Long-haul Transmission Research Articles

Nonlinear Mitigation of a 400G Frequency-Hybrid Superchannel for the 62.5-GHz Slot

We propose a 400G frequency-hybrid superchannel solution based on three carriers, two edge PM-16QAM, and a central PM-64QAM carrier, compatible with the 62.5-GHz grid slot (spectral efficiency of 6.4 b/s/Hz). The proposed superchannel is experimentally assessed in long-haul transmission by copropagation with other eight similar superchannels. The optimum power ratio between superchannel carriers is analytically determined in linear and nonlinear operation regimes using the enhanced Gaussian noise model and validated by experimental and simulation results. The 400G superchannel performance is evaluated in terms of maximum reach determining the optimum launch power and considering three distinct forward-error correction (FEC) paradigms: superchannel FEC (SC-FEC) where a single FEC is applied to the entire superchannel, independent carrier FEC (IC-FEC) where an independent FEC with fixed overhead is applied to each superchannel carrier, and independent carrier flexible FEC (Flex-FEC) where optimized FEC overheads are applied independently to each superchannel carrier with the constraint of a given total overhead. When compared to the IC-FEC approach, the SC-FEC or Flex-FEC approaches enable to extend the maximum transmission distance by more than 60%, while reducing the optimum power ratio by $\sim$ 6 dB, at the cost of 2 dB higher launched power. The system performance is also analyzed for the case of nonlinear compensation via digital backpropagation (DBP) techniques, assessing the improvement in reach and evaluating their impact on the optimum power ratio and launch power. For the proposed frequency-hybrid superchannel, we demonstrate that the application of DBP can be restricted to the carrier with higher QAM cardinality, thereby significantly reducing the overall computational effort, with a maximum reach reduction of only $\sim$ 2% over the application of DBP to all three carriers individually.

Photonics-Aided 32-Gb/s Wireless Signal Transmission Over 1 km at K-Band

We experimentally demonstrate a K-band (18–27 GHz) fiber-wireless-integration transmission system, which, compared with that at high-frequency W-band (75–110 GHz), can provide large-capacity long-haul wireless transmission with relatively low cost and simple architecture. In our demonstrated system, the generated up to 4-Gbaud (32-Gb/s) polarization-division-multiplexing 16-ary quadrature-amplitude-modulation (PDM-16QAM) radio frequency (RF) signal at 23 GHz can be delivered over 100-km single-mode fiber-28 and >1-km wireless distance with a bit error ratio less than the hard-decision forward-error-correction threshold of $3.8\times 10^{-3}$ . To the best of our knowledge, this is the first time to realize over 1-km wireless delivery for up to 32-Gb/s RF signal at K-band.

Physical-layer security analysis of PSK quantum-noise randomized cipher in optically amplified links

The quantitative security of quantum-noise randomized cipher (QNRC) in optically amplified links is analyzed from the perspective of physical-layer advantage. Establishing the wire-tap channel models for both key and data, we derive the general expressions of secrecy capacities for the key against ciphertext-only attack and known-plaintext attack, and that for the data, which serve as the basic performance metrics. Further, the maximal achievable secrecy rate of the system is proposed, under which secrecy of both the key and data is guaranteed. Based on the same framework, the secrecy capacities of various cases can be assessed and compared. The results indicate perfect secrecy is potentially achievable for data transmission, and an elementary principle of setting proper number of photons and bases is given to ensure the maximal data secrecy capacity. But the key security is asymptotically perfect, which tends to be the main constraint of systemic maximal secrecy rate. Moreover, by adopting cascaded optical amplification, QNRC can realize long-haul transmission with secure rate up to Gb/s, which is orders of magnitude higher than the perfect secrecy rates of other encryption systems.

Field Trial of 400-Gbps Transmission Using Advanced Digital Coherent Technologies

We conduct field trials to confirm that advanced digital coherent technologies can be employed to double the performance of 400-Gbps optical transmission compared to conventional technologies. Additionally, we show that 400-Gbps channel transmission does not affect existing 100-Gbps channels. We employ these technologies to realize a world-beating result in terms of long-haul transmission: 400-Gbps over 1900 km.

Golay sequences coded coherent optical OFDM for long-haul transmission

We propose to use binary Golay sequences in coherent optical orthogonal frequency division multiplexing (CO-OFDM) to improve the long-haul transmission performance. The Golay sequences are generated by binary Reed-Muller codes, which have low peak-to-average power ratio and certain error correction capability. A low-complexity decoding algorithm for the Golay sequences is then proposed to recover the signal. Under same spectral efficiency, the QPSK modulated OFDM with binary Golay sequences coding with and without discrete Fourier transform (DFT) spreading (DFTS-QPSK-GOFDM and QPSK-GOFDM) are compared with the normal BPSK modulated OFDM with and without DFT spreading (DFTS-BPSK-OFDM and BPSK-OFDM) after long-haul transmission. At a 7% forward error correction code threshold (Q2 factor of 8.5dB), it is shown that DFTS-QPSK-GOFDM outperforms DFTS-BPSK-OFDM by extending the transmission distance by 29% and 18%, in non-dispersion managed and dispersion managed links, respectively.

Space division multiplexing optical communication using few-mode fibers

Abstract To realize ultra-high capacity long-haul transmission, space-division multiplexing (SDM) has emerged as a promising solution, in which each data channel is modulated into an individual spatial/polarization modes in few-mode fibers (FMF) to increase the overall number of parallel channels. In this paper, we review the latest advances in SDM technology on the FMF, component, digital signal processing (DSP), as well as transmission demonstrations. First, we introduce the FMF characteristics, fabrication and manufacturing issues including modal dispersion, mode coupling, and nonlinearities. We next discuss in detail several key SDM components such as spatial multiplexers/demultiplexers (MUX/DeMUX), optical amplifiers, mode converters and SDM reconfigurable optical add-drop multiplexer (ROADM). Accordingly, we explore the DSP algorithms for SDM systems, covering least mean squares (LMS), recursive least squares (RLS), hardware complexity analysis, and mode dependent effects. Besides, a number of recent experimental validations are evaluated enabling higher transmission capacity for short, medium and long distances.

Analysis of chromatic dispersion compensation and carrier phase recovery in long-haul optical transmission system influenced by equalization enhanced phase noise

The performance of long-haul coherent optical fiber transmission system is significantly affected by the equalization enhanced phase noise (EEPN), due to the interaction between the electronic dispersion compensation (EDC) and the laser phase noise. In this paper, we present a comprehensive study on different chromatic dispersion (CD) compensation and carrier phase recovery (CPR) approaches, in the n-level phase shift keying (n-PSK) and the n-level quadrature amplitude modulation (n-QAM) coherent optical transmission systems, considering the impacts of EEPN. Four CD compensation methods are considered: the time-domain equalization (TDE), the frequency-domain equalization (FDE), the least mean square (LMS) adaptive equalization are applied for EDC, and the dispersion compensating fiber (DCF) is employed for optical dispersion compensation (ODC). Meanwhile, three carrier phase recovery methods are also involved: a one-tap normalized least mean square (NLMS) algorithm, a block-wise average (BWA) algorithm, and a Viterbi-Viterbi (VV) algorithm. Numerical simulations have been carried out in a 28-Gbaud dual-polarization quadrature phase shift keying (DP-QPSK) coherent transmission system, and the results indicate that the origin of EEPN depends on the choice of chromatic dispersion compensation methods, and the effects of EEPN also behave moderately different in accordance to different carrier phase recovery scenarios.

Effects of optical repeater lines for reshaping on communication systems

Analyses of ideal models for reamplifying and reshaping repeaters are presented to evaluate performance of such lumped repeater chains for long-haul transmission, compared with EDFA chains. A non-ideal case in real systems, which is based on interferometric configuration of semiconductor optical amplifiers (SOA), is addressed and measured and its noise characteristics are analyzed. System performance evaluations in a communication line are performed. © 2017 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:843–848, 2017

Power Consumption Analysis of Hybrid EDFA/Raman Amplifiers in Long-Haul Transmission Systems

We analyze the power consumption of optical amplifiers and the tradeoff between power consumption and system performance. The power consumption model includes erbium-doped fiber amplifiers (EDFA), backwards pumped Raman amplification, and monitoring and management electronics. Performance is studied using the Gaussian-noise model for nonlinear interference. We find that the power consumption of the monitoring and management electronics has a large impact on system configuration that gives the lowest overall power consumption, where a low value favors shorter spans and EDFA-only amplification, while a high value favors longer spans with Raman amplification. Long total system lengths and high requirements on the optical signal-to-noise ratio also favors Raman amplification. Furthermore, we compare the amplifier energy consumption per bit for polarization-multiplexed quadrature phase-shift-keying and 16-quadrature amplitude modulation (16QAM). Our results show that 16QAM has a lower energy consumption per bit due to its higher spectral efficiency. We also find that it may be more energy efficient to increase the signal quality by shortening the spans or using Raman amplification than using powerful forward error correction with high power consumption.

Laser Phase Noise Effects and Joint Carrier Phase Recovery in Coherent Optical Transmissions With Digital Subcarrier Multiplexing

In this paper, we numerically and experimentally investigate the impairments related to laser phase noise in coherent digital subcarrier multiplexing (SCM) systems. Based on the evaluation in both back-to-back and transmission scenarios and the performance comparison between single carrier (SC) and SCM systems, we show that even though SCM systems suffer from degraded performance of carrier phase recovery (CPR) algorithms, they can potentially achieve an improved laser linewidth tolerance compared with SC systems after long-haul transmission attributed to the mitigation of equalization-enhanced phase noise. In addition, the effect of dispersion-enhanced phase noise is discussed in digital SCM systems, and a joint CPR (J-CPR) algorithm is proposed for cycle slip detection and correction. The effectiveness of the proposed J-CPR algorithm is then demonstrated in a 16-quadrature amplitude modulation (QAM) digital SCM system by both simulations and experiments.

40 Gb/s DWDM Structure with Optical Phase Configuration for Long-Haul Transmission System

AbstractWe propose the experimental transport of 48 channels with 40 Gbit/s dense wavelength-division multiplexing (DWDM) system that uses single-mode fiber (SMF) in combination with dispersion compensation fiber (DCF) which is a dispersion compensation device, in C and L band wavelength range to solve the dispersion program. The DWDM system scheme employing single Mach–Zehnder modulation (MZM) return-to-zero differential phase-shift keying (RZ-DPSK) modulation format with hybrid Raman/EDFA (Erbium-doped fiber amplifier) configuration to improve transmission signal, and employing an optical phase conjugation (OPC) configuration in the middle line. That can compensate for dispersion impairment and improve nonlinear effects to investigate transmission distance performances.

Inter-Subcarrier Nonlinear Interference Canceler for Long-Haul Nyquist-WDM Transmission

For long-haul Nyquist-wavelength division multiplexing superchannel transmission, an inter-subcarrier nonlinear and linear interference canceler (INIC) is proposed. This approach consists in detecting the adjacent subcarriers, regenerating them thanks to the Volterra series model of optical fiber, and removing them from the subcarrier of interest. Different ways to implement the INIC are described and compared with the well-known techniques, such as digital backpropagation (DBP) and Volterra-based nonlinear equalizer (VNLE) implemented in a subcarrierwise manner. Significant performance gain (on either the $Q$ factor or transmission distance) is observed. In the context of 400 Gbps scheme, the transmission distance gain is up to 500 km compared with the DBP and VNLE.

The Compensation for Distorted WDM Signals in the Optical Long-Haul Transmission Link with the Artificially Distributed Lengths of Single Mode Fiber and Residual Dispersion Per Span

The possibility of the flexible configuration of dispersion-managed optical links combined with optical phase conjugation by adopting an artificial distribution of the lengths of single mode fiber (SMF) and residual dispersion per span (RDPS) of the fiber spans has been numerically investigated. We confirmed that ‘DA-DA’ pattern – i.e., the lengths of SMF and RDPS around the front and the rear of OPC are smaller, but the lengths of SMF and RDPS closer to the transmitter and the receiver are larger – is the best artificial pattern among the considered 16 artificial distribution patterns. Furthermore, the system performance in the optical link configured with this pattern is more improved than with the conventional uniform distribution of the SMF lengths and RDPS.

Design and comparative performance analysis of different chirping profiles of tanh apodized fiber Bragg grating and comparison with the dispersion compensation fiber for long-haul transmission system

Various dispersion compensation units are presented and evaluated in this paper. These dispersion compensation units include dispersion compensation fiber (DCF), DCF merged with fiber Bragg grating (FBG) (joint technique), and linear, square root, and cube root chirped tanh apodized FBG. For the performance evaluation 10 Gb/s NRZ transmission system over 100-km-long single-mode fiber is used. The three chirped FBGs are optimized individually to yield pulse width reduction percentage (PWRP) of 86.66, 79.96, 62.42% for linear, square root, and cube root, respectively. The DCF and Joint technique both provide a remarkable PWRP of 94.45 and 96.96%, respectively. The performance of optimized linear chirped tanh apodized FBG and DCF is compared for long-haul transmission system on the basis of quality factor of received signal. For both the systems maximum transmission distance is calculated such that quality factor is ≥ 6 at the receiver and result shows that performance of FBG is comparable to that of DCF with advantages of very low cost, small size and reduced nonlinear effects.

High-capacity dense space division multiplexing transmission

Abstract In this paper, we review space division multiplexing (SDM) transmission experimental demonstrations and associated technologies. In past years, SDM achieved high capacity transmission through increased spatial multiplicity, and long-haul transmission through improved transmission performance. More recently, dense SDM (DSDM) with a large spatial multiplicity exceeding 30 was demonstrated with multicore technology. Various types of multicore and multimode SDM fibers, amplification, and spatial multi/demultiplexers have helped achieve high-capacity DSDM transmission.

Few-mode multicore fibers for long-haul transmission line

Abstract Few-mode multicore fibers (FM-MCFs) that enable dense space-division multiplexing (DSDM) have the potential to drastically improve the fiber capacity. In designing the FM-MCFs, several issues that originate from multicore fibers and few-mode fibers must be considered. In this paper, these design issues such as inter-core crosstalk (IC-XT) and dispersion mode delay (DMD) are discussed. A three-mode 12-core fiber with low-DMD low-IC-XT achieves long-haul DSDM transmission over 500 km. The design concept, fiber design, and characteristics of the fabricated three-mode 12-core fiber are also described.

FPGA-Based Rate-Compatible LDPC Codes for the Next Generation of Optical Transmission Systems

In this paper, we propose a rate-compatible forward error-correcting (FEC) scheme based on low-density-parity check (LDPC) codes together with its software reconfigurable unified field-programmable gate array (FPGA) architecture. By FPGA emulation, we demonstrate that the proposed class of rate-compatible LDPC codes based on puncturing and generalized LDPC coding with an overhead from 25% to 46% provides a coding gain ranging from 12.67 to 13.8 dB at a post-FEC bit-error rate (BER) of 10−15. As a result, the proposed rate-compatible codes represent one of the strong FEC candidates of soft-decision FEC for both short-haul and long-haul optical transmission systems.

Optical amplification and reshaping based on the Peregrine rogue wave.

Based on the characteristics of the Peregrine rogue wave, the amplification and the reshaping of solitons are investigated. The numerical results show that the amplification and the reshaping of solitons can be realized by injecting a continuous wave (CW) and filtering the CW at suitable positions. The combination of a continuous-wave pump and a spectral filter placed suitably in fiber plays the role of the amplifier, which can be used to long-haul the transmission of solitons. As an example, a long-haul transmission with four amplification periods is demonstrated.

Information Rates of Next-Generation Long-Haul Optical Fiber Systems Using Coded Modulation

A comprehensive study of the coded performance of long-haul spectrally-efficient WDM optical fiber transmission systems with different coded modulation decoding structures is presented. Achievable information rates are derived for three different square QAM formats and the optimal format is identified as a function of distance and specific decoder implementation. The four cases analyzed combine hard-decision (HD) or soft-decision (SD) decoding together with either a bit-wise or a symbol-wise demapper, the last two suitable for binary and nonbinary codes, respectively. The information rates achievable for each scheme are calculated based on the mismatched decoder principle. These quantities represent true indicators of the coded performance of the system for specific decoder implementations and when the modulation format and its input distribution are fixed. In combination with the structure of the decoder, two different receiver-side equalization strategies are also analyzed: electronic dispersion compensation and digital backpropagation. We show that, somewhat unexpectedly, schemes based on nonbinary HD codes can achieve information rates comparable to SD decoders and that, when SD is used, switching from a symbol-wise to a bit-wise decoder results in a negligible penalty. Conversely, from an information-theoretic standpoint, HD binary decoders are shown to be unsuitable for spectrally-efficient, long-haul systems.

Simultaneous nonlinearity mitigation in 92 × 180-Gbit/s PDM-16QAM transmission over 3840 km using PPLN-based guard-band-less optical phase conjugation.

We experimentally demonstrated the simultaneous nonlinearity mitigation of PDM-16QAM WDM signals using complementary-spectrally-inverted optical phase conjugation (CSI-OPC). We achieved reserved-band-less, guard-band-less, and polarization independent OPC based on periodically poled LiNbO3 waveguides. By employing the CSI-OPC, 2.325-THz-band (93 × 25 GHz) complementary spectral inversion was achieved while retaining the original WDM bandwidth. A Q2-factor improvement of over 0.4 dB and a 5120 km transmission with a Q2-factor above the FEC limit were confirmed using a 10-channel WDM transmission at the signal band center and signal band edge. We then demonstrated the mitigation of the nonlinear impairments in a 3840 km long-haul WDM signal transmission for all 92-channel 180-Gbit/s PDM-16QAM quasi-Nyquist-WDM signals.