Few-mode Fiber Transmission Research Articles

Experimental study of intramodal XPM reduction by intramodal dispersion in weakly coupled FMF.

The understanding of nonlinear propagation effects in low-crosstalk few-mode fiber is crucial for a weakly coupled mode-division multiplexed system. In this Letter, we report the first, to the best of our knowledge, experimental verification of the advantage of intramodal dispersion on mitigating intramodal cross-phase modulation in a weakly coupled few-mode fiber transmission. The experimental system is established over a 70-km multiple-ring-core few-mode fiber accommodating 6 linearly polarized modes, based on which the influences of intramodal cross-phase modulation on transmission performances of each linearly polarized mode are evaluated. Experimental results show that the intramodal cross-phase modulation of degenerate linearly polarized modes with much larger intramodal dispersion values are significantly weaker than those of non-degenerate linearly polarized modes, in which the maximum suppression of intramodal cross-phase modulation noise is up to 9.7 dB. We believe that this work would be beneficial to practical applications of weakly coupled mode-division multiplexing technologies.

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.

A Comparative Study of Few-Mode Fiber and Coupled-Core Multi-Core Fiber Transmission

Few-mode fibers and coupled-core multi-core fibers are attractive transmission media for space-division multiplexed transmission systems as they enable a high spatial channel multiplicity whilst sustaining the current standard cladding diameter. This paper compares space-division multiplexed recirculating loop transmission using either a 3-mode few-mode fiber or a 3-core coupled-core multi-core fiber. After analyzing the spectral attenuation properties of both fibers, we investigate the wavelength and transmission distance dependence of the transmission channel characteristics. We find that the channel memory length is shorter and mode-dependent loss is lower in the coupled-core multi-core fiber, which can be attributed to the fundamental mode-structure differences between the two fiber types. This leads to a higher data rate at similar transmission distance for the coupled-core multi-core fiber.

Optical performance monitoring via domain adversarial adaptation in few-mode fiber

We propose and demonstrate an optical performance monitoring (OPM) scheme in few-mode fiber (FMF) based elastic optical networks (EONs). In this work, an adversarial transfer learning (TL) assisted by deep neural networks (DNN), which is used to learn the feature of signal constellations, is investigated for modulation format recognition (MFR) and optical signal-to-noise (OSNR) estimation. Because mode interference affects the projected results in FMF, the DNN is trained to increase link damage tolerance through domain adversarial adaptation (DAA). Five modulation formats have been considered for verifying the feasibility of the proposed scheme, including BPSK, QPSK, 8PSK, 8QAM, and 16QAM. The symbol rate of 12.5 Gbaud is experimentally tested for each scheme across a FMF transmission system. The results reveal that DAA scheme outperforms retraining DNN for requiring one-third training data without any accuracy penalty. Furthermore, the root mean square error (RMSE) of OSNR estimation in this paper can achieve less than 0.1 dB. We anticipate that our results can stimulate further research on the OPM tasks with a feasible deep learning scheme and contribute to the development of FMF-based EONs.

Few-Mode Fiber Transmission System Based on Probability Shaping and Interleaved Encoding

Few-Mode Fiber Transmission System Based on Probability Shaping and Interleaved Encoding

Optical modulation using digital image symbol patterns and grating light valve technology in a hybrid multiplexed superchannel

AbstractThe latest research exploits to increase optical channel bandwidth are termed polarization division multiplexing (PDM) and few mode fiber (FMF), which rely upon a complex multiplexed architecture together with customized electro‐optic modulated hardware elements, inhibited by cost implications and technical limitations thereof. The research pursued herein presents an alternate digital image pattern modulated (DIPM) approach to achieve higher bandwidth gains, spectral efficiency (SE), and transmission reach improvement. The solution focuses on reducing implementation cost with a scalable software approach and simplified conceptual hardware design using grating light valve (GLV) technology to address the drawbacks of PDM, FMF, and other conventional optical transmission technologies. Simulation test results for a DIPM‐GLV‐based fiber optic (FO) transmission system and comparisons against conventional and other novel techniques are graphically shown and interpreted, suggesting that DIPM using GLV is a viable high performance and cost efficient solution.

Fiber Vector Eigenmode Multiplexing Based High Capacity Transmission Over 5-km FMF With Kramers-Kronig Receiver

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.

Reduction of Nonlinear Mode Coupling in Mode Division Multiplexing Systems Based on Wavelength Interleaving Method

Mode division multiplexing (MDM) based few-mode fiber (FMF) transmission scheme is considered as one of the promising choices when one trying to achieve high bit rates in cascade to reduced cost per bit and upgrading the fiber capacity. Nonlinear mode coupling over FMFs can be addressed as the main degradation issue that can limit the efforts to reach their full capacity. In this work, we propose a new method to reduce nonlinear modal coupling by suppressing phase matching between different fiber modes using wavelength interleaving (WI) technique. This technique relays on interleave the optical carrier wavelength in a specific mode from those in adjacent ones. The five modes MDM system is analytically modeled and numerically demonstrated. Each spatial mode is modulated by a 4-ary quadrature amplitude modulation (4QAM) format with a symbol rate of 20 Gsymbol/s. The results reveal that the nonlinear phase (NLP) noise is reduced and the signal to noise ratio (SNR) is increased when applying the WI method. Additionally, it turned out that analytical and numerical results are in good agreement with each other.

High Spatial Density 6-Mode 7-Core Fiber Amplifier for L-Band Operation

We present a high spatial density space division multiplexed fiber amplifier (i.e., 6-mode 7-core multicore fiber amplifier) supporting 42 spatial channels. A high average modal gain of 23.1 dB is obtained in the L-band with an average differential modal gain of 3.2 dB and a minimum output power of 17.5 dBm per core per mode. The total power consumption of the amplifier is improved by a power reduction factor of 0.36 and a fully fiberized optical amplifier is realized in a cladding-pumped configuration. The assembled amplifier is then tested in a few-mode multicore fiber transmission link and simultaneously amplified 42 spatial channel fiber transmission is demonstrated with the highest spatial density and energy efficiency to date.

Tunable mode-selective characteristics of a mode-filter petal-fiber with liquid rods

In this paper, a novel tunable mode-filter optical fiber consisting of a high-index core and petal-shaped cladding surrounded by a high-index outer ring is proposed. The cladding of the fiber is formed with periodically arranged liquid rods that support cladding modes with effective indexes. These cladding modes form a two-super-mode group. The mode-selection is realized by the coupling between the core mode and the super-mode group. With the petal-shaped cladding, cladding mode can be transmitted at high loss. With the liquid rods, the index-band of super-mode group can be adjusted by external temperature field, thereby achieving the purpose of tunable mode-selective. The super-mode group formed by the LP&lt;sub&gt;11&lt;/sub&gt; mode of the liquid rods effectively increases its operating bandwidth and temperature tuning range. The numerical simulation results show that the mode-filter fiber with a length of only 71.4 mm can achieve a particular mode loss more than 20 dB, while other modes’ losses are below 1 dB. This special fiber can be used as a mode-filter in the few-mode fiber transmission system to reduce mode crosstalk of converters, multiplexer/demultiplexer, optical switch and optical routing.

Numerical Investigation of the Equalization Enhanced Phase Noise Penalty for M-Quadrature Amplitude Modulation Formats in Short-Haul Few-Mode Fiber Transmission Systems with Time-Domain Equalization

The equalization enhanced phase noise (EEPN), caused by the interaction of the chromatic dispersion (CD) with the phase noise of the local oscillator (LO), has been extensively studied for single-mode optical communication systems. Few-mode fiber (FMF) transmission systems introduce a new channel impairment, the differential mode delay (DMD), which also creates EEPN and hence limits the maximum transmission distance of those systems. In this work, we numerically investigate the optical signal to noise ratio (OSNR) penalties caused by the EEPN in a 3-mode FMF transmission system at 25 GBd for quadrature phase-shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 32-QAM and 64-QAM modulation formats when using the blind phase search (BPS) carrier phase recovery (CPR) algorithm, which has been demonstrated to be both robust and suitable for optical communication systems. Our numerical study assumes a short-span of FMF, modeled in the weakly-coupled regime, and includes two cases; the use of ideal mode-selective de/multiplexers at both ends of the FMF span (model A), and the use of ideal non-mode-selective de/multiplexers (model B). The results show that the EEPN has almost no effect in model A. However, EEPN produces a severe penalty in model B with the onset of the OSNR degradation starting for a DMD spread of the impulse response of about 100 symbols for all modulation formats investigated. The distribution ratio of the amount of phase noise between the transmitter and receiver lasers is also assessed for model B and we confirm that the degradation is mainly due to the phase noise of the LO.

DMD-Unmanaged Long-Haul SDM Transmission Over 2500-km 12-Core × 3-Mode MC-FMF and 6300-km 3-Mode FMF Employing Intermodal Interference Canceling Technique

We demonstrate differential mode delay (DMD) unmanaged 2500-km 12-core × 3-mode multicore few-mode fiber (MC-FMF) and 6300-km 3-mode FMF transmission. Mode-division multiplexed signals exhibit different transmission behavior depending on excited spatial modes in the presence of mode-relevant physical phenomena including DMD and mode-dependent loss (MDL). In weakly coupled FMF transmission over long distance where overall DMD and MDL grow almost linearly with fiber length, these phenomena restrict achievable information capacity and/or transmission reach. This paper presents a newly developed transmission scheme and a multiple-input multiple-output (MIMO) signal processing technique that are designed for achieving long-haul FMF transmission. Permutating spatial channels cyclically at each span induced quasi-strongly coupled transmission regime and hence significantly suppressed DMD-induced pulse broadening. Layered MIMO signal processing comprising multiple stages performing successive signal detection and intermodal interference canceling mitigated MDL impact with more than 2-dB Q-factor improvement. The combined use of these techniques enabled us to achieve the first-ever transoceanic-class long-haul FMF transmission.

High Capacity Transmission With Few-Mode Fibers

We experimentally investigate high-capacity few-mode fiber transmission for short and medium-haul optical links. In separate experiments, we demonstrate C + L band transmission of 283 Tbit/s over a single 30 km span and recirculating loop transmission of 159 Tbit/s over 1045 km graded-index three mode fiber. The first experiment reached a data-rate per fiber mode within 90% of the record data-rates reported in the same transmission bands for single-mode fibers. The second experiment demonstrated the feasibility of reaching high data-rates over long distance few-mode fiber transmission, despite strong impairments due to mode-dependent loss and differential mode delay.

257-Tbit/s Weakly Coupled 10-Mode C + L-Band WDM Transmission

Partial multiple-input multiple-output (MIMO)-based weakly coupled mode-division multiplexing (MDM) transmission is proposed to reduce the MIMO matrix size in MDM. This technique can mitigate the burden of MIMO processing for high-order few-mode fiber (FMF) transmission. In this paper, we first show that modal XT can be sufficiently suppressed over the C+L band using a weakly coupled FMF and a highly mode-selective multiplexer/demultiplexer. Next, 10-mode-multiplexed 336-WDM transmission of dual-carrier 12-Gbaud dual-polarization-quadrature phase-shift keying signals over a 48-km weakly coupled FMF using only 2 × 2 or 4 × 4 partial MIMOs is demonstrated. To the best of our knowledge, in this experiment, we achieved a record fiber capacity of 257 Tbps with an aggregate spectral efficiency of 30.5 bit/s/Hz in single-core fiber transmission.

Independent component analysis based digital signal processing in coherent optical fiber communication systems

In this paper, channel equalization techniques for coherent optical fiber transmission systems based on independent component analysis (ICA) are reviewed. The principle of ICA for blind source separation is introduced. The ICA based channel equalization after both single-mode fiber and few-mode fiber transmission for single-carrier and orthogonal frequency division multiplexing (OFDM) modulation formats are investigated, respectively. The performance comparisons with conventional channel equalization techniques are discussed.

A Sparsity Managed Adaptive MIMO Equalization for Few-Mode Fiber Transmission With Various Differential Mode Delays

It is often observed that various differential mode delays (DMDs) coexist in single multi-core fiber (MCF) and/or few-mode fiber (FMF) transmission. From a multi-input and multi-output (MIMO) equalization perspective, this indicates optimum equalization tap length for each multi-core and/or multi-mode signal varies according to its DMD. Correspondingly, complex calculation of finding each optimum tap length is necessary to obtain satisfactory performance. This paper presents a new adaptive MIMO equalization method to deal with various DMDs while avoiding such complex calculation. The method uses the same tap length for all multi-core and/or multi-mode signals according to the maximum DMD to reduce the calculation cost. To rectify negative effects such as noise enhancement due to the non-optimum tap length setting, the method applies the improved proportionate normalized least mean square (IPNLMS) with leveraged equalization coefficients update on the basis of the overall sparsity of coefficients. IPNLMS inherently updates equalization coefficients by proportionately promoting the previous coefficients in order that the coefficients of signal part are promoted while those of noise part are suppressed. To determine the IPNLMS parameters that govern the amount of the promotion, the presented method uses a simple sparsity metric that calculates the overall sparsity of coefficients. Then, the sparsity metric is mapped to IPNLMS parameters in a manner that the overall sparsity of coefficients is progressively facilitated as adaptation. Evaluations using experimental data of FMF transmission over 527 km end-to-end with 33.2 ns maximum DMD show the presented method effectively deals with various DMDs to suppress the noise and obtains 0.7 dB of Q-factor performance enhancement comparing to the conventional method.

Nonlinear Gaussian Noise Model for Multimode Fibers With Space-Division Multiplexing

Space-division multiplex has become a promising approach to overcome future capacity shortages in long-haul optical transmission systems. In this paper, we present an analytical approach to calculate the nonlinear interference between signals that travel in different fiber modes of a multimode fiber SDM system. We discuss several simplifications based on intra- and intermodal four-wave mixing theories, and compare the analytical model with simulations in an exemplary few-mode fiber transmission system with three and six spatial modes. To generate a deep understanding of the impact of intermodal nonlinear signal distortions, we focus our analysis on the relative strength of intra- and intermodal nonlinearities. A good agreement between the analytical model and numerical simulations is found, making the model an easy-to-use tool to estimate the overall nonlinear distortion in space-division multiplexed transmission systems with multimode fibers.

Transmission of 200 Tb/s (375 × 3 × 178.125 Gb/s) PDM-DFTS-OFDM-32QAM super channel over 1 km FMF

A few-mode fiber (FMF) is designed to support three spatial modes (LP01, LP11a, and LP11b) and fabricated through plasma chemical vapor deposition (PCVD)and rod-in-tube (RIT) method. Using PDM-DFTS-OFDM- 32QAM modulation, wavelength division multiplexing, mode multiplexing, and coherent detection, we successfully demonstrated 200 Tb/s (375 × 3 × 178.125 Gb/s) signal over 1 km FMF using C and L bands with 25 GHz channel spacing. After 1 km FMF transmission, all the tested bit error rates (BERs) are below 20% forward error correction (FEC) threshold (2.0 × 10–2). Within each sub-channel, we achieved a spectral efficiency of 21.375 bits/Hz in the C and L bands.

Near-ML detection for MDL-impaired few-mode fiber transmission.

Few-mode fiber transmission systems are typically impaired by mode-dependent loss (MDL). In an MDL-impaired link, maximum-likelihood (ML) detection yields a significant advantage in system performance compared to linear equalizers, such as zero-forcing and minimum-mean square error equalizers. However, the computational effort of the ML detection increases exponentially with the number of modes and the cardinality of the constellation. We present two methods that allow for near-ML performance without being afflicted with the enormous computational complexity of ML detection: improved reduced-search ML detection and sphere decoding. Both algorithms are tested regarding their performance and computational complexity in simulations of three and six spatial modes with QPSK and 16QAM constellations.

Optical chopper-based re-circulating loop for few-mode fiber transmission

An optical chopper-based re-circulating loop is presented for emulating long-haul transmission using few-mode fiber (FMF). A proper functioning of the loop is verified by transmission of three-mode-division multiplexed × 128 Gb/s DP-QPSK over more than 1000 km of FMF.