Recent progress in optical devices for mode division multiplex transmission system

Few-mode erbium-doped fiber amplifier (FM-EDFA) is a key element to realize signal gain compensation in a longdistance mode division multiplexing (MDM) system. The differential modal gain (DMG) between modes directly affects the communication quality of the MDM system. In this paper, the particle swarm optimization (PSO) method is applied to design the erbium ion doping profile for high gain and low DMG simultaneously. By adjusting the doping radius and concentration concurrently, both high signal gains and low DMG can be obtained. In the conditions of the core pump and cladding pump respectively, the erbium ion with a multi-layered doping profile is automatically optimized by the PSO for a few-mode erbium-doped fiber (FM-EDF). Results show that as a three-layered ion adjustment, the gain is higher than 20 dB and DMG is lower than 0.15 dB in a four-mode step-index fiber. PSO is easy to implement and simple to operate. Compared with the other intelligent methods, such as the genetic algorithm or gradient descent optimization algorithm, PSO has no "crossover" and "mutation". The optimization time is greatly reduced. The PSO-based fiber design provides new guidance for the improvement of fiber gain equalization.

A challenge is to measure the impairments of few-mode fiber (FMF) for mode division multiplexing (MDM) system, so as to quantify the few-mode fiber and compensate the impairments, and thus improves the transmission performance of MDM system. In this paper, the experimental setup of impairments measurement with the structure of Photonic Lantern and optical circulators is established. Not only the two kinds of mode coupling (MC) in 9.8 km 3-mode fiber and at splicing of two FMFs (0.9km and 9.8km) with an off-axis distance of 2.0μm and 2.4μm, but also the mode dependent loss (MDL) between LP 01 and LP 11 are successfully measured.

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.

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.

We describe the mode-division multiplexing(MDM) transmission using a conventional fiber coupler with negligible modal crosstalk. A 2×10Gbps MDM transmission was achieved over a 10km two-mode fiber without any signal processing.

As the most promising solution to sustain network capacity scaling beyond the fundamental nonlinear Shannon limits of standard single-mode fibers, space division multiplexing (SDM) has attracted enormous attention. To utilize the dimension of “space”, including mode-division multiplexing (MDM) using multi-mode (few-mode) fibers and core multiplexing using multi-core fibers, SDM can substantially increase the spectral bandwidth of a fiber-optic communication channel. In SDM optical networks, virtual network mapping becomes very complex, because the resource dimension has increased, meanwhile redundant resources and multi-dimensional resource fragments are more difficult to deal with, which will increase the blocking probability and decrease the utilization of spectrum resources. On the other hand, many connection requests with high quality of service (QoS) need physical network to provide high survivability for network resources to resist virtual link or virtual node failure. It determines the physical paths and the appropriate resources that are used to support a virtual network with predefined characteristics. A novel algorithm for the problem of shared backup path protected routing, spectrum and mode allocation (SBPP-RSMA) under dynamic traffic conditions is proposed, so as to protect virtual network and enhance the network resource utilization with lower blocking effectively and increase share ratio of spectrum resources. Finally, performance of our novel algorithm in terms of blocking probability and spectrum utilization is discussed with some simulation results.

A segment-coupled six-mode multiplexer based on multi-core fiber

Increasing demand for high data transmission rates and bandwidth availability driven by the rapid development of the broadband services becomes challenging in the context of constraints imposed by nowadays exploited telecommunication optical fibers. Based on single-mode fibers and multiplexing in the wavelength domain (wavelength division multiplexing, WDM), the current technology seems to have reached its fundamental limits. At present, two promising technologies are a subject of intensive research. One of them is the increase of the data transmission through multiplying the number of fiber cores, thus implementing the concept of Multi-Core Fibers (MCF), where each core is used as a separate data transmission channel. The second widely investigated technique is based on the idea of mode division multiplexing (MDM), where different transverse modes of a Few-Mode Fiber (FMF) can be used as different carriers for data channels. In this work, we demonstrate the results of R&D works and the comprehensive tests of the few-mode fibers developed within the project NMKM+. Developed few-mode fibers (both passive and active) and their commercially available counterparts have been tested with respect of transmission parameters and applicability to the real telecom systems. In particular, guided mode profiles, numerical apertures, OTDR, and dispersion characteristics for the passive fibers have been recorded and compared. Also, methods of selective excitation and detection of singular modes have been discussed and partially verified. The results of data transmission experiments have been performed and discussed with respect to the quality of transmitted signals. These have been complemented by the amplification experiments with the use of erbium-doped few-mode fibers in “classical” and microstructural geometry. The results were studied and analyzed, showing the potential for future optimization. Acknowledgement: This work has received support from the National Centre for Research and Development through project NMKM+ (TECHMATSTRATEG1/348438/16/NCBR/2018).

Multicore fibers and few-mode fibers are potential candidates for achieving high-capacity space division multiplexing and mode division multiplexing (MDM) systems. Mode converters are an essential part of any MDM for effectively launching light into the desired mode. Fiber-based mode converters will be more useful than bulk converters because of their compatibility and ease of use. In this contribution, a two-core fiber-based mode converter design is proposed that can effectively convert the fundamental LP01 mode into the LP11 mode by mode coupling. This device can also be used as a demultiplexer in the reverse operation. Such a mode converter can be fabricated by heating a piece of two-core fiber with the thermally diffused core technique. This leads to the diffusion of dopants into the cladding, resulting in an expanded core giving power coupling between the cores. The conversion efficiency of this converter is −0.5  dB at the optimum value. This all-fiber mode converter ensures an extinction ratio of more than 30 dB over 45 nm wavelength, which covers the entire C band. The robustness of the proposed design is analyzed by evaluating the effect of changes in design parameters on the conversion efficiency and extinction ratios.

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

Few-mode fibers (FMFs) have garnered significant attention in mode-division multiplexing (MDM) systems due to their parallel mode transmission capability. To achieve efficient mode control and characterization of FMFs, we present an adaptive spatial light modulation (SLM) system based on the genetic algorithm (GA) by combing the digital holography (DH). Single exposure DH technology can achieve precise decomposition of fiber modes, laying the foundation for subsequent precise mode control. The implementation of GA optimization can effectively select the phase distribution loaded onto SLM corresponds to the expected mode field in FMFs. Through closed-loop optimization of output mode fields via a SLM, this methodology achieves high-purity linearly polarized (LP) mode generation by solely monitoring the output intensity distribution. The experimental results show that in FMFs with 5.3 µm and 8.2 µm core diameters, LP mode excitation with purity approaching 90% is demonstrated. Simultaneous excitation of composite LP mode superposition fields generates customized intensity profiles showing similarity indices nearing 0.9. This self-adaptive mode control approach eliminates complex coupling requirements while overcoming static encoding limitations, providing critical technical support for advancing MDM systems.

We propose a mode-division multiplexing (MDM) transmission system with differential mode delay (DMD) independent low computational complexity MIMO processing. We adjusted the delay time of the delay units in the equalizer in accordance with the DMD value of the fiber, and the functional imperfection of the mode MUX/DEMUX can be compensated for with low computational complexity by using this technique. Finally, we investigated the crosstalk characteristics of the PLC-based MUX/DEMUX and achieved an experimental WDM-MDM transmission over the C-L band with low MIMO processing complexity even when using a few-mode fiber with a high DMD.

Design of precision spatial positioning scheme for mode division multiplexing system channels placement over optical fiber core end

Vector modes (VMs) with spatially nonhomogeneous polarization patterns, combining polarization and spatial-mode degrees of freedom together, are regarded as the eigenmodes of optical fiber. Such eigenmodes have potential to be used to implement the high capacity spatial division multiplexing (SDM)/mode division multiplexing (MDM) based optical fiber communication systems. Here, we respectively demonstrate 2 VMs of the same order (l = +2, EH11o and EH11e modes) and 2 VMs of different orders (l = 0, HE11o mode and l = +2, EH11e mode) multiplexing transmissions over 5-km few-mode fiber (FMF). The mode crosstalk after FMF transmission has been minimized to prepare high capacity signal transmission without multiple-input-multiple-output (MIMO) digital signal processing (DSP). To increase the transmission capacity and decrease the system cost, the Kramers-Kronig (KK) receiver has been employed and the single-wavelength two sets of eigenmodes multiplexed 360 Gbit/s and 400 Gbit/s signal transmissions over 5-km FMF link are realized, respectively. Then, to further burst the transmission capacity, 2 vector-mode-division-multiplexing channels (HE11o and EH11e modes) in combination of 5 wavelength-division-multiplexing (WDM) channels with a total of 10 parallel channels have been demonstrated. A total data rate of 1.12 Tbit/s with 28 GBaud 16-state quadrature amplitude modulation (QAM) signal has been realized over the 5-km FMF. In all data-carrying experimental demonstrations, the bit-error rate (BER) performance of each VM channel is below the 7% hard decision forward error correction threshold (FEC) at BER of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The experimental results highlight that the VMDM-based optical transmission technology could find enormous potential in huge-capacity short-reach optical interconnects.

&lt;p&gt;Mode-division multiplexing (MDM) and wavelength-division multiplexing (WDM) are regarded as effective solutions for enhancing the communication capacity of fiber-optic transmission system. To extend the transmission distance, a suitable amplification technique is required to amplify the signal light, with distributed Raman amplifier (DRA) offering a high-flat gain. In this paper, we experimentally demonstrate the transmission of 3 orbital angular momentum (OAM) mode-division multiplexing and 9 wavelength-division multiplexing ranging from 1530 nm to 1610 nm over 104-km self-designed ring-core fiber (RCF) with the assistance of DRA. The homemade all-fiber mode selective couplers (MSCs) are able to (de)multiplex OAM modes (OAM01, OAM11, OAM21) efficiently in long-distance MDM+WDM fiber-optic transmission system. The OAM-DRA provides high gain (with the on-off gain greater than 10 dB) as well as flat gain (with a differential wavelength gain less than 0.88 dB and a differential mode gain less than 0.28 dB) over the C+L band.&lt;/p&gt;