A study of green wavelength-division multiplexed optical communication systems using cascaded fiber bragg grating

This paper studies the performance analysis of wavelength-division multiplexed optical communication systems (WDM). First, flat-gain erbium doped fiber amplifiers (EDFAs) are seriously needed to obtain proper and equal amplification of all channels. Such amplifiers can be designed by intrinsically modifying the host material or extrinsically using proper filters. In this research, we benefit from both the intrinsic and extrinsic methods to achieve sharp flat EDFA output gain using cascaded fiber Bragg gratings (FBGs). Second, the performance of our technique has been evaluated through calculating the bit error rate (BER) and signal-to-noise ratio (SNR) of a WDM system embedded with the reported EDFA flattening system. The parametric simulations of the FWHM of FBGs, SNR, optical power and the transmission distance have shown a noticeable improved performance. Sending data via an optical WDM system will be proven from comprehensive simulations to achieve high quality signal transmission spectrums, increased transmission distances and low power consumption. By extension, the reported design using cascaded FBGs can also be generalized to equalize the gain of any arbitrary profile.     

Modeling of four-wave mixing and gain peaking in amplified WDMoptical communication systems and networks

A theoretical model is presented for analyzing the propagation of densely spaced WDM optical signals through a cascade of erbium-doped fiber amplifiers and single-mode optical fibers with nonuniform chromatic dispersion. By combining a numerical solution for the EDFA and an analytical expression for FWM components generated through the cascade, the model allows a realistic system analysis which includes gain peaking effect, amplified spontaneous emission accumulation and the effect of dispersion management on the four-wave mixing efficiency. The FWM power distribution at the end of the multi-amplifier transmission link is computed taking into account the phase relation between FWM light amplitudes generated within different sections of the link. The transmission of many WDM channels, evenly spaced around 1547.5 nm, has been analyzed for various dispersion management techniques and propagation distances. Numerical results point out the importance of such a model for a realistic design of WDM optical communication systems and networks. A proper choice of chromatic dispersion, amplifier characteristics, span length, input signal powers and wavelengths, combined with the use of gain equalizing filters, allows to maximize the transmission distance ensuring acceptable signal-to-noise ratio (SNR) and limited SNR variation among channels     

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     

The impact of optical amplifiers on long-distance lightwavetelecommunications

The potential of erbium-doped fiber amplifiers (EDFA) and wavelength-division multiplexing (WDM) technology for expanding transmission capacity in long-distance telecommunications is examined. Properties of EDFA are nearly ideal for application in lightwave long-haul transmission. Nonlinear effects in the transmission fiber and amplifier spontaneous emission noise limit the performance and therefore dictate the design of long-distance amplified systems, especially those employing WDM. The next-generation transoceanic system will use EDFA as repeaters, yielding a capacity almost ten times larger than what is available today. Multichannel WDM soliton transmission promises further substantial enhancement. Terrestrial long-haul networks will also benefit greatly from amplified WDM systems designed to mine the large inherent bandwidth in the embedded fiber. The ten- to fifty-fold capacity increase over present systems not only will provide for ample growth, but also will enable network operators to enhance operational flexibility and network functionality, and to facilitate a fast-recovery self-healing capability through cost-effective redundant routing     

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     

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     

Passive equalization of nonuniform EDFA gain by optical filteringfor megameter transmission of 20 WDM channels through a cascade ofEDFA's

Through the incorporation of optical filters in a cascade of erbium-doped fiber amplifiers (EDFA's), with each amplifier exhibiting nonuniform gain, we determine the optimal conditions for passively equalizing many wavelength division multiplexed (WDM) channels while maintaining a high SNR. For 20 WDM channels spaced 0.5 nm apart, it is found that 3-dB, 2-nm-wide notch filters with center wavelength at 1.560 μm will provide sufficient SNR equalization for potential megameter transmission when located after every 20 EDFA's. This performance is achieved with no a priori knowledge of the input or output signals     

EDFA-based DWDM lightwave transmission systems with end-to-end power and SNR equalization

An approximate analysis is presented which can be used to predict the performance of power and signal-to-noise ratio (SNR) equalization schemes when applied to dense wavelength-division multiplexing (DWDM) lightwave systems employing erbium-doped fiber amplifier (EDFA) cascades. Expressions are provided which relate the maximum number of amplifiers, EDFA gain imbalance, bit rate (R/sub b/), transmitter power, receiver dynamic range and number of channels. The relative advantages of these two equalization strategies are quantified by comparing the maximum number of amplifiers allowed by each scheme. It is shown that, while SNR equalization represents, on balance, the more desirable equalization strategy for future EDFA-based DWDM lightwave transmission systems, under certain conditions power equalization may be a better choice. When employing an APD receiver, for instance, power equalization can support 1.9 times more amplifiers than SNR equalization. However, when employing the more conventional preamplified PIN/FET receiver, SNR equalization can support 1.7 times more amplifiers than power equalization.     

Experimental characterization of a dynamically gain-flattened erbium-doped fiber amplifier

We have designed and experimentally characterized an erbium doped fiber amplifier (EDFA) which possesses a wavelength-independent gain spectrum, independent of the operating level of the gain (dynamic gain flatness), and without requiring any gain-level-dependent control of any parameters. In the wavelength range 1542-1552 nm, the gain was flat to within the experimental uncertainties of /spl plusmn/0.3 dB even as the gain level changed by 17 dB. The EDFA was based on a low-Al-content alumino-germanosilicate EDF and a Mach-Zehnder filter. We believe that this type of EDFA, which has not been demonstrated before, can significantly simplify the design of amplified wavelength-division multiplexing (WDM) transmission systems and increase the robustness of long-distance WDM transmission.     

Gain equalization by mitigating self-filtering effect in a chain ofcascaded EDFA's for WDM transmissions

A simple method of alternatively using high-inversion and moderate-inversion erbium-doped fiber amplifiers (EDFA's) in an optical amplifier chain is presented to mitigate the self-filtering effect and equalize both signal power and signal-to-noise ratio (SNR) of multiple wavelength channels in wavelength-division multiplexing (WDM) transmission systems. The performance of the compensated system with alternatively used high- and moderate-inversion amplifiers is compared with the uncompensated ones where only moderate- or high-inversion amplifiers are employed. The result shows that the compensated system has a flatter gain profile, a lesser signal power spread, and SNR degradation     

All-optical gain control of in-line erbium-doped fiber amplifiers for hybrid analog/digital WDM systems

Automatic gain control using an all-optical feedback loop in in-line erbium-doped fiber amplifiers (EDFA's) used in hybrid analog/digital wavelength division multiplexing (WDM) systems was studied. It is found that the signal level variation for the digital channels can be maintained within a range /spl les/3-dB between the presence and dropout of the analog channel when the narrowband feedback is centered at the amplified spontaneous emission (ASE) peak (/spl sim/1532 nm) with loop loss ranging between 13-22 dB. Robust transmission at 2.5 Gb/s without measurable power penalty was obtained for the digital channels when the EDFA was saturated by either the analog or the control lasing signal.     

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.     

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     

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     

Multi-wavelength all-optical networks with wavelengths outside theerbium-doped fiber amplifier bandwidth

Erbium-doped fiber amplifiers have become the dominating technology for signal amplification in all-optical networks. One constraint of EDFA's is that they have a much narrower bandwidth (≈25 nm) compared to the low-loss region (≈200 mn) of optical fiber. Instead of using only wavelengths within the bandwidth of EDFA's (i.e. the inband channels) for communication, we propose to include wavelengths outside the EDFA bandwidth (i.e. the outband channels) as well in order to increase the number of wavelengths and/or channel spacings that can be accommodated. Using outband wavelengths for sending messages presents a new constraint, namely that only if the power loss for transmitting a message is small enough can this message be transmitted on the outband wavelength. We develop wavelength-routing algorithms on arbitrary network topologies and wavelength assignments in hierarchical networks for sending messages subject to this constraint. We also analyze the SNR for inband/outband WDM signals     

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.     

掺铒光纤放大器黑盒模型在波分复用系统中的实验验证

介绍了掺铒光纤放大器的黑盒模型 ,随后给出应用黑盒模型的参数测量步骤 ;测量了一台内部参数和结构未知的商用 EDFA对六信道 WDM光信号的放大光谱图。通过实测结果与黑盒模型仿真计算得到的结果相比较 ,证明了黑盒模型具有很好的精度 ,适用于估算商用波分复系统的性能。     

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.     

Gain equalization of EDFA cascades

Investigates the impact of wavelength-dependent erbium-doped fiber amplifier (EDFA) gain spectrum on multichannel direct-detection lightwave transmission systems employing multiple amplifiers. An analysis is presented which quantifies the constraints imposed by received power imbalance, signal-to-noise ratio (SNR), and receiver sensitivity on an EDFA cascade. Expressions are derived which relate the system constraints to the EDFA gain imbalance, bit rate, number of channels, and receiver dynamic range. Results demonstrate that when four-wave mixing (FWM) is compensated in an 11-channel system, received power imbalance can impose a significant constraint on transmission distance when the EDFA gain imbalance is greater than 1 dB or when bit rate is less than 1.8 Gb/s. In addition, performance of the preemphasis gain equalization technique is studied for multichannel systems employing APD or p-i-n/FET direct-detection optical receivers. Simple expressions are derived which can be used to quantify the increase in transmission distance obtained when employing preemphasis equalization. Results indicate that equalization of the received power spectrum can provide a two- to four-fold increase in the transmission distance when using APD receivers, compared to a one- to two-fold improvement with p-i-n/FET receivers. Analytic results are compared with results obtained by proven simulation methods and found to be in good agreement     

Theoretical simulation and experimental investigation on a WDMsurvivable unidirectional open ring network using tunable channelselecting receivers

In this paper theoretical and experimental investigations on a wavelength-division-multiplexed (WDM) reconfigurable open ring network are reported. The theoretical study is focused on network component characteristics and ring network structure. These aspects will form the basis of numerical modeling. A powerful computer aided design software “COMSIS” is used for the simulation. The simulation takes into account the most important parameters: node structure, link losses, EDFA characteristics, optical powers of the channels, and signal wavelength arrangements. The power of a new channel added at each node and its wavelength with respect to those present before the node are two important parameters in the performance analysis of this open ring network. If the performance criterion is to receive the channels with high signal to noise ratios (SNR) and a narrow spread ΔSNR, the optimal length of this ring network can reach 366 km for 4 nodes and 565 km for 8 nodes. The experimental demonstrator is composed of 4 secondary nodes incorporating tunable channel selecting receivers (TCSR's) and erbium doped fiber amplifiers (EDFA's). The reconfiguration and the protection of the ring is computer controlled. Experimental results of a fully connected testbed demonstrator are also presented