分布拉曼光纤放大的实验研究

采用符合ITU-T标准的G波段40信道波分复用(WDM)光源对分布式拉曼放大器(DRA)的特性进行了实验研究。对不同抽运方式及不同光纤长度的分布式拉曼放大器性能作了较为详细的报道。在抽运功率相同的条件下，选用50km单模光纤对比研究了不同抽运方式的拉曼放大器增益和噪声。通过对不同光纤长度的分布式拉曼放大器的实验研究发现，在较低抽运功率且输入信号功率较低的情况下，随着光纤长度增加，拉曼增益也增加，有效噪声系数减小。研究了分布式拉曼放大器对波分复用通信系统信噪比的改善，实验发现不同抽运功率下，拉曼放大器对系统信噪比的改善随抽运功率增加而增加，但是不成线性关系，而且最终会出现饱和。     

Bidirectional higher order cascaded Raman amplification benefits for 10-Gb/s WDM unrepeated transmission systems

Benefits provided by higher order bidirectional Raman pumping schemes in 10-Gb/s unrepeated wavelength-division-multiplexing transmission systems are experimentally quantified in terms of BER performances at 10 Gb/s. By keeping under control double-Rayleigh-scattering-noise-induced transmission penalties, which can degrade system performance at very high ON-OFF Raman gain, as well as nonlinear propagation effects such as Brillouin scattering, self- and cross-phase modulations, four-wave-mixing, and Raman-induced crosstalks, we show a total unrepeated system reach enhancement up to 3.5 dB with respect to first-order bidirectional pumping. As confirmed by theory, the maximum reach enhancement is mainly limited by pump-to-signal relative intensity noise transfer induced by higher order copumping.     

Low-Noise Fiber Raman Amplifiers Based on a Raman Ring Fiber Oscillator

We propose two discrete fiber Raman amplifier schemes using Raman ring fiber oscillator, a two-stage scheme, and a forward pumping scheme, to reduce double Rayleigh backscattering noise at high gains over 20 dB. Both schemes have low noise characteristics compared to a previous scheme. Bit-error-rate measurements at 10 Gb/s show power penalty free operation for the two-stage scheme at a gain of 25 dB.     

Double Rayleigh scattering noise in Raman amplifiers using pump time-division-multiplexing schemes

The impact of signal double Rayleigh scattering (DRS) noise in time-division multiplexed counterpropagating Raman pumping schemes is theoretically and experimentally investigated. We show that, for small pump modulation duty cycles, DRS noise can be greatly enhanced with respect to continuous-wave pumping. By increasing the pump modulation frequency, however, this detrimental effect is effectively reduced, providing a powerful technique for noise figure equalization and pump four-wave mixing suppression in large bandwidth discrete and distributed amplifiers.     

Optimal design of multistage discrete Raman amplifiers incorporating midway isolators

Multistage discrete Raman amplifiers with midway isolators to suppress double Rayleigh backscattering are fully investigated at constant nonlinearity. The results show that placing Raman pumps at the last stage only is sufficient. For most conditions, two isolators can achieve adequate performance improvement, while more isolators only lead to slightly optical signal-to-noise ratio increase. The optimum positions of the isolators are insensitive to varied Raman gains, nonlinear phase shifts, and Rayleigh coefficients, but sensitive to different fiber lengths as well as Raman pumping schemes. Moreover, backward-pumping schemes can derive better noise performance than bidirectional pumping when using midway isolators.     

Design of bidirectionally pumped fiber amplifiers generating doubleRayleigh backscattering

Optimum pumping schemes for bidirectionally pumped distributed fiber amplifiers are evaluated that simultaneously minimize the beat noise between signal and amplified spontaneous emission, as well as between signal and double Rayleigh backscattering. A way of adding a constraint on signal power to describe operation in a strong Kerr fiber nonlinearity regime is included. We show that one can find an optimum Raman net gain, as well as a percentage of forward to total Raman pumping maximizing the signal-to-noise ratio at the end of a transmission line using bidirectionally Raman-pumped transmission fibers     

Raman amplification for fiber communications systems

Raman amplification has enabled dramatic increases in the reach and capacity of lightwave systems. This tutorial explains why, starting with the fundamental properties of gain from stimulated Raman scattering. Next, noise accumulation from amplified spontaneous emission is reviewed, and the merits of distributing Raman gain along a transmission fiber are explained. Other sources of noise that are particularly relevant for Raman amplifiers are summarized. Finally, novel Raman pumping schemes that have recently been developed are highlighted.     

基于混合抽运喇曼放大的准无损传输系统研究

为了实现信号光在光纤链路上的准无损传输,采用在光纤两端对称注入1阶和2阶喇曼抽运的高阶喇曼放大方法,对该结构下的光纤准无损传输系统建立了理论仿真模型,并对该系统进行了仿真优化,研究了在不同的1阶和2阶抽运功率组合方案下,系统总功率消耗和信号沿光纤功率平坦度的变化,以及信号光功率和光纤长度对系统性能指标的影响。结果表明,采用过高功率（大于1mW）的入射信号光,会恶化系统整体性能;当光纤的长度过长时（大于60km）,系统的平坦度指标恶化明显。采用该方案能实现60km的准无损传输,并且可以通过系统所需能耗和信号平坦度的实际需求,设计出不同的抽运功率组合方案。     

复杂条件下分布光纤拉曼放大器噪声特性的解析表达式

在同时考虑抽运光和信号光之间不同偏振态、多波长抽运以及抽运光和信号光损耗系数不同的情况下 ,导出了分布光纤拉曼放大器 (DFRA)的等效噪声系数解析公式 ,并据此定量研究了抽运光和信号光之间的偏振关系对DFRA特性的影响以及DFRA与掺铒光纤放大器 (EDFA)级联时系统光信噪比 (OSNR)的改善ΔOSNR。指出 :随着DFRA开关增益的增加 ,ΔOSNR会由线性增加逐渐变为对数增加而趋于饱和。解析结果与已发表文献的数值模拟结果基本吻合     

基于超长分布式2阶喇曼放大器的光载射频传输

为了提升光载射频传输链路的链路增益以及传输距离,采用了L波段的超长分布式2阶喇曼放大器结构对光信号进行放大。从理论上对分布式2阶喇曼放大器以及光载射频传输链路的原理进行了解释,利用信号光、1阶抽运光、2阶抽运光以及噪声之间的耦合方程组分析了它们之间的关系,并且得到了基于超长分布式2阶喇曼放大器的光载射频传输系统的1阶射频信号增益。通过数值仿真以及系统实验得到了抽运功率大小对超长分布式2阶喇曼放大器的开关增益的影响、光载射频传输系统在0GHz~7GHz范围内的频率响应及其射频增益以及该光载射频传输链路在应用超长分布式2阶喇曼放大器后的相位噪声情况。结果表明,光载射频传输在超长分布式2阶喇曼放大器的作用下获得了28.1dB的链路增益,在距离为80.94km的光链路上实现了近似无损传输,射频信号开关增益与射频信号频率无关。该研究在光载射频链路的长距离传输中有重要的应用价值。     

Noise and gain band management of thulium-doped fiber amplifierwith dual-wavelength pumping schemes

Three dual-wavelength pumping schemes (1550 nm + 1064, 1047, or 1400 nm) have been experimentally assessed for the thulium-doped fiber amplifier in terms of power and noise performance. It is shown that 1047- and 1550-nm pumping gives a better power performance than 1064- and 1550-nm pumping at fixed gain peak wavelength (GPW). In addition, the most efficient pumping scheme appears to be 1400- and 1550-nm pumping for GPW higher than 1475 nm, and 1047 and 1550 nm for lower GPW. Noise figures lower than 6 db (with -5-dBm signal input power) are exhibited regardless the pumping schemes (although a 0.8-dB increase is reported for the 1400- and 1550-nm pumping)     

Distributed fiber Raman amplifiers with incoherent pumping

Distributed fiber Raman amplifier (DFRA) with incoherent pumping is investigated and its performance is compared to that with the conventional coherent pumping. It is shown that increasing the spectral bandwidth of incoherent pumping source can reduce the Raman gain ripple significantly, and degrade noise figure slightly at short wavelengths, compared to coherent pumping. To achieve the same gain flatness, the number of pumps for broad-band DFRAs with incoherent pumping can be significantly reduced compared to the coherent pumping.     

不同抽运方式下掺Yb3+光纤放大器的增益和噪声特性

为了探讨不同抽运方式下掺Yb3+光纤放大器的增益和噪声特性,对910nm掺Yb3+光纤放大器进行数值模拟。结果表明,抽运方式对增益影响较小,但对噪声特性影响较大,光纤达最佳长度时,正向和双向抽运方式的掺Yb3+光纤放大器要优于反向抽运方式。这一结果对掺Yb3+光纤放大器的试验设计具有重要的作用。     

Raman fiber lasers for optical communication application

We review the principle and characteristics of cascaded Raman fiber lasers in their telecommunication applications. The fundamentals of Raman fiber lasers are described, such as pumping scheme, fiber type and reflectors. We explain simple equations forcw laser operation as well as a possible way to calculate spectral width. We investigate the common issues in the use of Raman fiber lasers: theoretical optimization, control of the linewidth, suppression level of the other Stokes orders and relative intensity noise. Finally the more complex multiple-wavelength Raman fiber lasers are reviewed.     

色散补偿型光纤拉曼放大器的增益和噪声指数的实验研究

对S波段的色散补偿型分布式光纤拉曼放大器(FRA)进行了研制。本放大器中采用了先进的光纤拉曼激光器作为抽运源,并对5 km DCF-50 km G652光纤色散补偿型分布式FRA分别进行了前向抽运和后向抽运,测量了其前向和后向增益光谱和噪声谱,结果表明后向抽运方式在增益和噪声特性方面优于前向抽运方式。对色散补偿型分布式FRA在后向抽运方式下的增益和噪声指数随抽运功率的变化而变化的规律进行了实验研究,结果表明其增益随抽运功率的增大而增大,噪声指数随增益的增大而减小。为比较色散补偿型分布式FRA和分布式FRA在增益和噪声方面的性能,分别测量了上述两种光纤拉曼放大器在后向抽运方式下的增益谱和噪声指数谱,结果表明: 5 km DCF-50 km G652光纤制成的色散补偿型分布式FRA的增益和噪声特性,均优于由50km-G652光纤制成的分布式FRA。本文对实验结果进行了理论分析。     

Influence of Raman effect on the noise characteristics in a dual pump FOPA

Influence induced by Raman effect on the noise characteristics of a depleted and lossy dual pump fiber optical parametric amplifier (FOPA) is investigated. Taking the Raman effect into account, the modified coupled amplitude equations are firstly built, then the influences of Raman effect on the stokes and anti-stokes wavelengths with different initial signal powers are further analyzed. Besides, the effect on the FOPA gain is also shown with the same initial signal power and constant noise figure (NF).     

多波长泵浦宽带拉曼放大器功率增益研究

受激拉曼散射（SRS）的Stokes波谱线型比较复杂,本文分别用Gauss线型和Lorentz线型,研究多波长泵浦宽带Raman放大器的增益特性,提出宽带增益平坦的多波长泵浦方案,研究表明增益平坦程序与泵清波频率间隔密切相关。对G．652、G．653、G．655光纤和新型大有效面积非零色散平坦光纤的Raman放大增益分别进行了研究,结果表明Raman增益与光纤种类和光纤传输性有关,特别是随光纤有效纤芯面积的增大而明显减小。     

不同光纤多波长抽运宽带Raman放大器研究

利用R.H.Stolen等的测量结果,经取样拟合得到SRS的Stokes谱线的多项式表达式,在此基础上,考虑到多抽运波之间的相互放大作用,研究了多波长袖运宽带光纤Raman放大器的增益与噪声指数。对ITU-T的G652,G653,G655和大有效面积(LEAF)光纤进行了数值分析,得到它们的Raman增益和噪声指数特性曲线。     

多波长泵浦宽带拉曼放大器功率增益的Lorentz近似研究

本文采用Lorentz线型近似,研究了多波长泵浦宽带Raman放大器的增益特性,并提出了宽带增益平坦的多波长泵浦方案。研究结果表明,增益平坦程度与泵浦波的频率间隔密切相关。对ITU－T的G．652、G．653及G．655光纤和新型的大有效面积非零色散平坦光纤的Raman放大增益特性研究表明,Raman增益与光纤的种类和光纤传输特性有关,特别是随光纤有效纤芯面积的增大而明显减小。     

Investigation on the Gain of Multi—wavelength Pumping Broadband Raman Amplifier

The Stokes spectrum in the stimulated Raman scattering(SRS)is very complicated.In this article,we use both Gaussian and Lorentz approximation to investigate the gain properties of multi-pumping broadband Raman amplifier,and present some schemes for flattening the gain.All the results show that the flatness of the gain spectrum is closely related to the pumping frequency grid.By researching the gain properties of Raman amplifiers of such ITU-T fibers as G.652,G.653,G.655 and large effective area non-zero dispersion flattening fiber,we find that the Raman gain is associated with the fiber type and its transmission characteristics,especially it decreases with the increase of the effective core area.