Dispersion-managed Fiber Links Research Articles

Modulational instability in randomly dispersion-managed optical fiber links

Modulational instability in randomly dispersion-managed optical fiber links

Various soliton molecules in fiber systems.

Generation and propagation of various soliton molecules (SMs) in fiber systems are reviewed. SMs can survive either in fibers or fiber lasers. A dispersion-managed (DM) fiber link is the only platform for SM demonstration, while various fiber lasers can support different SMs. The fundamental unit of SMs can be conventional solitons generated in an anomalous dispersion regime, stretched pulses in DM fiber lasers, parabolic pulses, or gain-guided solitons in a normal dispersion regime. SMs with typically close soliton separation are presented. In addition, we demonstrate a new kind of SM with nanosecond soliton separation. The narrow spectral filtering is required for the generation of SMs with such long-distance interaction.

Hybrid opto-digital signal processing in 112 Gbit/s DP-16QAM and DP-QDB transmission for long-haul large- $$\hbox {A}_\mathrm{eff}$$ A eff pure-silica-core fiber links

By means of numerical simulations, we demonstrated that all-optical signal processing methods (XPM-suppressor module and in-line nonlinear equalization) significantly increase the system performance of digital nonlinear compensation (digital backward propagation) and improve the system performance in five-channel 112 Gbit/s DP-16QAM and DP-QDB transmission over 2400 km large- effective-area pure-silica-core fiber ($$\hbox {LA}_\mathrm{eff}$$LAeff-PSCF). The system performance is quantified with the help of Q-factor (dB) for both dispersion-managed and nondispersion-managed fiber links.

Fiber looped phase conjugation of polarization multiplexed signals for pre-compensation of fiber nonlinearity effect

Compensation of nonlinear distortion of polarization-multiplexed (PolMux) signals in optical fiber is evaluated experimentally using all-optical signal pre-distortion and fiber loop phase-conjugation at the transmitter. An improved bit error rate is shown for high baud rate, 80 Gb/s RZ-DPSK PolMux signals before transmission in a 728 km long dispersion-managed fiber link employing a direct detection receiver. The partial compensation of nonlinear distortion for both single channel and 3 × 80 Gb/s WDM PolMux signals is observed, despite the impact from the inter-polarization nonlinearity and the associated polarization scattering. Evidence of the limited compensation of inter-polarization nonlinearity is shown.

Flexible Transponder Design Aided by Agile Binary Bit Encoder

In order to accommodate variable bit rates and achieve flexible transmission distances, a binary bit encoding scheme is incorporated in the dual-polarization (DP) quadrature phase-shift keying (QPSK) transmitter to manipulate the signal polarization multiplexing schemes into polarization-multiplexed (PolMux), polarization-switched (PolSw), and polarization-alternated (PolAl) schemes, such that the transmission bit rate could vary among 4B, 3B, and 2B, where B stands for the baud rate of the DP-QPSK signal. With the versatile polarization schemes, the trade-off between the transmission performance and system bit rate could be flexibly adjusted depending on the system requirements and conditions. The performance of these modulation formats has been evaluated experimentally at 32 Gbaud in a dispersion-managed fiber (DMF) link. At a 0.5 dB system margin, the transmission reach of PolSw and PolAl QPSK increased to ∼6400 and ∼8500  km, respectively, compared to the ∼4500  km transmission distance using the PolMux QPSK signal. This improvement is attributed to the lower bit rate of PolSw QPSK (96  Gbits/s) and PolAl QPSK (64  Gbits/s) and to the superior characteristics of the PolSw and PolAl schemes. For example, PolAl QPSK performs as good as the PolMux BPSK format to show similar receiver sensitivity. At the end, a flexible transponder platform with full software-defined optics features is discussed using the proposed flexible transmitter configuration and other techniques, such as optical multi-tone generation and adaptive forward error correction, to enable the transponder to carry standard 10G/40G/100G data.

Demonstration of digital phase-sensitive boosting to extend signal reach for long-haul WDM systems using optical phase-conjugated copy

We demonstrate a hybrid optical/digital phase-sensitive boosting (PSB) technique for long-haul wavelength division multiplexing (WDM) transmission systems. The approach uses four-wave mixing (FWM) to generate a phase-conjugated idler alongside the original signal. At the receiver, the signal and idler are jointly detected, and the phases of the idler symbols are conjugated and summed with the signal symbols to suppress noise and nonlinear phase distortion. The proposed hybrid PSB scheme is independent of modulation format and does not require an optical phase-locked loop to achieve phase matching required by conventional phase-sensitive amplifiers. Our simulation and experimental results of 112-Gb/s dual-polarization quadrature phase-shift-keying (DP-QPSK) transmission confirmed the principle of the PSB scheme, attaining a Q-factor improvement of 2.4 dB over conventional single-channel transmission after 4,800 km of dispersion-managed fiber (DMF) link at the expense of 50% reduction in spectral efficiency and extending the system reach by 60% to 7,680 km.

Performance comparison between digital back propagation (DBP) and pilot-aided method for fiber nonlinearity compensation in different fiber links

We numerically investigate and compare the performance of fiber nonlinearity compensation using digital back propagation (DBP) method and pilot-aided method in coherent optical transmission systems using different fiber links. Simulations for wavelength division multiplexed (WDM) 112Gb/s polarization division multiplexed quadrature phase shift keying (PDM-QPSK) systems with dispersion unmanaged (no DM) and dispersion managed (DM) fiber links are implemented. System Q-factor and maximum transmission distance at bit error rate (BER) of 3.8ÿ10⿿3 are calculated for performance comparison. The results show that, for system with no DM fiber link, DBP method outperforms pilot-aided method, because DBP method has better performance for intra-channel fiber nonlinearity compensation. However, for system with DM fiber link where inter-channel fiber nonlinearity plays an important role, pilot-aided method performs better than DBP method, because of its ability for inter-channel fiber nonlinearity compensation.

WDM Signal All-Optical Precompensation of Kerr Nonlinearity in Dispersion-Managed Fibers

Compensation of the Kerr nonlinearity in dispersion-managed optical fiber links is demonstrated for 100-GHz-spaced wavelength division multiplexing (WDM) 40-Gb/s return-to-zero-differential phase-shift keying signals using all-optical nonlinear predistortion and phase-conjugation at the transmitter. Measurements of the bit-error rate (BER) reveal a Q2-factor improvement from 1.3 to 2.9 dB for transmission of one to four WDM channels in a standard single-mode fiber link. Although signal distortion is primarily from intrachannel nonlinearity, the BER is also degraded by linear and nonlinear interchannel crosstalk.

A novel optical picosecond-duration NRZ-to-RZ format converter with simultaneous wavelength multicasting using a single-stage Mach–Zehnder modulator

We propose a novel and simple scheme to achieve NRZ-to-RZ format conversion and simultaneous wavelength multicasting based on a single-stage dual-arm electro-optic Mach–Zehnder modulator (MZM) and a short single mode fiber (SMF). The format conversion and wavelength multicast process are achieved by chirp compensation under the condition of generation of optical flat frequency comb. 40Gb/s NRZ-to-RZ conversion with one-to-five multiple-wavelength channel multicasting and transmission of the NRZ and the converted signals over 200km dispersion-managed fiber-link are successfully demonstrated by numerical simulation. Research results show that 40Gb/s 2ps RZ signal with wavelength-preserving can be obtained after format conversion. The converted RZ signal presents good transmission performance and can easily be multiplexed to 160Gb/s using optical time division multiplexing (OTDM) technology. All the multicast channels can be error free after 50km transmission. Besides, the conversion operation can also greatly reduce the timing jitter of the degraded NRZ signal due to the retiming function of the proposed scheme.

Nonlinearity compensation using dispersion-folded digital backward propagation

A computationally efficient dispersion-folded (D-folded) digital backward propagation (DBP) method for nonlinearity compensation of dispersion-managed fiber links is proposed. At the optimum power level of long-haul fiber transmission, the optical waveform evolution along the fiber is dominated by the chromatic dispersion. The optical waveform and, consequently, the nonlinear behavior of the optical signal repeat at locations of identical accumulated dispersion. Hence the DBP steps can be folded according to the accumulated dispersion. Experimental results show that for 6,084 km single channel transmission, the D-folded DBP method reduces the computation by a factor of 43 with negligible penalty in performance. Simulation of inter-channel nonlinearity compensation for 13,000 km wavelength-division multiplexing (WDM) transmission shows that the D-folded DBP method can reduce the computation by a factor of 37.

Proposal and simulation investigation of optical format conversion between quaternary amplitude-shift keying signals based on cascaded modulators

We propose a simple and novel format conversion scheme based on a polarization modulator (PolM) and a zero-chirp intensity modulator (IM) to perform NRZ to RZ conversion for quaternary amplitude-shift keying (4-ASK) signals. Simulation shows that the scheme is capable of realizing format conversion from 20-Gbit/s NRZ-4-ASK signal to RZ-4-ASK signal with tunable pulse-width for 4-level intensity modulation format. The converted signals can transmit over a dispersion-managed fiber link from 200km to 300km confirming the high quality conversion.

Suppression of parabolic pulse-pair interaction using dispersion-managed fiber links with non-zero dispersion

We report on the suppression of the parabolic pulse-pair interaction by using the dispersion-managed optical fiber links with non-zero dispersion maps. The numerical results show that both the interaction and pulse splitting can be efficiently suppressed when the integrated dispersion is non-zero, and the pulse-pair can maintain their parabolic characteristics through the whole distance and turn to its original shape at the end of each map. Moreover, we numerically investigate the influence of the initial pulse width and chirp on the parabolic pulse pair propagation, which shows that the higher chirp is not conducive to pulse transmission.

Interactions between dispersion-managed solitons with unequal powers

We revisit the problem of intrachannel interactions between solitons in dispersion-managed nonlinear fiber links, which is a major limiting factor for the use of solitons in fiber-optic networks. By means of systematic simulations, we demonstrate the existence of a well-defined maximum of the interaction length, zint, as a function of the ratio of peak powers of interacting solitons, while in the case of identical solitons the interaction length always attains a minimum. An explanation to these features is proposed. Interaction effects in three-soliton sets are studied too. The results suggest that the transmission characteristics of soliton streams in the dispersion-managed system may be improved by using alternating values of the peak powers whose ratio corresponds to the maximum of zint.

Investigation of all-optical nonreturn-to-zero-to-return-to-zero format converter based on a semiconductor optical amplifier and a reconfigurable delayed interferometer

A simple and robust all-optical nonreturn-to-zero to return-to-zero (RZ) format converter is demonstrated and investigated using a semiconductor optical amplifier (SOA) and a reconfigurable delayed interferometer. The scheme can be used to convert input signals at different bit rates and obtain converted signals with tunable duty cycles. Format conversion at 10 Gb/s is achieved with a negative power penalty of -4.9 dB at a bit-error rate of 10(-9). Transmission of the converted RZ signal over a dispersion-managed fiber link of 80 km results in only a small power penalty of 0.9 dB. In addition, the format converter is found to be insensitive to the power fluctuation of the optical clock, as the SOA works in the saturation regime.

Theoretical analysis of cross-phase modulational sideband instability in wavelength-division multiplexed system with periodic dispersion managed fiber links

Dispersion management is a technology that alternately and periodically uses fibers with opposite dispersion sign in wavelength-division multiplexing link, thus the path averaged dispersion coefficient over each period is close to zero. This paper studies the cross-phase modulational sideband instability in chain of periodic dispersion managed and lumped amplified fiber link. Analytisis and numerical simulation are applied to show that there are multi-sidebands in the dispersion managed fiber link if the path averaged dispersion exhibits the abnormal dispersion, while there is only the lowest order if the path averaged dispersion exhibits the normal dispersion. The results of analytical and numerical simulation methods are consistent.

Dynamics of subpicosecond dispersion-managed soliton in a fibre: a perturbative analysis

A model is studied which describes the propagation of a subpicosecond optical pulse in dispersion-managed fibre links. In the limit of weak chromatic dispersion management, the model equation is reduced to a perturbed modified NLS equation having a nonlinearity dispersion term. By means of the Riemann–Hilbert problem, a perturbation theory for the soliton of the modified NLS equation is developed. It is shown in the adiabatic approximation that there exists a unique possibility to suppress a perturbation-induced shift of the soliton centre at the cost of proper matching of the soliton width and nonlinearity dispersion parameter. In the next-order approximation, the spectral density of the radiation power emitted by a soliton is calculated.

Transmission of pulses in a dispersion-managed fiber link with extra nonlinear segments

We introduce an extended version of the dispersion-management (DM) model, which includes an extra nonlinear element, and consider transmission of return-to-zero pulses in this system (they are not solitons). The pulses feature self-compression, accompanied by generation of side peaks (in the temporal domain). An optimal transmission distance, zopt, is identified, up to which the pulse continues to compress itself (the eventual width-compression factor is ≃2), while the amplitude of the side peaks remains small enough. The distance zopt virtually does not depend on the strength S of the DM part of the system in the interval 1.5<S<11, but it is sensitive to the nonlinearity strength in the extra segment. The system provides essentially stronger suppression of the noise-induced jitter of the pulses than the ordinary DM model. The most important issue is interaction between adjacent pulses, which is a basic difficulty in the case of DM solitons. In a broad parameter region, the system provides effective isolation between pulses. The minimum initial temporal distance between them, necessary for the isolation, is quite small, slightly larger than 1.5 the pulse’s width. The transmission actually improves the quality of multi-pulse arrays, as it leads to deepening of hiatuses between originally overlapping pulses.

Optical 2R regeneration at 40 Gbit/s using saturable absorber in long-haul dispersion-managed fiber links

The optical regeneration is an attractive method to improve the performance of long-distance data transmission, though its application in high-speed fiber systems requires careful design consideration/optimization. In this letter we investigate 40 Gbit/s dispersion-managed fiber transmission with optical 2R regeneration based on quantum well saturable absorber and highly non-linear fiber. We demonstrate through numerical modeling a feasibility of a single channel transmission over 10,000 km using optimized system design.

Statistical analysis on stimulated raman crosstalk in dispersion-managed fiber links

Power impairments due to stimulated Raman scattering (SRS) in dispersion-managed (DM) fiber links are evaluated theoretically. We extend previous work on the statistical analysis of SRS crosstalk to the case of multiple fiber segments. Closed-form formulas are derived, and the applicable range is presented by comparison with simulation results. The SRS crosstalk in DM fiber links is evaluated using derived formulas, and the preferable configuration of DM fiber links for suppressing SRS crosstalk is discussed. We also evaluate the exact power penalty induced by the SRS crosstalk in consideration of the log-normal waveform distribution due to SRS and non-Gaussian noise and clarify the system bounds in some typical DM fiber links. The developed approach provides a design rule for DM fiber links from the viewpoint of SRS crosstalk suppression.

Coherence effect of four-wave mixing in a dispersion-managed fiber link

For an optical link using a dispersion-managed fiber (DMF) in a wavelength-division multiplexing system, the performance degradation due to inadvertent phase matching of four-wave mixing (FWM) products is investigated analytically and numerically. The analysis has shown that the phase mismatch parameters in all possible FWM products in a DMF link become integer multiples of a common divisor and that, if this divisor coincides with an integer multiple of π radian, the Q-penalty caused by FWM can be quenched in theory. Periodic fluctuation of the Q-penalty curves obtained by Monte-Carlo simulation is shown verifying such a coherence effect of the FWM. An optimized design taking advantage of this coherence effect is suggested based on the result from the present analysis.