Analysis of polarization-dependent gain in fiber amplifiers

Polarization-dependent gain (PDG) in fiber amplifiers is studied theoretically. A mathematical model is developed by treating the dopant ions in glass host as partially anisotropic oscillators, and both the signal and the pump effects are taken into account. It is illustrated that PDG includes two components. One arises from polarization-selective excitation of the dopant ions by the pump; the other originates from polarization-selective deexcitation of the lasing ions by the saturating signal, known as polarization hole burning (PHB). PDG depends strongly on the operating state of the amplifiers such as the level of gain compression, the anisotropy of the dopant ions, and the relative polarization orientations of the signal and the pump. In a small-signal regime, PDG is induced only by the pump while in large-signal condition it is the sum of the two components. The good agreement between the theoretical calculations and the reported results confirms the validity of the model     

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。     

双向多泵浦光纤拉曼放大器偏振相关增益研究

基于光纤拉曼放大器(FRA)的非线性耦合方程,采用数值计算的方法首次系统分析了双向多泵浦FRA系统中的偏振相关增益(PDG)特性。根据4个泵浦光源配置的不同,FRA有14种结构。首先整体分析了这14种不同FRA结构中PDG的特点;然后对其中一种结构FRA系统中PDG的特性进行了更具体的分析,包括泵浦-信号、泵浦-泵浦以及信号-信号拉曼相互作用所产生的PDG。研究结果表明,当泵浦总数目一定时,反向泵浦数目越多,其PDG均值越小;当泵浦总数目和反向泵浦数目都一定时,PDG的大小与反向泵浦光波长关系不大;双向多泵浦FRA系统中的PDG主要由泵浦-信号拉曼相互作用所产生的PDG决定。以上结果对有效降低双向多泵浦FRA系统中的PDG有重要的参考价值。     

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。     

Special erbium fiber amplifiers for short pulse switching, lasers,and propagation

We show that specially tailored erbium-doped fiber amplifiers (EDFA's) can be useful for short-pulse soliton lasers, switching, and propagation. EDFA properties include group-velocity dispersion, birefringence, doping level, and doping distribution; and changing each of these properties enables novel applications. Polarization-maintaining EDFA's can Re used in erbium-doped fiber lasers to avoid intensity-dependent or temperature-dependent state of polarization. We have demonstrated a passively mode-locked erbium-doped fiber laser that generates nearly transform-limited 320 fs pulses with 40 pJ energy using a polarization-maintaining EDFA near the zero dispersion wavelength and a bulk InGaAsP saturable absorber. Moderately birefringent EDFA's can be used to control the walkoff and interaction between orthogonally polarized solitons in all-optical switches. For example, through numerical simulations, we design an all-optical cascadable logic gate with a fanout of 2.7 and energy contrast of 5.5 based on interactions in a soliton period long EDFA. Furthermore, distributed EDFA's can be used for long-distance soliton propagation to shepherd the pulse using bandwidth-limited gain. For picosecond soliton pulses, we show that soliton self-frequency shift and Gordon-Hans effects limit propagation to below 100 km even for low-dispersion fibers. Bandwidth-limited amplification in EDFA may counteract frequency shift due to the soliton self-frequency shift, while frequency filters may suppress the Gordon-Hans effect. Also, because the gain spectral profile for the three-level EDFA changes with pump intensity, complications arise from pump attenuation, and the corresponding changes in the gain spectral profile along the length of the fiber     

L波段EDFA的优化设计和实验验证

基于Giles模型,对L波段掺Er光纤放大器(EDFA)的特性进行了数值模拟,分析了采用高掺杂Er纤放大器输出性能的改善。根据数值分析的结果进行了优化设计,使用9m长的高掺杂Er光纤进行了实验研究。实验结果表明．在泵浦功率为100mw时,小信号增益在10dB以上,噪声指数小于6dB。     

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.     

Pump wavelength-dependent spectral-hole burning in EDFAs

Gain-difference spectra are used to show that 980-nm band pump wavelength changes can significantly effect spectral-hole burning (SHB) in erbium-doped fiber amplifiers (EDFAs). Gain measurements made using a number of different signal and pump wavelengths help to characterize the underlying inhomogeneous spectral properties of EDFAs and a framework for modeling these effects is presented. Despite the difficulty of obtaining the needed subpopulation specific cross section spectra, this model can be used to explain seemingly anomalous changes in the gain spectra of EDFA as a result of changes in signal or pump wavelength     

Penalties in long-haul optical amplifier systems due topolarization dependent loss and gain

The impact of polarization dependent gain (PDG) and polarization dependent loss (PDL) in long-haul optical amplifier systems is studied by means of a power analysis accounting for PDG, PDL and polarization mode dispersion (PMD) in the saturation regime of the erbium-doped fiber amplifiers (EDFAs). System penalties are characterized in terms of mean values and standard deviations. Mean penalties on the bit-error-ratio (BER) are directly related to PDG. BER fluctuations are found to be greatly enhanced when PDG and PDL are combined. The impact of PMD is dependent on the values for PDG and PDL     

Gamma and proton radiation effects in erbium-doped fiberamplifiers: active and passive measurements

Commercially available Er-doped fibers were irradiated with 5.6 and 28 MeV protons and ⁶⁰Co gamma rays, up to levels of 50 krad. White-light transmission spectra under passive conditions (no pump or signal) were measured at several radiation levels for the six types of fibers that were tested. The spectra were used to evaluate the relative radiation sensitivity of the fibers and compare gamma versus proton-induced damage for two fiber types. The amount of radiation damage for the fibers was observed to scale inversely with the Ge concentration. Samples from three of the fiber types were configured as optical amplifiers using 980-nm and 1550-nm pump and input signals. In situ measurements of the gain, noise figure, and amplified spontaneous emission (ASE) were made as a function of pump power at several levels of radiation. A computer code, based on a conventional Er-doped fiber amplifier (EDFA) model, was written to simulate performance, using input data provided by the fiber vendor and anchored to measurements made prior to radiation. A comparison between the simulations and experimental data shows that, in certain fibers where the damage is significant, the radiation-induced loss determined from amplifier measurements can be substantially less than that determined from passive transmission spectra     

Recirculating loop for experimental evaluation of EDFA saturatedregime effects on optical communication systems

We demonstrate an optical-fiber recirculating loop for experimental simulation of long-haul optical communication systems using cascaded erbium-doped fiber amplifiers (EDFA's) operating in the gain saturation regime. The loop contains sections of dispersion shifted fibers (DSF's), standard fiber, and a set of in-line devices, such as tuning filters, optical amplifiers, polarization controllers, and a variable attenuator. The main results presented here are related to the observation of the effects due to the slow dynamics of the EDFA. We also discuss the validity of using an optical attenuator to simulate an extra length of fiber     

Double Clad Er-doped Fiber Amplifier

Presented is a theoretical study of double-clad Er-doped fiber power amplifier（EDFA）. Two kinds of double clad fibers（DCF） with rectangular and ＂flower＂ inner clad shapes are studied, and these fibers have different coupling constants and propagation losses. We calculate the effective pump power absorption ratio along the fiber with different coupling constants from the first cladding to the doped core and with different propagation losses for the power in the inner cladding. Then the gains of the double clad Er-doped fiber amplifiers versus fiber lengths are calculated using the EDFA model based on propagation and rate equations of a homogeneous, two-level medium.     

1477 nm LD泵浦掺铒光纤放大器的研究

报道了采用１４７７ｎｍ激光二极管（ＬＤ）泵浦的掺铒光纤放大器的实验结果。研究了放大器的增益和时域特性。对１５２０ｎｍ的信号光，获得了２３ｄＢ的增益，泵浦效率为２．２８ｄＢ／ｍＷ。低频脉冲信号经过放大器后未发生波形畸变。     

反向抽运光纤拉曼放大器偏振相关增益的数值计算

基于反向抽运方式的光纤拉曼放大器(FRA)的非线性耦合方程,采用数值计算的方法首次系统研究了单抽运多信号传输的FRA系统中各因素——光纤偏振模色散(PMD)、抽运光输入功率、抽运光波长、信号光输入功率、光纤长度及光纤损耗系数等对由抽运-信号拉曼相互作用所产生的偏振相关增益(PDG)的影响;比较了单抽运多信号FRA系统与单抽运单信号FRA系统中由抽运-信号拉曼相互作用所产生的PDG的大小;总结了各参数对降低由抽运-信号拉曼相互作用所产生的PDG的效果。研究结果表明,单抽运多信号FRA系统中的PDG主要由抽运-信号拉曼相互作用产生,并且光纤的PMD值和抽运光功率对PDG的影响较明显。     

Efficient Pump Depolarizer Analysis for Distributed Raman Amplifier With Low Polarization Dependence of Gain

This paper discusses a method of reducing the polarization dependence of gain (PDG) of a distributed Raman amplifier. Reducing its PDG is important for a Raman amplifier because it is higher than that of an erbium-doped fiber amplifier and can degrade transmission performance. Raman PDG is determined primarily by two factors, namely 1) polarization-mode dispersion (PMD) of the transmission fiber and 2) degree of polarization (DOP) of the pump source. The authors propose a simple analytical model to show the required pump light DOP for a given transmission fiber's PMD and the allowable PDG. For instance, a low pump DOP of 5% produces a low PDG of 0.2 dB under typical fiber PMD conditions, in which the analytical model agrees well with experiment. Subsequently, to achieve the required DOP, the pump source configuration is investigated in detail. The authors used one length of polarization-maintaining fiber (PMF) as an efficient pump depolarizer and evaluated its performance for various pump light spectra. It has been shown that the DOP following the depolarizer is determined simply by Fourier transformation of the pump light spectrum. The analysis in this paper has led to the important result that a Fabry-Perot laser diode pump with a short piece of PMF is effective in achieving a low pump DOP due to its multimode spectrum when the length of the PMF is properly adjusted for the longitudinal-mode spacing frequency. It has been verified that a Raman amplifier's PDG can be reduced by the proposed efficient depolarizer sufficiently for a PDG-reduced Raman amplifier repeater to be applicable to long-haul transmission systems     

Erbium-doped optical fiber amplifiers pumped in the 660- and 820-nmbands

State-of-the-art erbium (Er)-doped optical fiber amplifiers (EDFA's) pumped in the 660- and 820-nm bands are described. We have demonstrated highly efficient EDFA's incorporating optimized 664- and 827-nm pump wavelengths and an Er-doped high numerical aperture (NA) fiber with thermally diffused expanded core (TEC) fiber ends. Gain coefficients of 3.8 and 1.3 dB/mW at respective wavelengths of 664 and 827 nm were achieved at a signal wavelength of 1535 nm. Noise figures of 3.1 and 4.1 dB at respective pump wavelengths of 670 and 827 nm were obtained at a signal wavelength of 1535 nm. A highly efficient Er-doped fiber amplifier module, in which an AlGaInP visible laser diode (LD) was used as the pump source, was successfully developed as a practical application of this technology. A maximum overall gain coefficient of 3.0 dB/mW was achieved at a signal wavelength of 1535 nm. The EDFA module realized a maximum overall signal gain of 33 dB at 1535 nm with a saturated output power of -1 dBm. A maximum saturated output power of 3.9 dBm was obtained at a signal wavelength of 1552 nm. The present EDFA design using a low-cost laser diode for optical disk memory use and a high NA Er-doped fiber has great potential for providing inexpensive, high-performance EDFA's     

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     

Model for polarization-dependent gain due to pump depletion in a WDM system with forward-pumped Raman amplification

We study polarization-dependent gain (PDG) due to signal-induced pump depletion (SIPD) in a wavelength-division-multiplexing (WDM) system with forward-pumped Raman amplification. It is found that SIPD can polarize the pump significantly in fiber with very low polarization-mode dispersion (PMD). To quantify the impact of fiber PMD on SIPD-induced PDG for a practical WDM system with many signal channels and multiple Raman pumps, an approximate vector model has been developed. The developed model allows us to directly calculate PDG from both SIPD and signal-signal Raman interaction (SSRI) with greatly reduced computation time. Based on the developed model, detailed numerical investigations for two typical C-band WDM systems are presented. It is shown that significant PDG can be introduced by SIPD when the fiber PMD coefficient is lower than 0.01 ps/km/sup 1/2/ even if the pumps are fully depolarized. It is also shown that PDG due to SIPD and PDG due to SSRI are in phase at shorter wavelength channels but out of phase at longer wavelength channels.     

Transmission line with distributed erbium-doped fiber amplifier

We show the advantages of distributed erbium-doped fiber amplifiers (EDFAs) over lumped EDFAs. Using the distributed EDFA for 50%-100% of the transmission line lessens the signal degradation compared to the conventional transmission line. We also show the feasibility of the gain-flattened d-EDFA transmission line. An experiment shows that the gain peak is shifted and flattened at low pump powers     

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