Decreasing EDFA transients by power shaping

Multi-wavelength optical networks make extensive use of erbium-doped optical amplifiers (EDFAs) to offset the effects of fiber attenuation and signal power splitting. As optical networking moves towards burst and packet switching, the effects of changes of optical power levels on one wavelength, or channel, can have an impact on the behavior of other wavelengths passing through the same amplifier and impair network performance. We have observed in our simulations of EDFA operation that both the amplitude and duration of EDFA transients will decrease when gradual changes are made to the input power of the EDFA. It was also verified in [D.H. Richards, J.L. Jackel, M.A. Ali, A theoretical investigation of dynamic all-optical automatic gain control in multichannel EDFA’s and EDFA cascades, J. Select. Top. Quantum Electron. 3 (4) (1997)] that the switching time of the channel dropping operation can influence the amplitude and duration of EDFA transients. We show that this effect can be exploited to successfully suppress EDFA transients by power shaping. Our approach is to increase/decrease the power of an added/dropped channel gradually, rather than abruptly. Power shaping can be implemented by adapting existing link layer protocols. It is general and can be applied to amplifiers using different technologies, including EDFAs, solid state and Raman amplifiers.     

Analysis of channel addition/removal response in all-opticalgain-controlled cascade of erbium-doped fiber amplifiers

Application of the all-optical gain-controlled (AOGC) erbium-doped fiber amplifier (EDFA) for protection surviving channels in a multiwavelength network in which several EDFAs are cascaded is studied. The effects of addition and/or dropping of wavelength channels in a network comprising six concatenated EDFAs have been analyzed by numerical simulation. Only the first EDFA in the cascade is gain clamped using a ring laser configuration. A large-signal EDFA model which incorporates time variation numerical effects and the downstream propagation of signal and pump and downstream and upstream propagation of amplified spontaneous emission (ASE) has been used. In particular, the effects of relaxation oscillations from the gain-controlled EDFA on the surviving channel protection are investigated. It is shown that power excursions caused in an eight-channel wavelength division multiplexed (WDM) network by the loss/addition of 6 channels will be lower than 1 dB and free of relaxation oscillations if the gain-controlled EDFA is strongly inverted and the average normalized population density of the metastable level does not drop below 0.74     

Effects of nonlinear dispersion in EDFA's on optical communicationsystems

The signal-induced change of the refractive index in an erbium-doped fiber amplifier (EDFA) causes a phase modulation imposed on a signal when passing the EDFA. In this paper, we apply our extended EDFA model on an optical communication system. The model includes this phase modulation, by including the nonlinear dispersion in an EDFA, and the spontaneous emission noise. The influence of these effects on an optical communication system is examined by means of Q-factor and eye diagram. We assume an intensity modulated-direct detection (IM-DD) system operating at 193 THz (1552.5 nm) with a bit rate of 10 Gb/s in the anomalous dispersion regime and a total fiber length of 500 km. The fibers are assumed to be dispersion shifted ones, EDFAs are used to compensate for the fiber loss. By numerical simulation we obtain results for the influence of the phase modulation (nonlinear dispersion) due to the signal induced change of the refractive index in an EDFA and the spontaneous emission noise at different input peak powers. Neglecting the signal-induced change of the refractive index strongly underestimates the Q-factor in the anomalous dispersion regime. Therefore it should be included for reliable system simulations. This can be done with the numerical model presented here     

Modeling, measurement, and a simple analytic approximation for the return loss of erbium-doped fiber amplifiers

When fiber-optic systems contain erbium-doped fiber amplifiers (EDFA), reflections are enhanced by the gain of the amplifiers and can degrade system performance. We present a simple approximate analytic expression for the return loss of pumped EDFAs, with and without discrete reflections that applies at both the signal and pump wavelengths. A novel return loss measurement test set is used to verify the validity of this expression for a particular EDFA design. Using a computer simulation, the return loss is computed as a function of EDFA design to assess the range of validity of the approximate expression. The expression serves as an easy way to chain amplifiers and accurately predict the effects of reflections in long systems.     

Performance analysis of an optical serial-to-parallel converterwith erbium-doped fiber amplifier

In this paper, we develop a performance analysis of an surface-emitting second harmonic generation (SESHG) optical serial-to-parallel converter using an erbium-doped fiber amplifier (EDFA) as a preamplifier. The analysis is complicated by the fourth-order nonlinearity that acts on the signal plus amplified spontaneous emission (ASE) noise to create many beat noises at the binary decision device. However, we demonstrate that Gaussian approximation for the beat noise statistics is reasonable. We calculate the BER of the system as a function of the SHG nonlinear cross section (A^NL), EDFA gain, the bandwidth of the optical filter that band-limits the ASE noise, and the timing pulse-to-data pulse power ratio. We find that for reasonable values of these and other parameters, the EDFA/SESHG serial-to-parallel converter combination should be able to operate at or below a BER of 10^-12. We find that small increments (0-2 dB) in the signal power that is input to the EDFA are enough to compensate the effects of ASE noise for most of the parameter variations we consider. From this point of view, the ASE noise has little effect on system performance. However, when the input power is fixed, we show evidence in terms of BER that the ASE noise plays a significant role, particularly in the high A^NL, high gain case. Also in this case, we show that the optimal timing pulse-to-data pulse power ratio is somewhat different from the value that is optimal for the system without an EDFA     

Inherent enhancement of gain flatness and achievement of broad gainbandwidth in erbium-doped silica fiber amplifiers

We report new methods to inherently increase the flatness and bandwidth of erbium-doped silica fiber amplifiers from three perspectives: fiber design, pump-signal WDM coupler optimization, and amplifier structure. First, to achieve inherent control of the gain spectrum, a new type of composite fiber structure with an Er-doped core and a Sm-doped cladding ring is proposed and experimentally demonstrated. Interaction of the optical field with the Sm-doped cladding to produce evanescent wave filtering is modeled, which provides an in-line control of gain fluctuation in the erbium-doped flier amplifier (EDFA) C band, 1530-1560 nm. Second, the effect of the spectral characteristics of WDM couplers over the L band of an EDFA is explored. A fused taper fiber coupler for a 1480-nm pump is optimized for signals in the wavelength range of 1570-1610 nm by measuring the small-signal gain, gain tilt, and noise figure in an L-band EDFA. Finally, a new all-fiber structure for a wide-band EDFA, where the L and C bands were coupled serially, is demonstrated with optimized pump-signal couplers. Further optimization of the new composite fiber structure and the transient effects in the serially coupled EDFAs are also discussed     

带有前置EDFA卫星相干激光通信终端的信噪比分析

卫星激光通信链路是一项实现卫星大规模星座组网的关键技术。相干激光链路灵敏度高、抗背景干扰、速率升级空间大,在星间激光链路中应用广泛。文章建立了带有前置掺铒光纤放大器(EDFA)的卫星相干激光通信终端的信噪比分析模型,仿真分析了EDFA功率增益、EDFA噪声系数、本振光功率、信号光功率、光放后端光带宽、基带前端电带宽对终端输出信噪比的影响以及各种输出噪声功率占比的情况,得到了带有前置EDFA卫星相干激光通信终端的信噪比参数特性。     

980 nm抽运时掺铒光纤放大器中的上转换发光效应研究

采用980nm抽运的掺铒光纤放大器（EDFA）中存在上转换发光效应。能级分析和光谱扫描结果表明上转换辐射光为绿色荧光。波长为538nm和514nm，其产生机理为铒离子的激发态吸收效应（ESA）。从理论和实验两方面分析了抽运功率和信号功率这两个放大参量对上转换绿色荧光的影响，结果表明，存在一个特殊抽运功率值，当抽运功率小于该值时，上转换绿色荧光的抽运效率随抽运功率的增加而快速增大；抽运功率大于该值时，上转换绿色荧光的抽运效率变化缓慢，基本保持稳定。掺铒光纤放大器工作在线性放大状态下，输入信号的有无和功率大小对绿色荧光影响很小；掺铒光纤放大器工作在饱和状态下，绿色荧光功率随输入信号功率增加而增加。     

A low-noise and gain-flattened amplifier composed of a silica-based and a fluoride-based Er/sup 3+/-doped fiber amplifier in a cascade configuration

We propose a novel low noise and gain-flattened Er/sup 3+/-doped fiber amplifier (EDFA) with a cascade configuration for wavelength division multiplexing (WDM) signals. In this configuration, a 1480-nm pumped fluoride-based EDFA is joined to a 980-nm pumped silica-based EDFA through an optical isolator. By adjusting the silica-based Er/sup 3+/-doped fiber length in the silica-based EDFA, we realized an excellent flat gain EDFA with a gain excursion of less than 0.9 dB and noise figure of 5.7/spl plusmn/0.2 dB, and a low noise EDFA with a noise figure of 5/spl plusmn/0.2 dB and a gain excursion of less than 1.4 dB, for 8 channel WDM signal in the 1532-1560-nm wavelength region.     

Performance optimization of radio-on-fiber systems employing erbium doped fiber amplifier for optical single sideband signals considering intermodulation distortion

The performances of radio on fiber (RoF) systems with a dual-electrode Mach-Zehnder modulator and an erbium-doped fiber amplifier (EDFA) are optimized by numerical equations including the third order intermodulation (IM3) as well as amplified spontaneous emission (ASE) noise. We investigate a signal-to-noise-and-distortion ratio (SNDR) considering fiber dispersion with respect to an input signal power and an EDFA gain in both noise-dominant and third order intermodulation (IM3)-dominant cases. We also verify that the numerical analysis results are well matched with those of a commercial simulator, VPItransmissionMaker. In the analysis results, the optimum input signal power for the maximum SNDR of a RoF system with EDFA was reduced over 8 dB compared with that without EDFA. The dramatic reduction of IM3 power at a receiver was resulted from this decrement of input signal power. Thus, the maximum SNDR of the system with EDFA was obtained over 17 dB at 40 km fiber compared with that of the system without EDFA. In addition, the results showed that the SNDR was efficiently improved by EDFA in the noise-dominant case, while the SNDR improvement was negligible by EDFA in the IM3 dominant case.     

掺铒光纤放大器的非线性色散效应

给出一种模型来考虑非线性色散对光通信系统的影响。数值结果表明 ,考虑非线性色散时 ,光脉冲的传播速度比不考虑非线性色散时的传播速度要快 ,而脉冲展宽是一样的     

High-gain coefficient long-wavelength-band erbium-doped fiberamplifier using 1530-nm band pump

A 1530-nm band has been studied as a pump wavelength for the long-wavelength-band erbium-doped fiber amplifier (L-band EDFA). The pump source is built using a tunable light source and cascaded conventional-band (C-band) EDFA. The L-band EDFA uses a forward pumping scheme. Within the 1530-nm band, the 1545-nm pump demonstrates 0.45-dB/mW gain coefficient, which is twice better than that of conventional 1480-nm pumped EDFA. The noise figure of the 1530-nm pump is at worst 6.36 dB, which is 0.75 dB higher than that of the 1480-nm pumped EDFA. Such high-gain coefficient indicates that the L-band EDFA consumes low power     

一种新型EDFA构想--低噪声双腔高掺铝EDFA

提出了一种新型的EDFA构想——低噪声双腔高掺铝EDFA,并通过仿真和测试,对在单腔结构和双腔结构下产生的两种不同结果进行了比较分析。相对于传统的掺饵EDFA,它能同时减轻空间烧孔效应和张弛振荡在全光增益控制中的影响,并具有较好的增益平坦特性以及较低的噪声指数。     

Gain flattening of an erbium-doped fiber amplifier using ahigh-birefringence fiber loop mirror

We propose a flexible method for flattening the gain profile of an erbium-doped fiber amplifier (EDFA). The method is based on a high-birefringence fiber loop mirror (HiBi-FLM), which contains a number of high-birefringence fiber sections and polarization controllers. We show that, by setting the polarization controllers properly, the reflection spectrum of the loop mirror can be adjusted to compensate for the variations in the gain profile of the EDFA. The method is demonstrated experimentally by using a single-section HiBi-FLM and a two-section HiBi-FLM, respectively, With a two-section HiBi-FLM, we are able to flatten the gain profile of an EDFA to within ±0.9 dB over a bandwidth of 33 nm at the center wavelength of 1543 nm, under different operating conditions of the EDFA     

Bit error rate reduction by power shaping in WDM networks with EDFAs

The trend in multi-wavelength optical networks is toward dynamic route and wavelength assignment to support burst-switching or packet switching. Where erbium-doped optical amplifiers (EDFAs) are used for multi-wavelength amplifications, the time interval between traffic blocks can be long enough to induce EDFA optical power transients created by abrupt changes in the average input power to the EDFAs and can adversely affect the performance of the network. To mitigate the effects of EDFA optical power transients on optical networks, a method based on power shaping where heads and tails are joined to the beginning and end of a traffic block is proposed. A head (tail) gradually increases (decreases) the channel power by employing a pseudo-random bit sequence in which the probability of a “1” (“0”) increases from 0 to 0.5. This paper reports experimental results showing that EDFA optical power transients can be significantly reduced with adequate shaping periods. The effects of alternative shaping profiles are examined, and for a linear shaping profile, experiments show that the drop transient deviation is reduced by 0.5 dB while the add transient deviation is reduced by 0.71 dB. We show by lab experiments that the bit error rate induced by wavelength add-drop transients in a chain of EDFAs in an emulated fiber network can be reduced by 25% using power shaping techniques. Power shaping is an economical means of suppressing EDFA optical power transients compared to other physical layer approaches that require the addition of specialized components and can be applied to EDFAs as well as other solid-state and Raman optical amplifiers.     

掺饵光纤放大器中交叉相位调制效应的研究

本文建立了考虑交叉相位调制效应（XPM）的光放大模型,得到了（XPM）效应对小信号增益系数的影响,分析得到:掺铒光纤放大器（EDFA）中XPM引起的强度起伏和非线性系数、输入的泵浦功率以及放大器长度有关。仿真结果证明:EDFA中XPM效应的影响不能忽略,并且当γ≥0.01/W·m时,级联EDFA中第一个EDFA产生的XPM效应的影响也是不能忽略的。     

温度对L波段EDFA增益斜率影响的实验研究

实用的L波段EDFA(掺铒光纤放大器)常采用多段铒纤和多泵浦的复杂结构。对一种复杂结构的L波段EDFA的温度特性进行了实验研究,发现在不同的输入光功率下,L波段短波侧(约1 570～1 582nm)的增益谱可按相反的方式随温度而变化,并从理论上解释了此现象。实验结果表明,铒纤的温度和增益斜率之间仍然存在很好的线性关系。控制铒纤温度就可以调整L波段EDFA的增益斜率,获得平坦的增益。     

掺铒光纤放大器中交叉相位调制效应的研究

本文建立了考虑交叉相位调制效应(XPM)的光放大模型,得到了(XPM)效应对小信号增益系数的影响,分析得到掺铒光纤放大器(EDFA)中XPM引起的强度起伏和非线性系数、输入的泵浦功率以及放大器长度有关.仿真结果证明EDFA中XPM效应的影响不能忽略,并且当γ≥0.01/W@m时,级联EDFA中第一个EDFA产生的XPM效应的影响也是不能忽略的.     

Distributed temperature sensing with erbium-doped fiber amplifiers

The experimental verification of a novel fiber-optic sensor which performs distributed measurement of temperature by using a distributed erbium-doped fiber amplifier (EDFA) is presented. The sensor configuration is similar to that of a conventional optical time-domain reflectometer and detects the Rayleigh backscattered portion of a pulsed optical signal which is amplified by the EDFA, along with the backward amplified spontaneous emission (ASE) generated by the EDFA. The sensor utilizes the temperature dependence of the gain in an EDFA. The amplification provided by the erbium-doped fiber, which is pumped at 1.48 μm, significantly increases the magnitude of the optical signal reaching the receiver, thus leading to a simplified configuration and a potentially superior performance as compared to other types of distributed fiber-optic temperature sensors     

Gain stabilization in gain clamped EDFA cascades fed by WDMburst-mode packet traffic

This paper studies via simulation the stabilizing effect of all-optical gain-clamping (AOGC) in a chain of erbium-doped fiber amplifiers (EDFA) fed by wavelength-division multiplexing (WDM) burst-mode packet traffic. AOGC is necessary to suppress swings of output power and optical signal-to-noise ratio (OSNR). A case study is selected, in which only the first EDFA in a cascade of six amplifiers is clamped using a ring laser configuration. A numerical model which solves the transcendental equation for the average inversion at each EDFA is used for the analysis. The traffic is generated on the eight WDM channels by ON-OFF time-slotted sources, with statistically independent ON and OFF durations, randomly generated by a truncated Pareto distribution with infinite variance. The simulation model includes the generation of amplified spontaneous emission within each amplifier and the propagation of the lasing power generated in the AOGC EDFA through the cascade. It is shown that the sizable power and OSNR swings arising in an unclamped cascade of EDFA's can be effectively suppressed when a lasing signal a few decibels above the aggregate signal power develops in the AOGC EDFA and propagates along the cascade