Long-haul Transmission Research Articles

Power-aided probabilistic shaping parameter optimization for long-haul optical fiber transmission

A power-aided probabilistic shaping (PS) parameter optimization scheme is proposed to localize the optimal transmitting setup and maximize the transmission capacity in long-haul optical fiber transmission. Using a neural network and genetic algorithm (NNGA), the proposed scheme can select the optimal transmission parameter set, including the launching optical power (LOP) and three PS characteristics. We built a testbed using MATLAB and VPI to validate the proposed scheme. The proposed scheme was demonstrated in a dual-polarization coherent transmission system. The results show that the proposed scheme improves the generalized mutual information (GMI) by 0.3035 bits/symbol/pol and normalized GMI by 0.1022 in a 1000 km G.654E fiber transmission at 420-Gbit/s, compared to a traditional MB-based PS technique. The signal-to-noise ratio (SNR) achieves a gain of 1.003 dB, which validates the outperformance of the proposed scheme.

Quantum-limited transmission distance of optically repeating systems using a hybrid EDFA/Raman-amplifier.

The transmission distance of optically repeating systems is ultimately bounded by quantum noises arising from attenuation and amplification. It is known that fiber Raman amplifiers (FRAs) can achieve longer quantum-limited distances than other amplification schemes owing to their distributed amplification characteristics. However, all-FRA systems are difficult to implement in practice. Therefore, a hybrid erbium-doped fiber amplifier (EDFA)/FRA system may be a reasonable solution for long-haul transmission. In this study, the quantum-limited transmission distance of hybrid EDFA/FRA systems is investigated. A formula for the quantum fluctuation variance during transmission was derived based on quantum mechanical treatment. Calculation results obtained with the derived formula quantitatively indicate that the quantum-limited transmission distance increases as the EDFA/FRA hybrid ratio varies from EDFAs alone to FRAs alone.

Ant-colony optimization-based multi-input multi-output (ACO-MIMO) equalization for low-complexity mode-division multiplexing (MDM) processing

In mode-division multiplexing (MDM) systems, the computational complexity of the multi-input multi-output (MIMO) equalization module is a critical obstacle to practical development. The step size μ and the number of taps K are key parameters in the equalization algorithm, influencing the performance of finite impulse response (FIR) equalizers, including convergence speed and output signal quality. To alleviate the computational burden of locating the optimal μ-K combination, we propose two ant colony optimization (ACO) -based MIMO equalization schemes: the fixed ACO-MIMO and the random ACO-MIMO, corresponding to two optimization strategies. These schemes expedite the initialization process of both parameters. Subsequently, we conduct experiments to evaluate their performance in a 3-mode recirculating-loop transmission system. Our findings demonstrate that, compared to conventional schemes, such as genetic algorithm (GA) and steepest descent algorithm (SDA), the proposed ACO-MIMO schemes significantly reduce the number of calls to the equalization algorithm for locating optimal μ-K combination by up to 42.74% and 80.63%, reducing the complexity of the whole MIMO equalization for MDM systems. And the resulting hit-rate Phit for the optimal μ-K combination reaches up to 99.34%. Moreover, the ACO-MIMO schemes exhibit stable performance across different data collected from various round-trips, confirming the robust operation for the long-haul MDM transmission. Finally, we investigate the performance disparity between the two proposed ACO-MIMO schemes through bit-error-rate (BER) distribution, concluding that under a large dataset with various BER distributions, the performance of both schemes is essentially equivalent.

Detailed characterization of a high-gain, low differential modal gain and low gain flatness integrated FM-EDFA for multi-wavelength amplification.

In this paper, an integrated few-mode erbium-doped fiber amplifier (FM-EDFA), with high gain, low differential modal gain (DMG), and low gain flatness for multi-wavelength amplification, is constructed using homemade weakly-coupled ring-core few-mode erbium-doped fiber (FM-EDF) and low insertion-loss passive components. The gain characteristics of the FM-EDFA for multi-wavelength amplification are analyzed in detail and compared with single-wavelength amplification. The experimental results demonstrate that, although multi-wavelength amplification in different modes has an impact on the gain characteristics, through simple LP01 core pumping only, all guided modes can achieve a gain higher than 22.4 dB when simultaneously amplifying 8 wavelength channels. The DMG is less than 1.179 dB, the minimum DMG across the C-band is as low as 0.168 dB, and the wavelength gain flatness is less than 3.232 dB. Furthermore, when the three-signal mode is multiplexed for amplification, the DMG can be stabilised at less than 2 dB. The constructed FM-EDFA can achieve gain equalization in multi-wavelength amplification across the C-band, which has essential practical applications in long-haul WDM-MDM transmission systems.

Hybrid optical-electronic compensation of fiber nonlinearity for long-haul coherent optical transmission

Abstract From the concepts of the dispersion-folded digital backward propagation (DBP) and optical phase conjugation (OPC), a hybrid optical-electronic nonlinearity-compensation scheme is proposed to enhance the system performance of the dispersion-managed transmission. The computational complexity of the proposed scheme, compared with that of the conventional DBP method, is reduced significantly while the performance penalty is negligible. The compensation efficiency of the proposed scheme has been validated in a 5 (and 9)-channel PM-16QAM system at 256 Gbit/s.

High-Capacity Coherent WDM Networks Empowered by Probabilistic Shaping and End-to-End Deep Learning

To optimize the functionality of coherent optical fiber communication (OFC) systems and enhance their capacity related to long-haul transmissions, wavelength-division multiplexing (WDM) and probabilistic constellation shaping (PCS) techniques have been used. This paper develops an end-to-end (E2E) deep learning (DL)-based PCS algorithm, i.e., autoencoder (AE) for a high-order modulation format WDM system that minimizes nonlinear effects while ensuring high capacity and considers system parameters, in particular those related to the OFC channel. Only the AE of the central channel is trained to meet the specified performance objective, as the system design employs identical AEs in each WDM channel. The simulation results show that the architecture should consist of two hidden layers, with thirty two nodes per hidden layer and a ”32×modulation order” batch size to obtain optimal system performance, when designing AE using a dense layer neural network. The behavior of the AE is examined to determine the optimum launch-power ranges that enhance the system's performance. The developed AE-based PCS-WDM provides a 0.4 shaping gain and outperforms conventional solutions.

Auxiliary management and control channel aided crosstalk online monitoring in multi-core fibers

As a stochastic perturbation, the inter-core crosstalk (IC-XT) severely distorts the signal in multi-core fibers (MCF), especially for long-haul transmission. How to quickly measure and monitor the IC-XT online for an MCF-based space division multiplexing (SDM) system is of special importance. In this paper, we introduce the technology of auxiliary management and control channel (AMCC) to online monitor the IC-XT of MCF, in which the unique advantage of low-frequency auxiliary management and control signal is fully utilized with the limited influence on high-speed data transmission. Specifically, two orthogonal sequences are chosen as monitoring signals for the signal-channel core and the crosstalk-channel core, respectively, followed by digital signal processing (DSP) for the received signal to evaluate the real-time crosstalk accurately. The experimental verifications of XT online monitoring confirm the effectiveness of our proposed method with very small monitoring error (mostly < 0.5 dB) for both heterodyne XT and homodyne XT in the C and L bands, showing its great potential for future SDM systems.

Broadband dispersion compensation and high birefringence photonic crystal fiber for CWDM/DWDM networks

In this study, we propose a new design based on photonic crystal fibers (PCFs) for broadband dispersion compensation in telecommunication networks. The proposed design has a hexagonal structure arrangement of air-holes rings of different diameters between the silica core and the cladding. The PCF properties like effective area, nonlinearity, dispersion slope, confinement loss, and birefringence are reported and discussed. For the best performance we present three designs A, B and C. Simulation results show that the three designs cover the six-telecommunication optical bands O-, E-, S-, C-, L- and U- bands (wavelengths ranging from 1260 to 1675 nm). Design A achieves a large negative dispersion value of about − 1716 ps/(nm.km) with relative dispersion slope equals to that of conventional single-mode optical fibers (SMFs) of about 0.0036 nm−1, which makes it very suitable for long-haul DWDM transmission systems. With a little modification in the core, designs B and C achieve much higher confinement ability and achieve a very large birefringence value for polarization mode dispersion and sensing applications. Design C is engineered to have exact opposite dispersion of SMF with zero dispersion at the wavelength 1310 nm, which makes it a promising design in CWDM transmission system. The numerical values have been investigated using the full vector finite element method.

Magnon-photon coupling in an opto-electro-magnonic oscillator

The opto-electronic oscillators (OEOs) hosting self-sustained oscillations by a time-delayed mechanism are of particular interest in long-haul signal transmission and processing. On the other hand, owing to their unique tunability and compatibility, magnons—as elementary excitations of spin waves—are advantageous carriers for coherent signal transduction across different platforms. In this work, we integrated an opto-electronic oscillator with a magnonic oscillator consisting of a microwave waveguide and a yttrium iron garnet sphere. We find that, in the presence of the magnetic sphere, the oscillator power spectrum exhibits sidebands flanking the fundamental OEO modes. The measured waveguide transmission reveals anti-crossing gaps, a hallmark of the coupling between the opto-electronic oscillator modes and the Walker modes of the sphere. Experimental results are well reproduced by a coupled-mode theory that accounts for nonlinear magnetostrictive interactions mediated by the magnetic sphere. Leveraging the advanced fiber-optic technologies in opto-electronics, this work lays out a new, hybrid platform for investigating long-distance coupling and nonlinearity in coherent magnonic phenomena.

Extending the reach of multi-core fiber via Voronoi constellations with concatenated multilevel coding.

This Letter proposes a novel, to the best of our knowledge, coded modulation scheme for randomly coupled multi-core fiber (RC-MCF) via multidimensional (MD) constellation with concatenated two-level multilevel coding (MLC). In the proposed system, the 16-dimensional (16D) Voronoi constellation (VC), naturally fitting with the 16 degrees of freedom of a four-core fiber (two quadratures, two polarizations, and four cores), is generated by a latticed-based shaping method to provide higher shaping gains. Moreover, combining it with the concatenated two-level MLC can further achieve better performance-complexity trade-off. It is demonstrated by simulation results of long-haul multi-channel RC-MCF transmission that the proposed coded modulation scheme for four-core fiber transmission offers 77% reduction in the number of decoding operations and up to 21% (585 km) reach increase over the conventional bit-interleaved coded modulation scheme for quadrature amplitude modulation.

Co-LSTM-based fiber link modeling with ASE noise tracking for long-haul coherent optical transmission.

In this Letter, a novel, to the best of our knowledge, channel modeling scheme based on cascading chromatic dispersion-nonlinearity feature decoupling modules is proposed with the center-oriented long short-term memory (Co-LSTM) network structure adopted for modeling nonlinearity of each optical fiber span. By tracking the amplified spontaneous emission (ASE) noise at the output of each fiber span, the Co-LSTM-based channel modeling scheme achieves high waveform accuracy for long-haul coherent optical transmission compared with the conventional split-step Fourier transform method (SSFM) while saving calculation time by almost one order of magnitude.

1.12Tbps-16QAM Uncompensated Long-Haul Transmission Employing a New Self-Oscillating Optical Frequency Comb Generator Both at Transmitter and Receiver

1.12Tbps-16QAM Uncompensated Long-Haul Transmission Employing a New Self-Oscillating Optical Frequency Comb Generator Both at Transmitter and Receiver

Low-complexity fiber nonlinear distortion mitigation for long-haul optical transmission based on transformer and triplets

Low-complexity fiber nonlinear distortion mitigation for long-haul optical transmission based on transformer and triplets

Reliable edge-to-core optical networks: An optimal algorithm for maximal path diversity

With the emergence of IoT applications, 5G, and edge computing, network resource allocation has shifted toward the edge, bringing services closer to the end users. These applications often require communication with the core network for purposes that include cloud storage, compute offloading, 5G-and-Beyond transport communication between centralized unit (CU), distributed unit (DU) and core network, centralized network monitoring and management, etc. As the number of these services increases, efficient and reliable connectivity between the edge and core networks is of the essence. Wavelength Division Multiplexing (WDM) is a well-suited technology for transferring large amounts of data by simultaneously transmitting several wavelength-multiplexed data streams over each single fiber optics link. WDM is the technology of choice in mid-haul and long-haul transmission networks, including edge-to-core networks, to offer increased transport capacity.Optical networks are prone to failures of components such as network fiber links, sites, and transmission ports. A single network element failure alone can cause significant traffic loss due to the disruption of many active data flows. Thus, fault-tolerant and reliable network designs remain a priority. The architecture called “dual-hub and dual-spoke” is often used in metro area networks (MANs). A dual-hub, or in general a multi-hub network, consists of a set of designated destination nodes (hubs) in which the data traffic from all other nodes (the peripherals) should be directed to the hubs. Multiple hubs offer redundant connectivity to and from the core or wide area network (WAN) through geographical diversity. The routing of the connections (also known as lightpaths) between the peripheral node and the hubs has to be carefully computed to maximize path diversity across the edge-to-core network. This means that whenever possible the established redundant lightpaths must not contain a common Shared Risk Link Group (SRLG).An algorithm is proposed to compute the most reliable set of SRLG disjoint shortest paths from any peripheral to all hubs. The proposed algorithm can also be used to evaluate the overall edge-to-core network reliability quantified through a newly introduced figure of merit.

Digital Signal Processing for MDG Estimation in Long-Haul SDM Transmission

Digital Signal Processing for MDG Estimation in Long-Haul SDM Transmission

Chirped fiber Bragg grating enabled mode-dependent gain equalization of 2-mode EDFA

We report a few-mode chirped fiber Bragg grating (FM-CFBG) enabled mode-dependent gain (MDG) equalizer for the all-fiber FM-EDFA. Relying on the mode-dependent reflectivity of the FM-CFBG, the MDG arising in an all-fiber 2-mode EDFA can be effectively mitigated. With the LP01 pump light injection, the average gain is up to 17 dB while the MDG can be equalized to be less than 1.26 dB within the reflection bandwidth of FM-CFBG. The MDG equalizer based on a FM-CFBG is equipped with several merits of compact size, low insertion loss, and wavelength scalability, which is meaningful for long-haul MDM transmission.

Dynamic Secure Key Distribution Based on Dispersion Equalization and Cellular Automata for Optical Transmission

This paper proposes a dynamic secure key distribution scheme based on dispersion equalization and cellular automata (CAs). The scheme effectively eliminates the key inconsistency problem caused by imperfect channel reciprocity, and dynamic key sequences can be conveniently generated with large key space in long-haul optical transmission. In the process of communication, the legitimate parties obtain the secure core parameter from the frequency domain equalizer algorithm, and a final key sequence is generated through CA iterations on the basis of the core parameter. The randomness and reciprocity characteristics of the channel ensure the security and uniqueness of the core parameter and final key sequence. With 10G Baud 16 quadrature amplitude modulation over 400 km standard single-mode fiber transmission, the proposed scheme is verified with a free key error rate and an unlimited key generation rate. The security robustness of this scheme was theoretically analyzed and verified by sweeping the eavesdropper’s tapping position and improving CA operation processing. The proposed key distribution scheme is compatible with the existing transmission system for different signal modulation formats.

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.

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.

Research on the Low-loss Low-crosstalk Bending-insensitive Seven-core Fiber

Multicore fiber is regarded as the most efficient way to realize SDM networks. Fabrication of low-loss and low-crosstalk multi-core fiber is the key to achieving high-capacity communication systems. This paper designed and fabricated low transmission loss seven-core single-mode fiber with a standard 125μm cladding diameter based on modified chemical vapor deposition. The preform of MCF is fabricated using drilling method. The seven-core fiber has achieved a low transmission loss of 0.19~0.28dB/km throughout the 1250-1600nm wavelength range. The transmission loss is below 0.19dB/km at 1550nm wavelength. The crosstalk is below -54dB/100km from 1200nm to 1600nm wavelength by utilizing a trench-assisted structure. At 1550nm wavelength, the bending loss (BL) is lower than 0.45dB/m with a bending radius of 5 mm, and the BL is 0.05dB/m with a bending radius of 15 mm. The fabricated multi-core fiber can be used for long-haul transmission and space division multiplexing communication systems.