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     

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     

Second-order distortion improvements or degradations brought byerbium-doped fiber amplifiers in analog links using directly modulatedlasers

Within lightwave analog amplitude-modulated (AM) CATV systems using directly modulated lasers, erbium-doped fiber amplifiers (EDFA's) act upon the signal distortion because of the interaction between the laser chirp and the EDFA wavelength-dependent gain. This interaction is theoretically investigated in order to predict the EDFA-induced distortion. The relevant gain tilt characteristic for analog applications and the way to measure it are described. Expected and measured distortions at the EDFA output are in excellent agreement. Fiber amplifiers are found to decrease the signal distortion level when the gain tilt is negative, i.e., for wavelength above the gain maximum     

Optimization of large-scale fiber sensor arrays incorporatingmultiple optical amplifiers. I. Signal-to-noise ratio

We report on optimizing the signal-to-noise ratio (SNR) of large-scale fiber sensor arrays employing erbium-doped fiber amplifier (EDFA) telemetry with respect to the number of sensors per rung, the number of amplifiers per array, and the coupling ratio between the fiber buses and the rungs. Broad optimum regions are found, providing design flexibility to minimize pump power requirements. Simulations predict that 300 sensors can be multiplexed on a fiber pair while maintaining a high sensitivity (1 μrad/√(Hz)) for all sensors with a moderate input pump power (<1 W)     

Optimization of large-scale fiber sensor arrays incorporatingmultiple optical amplifiers. Part II: pump power

For part I see, ibid., p. 218, 1998. We discuss how to minimize the pump power required for large-scale fiber sensor arrays employing erbium-doped fiber amplifier (EDFA) telemetry with respect to the number of amplifiers per bus, number of sensors per rung, and the gain per amplifier. For a large array, the pump power requirement is dominated by passive component losses along the array. We investigate several methods, including alternative array topologies, to reduce the power requirement while minimizing the impact on the signal-to-noise ratio (SNR). We define an optimum topology which requires less than 1 W of 1480 nm pump power per bus to support 200 high sensitivity (1 μrad/√(Hz)) sensors on a pair of fiber buses, a power requirement that is reasonable and attainable with available laser diodes     

A Novel Interpolation Equalization Technique for Multicarrier Modulation/Demodulation Systems

This paper presents a novel multitap interpolation equalization technique for filter bank-based multicarrier modulation/demodulation systems. The proposed technique is based on the equalization of the channel fractional delay in each subchannel in time synchronization with the constituent receiver side decimator. The proposed synchronization is achieved by combining a subset of the polyphase components of the analysis filter output signal after having passed through a bank of interpolation equalizers. The resulting multitap interpolation equalization permits a trade-off between various equalization parameters, such as the number of used polyphase components, the length of the equalizer, and the interchannel interference terms, making it possible to achieve a high signal-to-noise ratio (SNR) involving moderate equalization cost or a moderate SNR involving very low equalization cost. Simulation results for the standard carrier serving area loop show that the proposed equalization technique gives rise to 15 dB improvement in SNR compared to the output combiner equalization technique and can achieve an SNR close to the matched-filter bound for the channel by employing a reasonable equalizer length. Compared to the output combiner equalization technique, the proposed equalization technique involves around three times less the storage requirement at the same computational cost or around three times less the computational cost at the same storage requirement for equalizer training. Two suboptimal solutions are also proposed to simplify the equalizer training at only a minor loss in SNR.     

Gain equalization of EDFA's with Bragg gratings

Flat-gain amplifiers are needed to ensure proper amplification of every channel in wavelength-division-multiplexed communication systems. Such amplifiers can be realized by combining a precisely tailored filter with an erbium-doped fiber amplifier (EDFA). In this letter, we show that Bragg gratings can lead to accurate EDFA gain equalization. Reflection and transmission gratings have been used to demonstrate gain equalization over 32 nm with excursion inferior to ±0.1 and ±0.3 dB, respectively. A maximum 0.04-dB noise-figure penalty results from this process. By extension, Bragg gratings could also equalize the gain of any arbitrary gain profile, over any arbitrary bandwidth     

Performance of coherent ASK lightwave systems with finiteintermediate frequency

The impact of finite intermediate frequency (IF) on the performance of heterodyne ASK lightwave systems is examined and quantified in the presence of laser phase noise and shot noise. For negligible linewidths, it is shown that certain finite choices of IF (R _b,3R_b/2,2R_b,5R_b/2, etc.) lead to the same ideal bit-error-rate (BER) performance as infinite choices of IF. Results indicate that for negligible linewidths the worst case sensitivity penalty is 0.9 dB for proper heterodyne detection and occurs when f_IF=1.25 R_b. For nonnegligible linewidths (e.g., when ΔνT⩾0.04) the sensitivity penalty is always less than 0.9 dB for finite choices of IF. The analysis presented does lead to a closed-form signal-to-noise ratio (SNR) expression at the decision gate of the receiver which can readily be used for BER and sensitivity penalty computations. The SNR expression provided includes all the key system parameters of interest such as system bit rate (R_b), the peak IF SNR (ξ), laser linewidth (Δν), and the IF filter expansion factor (α). The findings of this work suggest that the number of channels in a multichannel heterodyne ASK lightwave system can be increased substantially by properly choosing a small value for the IF at the expense of a small penalty <1 dB. On the negative side, IF frequency stabilization becomes a more critical requirement in multichannel systems employing small values of IF     

Theoretical analysis of system limitation for AM-DD/NRZ opticaltransmission systems using in-line phase-sensitive amplifiers

This paper shows the theoretically derived performance of single channel, amplitude modulation/direct detection optical transmission systems using in-line optical phase-sensitive amplifiers (PSA's). The calculations take into account the degradation of the signal-to-noise power ratio (SNR) and intersymbol interference (ISI) due to the distortion of transmitted signal pulses. The SNR is analyzed by considering not only amplifier noise and fiber loss but also noise enhancement by four-wave mixing in the transmission fiber. The ISI is estimated by eye-pattern degradation of the transmitted signal numerically calculated using the nonlinear Schrodinger equation. The regenerative repeater spacing of in-line PSA systems limited by SNR and ISI can be expanded by approximately 3 to 10 times that of in-line EDFA systems, in the case of |D|⩽0.1 ps/mn/km dispersion fiber systems transmitting a 40-Gb/s signal     

Observation of equivalent Rayleigh scattering mirrors in lightwavesystems with optical amplifiers

Experimental and theoretical studies that indicate that Rayleigh scattering acts as an equivalent mirror which can generate high levels of multiple reflection noise in lightwave systems employing optical amplifiers are discussed. The relative intensity noise can be calculated as a simple function of the amplifier gain and the ratio of total backscattered to incident power in the fibers. In high-speed digital systems, this noise limits the maximum usable repeater gain to less than 20 dB, unless optical isolators are employed     

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     

DWDM系统中OAS配置影响系统稳定性的研究

文章从密集波分复用(DWDM)系统抵抗恶意侵扰的角度,分析了光放大段(OAS)中掺铒光纤放大器(EDFA)和OAS长度的配置对系统稳定性的影响,认为EDFA累积噪声会随着放大器的级数增加而呈线性增长 ,而且还会随着OAS损耗的增大而呈指数增长.系统的OAS数量和接收机灵敏度确定时,选择恰当的EDFA配置和OAS长度,可以达到提高系统稳定性的效果.     

A new method for reducing the power consumption of portablehandsets in TDMA mobile systems: conditional equalization

In time-division multiple-access (TDMA) systems, the autonomy of portable handsets is a major constraint. We propose a new method, conditional equalization, which minimizes the power consumption due to equalization by equalizing only when it is most needed. At each time slot, the receiver estimates the need for equalization with a deterministic criterion. The proposed criteria use thresholds, chosen to provide a compromise between loss in performance and percentage of saved equalization. The performance of our method is theoretically analyzed for two-path Rayleigh fading channels and simulated for the recommended COST 207 channels. Conditional equalization can ensure a degradation in the average signal-to-noise ratio (SNR), providing a fixed bit error rate (BER with an average SNR=9 dB with systematic equalization), of less than 0.5 dB while providing a gain of 43% spared equalization for bad urban (BU), 62% for hilly terrain (HT), 79% for typical urban (TU), and 98% for rural area (RA) models, respectively. Conditional equalization is shown to greatly improve the autonomy of the receiver by drastically reducing the power consumption due to equalization, without a noticeable loss in the performance     

Power penalties due to multiple Rayleigh backscattering in coherenttransmission systems using in-line optical amplifiers

The power penalties due to double Rayleigh backscattering are calculated for coherent lightwave systems with optical in-line amplifiers. It is shown that coherent systems are less sensitive to Rayleigh noise than high-speed direct-detection systems. For PSK heterodyne systems, the maximum gain of in-line amplifiers that can be achieved without optical isolation is limited to about 25 dB     

Performance characteristics and applications of hybrid multichannelANI-VSB/M-QAM video lightwave transmission systems

The performance characteristics and applications of hybrid multichannel amplitude modulation vestigal sideband (AM-VSB)/M-quadrature amplitude modulation (QAM) video lightwave transmission systems operating at either 1310 or 1550 nm are reviewed. These systems can transport up to 80 AM-VSB video channels and more than 30 64/256-QAM digital video channels over a standard single-mode fiber (SMF) using a single laser transmitter. Five main transmission impairment mechanisms for these systems are reviewed as follows: (a) clipping-induced impulse noise, (b) bursty nonlinear distortions, (c) multiple optical reflections, (d) stimulated Brillouin scattering, and (e) self-phase modulation. For AM-QAM video lightwave trunking applications, the in-line erbium-doped fiber amplifier (EDFA) selection is discussed using a frequency-domain simulation model. Such lightwave trunking systems can provide an AM carrier-to-noise ratio (CNR) greater than 50 dB with composite second order (CSO) and composite-triple-beat (CTB) distortions less than -65 dBc, and nearly error-free transmission (BER⩽10^-9) for the 64-QAM channels with signal-to-noise ratio (SNR) of 30-dB or better. Comparison between 64-QAM and 256-QAM video channel transmission and the effect of the QAM channels on the AM-VSB channels are also presented. The implications of these results and others in hybrid multichannel AM-QAM video lightwave trunking systems are discussed     

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.     

双向时分复用光纤时间传递链路噪声分析

研究分析了双向时分复用(BTDM)光纤时间传递链路 的主要噪声及产生机理,建立了基于BTDM光纤时间传递 的链路噪声模型。仿真分析了激光器强度噪声、光放大器增益与个数、接收机带宽等对BTDM 光纤时间传 递接收信噪比(SNR)的影响。结果表明,BTDM光纤时间传递 接收SNR在光放大器达到最优增益时最大,且最 大SNR随光放大器个数的增加而增加并趋于稳定;相同长度光纤链路,光放大器个数越多 ,在一定范围 内,激光器相对强度噪声对BTDM光纤时间传递链路接收SNR 影响更大,接收机带宽对BTDM光纤时 间传递接收信号抖动的影响越小;BTDM光纤时间传递接收SNR随定时信号前 光持续时间的增加而减小,并趋于WDM方案的SNR。     

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.     

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     

一种新的优化喇曼掺Er混合光纤放大器结构

对于给定的非线性损伤，对喇曼／掺Er混合光纤放大器(HFA)结构进行优化，即优化光纤喇曼放大器(FRA)与掺Er光纤放大器(EDFA)的增益比和色散补偿光纤(DCF)长度比，得到一种最优HFA结构。分别比较了最优HFA、一般HFA、全FRA和全EDFA用作接收机前置放大器的噪声性能，以及用于多级链路在线放大器时系统所能到达的最大可传输距离。结果表明，最优HFA能提高系统性能。