Implications of fiber grating dispersion for WDM communication systems

For high bit-rate dense wavelength-division multiplexed (DWDM) applications fiber grating dispersion for the transmitted adjacent channels is shown to be detrimental and ultimately leads to a penalty. We consider design criteria for fiber grating filters in DWDM systems using both Gaussian pulses and super-Gaussian pulses that approximate square pulses that are more common in nonreturn-to-zero (NRZ) systems.     

Crosstalk in fiber Raman amplification for WDM systems

The crosstalk between channels in Raman amplification for two-channel WDM system is calculated. Theory shows that severe crosstalk can occur even in the linear amplification (or pump undepletion) region. To ensure small crosstalk, the signal gain and injected pump power should be limited to values well below the threshold of Raman amplification. As a numerical example, a 30-dB gain penalty and 3-dB pump power penalty occur when a 30-dB signal-to-interference ratio (SIR) is required. Thus, the conversion efficiency of Raman amplification in WDM systems is very low     

Impact of cross-phase modulation for WDM systems over positive andnegative dispersion NZ-DSF and methods for its suppression

The impact of cross-phase modulation (XPM) is measured on positive and negative dispersion fibre. The exponential build-up of XPM over positive dispersion fibre indicates amplification due to modulation instability. Methods for suppressing XPM crosstalk are also demonstrated     

Optimal dispersion management schemes for WDM soliton systems

Optimal dispersion profiles for a WDM soliton system are obtained by minimising the radiative noise and the collision-induced frequency shift. A two-step optimal profile without dispersion slaving to the ideal exponential profile allows a large increase in the amplifier spacing with no significant deterioration of the system performance     

Intrachannel four-wave mixing in dispersion managed RZ systems

We model the interaction of three consecutive pulses in a dispersion managed transmission system, and obtain an analytical expression for the energy exchange. The results show that interaction can be suppressed using unequally spaced pulses, which has been verified by the computer simulations. The central pulse pumps energy to the adjacent pulses if the phase shift is zero, and the energy exchange process is reversed if the central pulse has a phase shift of π/2     

Reduction of four-wave mixing crosstalk in WDM systems usingunequally spaced channels

Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A technique to design the channel frequency allocation in order to minimize the crosstalk due to FWM is presented. It is shown that suitable unequal channel separations can be found for which no four-wave mixing product term is superimposed on any of the transmitted channels. This is obtained at the expense of some expansion of the system bandwidth. Simulations are presented to show the effectiveness of this technique in a 10-channel, 10-Gb/s per channel, system     

波分复用系统中的高阶色散管理技术

光纤色散和非线性是影响密集波分复用(DWDM)系统性能的主要因素。高阶色散管理技术，可以有效地减小四波混频(FWM)和信号波形失真。介绍了该技术的基本原理及不同色散分布方法对系统性能的影响。     

Fiber four-wave mixing in multi-amplifier systems with nonuniformchromatic dispersion

Fiber fiber-wave mixing (FWM) is studied for multichannel multi-amplifier systems composed of short fibers with different zero-dispersion wavelength. An analytical expression describing FWM in these systems is derived. Using the expression, crosstalk is calculated for various combinations of fiber lengths, which are selected according to a random function, and allowable fiber input power is evaluated. The results show that the allowable input power for nonuniform dispersion is larger than that for uniform dispersion by several dB, depending on system conditions     

Optimum two-step dispersion management in WDM soliton systems

The first numerical simulations of a complete transoceanic wavelength-multiplexed soliton transmission system applying dispersion management are presented. Guidelines for management optimisation are given. The simple optimised two-level dispersion allocation scheme offers more than a doubling of the amplifier span without significant transmission degradation     

Four-wave-mixing suppression effect of dispersion varying fiber

This letter describes the four-wave-mixing (FWM) suppression effect of dispersion varying fiber (DVF) whose chromatic dispersion increases (or decreases) along its length. The FWM suppression performance is investigated for the dispersion variation rate and its cycle number in the DVF. Moreover, it is clarified experimentally that the DVF functions efficiently in wavelength division multiplexing (WDM) systems with an input power of more than 10 dBm/ch     

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     

Impact of spectral inverter fiber length on four-wave mixingefficiency and signal distortion

We compare the performance of spectral inversion through four-wave mixing in a long (13.5 km) and a short (1.5 km) dispersion-shifted optical fiber, respectively. While the effectiveness of the long fiber is limited by nonlinear phenomena such as stimulated Brillouin scattering and cross-phase modulation, the effectiveness of the short fiber is limited only by the available power, since the critical power levels where the deleterious nonlinear phenomena occur increase when the fiber length decreases. With the requirement of negligible spectral deformation of the inverted signal, the shorter fiber gives 60% higher conversion efficiency and four times more converted optical power. Measured with 12-nm wavelength conversion from the signal to the inverted replica     

General optical all-pass filter structures for dispersion controlin WDM systems

All-pass filters (APFs) are devices that allow phase correction or equalization without introducing any amplitude distortion. An optical implementation of such devices is very attractive since they can be used for dispersion compensation. In contrast to other dispersion control devices, optical APFs can correct any order of dispersion. This can be achieved by careful design of multistage APFs to approximate a target phase profile. However, large dispersion is usually narrow band or requires many filter stages. These performance tradeoffs and the general phase properties of optical APFs are reviewed and clarified in the first part of this paper. In the second part, a general design methodology of optical APFs is introduced. We show that any all-pass structure may be constructed from simple N-port devices (such as directional couplers or Mach-Zehnder interferometers) with N-1 outputs fed back to any of the N-1 inputs. The feedback paths may contain delays or further APFs (recursive design). This set of design rules allows for constructing complex all-pass filters of any number of stages starting with very simple elements. We use this technique to demonstrate a number of optical all-pass structures that may be implemented in planar waveguide or using thin-film filter technology     

WDM系统中色散走离条件下的拉曼串话研究

文章提出了一种考虑光纤色散的受激拉曼散射(SRS)效应的分析模型,它计算简便,且物理意义明确.依据该模型,分析计算了波分复用(WDM)系统的输出平均功率和SRS串话损害系数,以及由于SRS引起的输出功率波动的标准差,分析了色散走离可以降低SRS引起的输出功率波动,并对此进行了计算.     

Cross-phase modulation in multispan WDM optical fiber systems

The spectral characteristics of cross-phase modulation (XPM) in multispan intensity-modulation direct-detection (IM-DD) optical systems are investigated, both experimentally and theoretically. XPM crosstalk levels and its spectral features are found to be strongly dependent on fiber dispersion and optical signal channel spacing. Interference between XPM-induced crosstalk effects created in different amplified fiber spans is also found to be important to determine the overall frequency response of XPM crosstalk effects. XPM crosstalk between channels with different data rates is evaluated. The crosstalk level between higher and lower bit rate channels is found to be similar to that between two lower bit rate channels. The effect of dispersion compensation on XPM crosstalk in multispan optical systems is discussed and per span dispersion compensation was found to be the most effective way to minimize the effect of XPM crosstalk     

波分复用系统中色散系数选择优化设计

对传输容量为4 × 2.5Gb/s、传输距离为300km的波分复用系统进行了仿真实验.在考虑了非线性效应的情况下,分析了不同色散系数对系统性能的影响.仿真结果表明,在路径平均色散为零的波分复用光纤线路中,色散系数选择在20～30ps/(nm·km)左右系统性能最优.     

Effect of four-wave mixing on WDM optical systems: a statistical analysis

The dense wavelength-division-multiplexed system design requires an accurate knowledge of the four-wave mixing (FWM), which gives rise to intermodulation effects, especially when using very low-dispersion fibers. In this letter, a statistical analysis is presented that takes into account both the effect on the FWM intermodulation products due to dispersive propagation, and the statistics of such intermodulation terms, which must be considered as random processes. In particular, the autocorrelation function of the FWM process is calculated for the independent wavelength-division-multiplexing optical channel.     

Dispersion of cascaded fiber gratings in WDM lightwave systems

Fiber gratings operating in the transmission mode can provide high dispersion at wavelengths close to the Bragg resonance. When multiple gratings are cascaded for wavelength division multiplexing (WDM) applications, the net dispersion between the stop bands of any two consecutive gratings is significantly modified. We discuss the dispersion characteristics of such cascaded fiber gratings and propose a dispersion compensator for simultaneous compensation of group-velocity dispersion (GVD) for multiple channels of a WDM lightwave system. We also discuss the impact of the dispersion possessed by cascaded gratings on grating based add-drop multiplexers     

Nonlinear techniques for interference suppression inspread-spectrum systems

The binary nature of direct-sequence signals is exploited to obtain nonlinear filters that outperform the linear filters hitherto used for this purpose. The case of a Gaussian interferer with known autoregressive parameters is considered. Using simulations, it is shown that an approximate conditional mean (ACM) filter of the Masreliez type performs significantly better than the optimum linear (Kalman-Bucy) filter. For the case of interferers with unknown parameters, the nature of the nonlinearity in the ACM filter is used to obtain an adaptive filtering algorithm that is identical to the linear transversal filter except that the previous prediction errors are transformed nonlinearly before being incorporated into the linear prediction. Two versions of this filter are considered: one in which the filter coefficients are updated using the Widrow LMS algorithm, and another in which the coefficients are updated using an approximate gradient algorithm. Simulations indicate that the nonlinear filter with LMS updates performs substantially better than the linear filter for both narrowband Gaussian and single-tone interferers, whereas the gradient algorithm gives slightly better performance for Gaussian interferers but is rather ineffective in suppressing a sinusoidal interferer     

Modal dispersion control and distortion suppression of picosecondpulses in suspended coplanar waveguides

The propagation and dispersion characteristics of picosecond electrical pulses in a suspended coplanar waveguide (SCPW) are investigated, and it is shown that the SCPW is a very promising transmission structure for ultrashort pulses. Numerical results for the modal dispersion of the SCPW are presented and compared to those of the conventional CPW, and a field-coupling theory is used to explain the evolution in the dispersion behavior. An evaluation based on the numerical analysis shows that a SCPW with properly controlled dispersion can exhibit a five times improvement in pulse transmission capability compared to the conventional CPW. Both computer simulations and experimental measurements show a substantial suppression in pulse distortion as well, compared to conventional CPWs