Compensation for the self-steepening effects in optical fiber communication system using midway optical phase conjugation

Effects of self-steepening (SS) of chirped Gaussian pulses on optical fiber communication system using midway optical phase conjugation (OPC) are analyzed. Dynamic evolution of the ultrashort pulses is simulated numerically. It is found that OPC cannot compensate for pulse waveform distortion due to SS.The initial chirp of pulses and dispersion can counteract SS and improve the compensation performance for the distortion.     

Feasibility of 100-Gb/s 10000-km single-channel opticaltransmission by midway optical phase conjugation incorporated withthird-order dispersion compensation

In optical phase conjugation systems, the third-order dispersion of fibers almost linearly accumulates along the transmission distance, and the distortion induced from the third-order dispersion can be perfectly compensated by using a linear third-order dispersion compensator placed at any point of the system. We demonstrate by numerical simulations that 100-Gb/s single-channel transmission can be achieved over a 10000-km distance in midway optical phase conjugation transmission systems by compensating the third-order dispersion     

脉冲啁啾对相位共轭通信系统性能的影响

数值模拟了线性啁啾高斯脉冲在光纤中的传输演化过程,分析了线性啁啾高斯脉冲的传输特性。结果表明,脉冲啁啾不会影响中置相位共轭系统对色散和非线性效应导致的信号失真的完全补偿;单模光纤中啁啾将导致脉冲信号产生初期窄化和末期窄化过程,这种传输特性可用来优化相位共轭系统的总体结构设计,进一步改善和提高系统的补偿性能。     

Exact compensation for both chromatic dispersion and Kerr effect ina transmission fiber using optical phase conjugation

We propose a new method to compensate exactly for both chromatic dispersion and self-phase modulation in a transmission fiber, where the light intensity changes due to fiber loss and amplifier gain. This method utilizes optical phase conjugation (OPC). The pulse shape is precompensated before OPC by transmission through a fiber with large dispersion. A computer simulation demonstrates effective compensation for waveform distortion in a 40 Gb/s NRZ intensity-modulated light transmission     

Cancellation of the signal fading for 60 GHz subcarrier multiplexedoptical DSB signal transmission in nondispersion shifted fiber usingmidway optical phase conjugation

In millimeter-wave (mm-wave) optical double sideband (DSB) signal transmission systems, the received radio frequency (RF) power fades periodically because of the group velocity dispersion (GVD) and the self-phase modulation (SPM) of optical fibers. In this paper, cancellation of the signal fading by using midway optical phase conjugation in mm-wave subcarrier multiplexed (SCM) optical DSB signal is analyzed. Fading-free 60 GHz mm-wave optical DSB signal transmission over 100 km-long nondispersion shifted single-mode fiber at 1550 nm by using a semiconductor optical amplifier (SOA) optical phase conjugator (OPC) in the midway of optical link is experimentally demonstrated for the first time. Finally, the degradation factor of the OPC system is also discussed     

两种相位共轭技术对光纤中超短光脉冲传输失真的补偿比较

通过理论分析和数值计算,比较了时域相位共轭技术和频域相位共轭技术对光纤中由于色散和非线性引起的超短光脉冲传输失真的补偿效果。结果表明,在仅考虑群速度色散和自相位调制效应时,时域相位共轭技术与频域相位共轭技术的补偿效果一致;当需要考虑三阶色散时,频域相位共轭技术的补偿效果优于时域相位共轭技术;当需要考虑脉冲内拉曼散射时,时域相位共轭技术的补偿效果优于频域相位共轭技术;当上述四种效应同时考虑时,频域相位共轭技术的补偿效果略优于时域相位共轭技术。同时还对上述两种补偿技术的应用进行了讨论。     

Subpicosecond pulse transmission over 144 km using midway opticalphase conjugation via a cascaded second-order process in a LiNbO3 waveguide

We demonstrate for the first time the feasibility of 600-fs pulse transmission over 144 km using midway optical phase conjugation via a cascaded second-order process in a periodically poled LiNbO₃ waveguide. The output pulses are negligibly distorted and broadened to 680 fs. This result could not be obtained without complete compensation for fourth-order dispersion performed by midway optical phase conjugation     

WDM transmissions exploiting optical phase conjugation

Due to the frequency shift occurring in the process of optical phase conjugation (opc) generation, the application of opc in multiple-channel transmission is limited severely by the third-order dispersion. In this paper, taking into account the influence of the frequency shift and the compensation of nonlinear effects, we present an effective method to optimize the dispersion map for broad-band transmissions using optical phase conjugation. The numerical simulation results show that high efficiency wavelength-division-multiplexing (wdm) transmissions can be achieved by using the combination of opc with an optimized dispersion map.     

Long-span optical FSK heterodyne single-filter detection transmission experiment using a phase-tunable DFB laser diode

High receiver sensitivity (?51.9 dBm) and long span (243 km) transmission expriments have been achieved with a 140 Mbit/s optical FSK heterodyne single-filter detection system, using a phase-tunable DFB laser diode as a transmitter. This has enabled direct FSK modulation without waveform distortion. Also, a 280 Mbit/s 204 km transmission experiment has been carried out successfully.     

自陡峭效应对相位共轭系统脉冲传输的影响

光学相位共轭(OPC)技术能够同时且高效地补偿光纤传输过程中色散及非线性效应所导致的信号失真，且该技术同脉冲调制方式无关。从理论上分析了在自陡峭效应(SS)作用下高斯脉冲信号在中距相位共轭系统中的传输演化特性，数值模拟了在其作用下超短飞秒高斯脉冲的动态传输过程，讨论了自陡峭效应对中距相位共轭系统复原性能的影响。结果表明自陡峭效应将导致高斯脉冲信号发生峰值漂移和脉冲后沿变陡，相位共轭系统不能补偿由此导致的脉冲失真和畸变。引入合适的色散可以减小这种信号失真，并使得相位共轭系统能够同时补偿由于色散、自相位调制和自陡峭效应而引起的信号失真。     

利用相位共轭技术补偿光纤传输信号的失真

本文阐述了利用光学相位共轭技术进行补偿色散和复原非线性失真的理论基础，分析了其补偿方法和基本原理。     

Transmission performance of chirp-controlled signal by usingsemiconductor optical amplifier

We examine the fiber transmission performance of the optical signal whose chirp is controlled by utilizing phase modulation in semiconductor optical amplifier (SOA) with both simulations and experiments. This chirp control technique converts a positive chirp created by electroabsorption (EA) modulator into negative chirp, which reduces the waveform degradation due to the chromatic dispersion in transmission over standard single-mode fiber (SMF). It also provides an optical gain that is sufficient to compensate the insertion loss of the EA modulator. We investigate how the chirp control is affected by the input power to the SOA and the carrier lifetime of the SOA. As the SOA input power increases, the negative chirp becomes large, while the waveform is largely distorted due to gain saturation. However, the waveform distortion at high SOA input powers can be shaped by using a frequency discriminator. The acceleration of the carrier lifetime also reduces the waveform distortion due to gain saturation. We demonstrate that the chirp control technique is effective even for a high bit rate optical signal up to 10 Gb/s, when the carrier lifetime is expedited by optical pumping     

Simultaneous Cancellation of Fiber Loss, Dispersion, and Kerr Effect in Ultralong-Haul Optical Fiber Transmission by Midway Optical Phase Conjugation Incorporated With Distributed Raman Amplification

An alternative application of distributed Raman amplification (DRA) for ultralong-haul optical fiber transmission is proposed. In our study, the DRA is employed in a transmission system using midway optical phase conjugation (OPC) for amplifying an optical signal and, at the same time, for constructing signal power evolution, which is symmetrical with respect to the midpoint of the system where the OPC is performed. Then, the nonlinear signal waveform distortions that are caused by the Kerr effect, as well as fiber dispersion, are almost completely compensated by the OPC, whereas the fiber loss is compensated by the DRA. Three possible symmetrical signal power maps - a power map that has a reverse sign of the power map that is caused by lump amplification, a flat signal power map, and an arbitrary symmetrical signal power map - are numerically designed by using appropriate Raman pump powers. We show that the flat power map exhibits smaller difference from the target and a higher optical signal-to-noise ratio and requires lower pump power than the other two power maps. Numerical simulation results demonstrate that, by employing the flat power maps with a span of 40 km, a single-wavelength signal whose data rate is 160 Gb/s can be successfully transmitted over 5000 km, and the Kerr effect is sufficiently suppressed near limitation due to the nonlinear accumulation of noise. Finally, we study the feasibility of expanding our method to wavelength-division-multiplexed signal transmission by designing a DRA gain with multiple-wavelength pumping to simultaneously obtain a flat power map and a wide-and-flat gain bandwidth. By using four-wavelength Raman pumps while carefully choosing pump wavelengths and their powers, we achieve the DRA gain that simultaneously gives a fluctuation of the signal power of only 3.5%, a gain ripple of only 5.3%, and, at the same time, a gain bandwidth of as wide as 46 nm.     

Computationally Efficient Nonlinearity Compensation for Coherent Fiber-Optic Systems

Split-step digital backward propagation (DBP) can be combined with coherent detection to compensate for fiber nonlinear impairments.A large number of DBP steps is usually needed for a long-haul fiber system,and this creates a heavy computational load.In a trade-off between complexity and performance,interchannel nonlinearity can be disregarded in order to simplify the DBP algorithm.The number of steps can also be reduced at the expense of performance.In periodic dispersion-managed long-haul transmission systems,optical waveform distortion is dominated by chromatic dispersion.As a result,the nonlinearity of the optical signal repeats in every dispersion period.Because of this periodic behavior,DBP of many fiber spans can be folded into one span.Using this distance-folded DBP method,the required computation for a transoceanic transmission system with full inline dispersion compensation can be reduced by up to two orders of magnitude with negligible penalty.The folded DBP method can be modified to compensate for nonlinearity in fiber links with non-zero residual dispersion per span.     

Transmission of 2-ps optical pulses at 1550 nm over 40-km standardfiber using midspan optical phase conjugation in semiconductor opticalamplifiers

We demonstrate transmission of 2-ps optical pulses at 1550 nm over 40 km of standard fiber by employing midspan optical phase conjugation in semiconductor optical amplifiers (SOAs). The second-order group-velocity dispersion of the fiber is completely compensated and the third-order dispersion becomes a major transmission limitation. This experiment shows that the midspan optical phase conjugation system using SOAs is applicable to ultrahigh bit rates greater than 100 Gb/s     

Nonlinear distortion generated by DSF-based optical-phase conjugators in analog optical systems

In this paper, the nonlinear distortion induced by dispersion-shifted-fiber-based optical-phase conjugators (OPCs) in intensity-modulated subcarrier multiplexing optical systems is evaluated. In this study, it is shown that the nonlinear distortion mainly arises from the four-wave mixing (FWM) process during the phase conjugation, although there also exists influences from other nonlinear effects. Closed expressions for calculating the second- and third-order harmonic distortions due to FWM, self-phase modulation, cross-phase modulation and group-velocity dispersion effects in the dispersion-shifted-fiber-based OPC are also reported for the first time. The influence of several system design parameters, such as the optical modulation index, the number of channels, the input optical powers, and the effective area of the dispersion-shifted fiber on the compensation of fiber-induced nonlinear distortions employing the optical-phase conjugation technique, is considered.     

Fiber nonlinearity compensation of WDM-PDM 16-QAM signaling using multiple optical phase conjugations over a distributed Raman-amplified link

In this paper, multiple optical phase conjugation (OPC) devices were used along the optical link to improve the performance of an \(8\times 256\) Gbps polarization-division multiplexing 16-state quadrature amplitude modulation signaling, producing total bit rate of 2.048 Tbps. A 50-GHz spaced, eight-channel wavelength division multiplexing (WDM) communication system was considered using 912 km dispersion-unmanaged standard single-mode fiber link with backward distributed Raman pumps. The performance of a dual-pump highly nonlinear fiber-based OPC was investigated analytically using a set of eight nonlinear Schrödinger equations taking into account the effect of polarization. Simulation results were compared with the case of mid-span optical phase conjugation (MS-OPC) compensation scheme showing better performance in terms of achievable Q-factor, optimal signal launched power, and the total length of the transmission link. In 256 Gbps, single-channel scenario, a Q-factor improvement of 1.35 dB was achieved and the nonlinear threshold was increased by \(\sim \) 4 dB compared to the case of MS-OPC. Moreover, using multiple OPC led to increase the length of the transmission link by 30.7% compared with the case of MS-OPC. In 2.048 Tbps WDM system, a maximum Q-factor of 9.27 dB over the same link was obtained showing an improvement of 0.62 dB over the MS-OPC case. The simulation results were compared with published analogous experimental data showing very good agreement.     

Simultaneous suppression of third-order dispersion and sideband instability in single-channel optical fiber transmission by midway optical phase conjugation employing higher order dispersion management

In optical phase conjugation (OPC) systems, the third-order dispersion (TOD) of optical fibers and the nonlinear resonance at well-defined signal sideband frequencies called sideband instability (SI) mainly limit the transmission performance. We propose, for the first time, a scheme for simultaneous suppression of both TOD and SI in OPC systems using a periodic higher order dispersion-managed link consisting of standard single-mode fibers (SMFs) and reverse dispersion fibers (RDFs). Computer simulation results demonstrate the possibility of 200-Gb/s data transmission over 10 000 km in the higher order dispersion-managed OPC system, where the dispersion map is optimized by our system design strategies.     

A minimum distortion transmission scheme under combined effects of chromatic dispersion and SPM based on optical time-domain fractional Fourier transformation

In this paper, an optical time-domain fractional Fourier transformation (FRFT) system is proposed to achieve the minimum distortion transmission under combined effects of chromatic dispersion and self-phase modulation (SPM). In the new method, the pulses operated as FRFT will propagate in a new domain, in which the waveform in time domain will keep nearly unchanged through the transmission. The novel method achieves a 400 km optical transmission for an optical pulse with the full width at the 1/e point of peak power of 80 ps without any dispersion compensation and the pulse offers a nice performance with negligible nonlinear distortion. Compared with the soliton communication, this scheme shows more advantages on linear and nonlinear distortions without strict restriction to input pulses.     

Generation of optical phase-conjugate waves and compensation forpulse shape distortion in a single-mode fiber

The generation of optical phase-conjugate waves and the application of optical phase conjugation (OPC) to optical communication systems is described. The method of pulse shape distortion compensation by OPC is outlined including distortion due to both fiber dispersion and the optical Kerr effect. The generation of a forward-going phase-conjugate wave in a third-order nonlinear medium is discussed and that by a nondegenerate forward four-wave mixing in a zero-dispersion single-mode fiber (SMF) is investigated. Suppressing the stimulated Brillouin scattering (SBS) of a pump wave in the fiber prevents saturation of the generation efficiency of the phase-conjugate wave even when the pump power exceeds the SBS threshold. In transmission experiments through a 200-km standard SMF with a 16-Gb/s intensity-modulated signal and a 5-Gb/s continuous-phase FSK (CPFSK) modulated signal, it is shown the applicability of OPC is modulation independent and that OPC effectively compensates for both chromatic dispersion and the optical Kerr effect