Multicore Fiber Transmission Research Articles

Test Method of Splice Points for Constructing Uncoupled Multi-Core Fiber Transmission Line With Low Inter-Core Crosstalk

Test Method of Splice Points for Constructing Uncoupled Multi-Core Fiber Transmission Line With Low Inter-Core Crosstalk

Mitigating the Strong Inter-Core Crosstalk during MCF Transmissions by Neural-Network-Equalizer-Based MIMO-DSP and Approaches for Its Simplification

In this paper, we report our recent progress related to advanced digital signal processing (DSP) technologies to address the strong inter-core crosstalk (IC-XT) during multicore fiber (MCF) optical transmissions. MCF transmission technology has significant potential to break through the capacity crunch of single-mode fiber (SMF) transmissions. However, strong coupling among cores, namely, inter-core crosstalk (IC-XT), is unavoidable for high-density space-division multiplexing (SDM) transmissions using MCFs with the standard cladding size. To deal with this issue, we propose some novel DSP structures to eliminate IC-XT with considerable simplicity, based on the neural network equalizer (NNE)-based multiple-input and multiple-output digital signal processing (MIMO-DSP). The traditional NNE-based MIMO-DSP method has the ability to process the coupled SDM tributaries transmitted over MCFs; however, it exhibits complexity limitations for practical implementations. The implementation complexity of the NNE-based method is mainly attributed to the time-consumption of the training process and the large weight (neurons) numbers of the equalizers. Thus, we propose two main approaches to simplify NNE-based MIMO-DSP for the practical implementation of MCF transmissions: (1) To reduce the time-consumption of the training process in NNE-based MIMO-DSP, the idea of transfer learning (TL) is employed for initializing the weights, resulting in the faster convergence of the equalizers. (2) IC-XT cancellation is performed along with MIMO-DSP; thus, the dimensionality of MIMO-DSP could be reduced. To validate the performance improvement of the proposed machine learning DSP methods, both simulations and experiments related to transmissions and reception over MCFs were conducted. The results indicate that the proposed novel DSP structures possess the advantages of reduced complexity and improved robustness to IC-XT, which is beneficial for the next-generation high-density SDM transmissions.

Loss Imbalance Effect on Inter-Core Crosstalk in Bidirectional Uncoupled Multi-Core Fiber Transmission

The effect of core loss imbalance on inter-core crosstalk (XT) is studied theoretically and numerically in long-haul bidirectional uncoupled multi-core fiber transmission. The coupled power equations are solved for two cores with imbalance of loss coefficients, and the solution is used to obtain expressions for direct, indirect, backscattered and bidirectional XT powers. These expressions generalize expressions reported by other authors to arbitrary loss coefficients. It is shown that, in the range of the loss coefficient imbalance from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-0.04$</tex-math></inline-formula> dB/km to 0.04 dB/km, the XT powers in decibel show approximately a linear dependence on the loss coefficient imbalance. In that range, differences of the indirect XT power of nearly 1.2 dB relative to the indirect XT power with equal loss coefficients can occur for the larger loss coefficient imbalance; for the direct and backscattered XT powers, the differences are even larger, reaching around 2 dB. These values of 1.2 and 2 dB of difference translate directly to the bidirectional XT power of long-haul 2-core fiber links, in case of dominance of indirect and backscattered XT, respectively.

Cable Capacity and Cost/Bit Modeling of Submarine MCF Systems With MC-EDFA Alternatives

We model and examine various amplification options for trans-oceanic repeatered submarine systems using multicore fiber (MCF) transmission in the context of relative cable capacity and relative system cost/bit. First, we compare the optical performance of various configurations of parallel standard single-core erbium-doped fiber amplifiers (SC-EDFAs) and multicore erbium-doped fiber amplifiers (MC-EDFAs), including both the C/L-bands and core/cladding pump designs. Key amplifier parameters governing expected capacity and cost/bit performance include noise figure, amplifier bandwidth, relative electrical-to-optical conversion efficiency, and relative amplifier cost. We make our best estimates of these parameters through modeling, analysis, and component cost estimates and assess the sensitivity of the results to the nominal values. We find that MC-EDFAs using either core- or cladding-pumping might offer lower cost/bit than parallel SC-EDFAs but cladding-pumping may reduce cable capacities due to the higher noise figure and potentially smaller amplifier bandwidth.

Digital radio-over-fiber system based on differential pulse code modulation and space division multiplexing.

In this paper, we present and experimentally demonstrate a digital-radio-over-fiber (D-RoF) architecture based on differential pulse code modulation (DPCM) and space division multiplexing (SDM). At low quantization resolution, DPCM can effectively reduce quantization noise and obtain significant signal-to-quantization noise ratio (SQNR) gain. We experimentally study the 7-core and 8-core multicore fiber transmission of 64-ary quadrature amplitude modulation (64QAM) orthogonal frequency division multiplexing (OFDM) signals with a bandwidth of 100 MHz in a fiber-wireless hybrid transmission link. Compared to PCM-based D-RoF, the error vector magnitude (EVM) performance in the DPCM-based D-RoF is effectively improved when the quantization bits (QBs) are 3-5 bits. In particular, when the QB is 3 bits, the EVM of the DPCM-based D-RoF is 6.5% and 7% lower than that of the PCM-based system in 7-core- and 8-core-multicore fiber-wireless hybrid transmission links, respectively.

Crosstalk Noise in Bi-Directional Transmission on Multi-Core Fiber Link With Distributed Raman Amplifier

The crosstalk (XT) noise-to-signal ratio characteristics in a distributed Raman amplifier (DRA) used for bi-directional multi-core fiber (MCF) transmission are investigated. We propose a model for calculating XT noise by considering the noise contribution due to Rayleigh scattering. We verify the validity of our model both numerically and experimentally. Our model reveals that the XT noise-to-signal ratio ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">XT</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ) varies with the on-off gain. This feature is unique for bi-directional transmission, and it provides an on-off gain at which the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">XT</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> can be minimized, although the corresponding net gain is less than 0 dB. We also show that the minimum <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">XT</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> is relatively sensitive to the attenuation coefficient at both signal and pump wavelengths. Our model is beneficial for discussing the signal-to-noise ratio property in DRA-based MCF bi-directional transmission, and further study for hybrid DRA and discrete amplifier systems is expected.

Mitigating the Inter-Core Crosstalk of Multicore Fiber Transmission by Orthogonal Filtering

We for the first time proposed to utilize the orthogonal filtering technique to suppress the inter-core crosstalk (IC-XT) in multicore fiber (MCF) transmission. It could simplify the implementation of weakly-coupled MCF (WC-MCF) signal reception without multiple-input and multiple-output (MIMO) digital signal processing, by extracting the individual lanes separately. Effective IC-XT mitigation using a pair of orthogonal filters are demonstrated for MIMO-free 2-core MCF transmitted signal receptions. 600-Gbps 64-QAM (quadrature amplitude modulation) signal transmission over 100-km MCF is emulated by constructing a concrete transmission model based on coupled nonlinear Schrodinger equations. Maximum Q factor improvement of 6 dB can be achieved using orthogonal matching filters to suppress IC-XT at −10dB. The effect of spatial mode dispersion on the filter orthogonality performance is also analyzed.

High-security SCMA-OFDM multi-core fiber transmission system based on a regular hexagon chaotic codebook.

Sparse code multiple access (SCMA), a new code-domain non-orthogonal technology in the fifth-generation mobile communication (5G), can be modulated by orthogonal frequency division multiplexing (OFDM) to improve the link quality of a single user. In this paper, a high-security SCMA-OFDM multi-core fiber transmission system based on a regular hexagonal chaotic codebook is proposed for next-generation passive optical network (PON). The whole encryption process consists of a regular hexagon chaotic codebook design and frequency domain block scrambling. In designing the regular hexagon chaotic codebook, the optimization of constellation points on orthogonal resources are considered as the starting point. Firstly, the chaos factor generated by the four-dimensional Rossler chaos model is deployed to disturb the mother constellation, and then the corresponding chaotic book is formed by rotating the mother constellation and multiplying the sparse matrix. The designed codebook logically avoids the degradation of transmission performance caused by the rough scrambling of codebook constellation, to find a balance between codebook disturbance and bit error rate (BER). The security and reliability of the transmission system have been verified by performing 42 Gb/s encrypted SCMA-OFDM data transmission experiments in a 2km multi-core fiber. The key space of the encryption scheme can reach 10178, which effectively ensures the security of the transmission system. Furthermore, the performance of the transmission system with a regular hexagon chaotic codebook is improved by 2.5 dB compared with the traditional codebook when the BER is 1 × 10-3. Moreover, the SCMA-OFDM-based transmission architecture and the detection effects of different multi-user detection algorithms in the SCMA-OFDM multi-core fiber transmission system are also studied.

Optical Time Domain Reflectometry for Simultaneously Characterizing Forward and Backward Crosstalk Along Multi-Core Fibers

Uncoupled multi-core fibers (MCFs) are promising transmission media for realizing long-haul and large-capacity transmission using space division multiplexing. When the assignment of the signal transmission directions in adjacent cores is unidirectional or bidirectional, the forward and backward crosstalk respectively limits the capacity and distance of the transmission system using uncoupled MCFs. Understanding both the forward and backward crosstalk is crucial for determining an optimal transmission scheme according to traffic demands. In this paper, we propose a novel method based on optical time domain reflectometry for simultaneously characterizing the forward and backward crosstalk along uncoupled MCFs. We measured the forward and backward crosstalk along a four-core fiber by the proposed method. The forward and backward crosstalk obtained with the proposed method agreed well with those measured with unidirectional and bidirectional transmission methods. Since the proposed method enables us to simultaneously evaluate the forward and backward crosstalk along uncoupled MCFs from one end, it would be useful for testing those characteristics during the construction of MCF transmission lines.

Influence of the Width of Launch Beam Distribution on the Transmission Performance of Seven-Core Polymer-Clad Silica Fibers

We propose a space division multiplexing (SDM) in a newly constructed multicore polymer-clad silica fiber (PCSF) with seven cores arrayed in a hexagonal array, each carrying a centrally launched beam. This enables a higher SDM capacity at longer fiber lengths in the proposed seven-core PCSF if compared with previously proposed angular division multiplexing (ADM) in single-core (SC) PCSF. As a result, the SDM is not limited to short fiber lengths in the proposed seven-core PCSF, as it is in the case of the ADM channels due to mode coupling in the SC PCSF. In addition, the time-independent power flow equation (TI PFE) is used to analyze the effect of the width of the launch beam distribution on the equilibrium mode distribution (EMD) and steady state distribution (SSD) in each of the seven cores of the investigated PCSF. The width of the launch beam distribution has a considerable impact on the fiber length at which the EMD and SSD are attained, according to our numerical results. Thus, by decreasing the full width at half maximum (FWHM) of the launch beam distribution from 20 to 2°, the length at which EMD is established increases from Lc = 1020 to 1250 m, and the length at which SSD is attained increases from zs = 2650 to 3250 m. A narrow launch beam distribution leads to higher bandwidth at small and intermediate fiber lengths. On the other hand, at shorter fiber lengths, a wider launch beam distribution induces a bandwidth change from 1/z proportional to 1/z1/2 proportional curve, e.g., a slower bandwidth reduction. When building a multicore optical fiber transmission system for SDM, such characterization of multicore PCSFs under various launch conditions should be taken into account.

Demonstration of the high-security 3D-CAP scheme in multi-core fiber transmission

In this paper, the high-security three-dimensional (3D) carrier-less amplitude-phase (CAP) modulation based on 3D space constellation masking in multi-core fiber (MCF) transmission is proposed. Logistic Map model and Four-wing hyper-chaotic systems are employed to implement bit-level encryption, displacement masking, and space masking. This way, we achieved a key space of 1 × 1083 size, which resists violent attacks, and improves the physical layer security of the system. The superposition combined two chaotic models reduces the difficulty of implementation and enables more security for constellation encryption. A 280 Gb/s encrypted 3D-CAP32 signal transmission over 2.5 km MCF is experimentally demonstrated. The experimental results show that the proposed scheme can effectively resist illegal attack from an un-authorized optical network unit (ONU), and compared with an unencrypted 3D-CAP32 signal, the encrypted 3D-CAP32 signal has a receiver sensitivity penalty of 0.38 dB in a back-to-back configuration at a BER of 1 × 10−3. It is a strong candidate for the future highly-secured optical access network.

Densely packed 1.1 μm band vertical cavity surface emitting laser array for co-packaged optics

We demonstrated the 1.1 μm band 16-channel vertical cavity surface emitting laser (VCSEL) array for multi-core fiber (MCF) transmission towards co-packaged optics. Single-mode 16-ch top and bottom emitting VCSEL arrays were fabricated by either 3 inch or 6 inch wafer foundry process. The thermal crosstalk is estimated by wavelength shift. When the distance between adjacent VCSELs is 40 μm, the total thermal crosstalk is as low as 7 K. Bottom emitting metal-aperture VCSEL array were also fabricated via a full 3 inch wafer process. It shows low resistance and good single mode characteristic with an oxidation aperture of 7 μm thanks to an intra-cavity metal aperture. A large mode field diameter of 6 μm enables low coupling loss between bottom VCSEL array and MCF through a GaAs substrate for a flip-chip bonding process.

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.

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.

Long-Haul Coupled 4-Core Fiber Transmission Over 7,200 Km With Real-Time MIMO DSP

In this work, we present an experimental demonstration of real-time transoceanic space-division multiplexed transmission with coupled-core multicore fibers. To compensate for modal coupling in the coupled-core multicore fibers, we implement real-time multiple-input multiple-output (MIMO) digital signal processing based on field programmable gate array circuits. Using optical receivers with a real-time MIMO, we demonstrate 16-channel wavelength-division multiplexed coupled four-core fiber transmission over 7200 km. The results show the feasibility of real-time coupled-core multicore fiber transmission.

Trans-Pacific class transmission over a standard cladding ultralow-loss 4-core fiber

The total capacity of optical submarine cable systems as a global communication infrastructure must be continuously enlarged. Multi-core fibers (MCFs) have been studied as methods to maximize the total cable capacity under electrical power and cable space limitations. In particular, standard cladding MCFs, which are expected to have high productivity and mechanical reliability, are attractive for early deployment in submarine cable systems. In this paper, we demonstrate high-capacity trans-Pacific class transmission using standard cladding uncoupled 4-core fibers, achieving a transmission capacity of 55.94 Tbit/s over 12,040 km. In addition, based on the results of this and our previous coupled MCF transmission experiments, we summarize the characteristics of coupled and uncoupled MCFs applied to optical submarine cable systems.

Experimental demonstration of 448 Gb/s PS-PDM-PAM8 coherent transmission over multi-core fiber

The combination of pulse amplitude modulation (PAM) and coherent detection is a promising solution for short reach transmission systems which require the transmission rate of 100 Gb/s and above. In this paper, the transmission of probabilistic shaped polarization-division-multiplexed eight-level pulse amplitude modulated (PS-PDM-PAM-8) signal over multi-core fiber (MCF) using coherent detection is investigated. A modified signal-phase-based LMS (SP-LMS) algorithm is proposed to equalize both intensity-modulated and filed modulated PS-PAM signals. By employing PS and MCF transmission, the total rate of 448 Gb/s is achieved and compared with traditional PDM-PAM4 signal, the PS-PDM-PAM8 can achieve a 22.4 Gb/s gain in net bit rate.

Demonstration of the High-Security 3d-Cap Scheme in Multi-Core Fiber Transmission

Demonstration of the High-Security 3d-Cap Scheme in Multi-Core Fiber Transmission

Remote Photonic THZ Generation Using an Optical Frequency Comb and Multicore Fiber

This paper proposes and demonstrates a photonic THz generation technique based on an optical frequency comb and multicore fiber (MCF) transmission, with the advantage of remote generation with great reconfigurability and reduced digital signal processing (DSP). The feasibility of the proposed technique is evaluated experimentally comparing the performance when transmitting a data wavelength and a local oscillator for optical heterodyning over a single core or over different cores in a 1-km MCF link. The proposed remote photonic THz generation technique is demonstrated employing a 16QAM 12.5 GBd signal. A short wireless transmission at 183 GHz center frequency with 0.25 m antenna-separation is achieved after 1 km MCF, meeting the soft-decision decision forward error correction (SD-FEC) bit error rate (BER) recommendation of 210-2. The analysis includes the received photocurrent range for which the BER meets the standard hard-decision forward error correction (HD-FEC) recommendation of 3.810-3 BER. The BER performance is analyzed considering different DSP configurations, with and without frequency offset estimation (FOE) and decision-driven least mean squares (DD LMS) equalization. The performance of the remote photonic THz generation technique is evaluated comparatively against traditional free-running laser transmission over MCF, confirming the advantage of using comb generation to reduce the frequency offset fluctuation and simplify the DSP.

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.