Analysis of the cross-phase modulation in dispersion compensatedWDM optical fiber systems

Wavelength-division-multiplexed (WDM) optical systems are affected by cross-phase modulation (XPM) due to Kerr effect. In this letter, dispersion compensated optical WDM systems affected by XPM are investigated by an analytical model. The theoretical results fit rather well with previous simulations performed in the case of fully compensated and under compensated optical fiber links     

Dependence of cross-phase modulation on channel number in fiber WDMsystems

The phase term appearing in the expression for cross-phase modulation due to the optical Kerr effect depends on the sum of the powers carried by each wavelength channel. For this reason, one might expect that the amount of cross-phase modulation would increase with increasing channel number, causing increased interference among channels and hence limiting the total number of channels that a WDM system can support. However, computer simulations of multichannel systems have shown no change in signal distortion as the number of wavelength channels is increased from four to eight. In a simulated three-channel system, the signal distortion of the central channel approaches that of a single-channel system as the wavelength separation is increased to approximately 2 nm. Thus, even a moderate amount of dispersion tends to cancel out the influence of cross-phase modulation, so that beyond a certain wavelength spacing, additional channels do not interfere with the channel under consideration. From these observations, we conclude that cross-phase modulation does not limit the number of wavelength channels that a single optical fiber can support. However, self- and cross-phase modulation are not the only nonlinear effects influencing fiber lightwave systems. Stimulated Raman scattering tends to transfer optical power from short-wavelength channels to channels operating at longer wavelength, degrading their signal-to-noise ratio. The efficiency of this process increases with increasing wavelength spacing. Clearly, a compromise needs to be reached between the conflicting requirements imposed by the optical Kerr effect and by stimulated Raman scattering     

Limitations in 10 Gb/s WDM optical-fiber transmission when using avariety of fiber types to manage dispersion and nonlinearities

We analyze the system limitations of WDM transmission when using various types of optical fiber to manage dispersion and nonlinearities. In our model, from two to eight 10 Gb/s WDM channels are transmitted through a cascade of EDFA's experiencing dispersion, stimulated Raman scattering, and self- and cross-phase modulation. The fiber types modeled include: conventional single-mode fiber, dispersion shifted fiber, and dispersion-compensating fiber. These fibers have different dispersion spectral profiles and are combined to manage dispersion to produce a total zero dispersion for a certain fiber span while eliminating four-wave mixing. We find that a system using dispersion-shifted fiber and conventional single-mode fiber exhibits the best performance, with the combination of dispersion and cross-phase modulation as the dominant effects. Furthermore, conventional single-mode fiber combined with dispersion-compensating fiber system exhibits the worst performance, with the combination of dispersion and self-phase modulation as the dominant effects     

一种基于改进DD-RLS的非线性相位噪声追踪算法

在相干光纤通信系统中,为了提高接收端的相位追踪算法性能,提出了一种相位追踪算法——基于改进的判决导向递归最小二乘(DD-RLS)算法,将改进的DD-RLS算法和非线性前补算法结合应用于WDM相干光纤系统中抑制交叉相位调制(XPM).传输仿真结果表明:该算法能够有效抑制系统中的XPM效应,在1000 km的11个信道的波...     

Limitation to WDM transmission distance due to cross-phasemodulation induced spectral broadening in dispersion compensatedstandard fiber systems

We describe large spectral broadening due to the interaction of cross-phase modulation/self-phase modulation and fiber dispersion, and explain its contribution to the penalties in standard fiber systems pre- and postcompensated by dispersion compensating fiber. To our knowledge, this is the first measurement of this effect in a dense WDM transmission system at 10 Gb/s with this dispersion management scheme and good agreement is reported with the numerical modeling results     

Cross-phase modulation in an L-band EDFA

We measure the cross talk in a wavelength-division-multiplexed (WDM) system arising from cross-phase modulation (XPM) in an L-band erbium-doped fiber amplifier (EDFA). This effect has the potential of becoming the dominant nonlinear crosstalk mechanism in L-band WDM systems using standard fiber. We discuss how appropriate amplifier design can reduce the XPM in the EDFA     

Performance Comparison of WDM Systems with Different Duty Cycle

The performances of nonlinear WDM systems with different duty cycle are compared by means of numerical simulation.The numerical results show that the optical pulse with duty cycle of 0.5 is superior to the conventional NRZ modulation schene.The conclusion is different from that of some references.The reason is that inter symbol interference is not included in some references.In fact,inter symbol interference plays an important role in monlinear WDM system.Although the larger the duty cycle is,the stronger the effect of the cross-phase modulation and self-phase modulation on nonlinear WDM is however,the larger the duty cycle is ,the stronger the inter symbol interference is.     

Long-haul WDM transmission using higher order fiber dispersionmanagement

We propose a fiber dispersion management scheme for large-capacity long-haul wavelength division multiplexing (WDM) transmission systems that considers not only second- but also third-order dispersion characteristics using transmission fibers with opposite dispersion signs. It eliminates the waveform distortion of WDM signals that originates from the existence of third-order dispersion, which is a constraint placed on WDM capacity in conventional dispersion management, while reducing the interchannel interaction caused by the interplay of fiber nonlinearity and second-order dispersion. Design concept of the scheme is discussed to show the feasibility of using actual fiber parameters. An experimental investigation on transmission performance regarding the signal pulse format, nonreturn-to-zero (NRZ) and return-to-zero (RZ), and interchannel interaction caused by four-wave mixing (FWM) and cross-phase modulation (XPM) is described for optimizing WDM system performance. It is experimentally shown that RZ pulse transmission is possible without significant spectral broadening over a wide wavelength range in dispersion managed fiber spans. Using these results together with a wideband optical amplifier gain-bandwidth management technique, yields long-distance WDM transmission with the capacity of 25×10 Gb/s over 9288 km     

全光纤锁模腔结构的全光时钟提取实验研究

分析并实验验证了一种全光纤锁模激光器结构的全光时钟提取方案。方案中,采用高非线性光纤(HNLF)替代传统结构中的半导体光放大器(SOA),利用光纤中的交叉相位调制(XPM)效应实现腔内的非线性调制,避免了以SOA作为非线性光调制器件的锁模激光器全光时钟提取方案中,由于载流子恢复时间较长从而限制工作速率的缺点,以达到突破"电子瓶颈"的目的。理论分析了光纤中交叉相位调制的特性以及环形锁模腔的时钟提取原理,并通过实验,从40Gbit/s的光归零码(RZ)信号中提取出了高质量的光信号时钟。该方案可以直接在更高速率条件下工作。     

Carrier-induced phase noise in angle-modulated optical-fiber systems

Phase noise in angle-modulated optical-fiber communication systems arising from optical power fluctuations is analyzed. The nonlinear refractive index of silica is the physical mechanism which converts power fluctuations into phase fluctuations. The effects of self-phase modulation (an optical wave acting on itself) and cross-phase modulation in wavelength-division multiplexed (WDM) systems (one optical wave modulating a channel at a different wavelength) have been calculated. The phase noise generated in single-channel systems is negligible for laser fluctuations less than 1-mW rms. In WDM systems containing as few as four channels the phase noise exceeds tolerable levels (0.15 rad) for power fluctuation of 1 mW in each channel.     

Crosstalk due to optical fiber nonlinearities in WDM CATV lightwavesystems

Crosstalk in a two-wavelength 1550-nm standard fiber system at subcarrier frequencies 50-800 MHz is investigated. The dependence of the crosstalk on subcarrier frequency, wavelength spacing, and optical power is measured and analyzed. The observed crosstalk is attributed to three primary mechanisms: stimulated Raman scattering, cross-phase modulation, and the optical Kerr effect combined with polarization-dependent loss. At wavelength spacing greater than 9 nm, stimulated Raman scattering dominates. At wavelength spacing less than 5 nm, the primary contributor can be the optical Kerr effect with polarization dependent loss, except at higher modulation frequencies where cross-phase modulation also is significant. At even modest (by CATV standards) optical power, the crosstalk is as high as -40 to -45 dB     

Impact of Fiber Non-linearity in High Capacity WDM Systems and in Cross-Connected Backbone Networks

The rapidly increasing data traffic volumes will demand for very high transmission capacity and network nodes throughput. Wavelength division multiplexing (WDM) technology will be asked to support many channels on the same fiber, both in point-to-point links and in WDM optical networks. The transmission of a high number of wavelength channels in all these systems is a key issue. This paper analyzes this topic, in both high capacity links and optical networks, highlighting the impact of fiber non-linearity, and addressing the main source of impairments. This is done through the use of a semi-analytical model recently upgraded to account for all the contributions deriving from Kerr effects, particularly four-wave mixing and cross-phase modulation. The analysis reveals that more than one hundred of channels at 2.5 Gbit/s can be transmitted in point-to-point links whose length can span until the order of 1000 km, and 32 channels per fiber, at the previous bit rate, can be handled in WDM networks, without dispersion compensation. For a higher number of channels (e.g., 64) dispersion compensation is needed.     

Reduction of intrachannel nonlinear distortion in 40-Gb/s-based WDMtransmission over standard fiber

Intrachannel cross-phase modulation and four-wave mixing in high-bit-rate WDM transmission systems employing standard single-mode fiber are investigated. The effects of imperfect third-order dispersion compensation are included in the study and analytical expressions giving optimum values of dispersion precompensation minimizing the distortion due to the intrachannel nonlinear effects are derived     

A method of simulating intensity modulation-direct detection WDM systems

In the simulation of Intensity Modulation-Direct Detection WDM Systems,when the dispersion and nonlinear effects play equally important roles,the intensity fluctuation caused by cross-phase modulation may be overestimated as a result of the improper step size. Therefore,the step size in numerical simulation should be selected to suppress false XPM intensity modulation （keep it much less than signal power）. According to this criterion, the step size is variable along the fiber. For a WDM system,the step size depends on the channel separation. Different type of transmission fiber has different step size. In the split-step Fourier method,this criterion can reduce simulation time,and when the step size is bigger than 100 meters,the simulation accuracy can also be improved.     

Multicanonical Monte Carlo Modeling of Wavelength Division Multiplexed Differential Phase Shift Keying Systems

Differential phase shift keying (DPSK) modulation is being considered as a possible candidate for future optical wavelength division multiplexed (WDM) transmission systems. In a single channel link, the balanced interferometric DPSK receiver exhibits increased tolerance against amplified spontaneous emitting (ASE) noise and fiber nonlinear effects. In this paper, a model is presented that can be used to estimate the performance of a multichannel DPSK system taking into account the influence of interchannel phenomena, namely cross-phase modulation (XPM) and four wave mixing (FWM), in the phase noise statistics. The model is based on an approximate solution of the fiber propagation equation and the multicanonical Monte Carlo (MCMC) method. It provides an efficient tool that can be used to investigate the influence of many link design parameters such as channel spacing, launch power, and fiber dispersion. The model is illustrated in the comparison of the performance of multichannel DPSK to on-off keying (OOK) systems. It is verified that, even in the presence of interchannel effects, DPSK modulation greatly enhances the system performance compared to OOK.     

A 40-Gb/s/ch WDM transmission with SPM/XPM suppression throughprechirping and dispersion management

This paper proposes to combine prechirping with dispersion management scheme in such a way as to suppress the power penalty induced by self-phase modulation (SPM) and cross-phase modulation (XPM) in 40-Gb/s per channel wavelength-division multiplexed (WDM) transmission systems with long-amplifier spacing. First, we show that the optimum total dispersion to minimize SPM depends on prechirping and the local dispersion of the transmission fiber, unlike that for minimizing XPM. Next, it is shown that, by optimizing the combination of prechirping and local dispersion, these two optima can be made to match so as to improve the allowable maximum fiber input power. Finally, the operation of the proposed optimization scheme is confirmed experimentally, and 4×40-Gb/s WDM transmission over 400 km of nonzero dispersion-shifted fiber (NZDSF) is demonstrated successfully with the fiber input power of +10 dBm/ch and 250 GHz channel spacing     

160 Gb/s variable length packet/10 Gb/s-label all-optical label switching with wavelength conversion and unicast/multicast operation

We report on the first demonstration of all-optical label switching (AOLS) with 160 Gb/s variable length packets and 10 Gb/s optical labels. This result demonstrates the transparency of AOLS techniques from previously demonstrated 2.5 Gb/s to this 160 Gb/s demonstration using a common routing and packet lookup framework. Packet forwarding/conversion, optical label erasure/re-write and signal regeneration at 160 Gb/s is achieved using a WDM Raman enhanced all-optical fiber cross-phase modulation wavelength converter. It is also experimentally shown that this technique enables packet unicast and multicast operation at 160 Gb/s. The packet bit-error-rate is measured for all optical label switched 16 /spl times/ 10 Gb/s channels and error free operation is demonstrated after both label swapping and packet forwarding.     

Experimental investigation of signal distortions induced bycross-phase modulation combined with dispersion

Signal distortions in intensity modulated direct detection WDM systems induced by interaction of cross-phase modulation (XPM) and dispersion are investigated experimentally. The results quantify their reduction with increasing channel separation (walkoff) and are in good agreement with simulations. With the chosen setup it is shown that the limiting influence of XPM in standard single-mode fiber (SMF) systems is reduced efficiently for typical channel spacings (⩾0.8 nm)     

CF-RZ-DPSK for suppression of XPM on dispersion-managed long-hauloptical WDM transmission on standard single-mode fiber

We present a new optical modulation format chirp-free return-to-zero differential phase shift keying (CF-RZ-DPSK), which enables wavelength-division-multiplexing (WDM) transmission at 10 Gb/s/ch at a channel spacing of 100 GHz over 3000 km without significant impairments due to cross-phase modulation (XPM). A transmitter setup is presented, which allows a simple implementation of CF-RZ-DPSK with two Mach-Zehnder modulators in push-pull operation. The robustness toward XPM is shown theoretically with the help of a simple analytical model for the XPM-induced phase modulation. The superior performance of CF-RZ-DPSK over other modulation formats [RZ-ampfitude shift keying (ASK), nonreturn-to-zero (NRZ)-DPSK, and NRZ-ASK, respectively] is clarified. Finally, simulation results for CF-RZ-DPSK in comparison to RZ-ASK show the superior performance of the newly proposed modulation format in a dense WDM setup     

色散缓变光纤中基于交叉相位调制的不稳定性研究

研究了色散缓变光纤中基于交叉相位调制的不稳定性,得到了同时计及离散和光纤损耗效应时的色散关系式。发现在抽运功率、传输距离、光纤损耗相同的条件下,色散缓变光纤较常规光纤具有较宽的增益谱;研究同时发现,较大的色散缓变参量及两光束较小的离散均会使增益谱的谱宽加宽,振幅的增长速度加快。并用数值方法验证了利用色散缓变光纤更易产生超短脉冲。