Design of Dispersion Compensation and Nonlinear Distortion Self-compensation in Co-network Transmission Systems

Dispersion and nonlinear distortion have an effect on transmission performances of optical fiber transimission systems,The schemes of dispersion compensation and nonlinear distortion self-compensation in telecom-CATV co-network transmission systems are reported,followed by investigation on (1) the impact of dispersion compensation fiber (DCF) on fiber nonlinear effects with a cascade of erbium-doped fiber amplifiers (EDFAs) and different dispersion compensation schemes,(2) the impact of the complex on the total nonlinear distortion induced by EDFA gain tilt and the light source.As a result,dispersion compensation optimal scheme and EDFA negative gain tilt are suggested as a solution to dispersion compensation optimal scheme and EDFA negative gain tilt are suggested as a solution to dispersion compensation and the nonlinear distortion self-compensation.     

Gain tilt of erbium-doped fiber amplifiers due to signal-inducedinversion locking

A theoretical analysis describing an interpretation of the fundamental processes, responsible for the second-order distortion of AM signals in erbium-doped fiber amplifiers (EDFAs) is presented. It is shown that the locked-inversion gain tilt rather than the widely used continuous wave gain tilt is responsible for the distortion. The theoretical analysis is confirmed by experiments showing that the two types of gain tilt may differ by factors of up to 60. In particular, it is demonstrated that the locked-inversion gain-tilt may be significant even at the CW gain peak. Experimental results indicate that the relevant gain tilt can be reduced to acceptable levels by proper design of the EDFA     

Erbium-doped fiber amplifier second-order distortion in analog links and electronic compensation

The authors study the second-order distortion when an erbium-doped fiber amplifier (EDFA) is used to amplify the analog optical AM cable TV (CATV) multiple carrier signal from a directly modulated distributed feedback (DFB) semiconductor laser. Experimentally, it was seen that this second order distortion depends critically on the gain of the EDFA fiber amplifier. The authors attribute this distortion to the interaction between the frequency chirping of the DFB laser and the variable gain with wavelength of the amplifier. The authors describe an electronic predistorter that compensates the nonlinearity produced by the DFB-laser-EDFA combination. As a result, the high power advantage of the EDFA can be fully realized in spite of the potential for second-order distortion in the system.<>     

Second-order distortion of amplified intensity-modulated signalswith chirping in erbium-doped fiber

Second-order distortion produced in an erbium-doped fiber amplifier (EDFA) is investigated theoretically and experimentally. Rate equations for intermodulation products are led by the perturbation method. From numerical calculation, it is shown that optimum combinations of input power to EDFA and signal wavelength exist that give a distortion-free condition, Improvement of the composite second-order (CSO) distortion, which results from the cancellation of the laser distortion and the gain tilt induced distortion, was confirmed for VHF multichannel transmission. Furthermore, CSO distortion-free transmission in cascaded EDFA's has been demonstrated for a 42-channel AM-SCM signal by experimentally determining the optimum operating condition of the EDFA such that each amplifier generates no additive distortion     

Gain control in L-band EDFAs by monitoring backward travelingC-band ASE

A novel and simple technique for gain flatness control is reported for gain shifted, long wavelength band (L-band) erbium-doped fiber amplifiers (EDFAs). Utilization of the backward traveling amplified spontaneous emission (ASE) in the C-band is analyzed with respect to controlling the gain tilt observed in the L-band when the total input power of the EDFA is changed. It is shown that a gain flatness of 0.6 dB/30 nm can be achieved over a dynamic range greater than 10 dB by using the backward traveling ASE power in the C-band as a monitor to adjust the copropagating pump power of the EDFA. The proposed technique eliminates the need to extract the output signals from the monitored ASE signal, demonstrating the suitability and simplicity of the proposed technique for wavelength division multiplexed applications     

S-band erbium-doped fiber amplifiers with a multistage configuration /sub e/sign, characterization, and gain tilt compensation

We report an S-band erbium-doped fiber amplifier (EDFA) with a multistage configuration in terms of its design, gain, and noise characteristics for various pump powers and input signal powers, the temperature dependence of the gain spectra, and gain tilt compensation for changes in input signal power and temperature change. We show that there is a tradeoff between low noise and efficiency in the S-band EDFA and describe the development of an S-band EDFA with a flattened gain of more than 21 dB and a noise figure of less than 6.7 dB. We also show that there is a change in the gain spectra with changes in the pump power and input signal power that is different from that observed in C- and L-band EDFAs, and that our EDFA has a temperature-insensitive wavelength. Furthermore, we develop a gain tilt compensated S-band EDFA that can cope with changes in input signal power and temperature.     

Polarization dependent gain in erbium-doped fiber amplifiers:computer model and approximate formulas

A computer model for polarization dependent gain (PDG) in Er-doped fiber amplifiers (EDFA) is presented. The model assumes that each erbium ion possesses an ellipsoidal gain surface and that all ion orientations are equally likely. By dividing the ions into subsets based upon orientation and computing the inversion of each subset in the presence of polarized pump and signal waves, the model predicts the dependence of the PDG induced by this polarization hole-burning (PHB) on the design of the EDFA, the signal degree and state of polarization (SOP), and the pump SOP. For moderate gain amplifiers (made from the same fiber) with the same gain peak wavelength and the same compression level, the magnitude of the PDG is nearly independent of the EDFA gain. Internal and random fiber birefringence are included to model real fibers. In fibers which cause the signal SOP to walk rapidly around the Poincare sphere, the PDG is reduced by a factor of 2/3 when compared with a linear polarization-maintained signal. Scrambled signals and partially-polarized saturating tones are also considered. Simple rules are derived for predicting the PDG of a given EDFA     

Comparative study on temperature-dependent multichannel gain andnoise figure distortion for 1.48- and 0.98-μm pumped EDFAs

Temperature dependence of multichannel gain flatness and noise figure (NF) was compared for different pump wavelengths of 1.48 and 0.98 μm on silica-based erbium-doped fiber amplifiers (EDFAs) through measurement-based numerical simulation. Owing to its temperature sensitive pump emission cross section, the 1.48-μm pumping showed greater temperature sensitivity (maximum 0.75-dB gain flatness distortion with 0.57-dB average gain level shift, 0.3-dB NF variation for 25°C change) than the 0.98 μm pumping (maximum 0.5-dB gain flatness distortion with 0.015-dB average gain level shift, 0.05-dB NF variation for 25°C change). However, it was also found that distortion ripple spectra mainly coming from the changes of signal cross sections and asymmetric gain temperature dependence necessitate compensation techniques in the EDFA link, irrespective of pump wavelength     

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.     

Using equalizers to offset the deterioration in SCM videotransmission due to fiber dispersion and EDFA gain tilt

The carrier-to-noise ratio (CNR) and distortion of a 1.55 μm amplitude-modulated subcarrier-multiplexed (AM-SCM) video transmission system are worsened by the dispersion of the 1.3 μm zero-dispersion fiber. Distortion is also increased by the EDFA gain tilt. Both problems are shown to be corrected by the use of dispersion equalizers and gain tilt equalizers. The application of the equalizers to SCM video transmission systems is proposed     

空间光通信中空间辐射对光纤放大器性能的影响

以60Co为辐射源, 通过地面辐射模拟实验, 对掺铒和铒镱共掺两种光纤放大器的性能变化进行了对比分析。实验结果表明, 在总剂量为40 krad的低剂量轨道辐射环境中, 信号光通过这两种光纤放大器后, 其中心波长及半宽都没有发生显著变化, 这为光纤放大器能够应用于空间光通信提供了保证; 在辐照过程中掺铒光纤放大器的增益下降3.91 dB, 而铒镱共掺光纤放大器的增益下降17.60 dB, 表明镱离子的存在使得铒镱共掺光纤放大器的抗辐射性能要明显弱于掺铒光纤放大器, 这也为不同发射功率下的空间光通信系统在选择合适类型的放大器时提供了一个有益的参考。     

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.     

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     

The Correlation between Gain Slope, ASE Slope, and Second-Order Distortion of EDFAs in Directly Modulated AM-CATV Lightwave Systems

In the literature some disagreement and contradiction exist regarding the relationship between gain slope in EDFAs and the distortion this effect generates in analog multichannel CATV systems. We will try to identify these contradictions and consequently discuss the design criteria for an analog EDFA by showing that optimizing an EDFA for good gain slope can be done by sacrificing an insignificant amount of output power but enhancing the noise performance. The gain slope itself is measured with a new low-cost method: it will be shown from both theory and experiment that the ASE slope is a good approximation for the gain slope. We will finally demonstrate the predictive value of the gain-slope measurement by distortion measurement in a system evaluation.     

Analysis of AC gain tilt in erbium-doped fiber amplifiers

We analyze theoretically and experimentally the dependence of AC gain tilt variations on gain variations in homogeneously broadened erbium-doped fiber amplifiers. Theoretically, we find that this dependence can be evaluated from the fiber's absorption spectrum. This is experimentally verified for gain variations induced by a pump-power reduction. For AM CATV, AC gain tilt variations limit how much the gain can vary before composite second order distortions become too large. From this point of view, for an erbium-doped alumino-germanosilicate fiber, an operating wavelength between 1542-1555 nm is more suitable than other ones in the long wavelength range of the EDFA, normally considered for AM CATV.     

Second-order distortion generated by amplification ofintensity-modulated signals with chirping in erbium-doped fiber

A rate equation analysis for second-order distortion generated in an erbium-doped fiber amplifier is reported. It is shown that the relevant mechanism of distortion is the interaction between the frequency chirping of input light and the local gain tilt in the erbium-doped fiber. It is found that the output light distortion is highly affected by the phase difference between the input light distortion and the chirping-induced distortion. This indicates that second-order distortion is not always degraded by erbium-doped fiber amplifiers     

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     

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.     

双频激光泵浦掺饵光纤放大器的增益特性

从理论上研究了双频激光混合泵浦掺饵光纤放大器（ＨＥＤＦＡ）的增益特性。着重分析了ＨＥＤＦＡ的阈值和饱和行为，证明它比一般单频激光场泵浦的掺饵光纤放大器具有更高饱和增益和更低的单光阈值等优点。实验结果支持了上述结论。     

光纤长度优化的高功率铒镱共掺光纤放大器

为了研究不同增益光纤长度下1555nm高功率光纤放大器的输出功率,采用两级混合结构的方法,用掺铒光纤放大器和双包层铒镱共掺光纤放大器分别作为1级预放大器和2级主放大器。掺铒光纤放大器对信号光进行预放大,并提高放大器的信噪比;双包层铒镱共掺光纤放大器为主放大器,其双包层结构可以把更多的多模抽运光耦合进系统。对铒镱共掺光纤的最佳长度做了理论分析和实验验证,在信号光功率为10mW、掺铒光纤放大器的抽运功率为318.58mW、双包层铒镱共掺光纤放大器的抽运功率为11.71W、增益光纤长度为14m时,输出功率取得了2.11W的实验数据。在分析输出信号光谱时发现,L波段附近有放大自发辐射谱出现,这是选择的增益光纤过长导致的。结果表明,在光功率和信号光功率一定时,光纤放大器有一个最佳的光纤长度。这一结果对研究光纤放大器的高功率输出是有帮助的。