and reduce the average power of the constellation. Spatial mode multiplexer/demultiplexer is used to achieve high-capacity transmission. As a result, the proposed scheme can be used to improve the constellation figure of merit and bit error rate (BER) performance. Experimental results show that the 3D hybrid constellation has a 0.6dB receiver sensitivity gain compared with that of the traditional regular 3D constellation at the BER~3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> with the information entropy of 3.52 bits/symbol. Meanwhile, as the received optical power increases, the gain grows larger, indicating that the proposed scheme will provide more opportunities for short-distance optical transmission and 3D constellation modulation with more points in the future.

Inverse modeling of few-mode fiber for high-speed optical communication networks

Few-mode fibers have been used in contemporary communication with mode multiplexing and space-division-multiplexing techniques to enhance the capacity crunch in recent communication links. The design parameters of the proposed fiber are predicted through machine learning-based inverse design approach, using regression model. The proposed few-mode fibre profile parameters are predicted with an accuracy of 99.95% to guide five to ten modes with weak coupling among the guided modes. In second phase of this work, the authors used a finite difference method-based solver to obtain the modal characteristics of the proposed fibre with predicted parameters for six guided modes, namely, LP 01 , LP 11 , LP 21 , LP 31 , LP 41 , and LP 51 . The numerical simulation results show that the predicted profile parameter maximizes effective mode-area and minimizes the inter-channel crosstalk for mode division multiplexing transmission over C-band. Besides this, the proposed ring-core few-mode fiber also exhibits nearly zero-dispersion for LP 01 mode at 1550 nm along with low dispersion for other higher-order modes. Finally, an intensity-modulation and direct-detection mode multiplexed transmission link without erbium-doped fiber amplifier is established with six-spatial channels over 50 km and an attenuation of 0.18 dB/km to achieve minimum bit-error-rate of 4.45×10 -9 .

Orbital Angular Momentum Data Transmission Using a Silicon Photonic Mode Multiplexer

We designed and fabricated a silicon photonic orbital angular momentum (OAM) multiplexer (MUX) chip that enables simultaneous wavelength-division multiplexing (WDM) and OAM mode-division multiplexing (MDM) in optical fiber communication systems. We successfully utilized the fabricated MUX chip to demonstrate simultaneous 3 OAM MDM transmission as well as OAM MDM/WDM transmission with an 800-m OAM ring-core fiber. In both cases, 10 Gbit/s OOK signals were utilized and BERs below the 7% FEC limit were obtained.

Low-modal-crosstalk orthogonal combine reception for degenerate modes in IM/DD MDM transmission.

Weakly-coupled mode division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission is a promising candidate to enhance the capacity of short-reach applications such as optical interconnections, in which low-modal-crosstalk mode multiplexers/demultiplexers (MMUX/MDEMUX) are highly desired. In this paper, we firstly propose an all-fiber low-modal-crosstalk orthogonal combine reception scheme for degenerate linearly-polarized (LP) modes, in which signals in both degenerate modes are firstly demultiplexed into the LP01 mode of single-mode fibers, and then are multiplexed into mutually orthogonal LP01 and LP11 modes of a two-mode fiber for simultaneous detection. Then a pair of 4-LP-mode MMUX/MDEMUX consisting of cascaded mode-selective couplers and orthogonal combiners are fabricated with side-polishing processing, which achieve low back-to-back modal crosstalk of lower than -18.51 dB and insertion loss of lower than 3.81 dB for all the 4 modes. Finally, a stable real-time 4 modes × 4λ × 10 Gb/s MDM-wavelength division multiplexing (WDM) transmission over 20-km few-mode fiber is experimentally demonstrated. The proposed scheme is scalable to support more modes and can pave the way to practical implementation of IM/DD MDM transmission applications.