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     

Gain optimization of germanosilicate fiber Raman amplifier and itsapplications in the compensation of Raman-induced crosstalk amongwavelength division multiplexing channels

Spectral characteristics of the stimulated Raman scattering (SRS) process were theoretically investigated for step-index silica optical fibers with various GeO₂ concentrations. Optimal-fiber lengths and germanium concentration, where the first Stokes power reaches maximum, were calculated at various pump power levels for application in Raman amplifiers. Based on this analysis, we proposed and experimentally demonstrated a new channel-equalizing technique to simultaneously compensate Raman-induced crosstalk and amplify wavelength-division-multiplexing (WDM) signals using a discrete Raman amplifier in the 1.5-μm range. As a further application of SRS in germanosilicate glass fibers, we introduce an all-optical variable attenuator for channel equalization that could be used in dynamic optical power tilt control in WDM systems     

Extending transmission distance of high-density WDM systems usingpost transmitter fiber Raman amplifiers

The authors investigate the behavior of multichannel signals in a high-density wavelength division multiplexing (WDM) system with the consideration of stimulated Raman scattering. A parameter to evaluate the depth of Raman crosstalk is provided, and the transmission distance limitation imposed by Raman crosstalk is analyzed. To extend the system transmission distance, a post-transmitter fiber Raman amplifier is used. The allocation of signal channels on the pump gain profile is discussed. It is found that there exists an optimum pump power which results in a maximum amplifier gain. The example shows that the transmission distance limitation imposed by Raman crosstalk can be extended by 20 km for a 30-channel system with 0.25-dB/km fiber loss     

Performance limitation of hybrid WDM systems due to stimulatedRaman scattering

We report on the effect of stimulated Raman scattering (SRS) on the hybrid wavelength-division-multiplexing (WDM) system. The stimulated Raman scattering-induced crosstalk was measured by using a two-channel hybrid WDM system and compared with the theoretically calculated values. Using this result, we estimated the maximum number of WDM digital channels that a hybrid WDM system could support     

Limitation of stimulated Raman scattering cancellation in WDMsystems via spectral inversion

Crosstalk arising from stimulated Raman scattering may be cancelled by spectral inversion for low-speed wavelength-division-multiplexing (WDM) systems. By evaluating the crosstalk variance, the limitation of the method is provided in term of the allowable walkoff length, limiting to low-speed system with long walkoff length up to hundred of kilometers     

Effect of modulation statistics on Raman crosstalk in WDM systems

Stimulated Raman scattering crosstalk in intensity-modulated WDM systems is analyzed, including modulation statistics. The actual penalty is found to be lower than that expected from a worst-case analysis. For a large number of channels, the depletion due to stimulated Raman scattering becomes half of the value found with a worst-case analysis and assumes a deterministic behavior     

光纤色散对WDM系统中受激喇曼散射串扰的影响

本文研究了波分复用系统中光纤色散对受激喇曼散射串扰的影响,提出了在考虑信号调制几率特性和脉冲走离效应后,系统误码率和功率代价的理论计算方法,并对典型系统进行了数值模拟。研究结果表明,由光纤色散引起的信号脉冲走离可以降低受激喇曼散射串扰,尤其是在复用信道较多、信号传输速率较高的系统中,光纤色散的作用更加明显     

Effect of group velocity dispersion on stimulated Raman crosstalkin multichannel transmission systems

We investigate the effect of group velocity dispersion on stimulated Raman crosstalk in intensity modulated multichannel transmission systems. A theoretical model for bit-error rates (BERs) and power penalty calculation is provided, where the effects of modulation statistics and group velocity dispersion are taken into account. It has been found that walkoff effects may reduce stimulated Raman scattering (SRS) crosstalk, and can not be neglected for systems with a large number of channels     

Eliminating SRS channel depletion in massive WDM systems viaoptical filtering techniques

We demonstrate that the channel depletion due to stimulated Raman scattering in massive wavelength-division-multiplexed (WDM) systems can be eliminated using high-frequency pass filters. These filters, when inserted appropriately into the transmission link, can effectively suppress the SRS power flow from the WDM channels to lower frequency noise. Numerical simulations carried out on WDM systems indicate that the channel depletion penalties can be kept below 0.25 dB even for a total channel power of 2 W     

Nonlinear crosstalk and two countermeasures in SCM-WDM opticalcommunication systems

We investigate, theoretically and experimentally, crosstalk between wavelengths in subcarrier-multiplexed (SCM) wavelength-division multiplexed (WDM) optical communication systems. Crosstalk arises mainly from interactions between subcarriers on one wavelength and the optical carrier of another wavelength. In a dispersive fiber, crosstalk can be attributed to stimulated Raman scattering (SRS) and cross-phase modulation (XPM) combined with group velocity dispersion (GVD). We investigate the phase relationship between SRS-induced and XPM-induced crosstalks. Crosstalks induced by SRS and XPM add in the electrical domain and can interfere constructively or destructively. Experimental results show that the combined crosstalk level can be as high as 40 dBc after 25 km of SMF with two wavelengths and 18 dBm per wavelength of transmitted power. We propose two crosstalk countermeasures. The first countermeasure uses parallel fiber transmission. We show theoretically that both SRS-induced and XPM-induced crosstalks can be cancelled to the first order. We present an experimental demonstration of concept which has achieved 15 dB of crosstalk cancellation over 200 MHz. The second countermeasure uses optical carrier suppression. We show, theoretically and experimentally, that by suppressing the optical carrier, we can significantly reduce crosstalk while maintaining the same link budget and carrier-to-noise ratio (CNR) at the receiver, 20 dB of crosstalk reduction over 2 GHz has been demonstrated experimentally     

Waveform distortion due to stimulated Raman scattering in wide-bandWDM transmission systems

The influence of stimulated Raman scattering (SRS) taking into account the random modulation and the walk-off effect on wide-band wavelength division multiplexing (WDM) transmission systems are studied theoretically. First, it is shown that power depletion due to SRS can be separated into average power loss and waveform distortion. The waveform distortion is evaluated for various types of pulse shapes and fibers, and simple equations for evaluating the waveform distortion due to SRS are derived. These equations can be easily applied for designing wide-band WDM transmission systems from the viewpoint of the SRS waveform distortion. We also compare results obtained from our method with those from the split-step Fourier method, and confirm validity of our method. Our method should be useful in the design of wide-band WDM transmission systems, in which SRS is expected to be a serious limitation     

Performance of WDM ring and bus networks in the presence ofhomodyne crosstalk

This paper examines the effects of coherent and incoherent homodyne crosstalk in wavelength division multiplexed (WDM) ring and bus networks using reconfigurable optical add-drop multiplexers (OADM's). It is widely understood that incoherent homodyne crosstalk causes power penalties at the receivers in these networks. We show that coherent homodyne crosstalk causes a range of possible received powers, and that coherent and incoherent crosstalk together lead to a range of possible power penalties. A Monte Carlo simulation is used to examine the probability distribution of power penalties due to homodyne crosstalk under various conditions. We find that increasing the switch and multiplexer crosstalk within each OADM, and increasing the number of WDM channels, all produce increased probabilities of large power penalties. However, the number of nodes through which a signal is transmitted does not affect the power penalty distribution     

A generalized suboptimum unequally spaced channel allocationtechnique. I. In IM/DD WDM systems

Four-wave mixing (FWM) is the most serious fiber nonlinearity associated with low-input optical power levels in long-haul multichannel optical systems employing dispersion-shifted fiber. To reduce the crosstalk due to FWM, a generalized suboptimum unequally spaced channel allocation (S-USCA) technique is proposed and investigated. Even though the developed technique is useful in combating FWM crosstalk in wavelength division multiplexing (WDM) lightwave systems with up to 12 channels, its main virtue is in designing multichannel WDM lightwave systems with more than 12 channels. Comparisons of power penalty due to FWM between equal channel spacing (ECS) systems and the S-USCA systems are presented. It is shown that for an intensity modulation/direct detection (IM/DD) transmission system operating in an optical bandwidth of 16 nm with 0 dBm (1 mW) peak optical input power per channel, while a conventional ECS WDM system with 0.84-nm channel spacing cannot even achieve a bit-error rate (BER)=10^-9, the suboptimum technique developed in this paper, for the same minimum channel spacing, can achieve a BER=10^-9 with an FWM crosstalk power of less than 1 dB at the worst channel in a 20-channel WDM system     

Signal light amplification by stimulated Raman scattering in anN-channel WDM optical fiber communication system

The use of forward and backward Raman amplification in an N -channel wavelength-division-multiplexing (WDM) optical-fiber communication system is analyzed. Analytical expressions for the signals, the pumps, and the amplified spontaneous scattered power (ASSP) are presented. The crosstalk among the signal channels is analyzed in terms of system parameters. It is found that the crosstalk is negligible while low pump power is used and becomes significant if high pump power is used. The signal-to-ASSP ratio can be improved by increasing pump power; however, the improvement is small when pump power is already high. A 300-km repeater spacing with more than 20-dB signal-to-ASSP ratio is calculated     

Nonlinear crosstalk due to stimulated Raman scattering in a two-channel wavelength-division-multiplexed system

The effects of stimulated Raman crosstalk on two binary PCM channels, wavelength-multiplexed in a single optical fibre, are minimised by optimising the ratio of powers launched into each channel. The longest repeater spacings that could be achieved by increasing the launched powers are thereby obtained for two WDM channels of equal group velocity.     

DWDM系统中色散对受激喇曼散射的影响

研究了光纤色散对波分复用(WDM)系统中受激喇曼散射(SRS)非线性效应的影响.提出了一种简便且物理意义明确的分析计算模型,它适合于任意输入信号、任意信道数,以及不同的色散参量.基于该模型,深入分析了两信道光纤通信系统中,各种色散参量下,色散对SRS的影响,得出了色散能减小SRS效应所引起的输出功率的波动的结论,并且给出了在不同色散参量下,输出功率标准差随距离变化的曲线.     

Performance limitation due to statistical Raman crosstalk in a WDM system with multiple-wavelength bidirectionally pumped Raman amplification

A general theoretical model of statistical Raman crosstalk and its impact on system performance in a multiwavelength bidirectionally pumped Raman fiber amplifier (RFA) is developed for the first time, where we have taken modulation statistics, dispersion-induced pulse walk-off and signal-induced pump depletion into account. Two kinds of statistical Raman crosstalk, from signal-induced forward-pump depletion and from signal-signal Raman interaction, are included in one model. Formulas for normalized Raman crosstalk, Raman crosstalk-induced relative intensity noise spectral density, and its variance and system performance impact in terms of Q penalty are presented for both a single-span system and a dispersion-compensated multispan wavelength-division-multiplexed (WDM) link. Based on these formulas, we numerically investigate the impact of Raman crosstalk on system performance in a three-wavelengths bidirectionally pumped 40 /spl times/ 40-Gb/s WDM system for various fiber types. In addition, Raman crosstalk in a four-wavelength bidirectionally pumped RFA was experimentally measured. The results agree well with our theory.     

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.     

WDM光网络系统相干串扰与非相干串扰

同频串扰可以分为相干串扰与非相干串扰,本文提出了WDM光网络系统串扰相干性分析的数学模型,分析提出了相等的相干串扰代价和非干串扰代价下引起的WDM光网络系统功率代介的一致性,并与模拟相干串扰和非相干串扰结果进行了比较验证。实验结果表明,对应1dB功率代价下,WDM光网络系统对应相干串扰和非相干串扰代价容限步于－20dB和理论结果一致。串扰对于网络规模有一定的限制。     

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