Long-haul Optical Communication Research Articles

A 3.0 Gb/s Throughput Hardware-Efficient Decoder for Cyclically-Coupled QC-LDPC Codes

In this paper, we propose a new class of quasi-cyclic low-density parity-check (QC-LDPC) codes, namely cyclically-coupled QC-LDPC (CC-QC-LDPC) codes, and their RAM-based decoder architecture. CC-QC-LDPC codes have a simple structure and are constructed by cyclically-coupling a number of QC-LDPC subcodes. They can achieve throughput and error performance as excellent as LDPC convolutional codes, but with much lower hardware requirements. They are therefore promising candidates for future generations of communication systems such as long-haul optical communication systems. In particular, a rate-5/6 CC-QC-LDPC decoder has been implemented onto a field-programmable gate array (FPGA) and it achieves a throughput of 3.0 Gb/s at 100 MHz clock rate with 10-iteration decoding. No error floor is observed up to an $E_{b}/N_{0}$ of 3.50 dB, where all $1.14\times 10^{16}$ transmitted bits have been decoded correctly.

Modified Nonlinear Decision Feedback Equalizer for Long-Haul Fiber-Optic Communications

In a long-haul optical fiber communication system, fiber attenuation, dispersion, and nonlinearity combined with nondeterministic noise from optical amplifiers used for periodic regeneration cause adverse effects on system performance. Several optical and electrical signal processing techniques have been proposed, and implemented to extract the transmitted data; some provide better performance than others, but at a cost of higher computational complexity. We present a modified nonlinear decision feedback equalizer designed for use in a legacy optical communication system with periodic dispersion compensation. The effects of noise and nonlinearity on the equalizer coefficients are investigated, and a suboptimal convergence algorithm to reduce such effects is proposed and verified. Our equalizer provides performance comparable to that obtained using digital backpropagation, while being computationally simpler, compensating linear and nonlinear physical impairment effects effectively even at high power levels where fiber nonlinearity is significant. Performance prediction of the designed DFE is also discussed, using a numerical method, with and without error propagation.

Effect of Gain and Noise Figure on DPSK Modulated Long-haul Optical Communication System

AbstractIn this paper, the transmission of signal with DPSK modulation with variation of duty ratio to 66% and 33% is demonstrated and performance of the system is analysed by varying the gain and noise figure value of an EDFA amplifier.

Stochastic Digital Backpropagation

In this paper, we propose a novel detector for single-channel long-haul coherent optical communications, termed stochastic digital backpropagation (SDBP), which takes into account noise from the optical amplifiers in addition to handling deterministic linear and nonlinear impairments. We discuss the design approach behind this detector, which is based on the maximum a posteriori (MAP) principle. As closed-form expressions of the MAP detector are not tractable for coherent optical transmission, we employ the framework of Bayesian graphical models, which allows a numerical evaluation of the proposed detector. Through simulations, we observe that by accounting for nonlinear signal-noise interactions, we achieve a significant improvement in system reach with SDBP over digital backpropagation (DBP) for systems with periodic inline optical dispersion compensation. In uncompensated links with high symbol rates, the performance difference in terms of system reach for SDBP over DBP is small. In the absence of noise, the proposed detector is equivalent to the well-known DBP detector.

Ultra-high-density spatial division multiplexing with a few-mode multicore fibre

Single-mode fibres with low loss and a large transmission bandwidth are a key enabler for long-haul high-speed optical communication and form the backbone of our information-driven society. However, we are on the verge of reaching the fundamental limit of single-mode fibre transmission capacity. Therefore, a new means to increase the transmission capacity of optical fibre is essential to avoid a capacity crunch. Here, by employing few-mode multicore fibre, compact three-dimensional waveguide multiplexers and energy-efficient frequency-domain multiple-input multiple-output equalization, we demonstrate the viability of spatial multiplexing to reach a data rate of 5.1 Tbit s−1 carrier−1 (net 4 Tbit s−1 carrier−1) on a single wavelength over a single fibre. Furthermore, by combining this approach with wavelength division multiplexing with 50 wavelength carriers on a dense 50 GHz grid, a gross transmission throughput of 255 Tbit s−1 (net 200 Tbit s−1) over a 1 km fibre link is achieved. A few-mode, multicore fibre allows ultra-high-speed data transmission on a single wavelength of light.

Analysis of fiber Bragg grating with exponential–linear and parabolic taper profiles for dispersion slope compensation in optical fiber links

In this paper, the effect of various taper profiles on dispersion slope compensation in optical fiber links is studied. Theoretical and numerical investigation of the linear and nonlinear group delays of tapered fiber Bragg grating's (T-FBG) under strain is made. Calculation is performed using Matlab code based on solving the coupled mode equation using transfer matrix method. Our study shows that the linear tapered FBG profile provide the best result than the linear-exponential profile which can compensate up to 500km. As result, the spectral characteristics of tapered grating allow them to be used efficiently in high bit rates WDM and long-haul optical communication systems for chromatic dispersion of single-mode fiber.

Dynamic Single-Mode Lasers

The 40-year history of research on semiconductor lasers for high-speed long-distance optical fiber communications, so-called dynamic single-mode (DSM) lasers, is reviewed. DSM lasers include phase-shift distributed feedback (DFB) lasers, uniform DFB lasers, wavelength tunable (WT) distributed Bragg reflector lasers, WT distributed reflector lasers, and external reflector lasers. The vertical cavity surface emitting laser is also a type of DSM laser that is applied for rather short-distance communication. Photonic-integrated circuits with monolithic integration of DSM lasers with other devices, as well as photonic crystal lasers, have advanced significantly to support stable operation of photonic subsystems. The DSM laser is currently essential for most long-haul optical communications and transoceanic submarine cables, as well as medium-distance local area networks. Its application to sensing systems is another interesting area.

Digital Fiber Nonlinearity Compensation: Toward 1-Tb/s transport

The world is connected by a core network of long-haul optical communication systems that link countries and continents, enabling long-distance phone calls, data-center communications, and the Internet. The demands on information rates have been constantly driven up by applications such as online gaming, high-definition video, and cloud computing. All over the world, end-user connection speeds are being increased by replacing conventional digital subscriber line (DSL) and asymmetric DSL (ADSL) with fiber to the home. Clearly, the capacity of the core network must also increase proportionally.

Time-reversed twin

Co-propagating a signal with its phase conjugate along an optical fibre link makes it possible to mitigate unwanted nonlinear distortions and improve the signal-to-noise ratio in long-haul optical communication systems.

Large-effective-area dispersion-compensating fiber design based on dual-core microstructure

We present a microstructure-based dual-core dispersion-compensating fiber (DCF) design for dispersion compensation in long-haul optical communication links. The design has been conceptualized by combining the all-solid dual-core DCF and dispersion-compensating photonic crystal fiber. The fiber design has been analyzed numerically by using a full vectorial finite difference time domain method. We propose a fiber design for narrowband as well as broadband dispersion compensation. In the narrowband DCF design, the fiber exhibits very large negative dispersion of around -42,000 ps nm(-1) km(-1) and a large mode area of 67 μm(2). The effects of varying different structural parameters on the dispersion characteristics as well as on the trade-off between full width at half-maximum and dispersion have been investigated. For broadband DCF design, a dispersion value between -860 ps nm(-1)km(-1) and -200 ps nm(-1) km(-1) is obtained for the entire spectral range of the C band.

Comparative analysis on the performance of two concatenated codes for high-speed long-haul optical communication systems

After the development trend of high-speed long-haul optical communication systems and the theory of the concatenated code are analyzed, the comparative researches on the performances of the two concatenated codes of the inner-outer type and the improved interleaving type are performed in detail. The theoretical analyses and simulation results show that the inner-outer type concatenated code has the greater redundancy, and the improved interleaving type concatenated code is a superior concatenated code with the advantages of the better error correction performance, moderate redundancy and easy implementation. As a result, the improved interleaving type concatenated code can be better used in high-speed long-haul optical communication systems.

Modified Cramér–Rao Bound for Clock Recovery in the Presence of Self-Phase Modulation

We investigate the effect of nonlinear self-phase modulation (SPM) on clock recovery in long-haul coherent optical communication systems with low to moderate baud rates. We consider dual-polarization, multilevel quadrature amplitude modulation, and evaluate conventional timing estimators as well as a modified Cramer-Rao bound. As the latter is hard to compute in the presence of nonlinear channel impairments, we derive an approximate closed-form expression. Contrary to linear channel impairments, the nonlinear impairments can degrade the performance for higher input powers. In addition, the bounds exhibit oscillatory behavior due to the oscillations in the signal spectrum in the presence of SPM.

A novel RS BTC coding scheme for optical communications

A novel Reed Solomon (RS) block turbo code (BTC) coding scheme of RS(63,58)×RS(63,58) for optical communications is proposed. The simulation results show that the net coding gain (NCG) of this scheme at the sixth iteration is more than that of other coding schemes at the third iteration for the bit error rate (BER) of 10−12. Furthermore, the novel RS BTC has shorter component code and rapider encoding and decoding speed. Therefore, the novel RS BTC coding scheme can be better used in high-speed long-haul optical communication systems, and the novel RS BTC can be regarded as a candidate code of the super forward error correction (super-FEC) code. Moreover, the encoding/decoding design and implementation of the novel RS BTC are also presented

Optical orthogonal division multiplexing for long haul optical communications: A review of the first five years

Optical OFDM was proposed for dispersion compensation in long-haul optical communications systems in 2006 in two forms, one using direct-detection and the other using coherent detection. Since then there has been extensive innovation towards developing intermediate forms of optical OFDM that are more suited to specific applications. This review paper presents our view on the developments in optical OFDM for long-haul optical transmission applications. It covers the basic elements of radio OFDM before concentrating on direct detection optical OFDM and its development, followed by coherent optical OFDM. All-optical OFDM is then considered, together with optical methods of generating and separating the OFDM subcarriers. The paper then discusses the critical issue of nonlinear degradation due to the Kerr effect in optical fibers and reviews recent innovations to mitigate the effects of fiber nonlinearity. Finally some future research directions are discussed.

Scintillation behavior of Laguerre Gaussian beams in strong turbulence

In strong atmospheric turbulence, the asymptotic on-axis scintillation behaviors of Laguerre Gaussian (LG) beams are examined. To arrive at the strong-turbulence solution, we utilize the existing filtering approach for weak-turbulence solutions and our recently reported weak-turbulence scintillation index formula for LG beams. In the limiting case, our solution correctly predicts the asymptotic strong-turbulence behavior of Gaussian beam wave scintillation. Investigation of the scintillations versus the propagation distance, source size, wavelength and refractive index structure parameter lead to the conclusion that the LG beams with higher order radial modes can provide less scintillation. The results are applicable to long-haul atmospheric optical communication links.

Pilot-based cross-phase modulation compensation for coherent optical orthogonal frequency division multiplexing long-haul optical communications systems

A pilot-based nonlinearity compensator (PB-NLC) is shown in this Letter to be an effective method for compensating cross-phase modulation (XPM) in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. An unmodulated pilot tone is transmitted at the center of each OFDM channel to detect phase errors caused by the Kerr effect, which converts intensity fluctuations from all channels to phase errors. The pilots are then used to cancel the XPM phase errors for each OFDM channel at the receiver after each channel's self-phase modulation (SPM) has been compensated, using its intensity waveform to determine its SPM. Numerical simulations of a 58 Gb/s single polarization 2375 km system with inline dispersion compensation show that the signal quality, Q, at the optimal launch power is increased by 2.4 dB if SPM compensation is used before the PB-NLC. This contrasts with only a 0.9 dB improvement if the PB-NLC is used without an SPM compensator for the same link. This shows the PB-NLC can effectively mitigate XPM but not SPM.

OFDM versus Single-Carrier Transmission for 100 Gbps Optical Communication

We analyze the orthogonal frequency division multiplexing (OFDM) technique in long-haul next generation optical communication links and compare it with the well-established single-carrier (SC) data transmission using high-level modulation formats and coherent detection. The analysis of the two alternative solutions is carried out in the 100 Gbps scenario, which is commonly considered to be the next upgrade of existing optical links, with special emphasis on quaternary phase-shift keying (QPSK) modulations. The comparison between OFDM and SC takes into account the main linear and nonlinear impairments of the optical channel, e.g., group velocity dispersion (GVD), polarization mode dispersion (PMD), self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), as well as the phase noise due to transmit and receive lasers, their relative frequency offset, other synchronization aspects, the overall complexity, the power and spectral efficiency, and the technological constraints.

Analytical approach of developing wavelength encoded AND, NAND and X-OR logic operations and implementation of the theory using semiconductor optical amplifiers

Frequency encoding technique is a very promising and faithful technology for very fast long-haul optical communication and super fast computation. Implementation of different logic gates based on the principle of frequency conversion is the key mechanism of frequency encoded data processing and networking. It is established that semiconductor optical amplifiers (SOA) have been used successfully for the purpose of frequency conversion. One of the important techniques of frequency conversion is the conjugate beam generation by four wave mixing (FWM) in SOA and ultimately conversion of it into desired frequency by means of reflecting semiconductor optical amplifier (RSOA). However the efficiency of conjugate beam generation is restricted by polarization dependent gain saturation of SOA. This dependency can be successfully removed using polarization diversity scheme. Another technique of the frequency conversion is based on nonlinear rotation of the state of polarization of the linearly polarized probe beam. An important advantage of using polarization rotation in SOA is that a small change in rotation of the state of polarization will lead to a large difference in output power. Here in our present communication we propose a method of developing wavelength encoded AND, NAND and X-OR logic operations exploiting the above mentioned functions of SOA. For this purpose we have developed an analytical treatment based on which above mentioned three logic gates are conducted. The satisfactory simulation result proves also the validity of the developed theory.

Improved single channel backpropagation for intra-channel fiber nonlinearity compensation in long-haul optical communication systems

Backpropagation has been shown to be the most effective method for compensating intra-channel fiber nonlinearity in long-haul optical communications systems. However, effective compensation is computationally expensive, as it requires numerous steps and possibly increased sampling rates compared with the baud rate. This makes backpropagation difficult to implement in real-time. We propose: (i) low-pass filtering the compensation signal (the intensity waveform used to calculate the nonlinearity compensation) in each backpropagation step and (ii) optimizing the position of the nonlinear section in each step. With numerical simulations, we show that these modifications to backpropagation improve system performance, reducing the number of backpropagation steps and reducing the oversampling for a given system performance. Using our 'filtered backpropagation', with four backpropagation steps operating at the same sampling rate as that required for linear equalizers, the Q at the optimal launch power was improved by 2 dB and 1.6 dB for single wavelength CO-OFDM and CO-QPSK systems, respectively, in a 3200 km (40 x 80 km) single-mode fiber link, with no optical dispersion compensation. With previously proposed backpropagation methods, 40 steps were required to achieve an equivalent performance. A doubling in the sampling rate of the OFDM system was also required. We estimate this is a reduction in computational complexity by a factor of around ten.

Novel orthogonal modulation format DRZ-FSK/DPSK for high-speed long-haul optical communication

We propose a novel advanced orthogonal modulation format dark return-to-zero frequency shift keying/differential phase shift keying (DRZ-FSK/DPSK) and its realization scheme. The DRZ-FSK/DPSK is generated by the combination of a 40-Gb/s return-to-zero (RZ) signal and a DRZ signal which is converted from the RZ using a semiconductor optical amplifier (SOA) based on nonlinear cross polarization rotation (XPR) and then re-modulated by high-bit-rate DPSK at 40 Gb/s. The feasibility of the scheme is experimentally demonstrated. Bit error rate (BER) results of the total 80-Gb/s DRZ-FSK/DPSK orthogonal modulation signal with a subsequent 100-km single-mode fiber (SMF) transmission link show its potential for future high-speed long-haul optical communication.