Optimization of pulse amplification in ytterbium-doped double-clad fiber amplifiers

In the present paper, high-power continuous-wave (CW)-pumped ytterbium-doped double-clad (YDDC) fiber amplifiers for the amplification of nanosecond pulses with tens of kilohertz repetition rates are numerically studied. Targeting pulse energy and average output power, the fiber amplifiers under forward, backward, and bidirectional pump are systematically analyzed and optimized in consideration of pump and signal wavelengths, pulse repetition rate, fiber structure and length, dopant concentration, and Raman effect, etc.     

Dynamic characteristics of double-clad fiber amplifiers for high-power pulse amplification

Based on numerical modeling, the dynamic characteristics of high-power ytterbium-doped double-clad fiber amplifiers during the amplification of nanosecond pulses with kilohertz repetition rates are comprehensively analyzed. The temporal pulse energy, power, upper-level population distribution, amplified spontaneous emission, stored energy, pulse waveform evolution, etc., are emphasized in this paper. Some practical issues in amplification processes, such as the impacts of reflected pulses from the external surfaces and the fluctuation of input pulse energy on the amplifier performance, are also discussed. The models and results are important for the design and development of high-power double-clad fiber amplifiers.     

Amplified spontaneous Raman scattering in fiber Raman amplifiers

Amplified spontaneous Raman scattered light power in forward and backward Raman amplifiers is theoretically and experimentally studied under the assumption of the constant Raman gain coefficient in the bandwidth of an optical filter. On the basis of the results, the power ratio of the amplified signal to amplified spontaneous scattering is discussed. As a result, it is clarified that the effect of amplified spontaneous Raman scattering on the signal to noise ratio for backward Raman scattering is larger than for forward Raman scattering.     

Amplified spontaneous Raman scattering and gain in fiber Raman amplifiers

The spectrum of amplified spontaneous Raman scattering and gain in a fiber Raman amplifer has been calculated analytically as a function of distance and pump power. The model used makes no assumptions on the magnitude of the gain and considers the pump nondepletion region. From the results, the on/off ratio has been calculated and is found to have a variation with length that depends on frequency detuning. An enhancement in on/off ratio is also found for small fiber lengths but at limited gain. The use of a narrow-bandpass optical fiber leads to quite different behavior of the on/off ratio than for the unfiltered case.     

玻璃光纤中的瞬态受激喇曼散射

用峰功率大于10~7瓦的0.53微米微微秒光脉冲序列激发一根20米长的锗磷硅玻璃光纤,观测到稳定的15级以上斯托克斯和8级以上反斯托克斯散射。喇曼频移为430厘米~(-1)。散射光频谱范围为0.45～1.00微米。受激喇曼散射的转换效率大于50％。     

Control of Stokes pulse duration in stimulated Raman scattering

The generation of long Stokes pulses by stimulated Raman scattering, which is made efficient by a short seed pulse at the Stokes frequency, is demonstrated both in experiments and in simulations for pump powers below the threshold power for unseeded scattering. It is observed that a second seed pulse with appropriate phase and intensity can extend, switch-off or briefly interrupt the Stokes pulse     

Novel delayed pulse techniques for stimulated Raman scattering

The outputs of two independent, incoherent lasers are spatially overlapped in a Raman-active medium. When the probes are separated in time, the stimulated Raman scattering (SRS) efficiency of the delayed pulse is increased from 14 to 27%. This occurs because of the presence of the decaying coherent excitation induced by the SRS of the first pulse. The influence of saturable losses is removed by prepulsing the medium before applying the pulse pair. The temporal evolution of the enhancement and phase-matching conditions are investigated. Surprisingly, the pulses interact only when they are separated in time, with the enhancement vanishing when they are temporally overlapped. The outputs of several independent lasers may be combined to generate significantly higher SRS efficiencies, without increasing the pump intensity incident on the medium. The technique provides a means for recovering efficient SRS in the transient (short-pulse) regime     

Suppression of stimulated Brillouin scattering in high-power single-frequency multicore fiber amplifiers

To describe the interplay between the first-order Stokes, pump, and signal in the multicore fiber amplifier, a set of differential equations is presented. The model takes into account the dependence of the stimulated Brillouin scattering (SBS) on the pump-induced temperature distribution along the fiber. The influence of pump schemes, pump powers, convective coefficient, density of the rare earth dopant (N) and fiber length on suppression of the SBS is studied. The backward pump scheme has advantage over forward pump scheme in suppression of SBS. For the SBS gain spectrum, reducing convective coefficient can decrease the maximum SBS gain and increase the frequency detuning corresponding to the maximum SBS gain. Increasing N can lead to higher slope efficiency and shorter optimal fiber length, which can further suppress SBS. The SBS gain (G) and maximum operating temperature are compared between the single core fiber amplifier and multicore fiber amplifier with the same optimal fiber length. Compared to the single core fiber amplifier, the multicore fiber amplifier has lower maximum operating temperatures, which provide more space for further increasing output power.     

Gain saturation in fiber Raman amplifiers due to stimulatedBrillouin scattering

The effects of stimulated Brillouin scattering (SBS) on Raman gain are explored. Measurements of gain in a fiber Raman amplifier show a saturation at low gain levels. Experimental data and a theoretical model are presented, demonstrating that this saturation is due to pump depletion by SBS. This effect also leads to Raman gain fluctuations arising from mode partitioning in a multimode pump laser. Two ways to avoid the deleterious effects of SBS on the performance of Raman amplifiers are suggested. One is to use a multimode semiconductor laser with a modal linewidth of the order of a few hundred megahertz, which leads to a strongly reduced Brillouin gain coefficient. The second approach is to use very short pump pulses in a backward configuration     

基于瑞利散射和布里渊散射的自调Q双包层掺镱光纤激光器研究

结合光纤干涉环原理和受激布里渊散射效应数值求解描述双包层掺镱光纤激光器的速率方程,得到构建光纤干涉环的耦合器耦合率及泵浦功率与输出脉冲重频的关系;进一步采用掺镱双包层光纤激光器中光子数守恒的半数值模型得到耦合器耦合率、泵浦功率与输出平均功率、脉冲能量的关系。实验测试了不同泵浦功率对输出平均功率和脉冲能量的影响,实验结果与仿真结果吻合。研究表明:提高泵浦功率只能提高脉冲重复频率和平均功率,并不能提高脉冲能量;选择合适耦合率的耦合器构建光纤干涉环才能获得较高脉冲能量;泵浦功率较高时会激发二阶斯托克斯光脉冲。     

Numerical modeling of Tm-doped double-clad fluoride fiber amplifiers

Theoretical modeling of Watt-level average power Tm-doped fluoride glass fiber amplifiers operating at 1.87 /spl mu/m is presented. To characterize and optimize these devices a computer model has been developed taking into account the full spectral information of the laser transition as well as all important ionic levels, their decay schemes and important cross-relaxation rates, being capable of modeling steady-state and especially transient characteristics of an optically pumped fiber as is needed for the amplification of short pulses. As a result, optimum fiber lengths and core sizes for maximum output power can be determined. It is shown that the influence of amplified spontaneous emission (ASE) onto amplifier efficiency and gain strongly depends on the fiber length for a given amplifier geometry, thus realistic modeling of the ASE background and its wavelength shift with respect to the fiber length is a key issue for the layout of amplifier fibers. The model is compared with experimental results obtained by amplification of 20-30-ns pulses at repetition rates in the range of 5-60 kHz. A good agreement between experiment and numerical results was reached without a substantial adjustment on the input parameters concerning amplification as well as continuous-wave ASE output power of an unseeded fiber.     

The effect of pulse walkoff on stimulated Raman scattering in fibers

We have experimentally investigated stimulated Raman scattering in single-mode fibers in the regime of large Raman Stokes pulse walkoff from the pump pulse by the effect of group-velocity dispersion. Measurements are made with 36 ps duration pulses at 532 nm produced from a frequency-doubled and harmonically mode-locked Nd:YAG laser. We find that for 20 percent conversion, the Raman output is produced about two walkoff lengths into the fiber as a pulse of approximately the same duration as the input pulse. We also find that the Raman pulse is produced with a strong frequency chirp.     

传输光纤对单频光纤放大器中受激布里渊散射的影响

在全光纤的单频光纤放大器系统中,泵浦耦合器带有的传输光纤将会对放大器的性能产生影响,为了研究其影响的规律,本文从含有SBS的速率方程出发,建立模型,讨论在含有传输光纤的情况下的泵浦方式,传输光纤长度,光纤结构对SBS的影响,所得结论为从实验上研究单频光纤放大器中SBS的变化规律提供理论依据。     

Limitations imposed by stimulated Raman scattering of 1550 nmhigh-energy pulse transmission

The limitations imposed by stimulated Raman scattering on 1550 nm high-energy pulse transmission in single-mode optical fiber are investigated. The generation of a Stokes pulse from an input pulse with peak power up to 60 W and duration on the order of 10 ns is experimentally studied and numerically simulated. This is the first demonstration of Stokes pulse generation in optical fiber from pump pulses within the 1550 nm telecommunications window. Experiment and theory show that there is an optimum input power for a given pulse duration and distance that maximizes the energy transmitted over long fibers     

Practical design of double-clad ytterbium-doped fiber amplifiers using Giles parameters

We present a simple and accurate method for measuring the Giles parameters of a double-clad ytterbium-doped fiber. The characterization is performed by cut-back on the doped fiber under constant pumping. Using nonlinear curve-fitting of the amplified spontaneous emission (ASE) power-density spectra, along with iterative solution of the photon balance model, we compute both the small-signal gain at complete population inversion and the small-signal absorption of the fiber. The method successfully predicts the extraction efficiency of an amplifier operating at 1064 nm. The ratio between the signal power and the out-of-band ASE power at the output of the amplifier is also accurately predicted by introducing spurious feedback from the fiber facets in the photon balance model. This work shows that a fiber facet reflectivity of a few thousandths of a percent (-40 to -50) dB can significantly enhance the out-of-band ASE power.     

单频脉冲光纤放大器中的SBS效应实验研究

为了研究受激布里渊散射(SBS)效应对单频脉冲光纤激光放大器中脉冲波形的影响,搭建了单频脉冲光纤激光放大器系统。实验中观测到脉冲波形的畸变,继续提高功率,畸变处出现峰值功率极高的尖峰,分析认为,光纤中的二次受激布里渊散射导致了尖峰脉冲出现。为验证这一结论,构造包含二阶Stokes波的耦合方程组,进行了模拟仿真。结果表明,该峰值功率极高的尖峰正是SBS效应的二阶Stokes波。     

用光纤获得十级受激喇曼散射

本文报导了用多模光纤获得受激喇曼散射的实验研究。用480米长、多模、低损耗的梯度型石英光纤,用重复率为5次/秒和10次/秒、脉宽7毫微秒、功率0.7兆瓦的 YAG:Nd 倍频532毫微米的激光泵浦,获得了从546～702毫微米的十级斯托克斯受激散射。     

背向受激喇曼散射(BSRS)

采用染料Q开关YAP倍频输出的光脉冲(波长0.5398μm、脉宽～30ns),聚焦注入CS2液体盒.在背向获得了四级斯托克斯散射线和两级反斯托克斯散射线,其脉宽压缩至约5ns.     

高功率光纤激光器受激拉曼散射致模式不稳定研究

通过稳态速率方程和热传导方程建立了高功率光纤放大器热光耦合的有限元模型,利用该模型对受激拉曼散射致模式不稳定效应进行了数值研究。对增益光纤为25/400μm的正向泵浦激光放大器,受激拉曼散射导致激光器信号光高阶模比例上升,当高阶模的比例达到5左右时,模场发生畸变,信号光出现了百毫秒量级的模式耦合,这与实验中观察到光强波动现象一致。本文仿真模型为研究高功率光纤放大器中受激拉曼散射和模式不稳定的关系提供了分析手段。     

喇曼光纤放大器的几项关键技术

喇曼光纤放大器具有优越的宽带性能 ,是现代宽带通信的较理想放大器。它的实用化得益于一些关键性技术的发展。对喇曼光纤放大器的最新发展进行了研究 ,对其中的关键性技术进行了分析、对比 ,以期有利于今后的实用化研制工作