Model for polarization-dependent gain due to pump depletion in a WDM system with forward-pumped Raman amplification

We study polarization-dependent gain (PDG) due to signal-induced pump depletion (SIPD) in a wavelength-division-multiplexing (WDM) system with forward-pumped Raman amplification. It is found that SIPD can polarize the pump significantly in fiber with very low polarization-mode dispersion (PMD). To quantify the impact of fiber PMD on SIPD-induced PDG for a practical WDM system with many signal channels and multiple Raman pumps, an approximate vector model has been developed. The developed model allows us to directly calculate PDG from both SIPD and signal-signal Raman interaction (SSRI) with greatly reduced computation time. Based on the developed model, detailed numerical investigations for two typical C-band WDM systems are presented. It is shown that significant PDG can be introduced by SIPD when the fiber PMD coefficient is lower than 0.01 ps/km/sup 1/2/ even if the pumps are fully depolarized. It is also shown that PDG due to SIPD and PDG due to SSRI are in phase at shorter wavelength channels but out of phase at longer wavelength channels.     

Comparison of EDFA and bidirectionally pumped Raman amplifier in a40-Gb/s RZ transmission system

We numerically compare the performance of a bidirectionally pumped Raman and an erbium-doped fiber amplifier (EDFA) system in a 40-Gb/s dispersion-managed return-to-zero (RZ) transmission. The Raman amplifier system shows a substantially higher Q-value compared to that of the EDFA     

Performance comparison of an 80-km-per-span EDFA system and a 160-km hut-skipped all-Raman system over standard single-mode fiber

In this paper, we present a comprehensive comparison of the performance of an 80-km-per-span erbium-doped fiber amplifier (EDFA) system and a hut-skipped (160-km-per-span) all-Raman system over standard single-mode fiber (SSMF) for the first time, using semianalytic models. The numerical results reveal that a hut-skipped all-Raman system (using one-order Raman pumping) can achieve comparable performance as the conventional 80-km-per-span EDFA system for a common 50-GHz-spaced 80 /spl times/ 10 Gb/s nonreturn-to-zero (NRZ) wavelength division multiplexing (WDM) system at typical fiber loss of 0.22 dB/km. For 100-GHz-spaced 40 /spl times/ 40 Gb/s carrier-suppressed return-to-zero (CS-RZ) WDM transmission, it was found that a hut-skipped all-Raman system can achieve even better performance than the current 80-km-per-span EDFA system. It was also found that the impact of pattern-dependent Raman crosstalk is more severe than interchannel cross-phase modulation (XPM) in a hut-skipped all-Raman system with 80 /spl times/ 10 Gb/s capacity.     

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     

XPM and SPM induced crosstalk in WDM system employing distributed Raman amplifier for DPSK and OOK modulation format

Cross-phase modulation (XPM) and self-phase modulation (SPM) induced nonlinear crosstalk has been studied analytically for coherent DPSK and OOK signal in NRZ- and RZ-modulation format for WDM system employing distributed Raman amplifier. The study shows that 40 Gb/s RZ-DPSK signal with 33.3% duty cycle experiences minimum XPM and SPM induced crosstalk. The results also reveal that minimum crosstalk was induced in backward pumped DRA among the three pumping schemes i.e. forward, backward and bi-directional. Results assume importance for minimizing deleterious XPM and SPM effects in optical communication system.     

基于自发Raman散射分布式光纤测温系统设计

光纤传感器是一种新发展起来的传感技术。文章介绍了基于自发Raman散射的分布式光纤测温系统的应用及其结构设计,并着重分析了在脉冲光作用下系统的测温原理,同时设计了一个新型的基于薄膜滤光片的用于Raman散射的波分复用（WDM）组件。     

Research on Raman Crosstalk in Broadband Wavelength Division Multiplexed Systems

With the increasing of bandwidth and communication capacity of optical fiber communication system, fiber nonlinearities are known to limit the performance of optical fiber communication system badly[1]. In many systems, innovative engineering design may substantially reduce the the nonlinear effects on stimulated Brillouin scattering(SBS), self-phase modulation(SPM), cross-phase modulation(XPM) and four-photon mixing(FWM)[2,3]. Ultimately, the system performance is expected to be limited …     

On distributed Raman amplification for ultrabroad-band long-haulWDM systems

We present an analysis and design method for multiwavelength pumped fiber Raman amplifiers for massive wavelength division multiplexing (WDM) systems with signal bandwidths of 10 to 12 THz and a very large number of channels. We solve self-consistently the optimal design problem for backward-pumped distributed Raman amplifiers with special gain profiles that compensate for the Raman tilt and wavelength-dependent fiber loss. We study the noise performance of these optimally designed amplifiers and address the issues of multispan amplification for long-haul transmission of such broadband signals     

瑞利散射对拉曼光纤放大器性能的影响及抑制措施

瑞利散射是影响拉曼光纤放大器性能的重要因素。本文系统地分析了瑞利散射对拉曼光纤放大器噪声、串扰和功率代价等性能的影响，指出优化选择泵浦功率和泵浦方式以及插入光隔离器等措施可以降低瑞利散射的影响，从而为拉曼光纤放大器的优化设计提供了依据。     

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.     

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     

Channel addition/removal response in Raman fiber amplifiers: modeling and experimentation

In this article, we investigate, both theoretically and experimentally, transient effects in Raman fiber amplifiers (RFA) caused by channel addition/removal. A comprehensive large-signal numerical model of RFA, which incorporates time variation effects and the downstream and upstream propagation of multiple signals, pumps, and amplified spontaneous emission components, has been used for the theoretical analysis. The effect of pumping scheme, pump power, the length and type of Raman fiber, and number of added and/or dropped channels on the dynamics of surviving channel power fluctuations has been studied. In our experimental setup, signals from two laser diodes (LDs) were transmitted through counterdirectionally pumped RFA consisting of 15.6 km of dispersion compensating fiber (DCF). To simulate channel addition/removal, one signal was square-wave-modulated at 500 Hz. At the output of the RFA, the signal of the continuous-wave LD was selected with an optical bandpass filter, and power fluctuations of the surviving channel were recorded with a high-speed digital oscilloscope. Power fluctuations as high as 0.45 dB with typical saturated amplifier overshoots were observed.     

Ultra-wide-band tellurite-based fiber Raman amplifier

We describe the first wide-band tellurite-based fiber Raman amplifier (T-FRA) for application to seamless ultra-large-capacity dense wavelength-division multiplexing (WDM) systems. First, we confirmed that the Raman scattering characteristics of the tellurite-based fiber has so large a gain coefficient and Stokes shift that we can achieve a wide-band tellurite-based fiber Raman amplifier with a shorter fiber length than when using silica-based fiber. Second, we investigated the small signal gain and the signal transmission characteristics for a high gain and high output power operation with a single-stage amplifier. Focusing on double Rayleigh scattering, we compared the high gain limit of tellurite- and silica-based fibers. We then studied the impact of nonlinear effects by measuring the bit error rate (BER) when using a two-stage amplifier with a high output power of 18.8 dBm in which we simultaneously amplified eight channel signals in the L-band located on the ITU 100-GHz grid. Finally, we designed a wide-band tellurite-based fiber Raman amplifier with a multiwavelength band pumping scheme. We constructed this amplifier with a tellurite-based fiber only 250 m in length pumped by four-wavelength-channel laser diodes, and it provided a 160-nm bandwidth with a gain of over 10 dB and a noise figure below 10 dB from 1490 to 1650 nm. We also measured the BER to confirm the transmission characteristics of the amplifier for single channel operation over the whole signal wavelength range of 160 nm. We thus confirmed that the amplifier could be employed in ultra-high-capacity WDM 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     

拉曼放大器的交迭因子模型及其性能特性仿真

通过引人交迭因子的概念，提出了光纤拉曼放大器(FRA)仿真的改进模型。采用此模型进行仿真，可以在保证精度的基础上，显著缩短仿真时间。在此模型的基础上，仿真了色散和非线性效应对超高斯脉冲抽运拉曼放大器性能的影响。结果表明，色散对其影响较大，而非线性系数对其影响相对较小。     

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     

Statistical properties of stimulated Raman crosstalk in WDM systems

The crosstalk variance of stimulated Raman crosstalk in wavelength-division-multiplexing (WDM) systems is derived analytically in a closed-form formula for all systems with different walkoff length. The probability density function (pdf) of stimulated Raman crosstalk is found to be lognormal distribution (Gaussian distribution in decibel scale). Both power penalty and power limit induced by Raman crosstalk are evaluated and can be applied to single- and multispan WDM systems     

Cross-gain modulation in Raman fiber amplifier: experimentation and modeling

The effects of channel loss on the performance of Raman fiber amplifiers (RFA) are investigated both experimentally and theoretically. Signals from one distributed-feedback (DFB) laser and an external cavity laser (ECL) were transmitted through counterdirectionally pumped RFAs consisting of 15.6 km of dispersion compensating fiber (DCF). The ECL light was square-wave modulated at 500 Hz. At the output of the RFA, the signal of the modulated channel was eliminated with an optical band pass filter, and power fluctuations of the surviving channel were recorded with a high-speed digital oscilloscope. Power fluctuations as high as 0.45 dB with typical saturated amplifier overshoots were observed. The experimental results were confirmed by a large signal numerical analysis.     

Channel addition-removal response in all-optical gain-clamped lumped Raman fiber amplifier

Application of all-optical gain-clamped (AOGC) lumped Raman fiber amplifier (RFA) for protection of surviving channels in multiwavelength networks is investigated experimentally and theoretically. Channel addition-removal was simulated by transmitting signals of two lasers through a counterdirectionally pumped RFA consisting of 16 km of dispersion compensating fiber. Light of one of the lasers was square-wave modulated at 500 Hz; power fluctuations of the other laser caused by cross-gain modulation of the RFA were monitored at the output of the amplifier with a digital oscilloscope. An all-optical feedback loop was implemented in the form of a ring laser. Theoretical analysis of the AOGC lumped RFA is based on numerical solution of coupled propagation equations for forward and backward-propagating pumps, signals, and spectral components of amplified spontaneous emission powers.     

（英）基于增益谱多项式拟合的双泵浦碲基光纤级联拉曼光纤放大器

光纤拉曼放大器的带宽、增益及增益平坦度直接影响了光纤通信系统的传输质量.针对这些参数的优化,根据碲基光纤的拉曼增益谱特性提出了一种双泵浦级联碲基光纤的拉曼放大器结构.并推导了实现增益谱平坦时光纤长度和泵浦参数满足的约束条件.经过对拉曼增益谱的5次多项式拟合,更准确地反映了拉曼增益谱的信息,同时也简化了其实现增益谱平坦的条件.通过Matlab仿真分析得到,当两段光纤分别取0.339 km,0.16 km时,其最大增益为17.81 dB,增益平坦度为0.66 dB ,放大带宽为48 nm.该方案为宽带宽、高增益、增益平坦度小的拉曼光纤放大器设计提供了一种新的思路.