放大器噪声对色散管理光孤子传输的影响

采用变分法讨论了放大器噪声对色散管理光孤子脉冲参数的影响。讨论了ASE噪声对脉冲相位、脉冲中心时间和中心频率的影响，并且对比讨论了采用密集色散管理系统和降低平均色散值后，ASE噪声对脉冲的影响，结果表明：密集色散管理孤子系统和平均色散接近于零的色散管理系统更有利于降低放大器噪声对脉冲的扰动。     

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.     

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.     

普通单模光纤中光孤子传输问题的研究

在使用普通单模光纤的光孤子传输系统中,放大器自发辐射（ASE）噪声和色散波逸出是限制光孤子传输的两个主要因素,它们将引起孤子能量科动和定时抖动,从而导致系统误码。本文分别研究了带限光滤波器对ASE噪声的抑制和非线性增益对色散波的抑制以及对孤子传输系统的稳定作用,并设计了使用普通单模光纤的光孤子传输系统。     

Optical phase conjugation for ultra long-haul phase-shift-keyed transmission

Nonlinear phase noise (Gordon-Mollenauer phase noise) can limit the transmission distance for phase-shift-keyed modulation formats. In this paper, the compensation of nonlinear phase noise by a midlink optical phase conjugation (OPC) is studied. A proof-of-principle experiment is presented showing an over 4-dB improvement in Q factor when OPC is employed in a differential phase-shift-keying (DPSK) system. Also, an ultra long-haul OPC-based differential quadrature phase-shift-keying (DQPSK) transmission experiment is studied to show the impact of self-phase modulation (SPM)-induced impairments, including nonlinear phase noise, in a transmission line. OPC results in a 44% increase in transmission distance when compared to a "conventional" transmission system using dispersion compensating fiber (DCF) for chromatic dispersion compensation.     

Parametric-gain approach to the analysis of single-channel DPSK/DQPSK systems with nonlinear phase noise

This paper presents a novel method based on a parametric gain (PG) approach to study the impact of nonlinear phase noise in single-channel dispersion-managed differentially phase-modulated systems. This paper first shows through Monte Carlo simulations that the received amplified spontaneous emission (ASE) noise statistics, before photodetection, can be reasonably assumed to be Gaussian, provided a sufficiently large chromatic dispersion is present in the transmission fiber. This paper then evaluates in a closed form the ASE power spectral density by linearizing the interaction between a signal and a noise in the limit of a distributed system. Even if the received ASE is nonstationary in time due to pulse shape and modulation, this paper shows that it can be approximated by an equivalent stationary process, as if the signal were continuous wave (CW). This paper then applies the CW-equivalent ASE model to bit-error-rate evaluation by using an extension of a known Karhunen-Loe/spl acute/ve method for quadratic detectors in colored Gaussian noise. Such a method avoids calculation of the nonlinear phase statistics and accounts for intersymbol interference due to a nonlinear waveform distortion and optical and electrical postdetection filtering. This paper compares binary and quaternary schemes with both nonreturn- and return-to-zero (RZ) pulses for various values of nonlinear phases and bit rates. The results confirm that PG deeply affects the system performance, especially with RZ pulses and with quaternary schemes. This paper also compares ON-OFF keying (OOK) differential phase-shifted keying (DPSK) systems, showing that the initial 3-dB advantage of DPSK is lost for increasing nonlinear phases because DPSK is less robust to PG than OOK.     

色散补偿方式对相位调制系统中相位噪声的影响

分别推导了相位调制系统中采用色散预补偿方式和后补偿方式下非线性相位噪声的计算公式,基于此对这两种色散补偿方式下的相位噪声、功率容限以及最优信号峰值功率进行了详细的分析和讨论,结果表明:采用色散预补偿方式较后补偿方式能更有效地抑制非线性相位噪声,其对非线性相位噪声的抑制能力随着信号能量、放大自发辐射(ASE)的功率谱密度以及传输距离的增加而提高;同时,色散预补偿系统具有更高的功率容限;色散的作用使系统的最优信号峰值功率增大,最佳相移大于1rad;色散预补偿系统的最优信号峰值功率大于色散后补偿系统。     

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     

基于OPC的高速PDM-CO-OFDM系统研究

通过理论分析和数值仿真,搭建了100Gbit/s的PDM(偏振复用)-CO-OFDM(相干光正交频分复用)系统,并使用OPC(光学相位共轭)技术对100Gbit/s的PDM-CO-OFDM系统的传输性能进行了补偿。研究表明,相位共轭技术不仅能够补偿系统的各阶色散,还能补偿非线性效应,使用OPC技术能使系统的传输距离达到1 200km,在最佳补偿距离上,使用OPC的系统Q值比使用传统的DCF(色散补偿光纤)的系统Q值大6.3dB。     

Performance analysis of an optical serial-to-parallel converterwith erbium-doped fiber amplifier

In this paper, we develop a performance analysis of an surface-emitting second harmonic generation (SESHG) optical serial-to-parallel converter using an erbium-doped fiber amplifier (EDFA) as a preamplifier. The analysis is complicated by the fourth-order nonlinearity that acts on the signal plus amplified spontaneous emission (ASE) noise to create many beat noises at the binary decision device. However, we demonstrate that Gaussian approximation for the beat noise statistics is reasonable. We calculate the BER of the system as a function of the SHG nonlinear cross section (A^NL), EDFA gain, the bandwidth of the optical filter that band-limits the ASE noise, and the timing pulse-to-data pulse power ratio. We find that for reasonable values of these and other parameters, the EDFA/SESHG serial-to-parallel converter combination should be able to operate at or below a BER of 10^-12. We find that small increments (0-2 dB) in the signal power that is input to the EDFA are enough to compensate the effects of ASE noise for most of the parameter variations we consider. From this point of view, the ASE noise has little effect on system performance. However, when the input power is fixed, we show evidence in terms of BER that the ASE noise plays a significant role, particularly in the high A^NL, high gain case. Also in this case, we show that the optimal timing pulse-to-data pulse power ratio is somewhat different from the value that is optimal for the system without an EDFA     

基于沃尔泰拉理论的集总光纤非线性噪声补偿

通过沃尔泰拉(Volterra)级数理论求得了非线性薛定谔方程(NLSE)的半解析解,在考虑光纤损耗、色散及非线性效应的情况下,推出了长距离在线级联掺铒光纤放大器(EDFA)光纤通信系统中信号和自发辐射噪声(ASE)之间耦合串扰的半解析表达式,得到了多跨距传输接收端输出信号和发射端输入信号之间的关系式.根据维纳(Wiener)滤波理论的时域滤波原理,在多跨距传输系统接收端设计了对非线性乘性噪声有滤波作用的维纳滤波器,并对预集总色散补偿、后集总色散补偿、分布链路色散补偿系统及一对三"跨距成比例平移对称的色散非线性同步补偿系统进行了仿真模拟研究.结果表明了提出的设计思路及方法的可行性,为进一步提高传输距离增大入纤功率提供了新的思路.     

光孤子在分布式光纤放大器中传输特性的研究

光纤放大器用于补偿光信号在光纤中的传输损耗,是全光通信网中的核心器件。建立了光孤子在分布式光纤放大器中传输的物理模型,采用分步傅里叶变换法数值模拟了光孤子的传输放大特性,讨论了增益色散对光孤子形状和频谱的影响。结果表明：在放大器的反常色散区,随着光孤子的放大,会不断地产生啁啾孤子,孤子频谱会加宽并且产生振荡结构。放大介质的增益色散将会使光孤子幅度下降,宽度展宽,频谱窄化。因此,光纤放大器的色散、非线性效应和增益色散均会对光孤子的传输特性产生影响。     

Compensation for fiber-induced composite second-order distortion inexternally modulated lightwave AM-SCM systems using optical-phaseconjugation

Compensation for composite second-order (CSO) distortion due to the interplay of fiber chromatic dispersion and self-phase modulation in externally modulated amplitude modulated subcarrier multiplexing (AM-SCM) systems employing optical-phase conjugation (OPC) is investigated. The simulation results show that significant reductions (>20 dB) of fiber-induced CSO may be achieved using OPC, which results in an increase of the transmission distance in AM-SCM video distribution systems operating near 1550 nm. In addition, a comparison with another compensation technique, the phase modulation technique, is established, showing that OPC is much less sensitive to variations in the fiber length. However it is also calculated that for practical implementation of the OPC-based compensation technique, 25 to 30 dB reduction in the amplified spontaneous emission (ASE) associated with the OPC will be required     

RF response of analog optical links employing optical phaseconjugation

The RF transfer function of analog optical links employing optical phase conjugation (OPC) for the compensation of the carrier suppression effect is investigated theoretically. It is well known that the ultimate performance of the OPC technique depends on several system parameters such as fiber span lengths, dispersion characteristics, input optical powers, or optical transmitter chirp. The influence of these parameters on the RF system response is studied by means of simulation, showing that the main degradation of the equalized response is due to the joint action of nonlinear effects and asymmetric optical power change along the fiber spans     

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     

A comparison study of the Shannon channel capacity of various nonlinear optical fibers

A comparative study of the Shannon channel capacity is presented for a dispersion-free fiber, a fiber with constant dispersion, and a fiber with variable dispersion. Improvement of the capacity by optical phase conjugation (OPC) is also investigated. Simple scaling laws are prescribed for the dependence of the optimal capacity on various system settings such as number of spans, number of channels, noise power, channel width, strength of chromatic dispersion, bandwidth of an OPC device, etc.     

Nonlinear Evolution of Gaussian ASE Noise in ZMNL Fiber

This paper investigates the evolution of kurtosis of the input Gaussian amplified spontaneous emission (ASE) noise in a nonlinear fiber with negligible dispersion. The nonlinear Schrodinger equation (NLSE) describing propagation in optical fibers is simplified such that the fiber represents a zero memory nonlinear (ZMNL) system, and this approximation allows the development of analytical formulas for the statistical moments of the output noise. It is possible to calculate moments of all integer orders and the explicit expressions for the first four moments are given. The investigations show that the ASE noise does not preserve its Gaussian character when Kerr nonlinearity is significant. This observation proves that the common assumption of the Gaussian output ASE is not necessarily valid. Numerical simulations are provided to support the derivation. Kurtosis deviating significantly from the value typical for Gaussian noise is also an indicator that BER calculation in the coherent systems based on the assumption that ASE is Gaussian is likely to be inaccurate.     

La transmission par solitons sur fibre optique soliton transmission on optical fibres

Optical amplification has fundamentally changed the design of optical fibre transmission systems, especially transoceanic and other such long-haul systems. These changes have made it necessary to consider the cumulative effects of chromatic dispersion and self phase modulation as well as the effect of amplified spontaneous emission (ase) noise. In this context, transmission using solitons is an attractive alternative for high bitrate transmission. Its practical implementation is subject to limitations, however, due primarily to non-linear interaction between signal and noise. Current research work is focused on overcoming these limitations, in particular through the introduction of on-line optical processing techniques of varying degrees of sophistication. This article looks at results achieved by cnet.     

Noise figure monitoring of optical amplifiers via backwardamplified spontaneous emission

A new configuration of an optical amplifier is proposed that provide in-service noise figure (NF) monitoring. It is difficult to determine the NF of traditional optical amplifiers in operation because the amplified spontaneous emission (ASE) power cannot be measured separately from the output signal power. The proposed technique measures the NF from the backward ASE power. The discrepancy between the NF measured from forward ASE (using the traditional technique) and backward ASE (proposed technique) is found to be less than 1.0 dB at input powers below -6.2 dBm. A prototype optical amplifier is successfully used as a linear repeater, in a 10 Gbit/s 200 km transmission experiment     

On the joint effects of fiber parametric gain and birefringence andtheir influence on ASE noise

Parametric gain (PG) in optical fibers may substantially enhance amplified spontaneous emission (ASE) noise, especially in long-haul amplified links. We present new results that permit to accurately characterize the effects of PG on ASE noise, as well as the interplay of PG and ASE noise with fiber dispersion in both the anomalous and normal dispersion regions. In a recent letter, we introduced the concept of a PG transfer matrix, that allows the easy evaluation of ASE noise enhancement over chains of amplified fiber spans. In the same letter we showed that the transfer matrix can be expressed in analytical closed-form when a scalar (single polarization) fiber is assumed. In this paper we extend the analysis to a more realistic two-polarization fiber model that accounts for both linear and nonlinear polarization coupling effects. We show that the new transfer matrix does not have an analytical expression, but can be easily evaluated using standard numerical algorithms. ASE noise enhancement due to PG turns out to be slightly lower in a realistic birefringent two-polarization fiber than a single-polarization fiber. An interesting result is that the single polarization model yields a convenient approximation to ASE noise enhancement, that can be evaluated analytically