Effects of signal power control strategies and wavelength assignment algorithms on circuit OSNR in WDM networks

Software-defined networking is enabling wavelength-division multiplexed (WDM) networks to be programmable down to individual components. While taking into account typical gain and noise figure profiles of erbium-doped fiber amplifier (EDFA) components, the authors consider a number of signal power control strategies and compare their performance in terms of achievable lightpath optical signal-to-noise ratio (OSNR). These strategies are applied network-wide to concurrently control the gain of each individual amplifier and the signal power equalization at each reconfigurable optical add/drop multiplexer. Simulation and (in part) experimental results show that the lightpath OSNR is affected by three factors: the EDFA gain control strategy, power equalization strategy and wavelength assignment (WA) algorithm. A trade-off between lightpath average OSNR and OSNR variance across the WDM channels is also noted. Experimental work is conducted using a five-node meshed WDM network testbed proving both feasibility and effectiveness of a coordinated use of signal power control strategies and WA algorithms.     

Application of preemphasis to achieve flat output OSNR intime-varying channels in cascaded EDFAs without equalization

In this paper, we present a new method for optical signal-to-noise ratio (OSNR) equalization of wavelength division multiplexed (WDM) channels at the end of a cascade of several erbium-doped fiber amplifiers (EDFAs) by use of preemphasis, as well as the proper choice of EDFA design parameters. Identical OSNR at the end of the cascade ensures better signal detection and quality of service. The dynamics of the equalizing method have been demonstrated by simulation for single- and double-stage amplifier designs using a numerical model incorporating time variation effects in EDFA. Calculations are based on the solution of a transcendental equation describing the dynamics of the reservoir, i.e., the total number of excited ions, for each EDFA. Traffic on eight WDM channels is modeled as statistically independent ON-OFF time-slotted sources. In addition, we investigate the effect of gain clamping of the first amplifier in the cascade-by implementing a ring laser and propagating the lasing power through the cascade-on the statistics of OSNR variation. We show that it is possible to achieve dynamic OSNR equalization for a WDM system by the use of preemphasis and an appropriate choice of EDFA parameters, without resorting to optical equalization filters. Most previous equalization methods are static with flat gain for a given inversion level in the amplifier. Changes in the input power (due to network reconfiguration or packetized traffic) will lead to a varying inversion level and hence non optimal equalization     

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     

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     

Output power and SNR swings in cascades of EDFAs for circuit- andpacket-switched optical networks

A simple dynamic model of the erbium-doped fiber amplifier (EDFA) that includes self-saturation by amplified spontaneous emission (ASE) is used to analyze the power and signal-to-noise ratio (SNR) transients in wavelength division multiplexed (WDM) optical networks in which signals cross chains of EDFAs from source to destination. The model, which consists of solving sequentially one ordinary differential equation per amplifier, is used to (1) determine power and SNR excursions in the surviving channels along a chain of 35 EDFAs during isolated add-drop events in a 16-channel WDM circuit switching scenario and (2) run Monte Carlo simulations of the first five EDFAs of the same chain fed by burst-mode packet switching traffic on each of the 16 channels. Each packet source is modeled as an ON-OFF asynchronous transfer mode (ATM) source, with ON and OFF times having a heavy-tailed Pareto distribution. The aggregate source model is asymptotically self-similar, and well describes multimedia packet communications. The results are used to examine the influence of average network utilization and source ON-OFF time variance on the probability density function of signal power and SNR at each EDFA output. We demonstrate that self-similar traffic generates sizable power and SNR swings, especially at low network utilization. The simulations also indicate sizable broadening of the power and SNR density functions along the cascade of EDFAs, reaching levels in excess of 9 dBm and 4 dB for the power and SNR swings, respectively, at the 5th EDFA. The effect becomes more pronounced for longer EDFA chains. Such a large broadening may imply serious system impairments in burst-mode WDM packet networks     

Output power excursions in a cascade of EDFAs fed by multichannelburst-mode packet traffic: experimentation and modeling

A serious problem facing wavelength-division multiplexed (WDM) networks with fiber amplifier cascades is transient cross-gain saturation or gain dynamics of fiber amplifiers. Attention has been focused primarily on circuit-switched scenarios. When the number of WDM channels transmitted through a circuit-switching network varies, channel addition/removal will tend to perturb signals at the surviving channels that share all or part of the route. Even more serious bit error rate deterioration can arise in WDM packet switched burst mode networks. In this paper, we present experimental and theoretical results demonstrating the effect of fast power transients in erbium-doped fiber amplifiers (EDFAs) on packetized traffic transmitted through a chain of five EDFAs. Traffic of a local-area network has been transmitted over three channels. The effect of EDFA cross-gain saturation due to the burstiness of the traffic has been observed in a continuous-wave monitoring channel. The stabilizing effect of gain clamping of the first EDFA in the cascade has been investigated. The experimental results are extended to eight-channel WDM system using a large signal numerical analysis     

Applications of active arrayed-waveguide gratings in dynamic WDMnetworking and routing

We describe how active arrayed-waveguide gratings (AWGs) may find a diverse range of applications in future dynamic wavelength division multiplexed (WDM) networking and routing. Our initial simulations indicate that these applications include dynamic signal power and erbium-doped fiber amplifier (EDFA) gain equalization with a dynamic range of 12 dB, and interchannel amplified spontaneous emission (ASE) suppression by more than 20 dB; optical add/drop multiplexing with passband-flattened channels and suppressions of 15 dB; and dynamic dispersion compensation of up to ±300 ps/nm     

Transmission of many WDM channels through a cascade of EDFA's inlong-distance links and ring networks

We analyze the transmission of many wavelength-division-multiplexed (WDM) channels through a cascade of erbium-doped fiber amplifiers (EDFA) in both long-distance links and ring-based networks. For a megameter long-distance system, optimal operating conditions are found for achieving a high signal-to-noise ratio (SNR) per channel with as small an SNR differential as possible between 20 WDM channels spaced 0.5 nm apart. Critical issues addressed in this paper include: (a) the non-uniformity of the EDFA gain with wavelength: (b) the link loss between amplifiers; (c) the small-signal gain per amplifier; and (d) the input signal power     

基于喇曼光纤的可变增益光放大器研究

通过分析WDM系统中全光增益控制EDFA(GC-EDFA)的优缺点,提出了一种新的可用于WDM城域网中的基于分布式喇曼光纤放大器的可变增益光放大器结构,并分析了通过监视前馈增益控制喇曼放大器的ASE信号来改善跨距损耗的扩展结构,使增益动态范围可从17dB变化到28dB以补偿跨损,当跨损变化从 5dB到-5dB时,能使输出光功率保持在0.15dB.     

An EDFA gain control and power monitoring scheme for fault detection in WDM networks by employing a power-stabilized control channel

Schemes are proposed for the highly reliable gain control of erbium-doped fiber amplifiers (EDFAs) and for power monitoring to detect faults in wavelength-division-multiplexing (WDM) networks. These schemes employ one WDM channel (a control channel). The EDFA gain and output power levels are controlled by monitoring the control channel power that is automatically controlled and stabilized in the node. This prevents the uncontrolled EDFA operation that might result from any serious change in the control channel power. The use of a power stabilized control channel for power monitoring makes it possible to detect transmission system faults correctly because the monitoring of the control channel power is unaffected by the amplified spontaneous emission (ASE) generated in the EDFA. We also report experimental results on the dependence of the transient response of the EDFA gain and output power on the signal channel power and channel number input into the EDFA, when the power of the control channel changes due to problems with its light source. Numerical calculation of the gain transience explains the experimental results.     

8 × 10 Gb/s transmission system over 2 015 km with dispersion compensation by fiber Bragg Grating

The cascaded chirp fiber Bragg gratings（CFBGs） with ITU-T standard wavelengths and wavelength grid are applied to compensate the dispersion of 8×10 Gb/s WDM system. The ASE of the EDFA could be reduced, the OSNR of the transmitted signal can be increased and the fluctuation of the EDFA gain can be restrained in a certain scope by the CFBG employed in the system. Experiment of error-free 8×10 Gb/s 2015 km transmission without FEC and electric regeneration is demonstrated in this paper. In this system, only EDFA is used as amplifier,and no other form of dispersion compensator is adopted except CFBG. The experimental result showed that after 2 015 km transmission,the consistency of the dispersion compensating for each channel is perfect.     

Scalability of a Metropolitan Bidirectional Multifiber WDM-Ring Network

We analyze the scalability of a metropolitan bidirectional multifiber wavelength-division-multiplexed (WDM) ring network. The analysis is carried out by using a bidirectional transmission model for optical networks and by building an experimental network. The model includes major limiting factors in WDM-ring networks such as relative intensity noise (RIN) due to multiple Rayleigh backscattering, amplified spontaneous emission (ASE) accumulation in a cascade of bidirectional erbium-doped fiber amplifiers (EDFA), tilting of the EDFA gain spectrum and input saturation power of the EDFA. We found that in metropolitan areas the scalability of a WDM-ring network using bidirectional transmission is not mainly limited by the RIN arising from the Rayleigh backscattering. The result was verified experimentally. The maximum size of the demonstrated network is 33–43 nodes with a spacing of 5–10 km between nodes. With this spacing, which is typical in metropolitan areas, the scalability of the network is mainly limited by the gain tilt and the input saturation power of the EDFA.     

ASE Analysis and Correction for EDFA Automatic Control

A theoretical model is described that can be used for calculating the amplified spontaneous emission (ASE) at the output of an optical amplifier. The technique can be used to determine the usable optical signal power at the output of an erbium-doped fiber amplifier (EDFA) operating in the constant-output-power control mode. The model was experimentally verified by automating an EDFA to operate in the constant-signal-power control mode using an ASE correction based on our model. Calculated signal-output powers were then compared to the measured values that were obtained using an optical spectrum analyzer.     

Enhancement of the Performance of a Reflective SOA-Based Hybrid WDM/TDM PON System With a Remotely Pumped Erbium-Doped Fiber Amplifier

A passive optical network (PON) architecture based on a hybrid wavelength-division multiplexing (WDM) and time-division multiplexing (TDM) PON system with a remotely pumped erbium-doped fiber amplifier (EDFA) is presented as an excellent candidate for use in a next-generation optical access network. The remotely pumped EDFA operates as a bidirectional amplifier and provides a 15-dB gain to both upstream signals and seed light sources, so the sensitivity of upstream transmission is greatly improved. An upstream transmission of 1.25 Gb/s with a low seed channel power of -14 dBm is made feasible over a total reach of 25 km for 32-WDM channels and 16-TDM splits by the use of the remotely pumped EDFA. This scheme has advantages that it uses a single transmission fiber for both down-and up-stream signals and that it reduces the Rayleigh scattering contribution.     

Experimental and theoretical analysis of relaxation-oscillationsand spectral hole burning effects in all-optical gain-clamped EDFA's forWDM networks

Experimental and theoretical analysis of the gain dynamics of all-optically stabilized multichannel erbium-doped fiber amplifier (EDFA) and the impact on wavelength division multiplexed (WDM) networks performance requirements is presented. In particular, we focus on precise analysis of the detailed transient response of the surviving channel and the relaxation oscillations of the compensating (lasing) signal. The main objective of this work is to experimentally and theoretically analyze and examine some of the critical factors such as, lasing wavelength, gain recovery time, relaxation oscillation frequency of the feedback loop, and the number of channels dropped/added, that affect the transient power excursions in the surviving channel. First, we consider the applicability of laser automatic gain control (AGC) to control fast power transients in WDM optical networks and reports the first high resolution measurements of transients in such gain controlled EDFAs. Second, the experimental results are compared with those predicted from a numerical simulation of the dynamic of the gain controlled EDFA     

Demonstration of performance enhanced pre-spectrum sliced seed light for low-cost seeded WDM–PON architecture

We proposed and demonstrated high efficient pre-spectrum sliced seed lights (PS-SL) for serving low-cost seeded wavelength division multiplexing passive optical network (WDM–PON). The PS-SLs reused backward amplified spontaneous emission light which were generated by an Erbium doped fiber amplifier (EDFA) to improve output characteristics. The power, flatness and bandwidth of seed signal from the proposed PS-SL were sufficient to serve multiple seeded WDM–PON OLTs. We experimentally demonstrated the feasibility of the PS-SL with multiple seeded WDM–PONs over 20 km transmission. In addition, the PS-SL was tolerated to these ranges of environmental conditions since no data error was observed after 48 h. We anticipated a considerable reduction in the seed light cost per channel for multiple WDM–PONs because the seed light cost was shared by multiple WDM–PON OLTs.     

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     

Fast power transients in concatenated Pr3+-dopedfluoride fiber amplifiers

Amplified multiwavelength optical communication networks are prone to substantial saturation induced power fluctuations when a channel is turned on or rerouted. This perturbs amplifier gains at other wavelengths. The effects of addition and/or dropping of wavelength channels due to network reconfigurations or line failures in a multiwavelength network comprising eight concatenated Pr³⁺-doped fluoride fiber amplifiers (PDFFAs) have been studied theoretically. A large-signal numerical model which incorporates time variation effects and the downstream and upstream propagation of signal, pump and amplified spontaneous emission has been used. In an eight channel WDM network the loss of channels will cause very fast power excursions of the surviving channels. The necessary speed of the link control circuit for surviving channel protection has been investigated     

Dynamic power-equalized EDFA module based on strain tunable fiberBragg gratings

An adaptive power-equalized erbium-doped fiber amplifier (EDFA) module is proposed and experimentally demonstrated by using strain tunable fiber Bragg gratings (FBGs). In a demonstration for a five-channel wavelength-division-multiplexed (WDM) system, the EDFA module can effectively reduce the power variation from 11 dB to 0.3 dB. Measured power penalty for 2.5-Gb/s data is less than 0.5 and 1.1 dB for 5 and 17 dB of signal attenuation by tunable FBGs, respectively. The power-equalized EDFA module can find wide applications in WDM lightwave transmission systems     

波分复用传输系统中EDFA-BDE模块的仿真研究

本文在ＣＯＭＤＩＳＣＯ信号处理工作系统中研究了用于波分复用传输系统的掺铒光纤放大器块图编辑器仿真模块。仿真模块考虑了前向及后向放大的自发辐射噪声。通过信息计算ＥＤＦＡ－ＢＤＥ模块,优化设计了掺铒光纤的长度。在含８级ＥＤＦＡ的ＷＤＭ光传输系统的仿真中论证了模块的实际应用。本文在输出信号幅度及ＡＳＥ噪声谱上都获得了与前期发表的实验相一致的仿真结果,证明了ＥＤＦＡ－ＢＤＥ模块在高饱和状态下的实际可用性。