Long-haul Optical Communication Research Articles

Coherent Optical Single Carrier Transmission Using Overlap Frequency Domain Equalization for Long-Haul Optical Systems

In this paper, we present coherent optical single carrier transmission (COSC) using overlap frequency domain equalization (OFDE) for the chromatic dispersion (CD) compensation. Residual inter-symbol interference is suppressed by time domain equalization with small tap size. In single polarization transmission, 25-Gb/s COSC transmission with OFDE is demonstrated over 800 km to 4320 km of single-mode fiber (SMF) without optical dispersion compensation. The measured results show that the proposed configuration improves the transmission performance by setting adequate overlapped size of FFT windows and that it is suitable for long-haul optical communication systems.

Maximum-Likelihood Detection and Constrained Coding on Optical Channels

The most probable error events for a long-haul optical communication system using sequence estimation are characterized, and the performance using a modulation code is analyzed. The modulation code is restricted to be a constrained code that enhances the minimum distance between received signal sequences. We investigate several constrained codes used in combination with 4- and 8-state Viterbi algorithms and apply them to a standard noncoherent NRZ system. The improvement using a rate 0.85 code can be as high as 5 dB relative to an uncoded system operating at the same symbol rate. If the coded and uncoded systems are constrained to have the same information rate, a more modest 1 dB improvement can be obtained for specific fiber distances. We also determine the effect of the code on the number of states of the Viterbi algorithm and the sensitivity of the sampling point. Using these codes, the tolerance of the receiver to sampling errors can be increased by a nearly a factor of two for specific distances. The performance improvements that these codes can provide make them attractive candidates when used in conjunction with sequence estimation.

State-Complexity Reduction in MLSD Receivers for Optical Communications With Direct Photodetection

We investigate the impact of state-complexity reduction on the performance of maximum likelihood sequence detection (MLSD) receivers for direct-photodetection long-haul optical communication systems affected by uncompensated chromatic dispersion (CD). We directly compare two possible approaches: (i) detection through a simple ldquobrute-forcerdquo state-complexity reduction strategy and (ii) a more structured reduced-state sequence detection (RSSD) strategy. The performance of both state-complexity reduction techniques is evaluated considering two realistic optical transmission schemes, based on on-off keying (OOK) and optical duobinary modulation (ODBM), respectively. The detection algorithms are characterized considering the impact of the timing offset, the quantization scheme, and the amount of uncompensated CD. As one would expect, for a given number of states in MLSD receivers, the schemes based on RSSD exhibit better performance with respect to those based on simple brute-force state-complexity reduction. However, we show that MLSD schemes based on the use of brute-force state-complexity reduction are characterized by a better complexity/performance trade-off for low/medium CD values, whereas RSSD is the best choice for high CD values.

Optimum algorithm for WDM channel allocation for reducing four-wave mixing effects

A novel channel allocation method, based on optical Golomb ruler (OGR), that allows reduction of the FWM effect while maintaining bandwidth efficiency along with the algorithms has been presented in this paper. Very high-capacity, long-haul optical communication systems can be designed by wavelength division multiplexing (WDM) of high-bit-rate channels and by using erbium-doped fiber amplifiers (EDFAs) to periodically compensate the fiber loss. In such all-optical systems, the effects of chromatic dispersion and nonlinearities accumulate during light propagation, imposing limits on the achievable performance. Chromatic dispersion at 1.55 pm can be effectively reduced by using dispersion-shifted fiber (DSF). The use of very-low-dispersion fiber, however, enhances the efficiency of generation of four-wave mixing (FWM) waves by reducing the phase mismatch naturally provided by the fiber dispersion. For this reason, crosstalk due to FWM is the dominant nonlinear effect in long-haul WDM systems using DSFs. To reduce four-wave-mixing crosstalk in high capacity long-haul repeater less WDM light wave systems, the use of the channel allocation method that involves unequal spaced channels has been proposed.

A novel block turbo code for high-speed long-haul DWDM optical communication systems

A novel super forward error correction (SFEC) coding scheme, based on the block turbo code (BTC) of Bose–Chaudhuri–Hocguenghem (BCH)(64,57)×BCH(64,57), in high-speed long-haul dense wavelength division multiplexing (DWDM) optical communication systems is proposed. The simulation results and its analyses show that the net coding gain (NCG) of the novel SFEC code at iteration 6 is, respectively, 0.31 and 0.34 dB more than those of the BCH(3860, 3824)+BCH(2040, 1930) code and Reed–Solomon (RS)(255,239)+convolutional–self-orthogonal code (CSOC)( k 0/ n 0=6/7, J=8) code in the Recommendation of ITU-T G.975.1 at iteration 3 for the bit error rate (BER) of 10 −12. The performance analyses for the novel SFEC code show that it has excellent advantages such as the shorter component code and rapid encoding/decoding speed; thus, both the complexity to implement its software/hardware and the delay time for its encoding/decoding can be greatly reduced. As a result, the novel SFEC coding scheme can better be applicable in high-speed long-haul DWDM optical communication systems. In addition, the design and implementation of the novel BTC are also analyzed and probed.

Nanoscale signal regeneration

The regeneration of weak and distorted optical signals is vital in long-haul optical communication systems. Now scientists at Cornell University have developed an all-optical scheme that performs the task and is small enough to fit on a chip.

Characterisation of a parallel optical all pass filter for chromatic dispersion equalisation in 10 Gb/s system

The chromatic dispersion (CD) that occurs in single mode fibres (SMFs) is an important issue that needs to be addressed in long-haul optical communication links. The effect of CD is pulse spreading which in turn leads to inter-symbol interference, thus resulting in the deterioration of the bit error rate (BER) performance of the system. An alternative CD compensation technique that utilises a parallel optical all pass filter (p-OAPF) is presented, where the p-OAPF design is based on the inverse phase response of the SMF. The p-OAPF is based on a class of all-pass filter. Simulation results of the proposed technique show an increase in the repeater-less length of a point-to-point optical communication system by up to three and four times using a non-return-to-zero data format with a rectangular and Gaussian pulse shape, respectively, at error-free condition (BER < 10 -9 ). The results also show that the p-OAPF is robust in performing dispersion equalisation at a wide range of SMF lengths to attain error-free communication.

NET EFFECTIVE GAIN OF MULTI-PUMPING RAMAN AMPLIFIER IN ULTRA-WAVELENGTH DIVISION MULTIPLEXING AND ULTRA-LONG – HAUL OPTICAL COMMUNICATION SYSTEMS

In the present paper, a net effective gain of multi -pumping Raman amplifier in ultra-wavelength division multiplexin g (UW-WDM) and ultra long-haul (ULH) optical communication systems has been modeled and parametrically investigated over wide ranges of affecting parameters, taking into account the polarization effect. NR pum ps are processed with or without equal spectral spacing. The effective fi ber core area, the gain coefficients and the amplified spontaneous emission power ASE (as a measure of the minimum detectable power) are calcul ated over the operating wavelength. The above three quantities ar e cast in simple polynomial forms. The average net Raman gain, the o n-off Raman gain, the signal-to-noise ratio at the effective length has been investigated also. The obtained results are employed to find the avera ge repeater spacing over the spectral range under the processing. It is concluded that for repeater spacing, the total injected Raman powers athe vital factor rather than the number of amplifiers, while for the gain spectral width, the number of amplifiers is the vital factor. The centr al wavelength of the subset of channels that propagates in a link indica tes the order of link. The total number of links in the core determines th e ultimate values of different effects.

Accuracy of Gaussian Approach for the Performance Evaluation of Direct-Detection Receiver with Partially Polarized Noise

We investigate the accuracy of a Gaussian approach (GA) developed to estimate the performance of a direct-detection optical receiver with arbitrary optical and electrical filtering and in the presence of partially polarized noise due to polarization-dependent loss (PDL). The accuracy is assessed by comparison of the performance estimates obtained from the GA with the estimates obtained from a rigorous method (RM) based on the calculation of the moment-generating function of the current at the optical receiver output. We show that the GA has a good accuracy when considering the variation of the optical filter bandwidth, extinction ratio, degree of polarization of the noise (DOP), and angle between signal and noise polarizations. However, it fails to predict the receiver sensitivity within 2 dB of the RM when DOP is greater than 0.7 and signal and noise polarizations are orthogonal in the Jones space. Nevertheless, it is shown that the GA provides receiver sensitivity estimates with good accuracy in most cases of long-haul optical communication systems influenced by PDL, where the typical average DOP is below 0.15. Due to its simplicity, shorter computation time, and good accuracy, the GA is a good tool to assess the performance of such optical systems.

Fiber soliton-form 3R regenerator and its performance analysis

A novel fiber optical 3R regenerator based on optical soliton-effect using highly nonlinear fiber is constructed and investigated for the needs of the high rate and long-haul optical communications. The propagation equation of the pulses in the proposed optical 3R regenerator with the control of optical modulator and filter is established. By the use of the variational approach, the evolution of the distorted optical pulses in the regenerator and the functions of reamplification, reshaping, and retiming are investigated. The relation between the construction parameters and the output performance of the regenerator is discussed. The stable operation condition of the regenerator is revealed.

A novel super-FEC code based on concatenated code for high-speed long-haul optical communication systems

The structures of the novel super forward error correction (Super-FEC) code type based on the concatenated code for high-speed long-haul optical communication systems are studied in this paper. The Reed–Solomon (RS) (255, 239) + Bose–Chaudhuri–Hocguenghem (BCH) (1023, 963) concatenated code is presented after the characteristics of the concatenated code and the two Super-FEC code type presented in ITU-T G.975.1 have theoretically been analyzed, the simulation result shows that this novel code type, compared with the RS (255, 239) + convolutional-self-orthogonal-code (CSOC) ( k 0/ n 0 = 6/7, J = 8) code in ITU-T G.975.1, has a lower redundancy and better error-correction capabilities, and its net coding gain (NCG) at the third iteration is 0.57 dB more than that of RS (255, 239) + CSOC ( k 0/ n 0 = 6/7, J = 8) code in ITU-T G.975.1 at the third iteration for the bit error rate (BER) of 10 −12. Therefore, the novel code type can better be used in long-haul, larger capacity and higher bit-rate optical communication systems. Furthermore, the design and implementation of the novel concatenated code type are also discussed.

Lithium niobate optical modulators: Devices and applications

Optical modulators, using acousto-optic, magneto-optic or electro-optic effects, as the principal components for external modulation of lightwaves, have presently played the important role in modern long-haul ultra-high speed optical communications and photonic signal processing systems. LiNbO 3 is an ideal host material for such modulators. In this paper we present: (i) a brief overview the electro-optic interaction in LiNbO 3 optical waveguides and the travelling electric waves for modulation; (ii) fabrication techniques of optical waveguides and electrodes for the excitation of microwave and millimeter waves, including the prospects of the uses of ultra-thick “brick-wall like” type by synchrotron deep X-ray LIGA process; (iii) a comprehensive modelling of the travelling wave electrodes for the design of interferometric optical modulators, including different electrode structures. Tilted and thick and “brick-wall” like electro-plated electrodes are modelled and confirmed with implemented modulators operating up to 26 GHz in diffused LiNbO 3 optical interferometric waveguide structures; (iv) the design and implementation of LiNbO 3 single or dual drive as modulators for modulating lightwaves in modern transmission systems as ultra-high speed pulse carvers, generators of modulation formats (NRZ, RZ, CS-RZ, DPSK, DQPSK, etc.) and photonic signal processors.

Broadband all-order polarization mode dispersion compensation using liquid-crystal modulator arrays

Polarization mode dispersion (PMD) is a potential limiting factor in long-haul high-speed optical communications, especially in systems beyond 10 Gb/s. Although considerable effort has been devoted to compensation methods for PMD, most of the research is restricted to a small bandwidth, within the limits of the first- and second-order PMD approximations. For the first-order approximation to be valid, the distortions induced by PMD must remain less than a few tenths of the pulse duration or the bit period for return-to-zero (RZ) or non-return-to-zero (NRZ) systems, respectively. In this work, as far as the authors know, the application of ultrafast optical pulse-shaping techniques for experimental broadband all-order PMD compensation is demonstrated for the first time. PMD is treated as arbitrary variations of state of polarization (SOP) and phase versus wavelength, in an all-order sense. Two fiber-pigtailed pulse shapers are implemented in a serial manner to compensate the polarization and phase spectra independently. The first step corrects the wavelength-dependent polarization states to a fixed wavelength-independent state. This reduces the PMD compensation problem to a generalized chromatic dispersion compensation problem. Consequently, equalization of the spectral phase in the second step restores the clean broadband pulse signal. In the experiments, compensation of subpicosecond pulses (14 nm bandwidth around 1550 nm) that are anomalously spread to more than 2 ps due to PMD is demonstrated. These results are potentially relevant for future ultra-high-speed time-division-multiplexed (TDM) systems. Research challenges that must be addressed to bring this approach to practical application are also briefly enumerated, and the potential for scaling to a range compatible with wavelength-division-multiplexed (WDM) systems is discussed.

High-speed architectures for parallel long BCH encoders

Long Bose-Chaudhuri-Hocquenghen (BCH) codes are used as the outer error correcting codes in the second-generation Digital Video Broadcasting Standard from the European Telecommunications Standard Institute. These codes can achieve around 0.6-dB additional coding gain over Reed-Solomon codes with similar code rate and codeword length in long-haul optical communication systems. BCH encoders are conventionally implemented by a linear feedback shift register architecture. High-speed applications of BCH codes require parallel implementation of the encoders. In addition, long BCH encoders suffer from the effect of large fanout. In this paper, three novel architectures are proposed to reduce the achievable minimum clock period for long BCH encoders after the fanout bottleneck has been eliminated. For an (8191, 7684) BCH code, compared to the original 32-parallel BCH encoder architecture without fanout bottleneck, the proposed architectures can achieve a speedup of over 100%.

Novel hollow optical fibers and their applications in photonic devices for optical communications

Novel photonic devices based on a new type of waveguide, hollow optical fibers (HOF), are described. Utilizing unique three layered structure of HOF, the central air hole, germanosilicate ring core, and silica cladding along with its adiabatic mode transformation capability we demonstrated versatile applications in short-haul, long-haul optical communications, and tunable wavelength selective devices. Detailed design parameters, fabrication arts of the fibers, and operation principles of the devices are discussed.

Chaos Synchronization in Semiconductor Laser Systems and its Application to Optical Fiber Secure Communications

Chaotic laser synchronization applied to optical fiber secure communications is proposed. A synchronous error equation is achieved to the first order approximation. A model of optical fiber chaotic communication is presented by coupling the chaotic laser synchronous system with optical fiber propagation system. The chaotic synchronous system has been successfully stimulated in a long-haul optical fiber communications. The synchronization is limited by the self-phase modulation in the fiber, and the propagation phase-limited formula and the propagation distance-limited formula are demonstrated. Synchronous errors with optical fiber lengths are calculated. The influences of intensity and phase noises on the synchronous error are also numerically analyzed in detail. Analog and digital optical fiber chaotic secure communications in the system at wavelength of 1.31 μm are studied by numerically simulations. The systems show a good ability of robust security and anti-uncover in time-domain and frequency-domain.

Theoretical studies on optical fiber secure communication using chaotic phase encoding

In this paper, we present the synchronization of chaotic injected semiconductor lasers in an optical fiber system via phase-controllers controlling its lasing p hase shift, and a theoretical model for optical fiber chaotic secure communicati on system via chaos phase shift modulation by coupling a chaotic laser phase con trolled synchronization system and an optical fiber channel. Chaotic synchroniza tion is realized numerically under phase-controllers controlling in a long-haul optical fiber system. Optical fiber chaos propagation formula is demonstrated by analyzing theoretically the effect of optical fiber self-phase modulation on ch aotic signal and synchronization. Chaotic laser phase shift is controlled and it s signal is transmitted via continuous chaos shift keying modulation on end for optical fiber communication encoding. Chaotic laser demodulation is achieved via setting a phase shift of the receiver phase-controllers for optical fiber commu nication decoding. Application of chaotic digital encoding of 50Mbit/s rate is simulated numerically for long-haul optical fiber secure communication. The system parameter mismatch and robustness of anti-noise are analyzed numerically in long-haul optical fiber propagation, in particular.

Eliminating the Fanout Bottleneck in Parallel Long BCH Encoders

Long BCH codes can achieve about 0.6-dB additional coding gain over Reed-Solomon codes with similar code rate in long-haul optical communication systems. BCH encoders are conventionally implemented by a linear feedback shift register architecture. Encoders of long BCH codes may suffer from the effect of large fanout, which may reduce the achievable clock speed. The data rate requirement of optical applications require parallel implementations of the BCH encoders. In this paper, a novel scheme based on look-ahead computation and retiming is proposed to eliminate the effect of large fanout in parallel long BCH encoders. For a (2047, 1926) code, compared to the original parallel BCH encoder architecture, the modified architecture can achieve a speedup of 132%.

Chaotic laser synchronization and its application in optical fiber secure communication

In this paper, optical fiber chaotic secure communication is proposed by coupling chaotic laser synchronous system with optical fiber propagation channel. Feedback synchronous system of chaotic semiconductor lasers is presented and synchronous error and decoding formulae are demonstrated. Synchronization between two chaotic laser systems with distributed feedback semiconductor lasers at wavelength of 1.31 µm is simulatively achieved with almost zero synchronous error. Parameter mismatch, synchronous transient response and noise effect on the system are studied. Robustness of synchronization and anti-perturbation can increase by increasing the feedback coefficient of the system. Influence of group-velocity-dispersion and self-phase modulation of optical fiber on chaotic laser signal and synchronization are analyzed, and it is found that group-velocity-dispersion affects pulse shape, synchronization and decoding, and limits optical fiber propagation distances, and self-phase modulation does not affect pulse shape, while its product of nonlinear phase shift can affect synchronization and decoding, and restrict propagation distances of optical fiber chaotic communication system and hence the maximum optical power formula is educed. Injecting parameter modulation and amplitude modulation are numerically simulated in long-haul optical fiber secure communication systems.

Hole-Assisted Multiring Fiber With Low Dispersion Around 1550 nm

Due to large index contrast between air and silica, many properties of optical fiber unachievable through doping can be realized easily by utilizing air holes. We present a nonzero dispersion-shifted fiber design based on multiring index profile with the assistance of air holes. The proposed fiber exhibits small dispersion values over nearly 100-nm-wide wavelength range with very low dispersion slope. It can have applications in future long-haul optical communication network.